Piston pumps. Piston liquid pump
The liquid piston pump is one of the oldest devices, the purpose of which is to pump liquid media. Piston pumps operate on the basis of the simplest principle of displacement of liquids, which is carried out mechanically. Compared with the first models of such devices, modern piston-type liquid pumps differ significantly more complex design, they are more reliable and efficient to use. So, piston pumps produced by modern manufacturers have not only an ergonomic and durable housing, but also a developed element base, and also provide more opportunities for installation in pipeline systems. Due to this versatility, piston-type liquid pumps are actively used in pipeline systems for both industrial and domestic purposes.
Design features
The main element of the liquid piston pump is hollow metal cylinder, in which all the working processes carried out with the pumped liquid take place. The physical effect on the liquid is carried out by a plunger-type piston. Thanks to this element, this liquid pump got its name.
The principle of operation of a piston pump is based on the reciprocating movement of its working body, acting as. At the same time, in the design of such a machine, unlike classical hydraulic devices, there is a valve distribution mechanism, as well as a number of additional structural elements(in particular, the crank and connecting rod, which form the basis of the power part of the liquid piston type pump).
Principle of operation
From most of those who select technical devices for equipping pipeline systems, experts hear: "Explain the operation of a piston pump with an air chamber." It should be said right away that the principle by which the liquid piston pump, invented several centuries ago, operates is quite simple. It consists in the following: making a translational movement, the piston creates a rarefaction of air in the working chamber, due to which liquid is sucked into the chamber from the supply pipeline. With the reverse movement of the piston of such a pump, which, according to some historical data, was invented by an ancient Greek mechanic, the liquid from the working chamber is pushed into the discharge line. Piston pumps, as mentioned above, are equipped with a valve mechanism, the main task of which is to prevent the pumped liquid from falling back into the suction channel at the moment when it is pushed into the discharge line.
The principle on which piston pumps work is explained by the fact that the flow created by such devices moves through the pipeline with different speed, jumps. To avoid this negative phenomenon, pumps are used that are equipped with several pistons at once, operating in a certain sequence. Another advantage of using multi-piston fluid pumps is that such devices are able to pump fluid even when their working chamber is not filled with it. This quality of the multi-piston plunger pump, which is called "dry suction", is relevant in many areas where such devices are used.
Double acting pumps
The main reason why the double-acting piston pump was developed and began to be actively used is the desire of manufacturers to reduce the level of pulsation of the fluid flow injected into the pipeline system. In order to understand the advantages of using a double-acting pumping device, it is enough to understand how a positive displacement liquid pump works. of this type.
A feature of the device of the double-acting liquid piston pump is that the rod and piston cavities of this machine are equipped with individual valve systems. Such a design of a double-acting piston pump, the uniqueness of which can be seen even from the photo, allows not only to eliminate flow pulsations in the pipeline system, but also to significantly increase the efficiency of the machine itself. Meanwhile, single-acting piston pumps, when compared with double-acting models, due to their simple design, are more reliable and durable.
There is another structural diagram piston pump, using which it is possible to achieve the elimination of pulsation processes in pipeline systems. Pumping equipment made according to this scheme involves the use of a special hydraulic accumulator. The main purpose of such hydraulic accumulators used to equip pumping stations is to accumulate the energy of the fluid flow at the moments of peak pressure in the pipeline and release it when such pressure for normal operation system is not enough.
However, no matter what types of piston pumps are used and no matter what additional technical devices No matter how pumping stations were equipped, it is not always possible to eliminate pulsation processes in pipelines. In such situations, it is often used optional equipment providing efficient removal excess fluid beyond the borders pumping station.
Applications
The scope of liquid piston pumps is quite wide, which is explained by their high versatility. Meanwhile, the design of such machines does not allow their use in cases where it is necessary to pump significant volumes of water or other liquid. One of the main advantages of these hydraulic machines is that their pistons, displacing liquid through the discharge line, simultaneously suck in a new portion of it through the supply channel, which is very important in a dry cylinder. This quality predetermines the appointment of piston liquid pumps as the most effective devices used in the chemical industry.
The areas of application of piston-type liquid pumps are also expanding due to the fact that such equipment can be successfully used to work with chemically aggressive media, some types of fuel and explosive mixtures. Pumps of this type are also actively used for domestic purposes; they can be used to create pipeline systems for autonomous water supply to private buildings and for irrigation. Meanwhile, having decided to use such a device, do not forget that it is not intended for pumping large volumes of liquid.
Another area in which piston-type liquid pumps are actively used is food industry. This is due to the fact that such devices are distinguished by a very delicate attitude to the liquid pumped through them.
Advantages and disadvantages
If we talk about the advantages that piston-type pumps have for pumping liquid media, then the most significant include:
- simplicity of design, which is demonstrated even by pictures and a schematic representation of such devices;
- high reliability, which is determined not only by the use of high-strength materials for the production of such machines, but also by the principle of operation of a piston pump;
- the ability to work with media, the use of which imposes special requirements on the conditions for starting pumping equipment.
The main disadvantage of the pumping equipment under consideration, mentioned above, is its low productivity. Of course expand technical capabilities such devices are possible, but why do this if this task is solved at a lower financial cost by means of another type of pumping equipment.
When choosing piston-type liquid pumps, first determine what such equipment will be used for. If you do not intend to pump too large volumes of liquid, then affordable and reliable piston-type liquid pumps will be the best fit for your needs.
Piston pumps are among the volumetric pumps in which the movement of fluid is carried out by displacing it from the stationary working chambers with displacers. working chamber volumetric pump called a limited space, alternately communicating with the inlet and outlet of the pump. displacer called the working body of the pump, which performs the displacement of fluid from the working chambers (plunger, piston, diaphragm).
Piston pumps are classified according to the following indicators: 1) according to the type of displacers: plunger, piston and diaphragm; 2) by the nature of the movement of the leading link: reciprocating motion of the leading link; rotational movement of the leading link (crank and lobe pumps); 3) by the number of injection and suction cycles in one double stroke: single-acting; double action. 4) by the number of pistons: single-piston; two-piston; multi-piston.
Rice. 7.3. Piston pump simple action
Single acting pump . A diagram of a single-acting pump is shown in fig. 7.3. Piston 2 connected to the crank mechanism through the rod 3 , as a result of which it reciprocates in the cylinder 1 . The piston, when moving to the right, creates a vacuum in the working chamber, as a result of which the suction valve 6 rises and the liquid from the supply tank 4 through the suction pipe 5 goes to working chamber 7 . When the piston moves back (to the left), the suction valve closes and the discharge valve 8 opens and liquid is forced into the discharge pipe 9 .
Since each revolution of the engine corresponds to two strokes of the piston, of which only one corresponds to the injection, the theoretical productivity in one second will be
where F- piston area, m²; l- piston stroke, m; n- number of engine revolutions, rpm.
To improve the performance of piston pumps, they are often made in twin, triple, etc. The pistons of such pumps are driven by a single offset crankshaft.
Actual pump performance Q less than theoretical, as leaks occur due to untimely closing of valves, leaks in valves and piston and rod seals, as well as incomplete filling of the working chamber.
Actual feed ratio Q to theoretical Q T is called the volumetric efficiency of the piston pump:
Volumetric efficiency - the main economic indicator that characterizes the operation of the pump.
Rice. 7.4. Double acting piston pump
Double acting pump . A more uniform and increased fluid supply, compared with a single-acting pump, can be achieved with a double-acting pump (Fig. 7.4), in which each piston stroke corresponds to both suction and discharge processes. These pumps are horizontal and vertical, the latter being the most compact. The theoretical capacity of a double acting pump will be
where f- stock area, m 2.
Rice. 7.5. Scheme of a piston pump with a differential piston
Differential pump . In a differential pump (Fig. 7.5), the piston 4 moves in a smoothly machined cylinder 5 . The piston is sealed with an oil seal 3 (option I) or a small gap (option II) with the cylinder wall. The pump has two valves: suction 7 and delivery 6 , as well as an auxiliary camera 1 . Suction occurs in one stroke of the piston, and injection occurs in both strokes. So, when the piston moves to the left from the auxiliary chamber to the discharge pipeline 2 a volume of fluid is displaced equal to (F - f)l; when the piston moves to the right, a volume of liquid is displaced from the main chamber equal to f l. Thus, for both strokes of the piston, a volume of liquid equal to
(F - f)l + fl = Fl
those. as much as is supplied by a single-acting pump. The only difference is that this amount of liquid is supplied for both strokes of the piston, therefore, the supply occurs more evenly.
Pumps according to GOST 17398, according to the principle of operation and design, are divided into two main groups - dynamic and volumetric (table).
Dynamic pumps include pumps in which the liquid in the chamber moves under the influence of force and has constant communication with the inlet and outlet nozzles. This force effect is carried out with the help of an impeller that imparts kinetic energy to the liquid, which is transformed into pressure energy. Dynamic pumps are vane, electromagnetic, friction and inertia.
Volumetric pumps include pumps in which the energy of the liquid is communicated according to the principle of mechanical periodic displacement of the liquid by the working body, which creates a certain pressure of the liquid in the process of movement. In positive displacement pumps, the fluid receives energy as a result of a periodic change in a closed volume, which alternately communicates with the inlet and outlet of the pump. Positive displacement pumps are piston, plunger, diaphragm, rotary and gear.
Vane pumps are called pumps in which energy is transferred using a rotating impeller (which serves as their working body), by dynamic interaction of the wheel blades with the fluid flowing around them. Vane pumps are centrifugal, axial and diagonal.
Centrifugal pumps are called vane pumps with the movement of fluid through the impeller from the center to the periphery, axial - vane pumps (GOST 9366) with the movement of fluid through the impeller in the direction of its axis. The impellers of axial pumps consist of several helical cavities in the form of propeller blades.
Friction and inertia pumps are a group of dynamic pumps in which the transfer of fluid energy is carried out by friction and inertia forces. These include vortex, screw, labyrinth, worm and jet pumps. Vane pumps are also classified by pressure, power and speed factor.
By pressure(m st. liquid) pumps are distinguished: Low-pressure up to 20 m, medium-pressure from 20 to 60, high-pressure over 60.
By power(kW) pumps can be micropumps up to 0.4, small up to 4, small up to 100 with a flow of 0.5 m 3 / s, medium up to 400, large over 400 with a flow above 0.5 m 3 / s, unique over 8000 when supplying more than 20 m 3 / s.
By speed factor
,
where n - speed, rpm; Q - feed, m 3 / s; H-head, m
In this formula, the pressure H for multistage pumps is understood as the pressure developed by one wheel (stage). If the pump has an impeller with a double-sided inlet, substitute a Q value equal to half their supply. The speed coefficient is the most complete hydraulic characteristic centrifugal pumps, allows you to classify pumps not according to one particular parameter (flow, head or speed), but according to their combination and provides a basis for comparing different types of pumps and choosing the pump that is most suitable for operation under given conditions. For various types of vane pumps, the values of ns rpm are given below:
Centrifugal are low-speed 50 ... 80, normal 80 ... 150, high-speed 350 ... .500. For diagonal pumps, the coefficient speed is in the range of 350 ... 500, and for axial 500 ... 1500.
The speed factor ns also determines the shape of the pump impeller. As an example, consider pump wheels of various speeds. The low-speed wheel is characterized by the fact that the outlet diameter is much larger than the inlet one and the wheel has a relatively small width. With an increase in speed, this difference decreases, the width increases, and then the wheel becomes diagonal and axial.
Classification of pumps by design and purpose.
When classifying vane pumps by design, the following features are taken into account: the location of the axis of rotation (vertical, horizontal), the location and execution of supports (cantilever, with external or internal supports, etc.), the number of wheels (one-, two- and multi-stage), execution of inlet and outlet (with semi-spiral or chamber inlet, with spatula outlet, etc.), availability of regulation, body design (with a longitudinal split, sectional, etc.), immersion under the level, type of seal (with soft stuffing box, mechanical seal, etc.), impeller design (open impeller, closed impeller, rotary vane, double inlet, etc.), self-priming capability, tightness, structural integration with the motor, heating systems or cooling, upstream auger, purpose (for installation in a well, capsule, etc.).
When classified by purpose, pumps are distinguished: general purpose(table) for pumping clean water with a low content of suspended particles; for pumping pulp or soil - dredgers, soil and mud; for supplying water from wells - electric submersible with an engine located under the water level, and deep, in which the engine is installed above the well, and the pump is located in the well under water (a sectional shaft goes from the pump to the engine, held in guide bearings installed in the crosses between sections of water pipes); for pumping gasoline, kerosene or oils, chemicals, etc.
Type K and KM pumps are single-stage cantilever pumps with fluid inlet to the impeller on one side. They have the following characteristics: head 8.8 ... 9.8 m, suction height up to 8 m and flow 4.5 ... 360 m / h.
Depending on the size, each pump has its own brand, which indicates the diameter of the inlet pipe, the speed factor and the type of pump. So, the number 8 for a cantilever pump brand 8K-18 means the diameter of the inlet pipe (mm), reduced by 25 times, the cantilever type of pump is indicated by the letter K, and the number 18 means the speed coefficient of the pump reduced by 10 times.
Pumps of the ND type are single-wheel horizontal pumps with a two-way supply of fluid to the impeller. There are three types of such pumps: NDn (low pressure), NDs (medium pressure) and NDv (high pressure). Each of the three varieties has several sizes. The diameter of the discharge pipe (mm), reduced (rounded) by 25 times, is indicated by a number in front of the letters in the brand of the pump. The suction height of such pumps does not exceed 7 m.
Pumps of the NDn type have a flow rate of 1350...5000 m3/h and a head of 10 to 32 m;
NDs type pumps - delivery 216...6500 m3/h and head 18...90 m,
pumps type NDv supply from 90 to 720 m3 / h and head 22 ... 104 m.
Pumps of the NMK, TsNS, TsNNM, TsK types are multistage horizontal pumps where liquid is supplied from two sides to the first impeller. These pumps have several varieties with the number of wheels from 2 to 11. They have a head of up to 2000 m and a flow of 3600 m3 / h.
The group of horizontal centrifugal pumps includes single-wheel pumps of type D with a flow of 380 ... 12,500 m3 / h and a head of 12 ... 137 m, four-stage pumps of type M with a flow of 700 ... 1200 m3 / h and a head of 240 ... 350 m three- and five-stage pumps of the MD type with a flow of 90...320 m/h and a head of 138...725 m four- and six-stage sectional pumps of the NGM type with a flow of 54...90 m/h and a head of 102.. .210 m
Let's consider vertical centrifugal and axial pumps for pumping water and clean liquids.
Pumps of the NDsV type - they are produced in two sizes 207 DV and 24 NDv. These are single-stage vertical medium-head pumps with double-sided liquid inlet to the impeller. The flow is 2700...6500 m3/h, the head is 40...79 m.
Type B pumps are the largest pumps, single-stage vertical with one-way fluid inlet to the impeller. They are produced with a supply of 3000 to 6500 m3 / h, a head of 18 ... 72 m of several sizes.
Axial pumps. Vane pumps, in which the liquid moves through the impeller parallel to its axis, are called axial.
Such pumps are designed to deliver large quantities of fluid at relatively low heads. In axial pumps, the flow of fluid leaving the channels of the impeller has a vortex structure with a swirl, and, getting into the fixed channels of the straightening device, it unwinds, gradually turning into an axial direction.
Advantages of axial pumps: simplicity and compact design. The compact design has crucial at high flows, and consequently, at large diameters of pipelines. Axial pumps can be mounted on a vertical, horizontal or inclined pipe.
In axial pumps, the liquid, moving forward, simultaneously receives rotational motion created by the impeller. To eliminate the rotational movement of the liquid, a guide vane is used, through which the liquid flows before exiting into the pressure pipeline.
Diagonal pumps. By design, diagonal pumps are similar to axial pumps, their main difference is the shape of the impeller. The liquid medium moves in the impeller at an angle to the pump axis (diagonally), which determines the name of these pumps.
The diagonal rotary-vane pump with an impeller with a diameter of 2 m (Fig.) is designed for a head of 30 m. The impeller blades can be rigidly mounted and can be swivel, i.e. their installation is adjustable.
Liquid ring pumps belong to the group of self-priming or vacuum pumps.
Their device is such that they can suck in both air and water. A big drawback of centrifugal pumps of conventional designs is their inability to self-suck liquid, since the air initially located in the suction pipe, due to its small mass, cannot be pumped out to create a sufficiently deep vacuum to ensure that the liquid rises until it fills the pump impeller. Liquid ring pumps can create significant vacuums in air environment, and, consequently, raise the liquid along the suction pipe to a sufficiently high height, i.e., they can themselves suck up the liquid without first filling the pump. This phenomenon is called self-priming.
Water ring pumps are used as independent units for pumping gases or liquids, but more often as auxiliary units for filling large centrifugal pumps, as well as for creating and maintaining vacuum in various containers and apparatuses.
The head of a vortex pump is 4...6 times greater than that of a centrifugal pump, with the same dimensions and speed. Vortex pumps are produced in single-stage and two-stage. In addition, peripheral pumps are self-priming, which allows them to be used as vacuum pumps when filling large centrifugal pumps. The vortex pumps have relatively low efficiency (25...55%). Combined pumps are produced, in which both vortex and centrifugal wheels are placed in one housing.
Comparison of the technical data of vortex and centrifugal-vortex pumps shows that with the same flow, vortex and centrifugal-vortex pumps operate at higher heads, but relatively low efficiency.
Airlifts ( emulsion water lifts). Airlifts are used in the sewerage system to lift household and fecal and waste industrial wastewater.
Typically, an airlift is a riser tube designed to lift a mixture of water and air. The pipe is lowered into the well, to which is supplied through another pipe compressed air. Both pipes are inserted into the well casing and lowered to the water level.
The principle of operation of the airlift is as follows. When immersed in water, the riser pipe fills with water. The air with water supplied to the pipe forms a water-air mixture, which has a lower density compared to water and, therefore, rises to a higher level. Thus, water is transported from the well to the water-air reservoir. Here the water is freed from the air and goes by gravity to the consumer.
In the case of temporary use of airlift installations (for example, in construction during dewatering or in surveys during test pumping), it is possible to do without water-lifting pipes. In this case, the air supplied through the riser pipe 4 is discharged directly into the casing pipe, where it mixes with water. The resulting water-air emulsion will flow directly through the casing.
The advantages of airlifts: the absence of rubbing and blocking parts in the well, the possibility of passing contaminated water and using deviated wells, the simplicity of the device, etc.
The main disadvantages: low efficiency of the airlift installation (10 ... 15%), the need for a second rise of water from the collection tank to the consumer using a centrifugal or other pump, and the need for a significant (at least 50% of the total height) immersion of the airlift nozzle under the dynamic water horizon ( DGV), formed during the operation of the airlift.
Question No. 36. Head of a dynamic pump.
The dynamic head of the pump is the increase in the kinetic energy of a unit mass of liquid in the pump.
This is the part of the total head related to the fluid velocity. The dynamic head Hd is determined by the following formula: Hd = v/2g where: V is the fluid velocity measured at the inlet (in m/s); g is the free fall acceleration (in m/s?). If the inlet and outlet pipes have different diameters, the dynamic head is the difference between the dynamic heads at the suction and at the outlet. If the inlet and outlet pipes have the same diameter, then there is no dynamic pressure.
Question number 37. Performance, power and efficiency of a dynamic pump.
Productivity (Q) is usually expressed in terms of cubic meters x per hour (m 3 / hour). Since liquids are absolutely incompressible, there is a direct relationship between the capacity, or flow, pipe size, and fluid velocity. This relationship has the form: Where ID is the inner diameter of the pipeline, inch V is the fluid velocity, m / s Q is the productivity, (m 3 / h)
Rice. 1. Suction head - shows the geometric heads in the pumping system, where the pump is above the suction reservoir (static head)
Power and efficiency The work performed by a pump is a function of the total head and weight of the fluid pumped over a given period of time. As a rule, the pump performance parameter (m 3 / h) and the density of the liquid are used in the formulas instead of weight. The pump power (bhp) is the actual shaft power supplied to the pump by the motor. Pump output or hydraulic power (whp) is the power supplied by the pump to a liquid medium. These two definitions are expressed by the following formulas. 
The power at the pump inlet (power consumption) is greater than the power at the pump outlet or hydraulic power due to the mechanical and hydraulic losses that occur in the pump. Therefore, pump efficiency (COP) is defined as the ratio of these two values. 
Pump speed and type Speed is design factor used to classify pump impellers according to their type and size. It is defined as the rotational speed of a geometrically similar impeller delivering 0.075 m 3 /s of liquid at a head of 1 m. (In US units, 1 gpm at 1 foot of head) However, this definition is used only in engineering design, and rapidity should be understood as a factor to calculate certain pump characteristics. To determine the speed factor, the following formula is used: 
Where N - Pump speed (in revolutions per minute) Q - Productivity (m 3 / min) at the point of maximum efficiency. H - Head at the point of maximum efficiency. The speed determines the geometry or class of the impeller, as shown in Fig. 3 
Rice. 3 Impeller shape and speed As the speed increases, the ratio between the impeller outer diameter D2 and the inlet diameter D1 decreases. This ratio is 1.0 for an axial flow impeller. Impellers with radial vanes (low Ns) generate head through centrifugal force. Pumps with higher Ns generate head partly with the same centrifugal force and partly with axial forces. The higher the coefficient of speed, the greater the proportion of axial forces in the creation of pressure. Axial flow or propeller pumps with a speed factor of 10,000 (in US units) and above generate head solely from axial forces. Radial flow impellers are typically used when high head and low flow rates are required, while axial flow impellers are used for high volume applications at low heads. Net suction head (NPSH), inlet pressure and cavitation The Hydraulic Institute defines NPSH as the difference between the absolute head of the fluid at the impeller inlet and the saturation vapor pressure. In other words, this is the excess of the internal energy of the liquid at the inlet to the impeller by its saturated vapor pressure. This ratio allows you to determine whether the liquid in the pump boils at the point of minimum pressure. The pressure that a liquid exerts on its surrounding surfaces depends on the temperature. This pressure is called vapor pressure and is a unique characteristic of any liquid that increases with temperature. When the saturation vapor pressure of a liquid reaches ambient pressure, the liquid begins to evaporate or boil. The temperature at which this evaporation occurs will decrease as the ambient pressure decreases. During evaporation, the liquid increases significantly in volume. One cubic meter of water at room temperature turns into 1700 cubic meters of steam (evaporation) at the same temperature. It can be seen from the above that if we want to pump liquid efficiently, we need to keep it in liquid state. Thus, NPSH is defined as the value of the effective suction head of the pump, at which there will be no evaporation of the pumped liquid at the point of lowest possible liquid pressure in the pump. Required NPSH (NPSHR) - Depends on pump design. As the fluid passes through the suction port of the pump and hits the impeller guide vanes, the velocity of the fluid increases and the pressure drops. There are also pressure losses due to turbulence and uneven fluid flow, as liquid hits the wheel. The centrifugal force of the impeller blades also increases the speed and reduces the fluid pressure. NPSHR is the required head pressure at the suction port of the pump to compensate for all pressure losses in the pump and keep the liquid above the saturated vapor pressure level and limit the head loss resulting from cavitation to 3%. A 3% head drop margin is a common NPSHR criterion adopted to facilitate calculation. Most pumps with low suction capacity can operate with low or minimal NPSHR without seriously affecting their service life. NPSHR depends on the speed and performance of the pumps. Typically, pump manufacturers provide information on the NPSHR. Allowable NPSH (NPSHA) - is a characteristic of the system in which the pump operates. This is the difference between atmospheric pressure, pump suction head and saturated vapor pressure. The figure shows 4 types of systems, for each the formulas for calculating the NPSHA system are given. It is also very important to take into account the density of the liquid and bring all quantities to the same unit of measurement. 
Rice. 4 Calculation of the liquid column above the suction pipe of the pump for typical suction conditions Pv - atmospheric pressure, in meters; Vp - Saturated vapor pressure of the liquid at the maximum operating temperature of the liquid; P - Pressure on the liquid surface in a closed container, in meters; Ls - Maximum suction height, in meters; Lн - Maximum height of backwater, in meters; Hf - Friction loss in the suction pipe at the required pump capacity, in meters. In a real system, NPSHA is determined using the pressure gauge on the suction side of the pump. The following formula applies: Where Gr is the pump suction pressure gauge reading, expressed in meters, plus (+) if the pressure is above atmospheric and minus (-) if below, corrected for the pump centerline; hv = Dynamic head in the suction pipe expressed in meters. Cavitation is a term used to describe the phenomenon that occurs in a pump when the NPSHA is insufficient. In this case, the liquid pressure is lower than the saturated vapor pressure, and the smallest liquid vapor bubbles move along the impeller blades; in the high pressure area, the bubbles quickly collapse. The collapse or "explosion" is so rapid that it may sound like a rumble to the ear, as if gravel had been poured into a pump. In pumps with high suction capacity, the bubble explosions are so strong that the impeller blades are destroyed in just a few minutes. This effect can be increased and under certain conditions (very high suction capacity) can lead to severe impeller erosion. The cavitation that has arisen in the pump is very easy to recognize by the characteristic noise. In addition to damage to the impeller, cavitation can lead to a decrease in pump performance due to the evaporation of liquid in the pump. During cavitation, the pump head may decrease and/or become unstable, and the power consumption of the pump may also become unstable. Vibrations and mechanical damage, such as damage to bearings, can also result from running a pump with high or very high cavitation suction capacity. To prevent the undesirable effect of cavitation on standard low suction pumps, it is necessary to ensure that the NPSHA of the system is higher than the NPSHR of the pump. High suction pumps require a margin for NPSHR. The Hydraulic Institute standard (ANSI/HI 9.6.1) suggests increasing NPSHR by a factor of 1.2 to 2.5 for high and very high suction pumps when operating within the allowable performance range.
Question number 38. The basic equation for the operation of centrifugal pumps.
The basic equation of a centrifugal pump for the first time in the general view was received in 1754 by L. Euler and bears her name.
Considering the movement of fluid inside the impeller, we make the following assumptions: the pump pumps an ideal fluid in the form of jets, i.e., there are no all types of energy losses in the pump. The number of identical pump blades is infinitely large (z = µ), their thickness is zero (d= 0), and the angular speed of the impeller is constant (w= const.).
To the impeller of a centrifugal pump with a speed Vo, the liquid is supplied axially, i.e., in the direction of the shaft axis. Then the direction of the liquid jets changes from axial to radial, perpendicular to the axis of the shaft, and the speed due to centrifugal force increases from the value V1 in the space between the impeller blades to the value V2 at the outlet of the impeller.
In the interblade space of the impeller during the movement of fluid, absolute and relative flow rates are distinguished. Relative speed flow - speed relative to the impeller, and absolute - relative to the pump housing.
Rice. Scheme of the movement of fluid in the impeller of a centrifugal pump
The absolute speed is equal to the geometric sum of the relative speed of the fluid and the circumferential speed of the impeller. The circumferential velocity of the liquid exiting between the vanes of the impeller coincides with the circumferential speed of the impeller at a given point.
Circumferential fluid velocity (m/s) at the impeller inlet
Circumferential velocity of fluid at the outlet of the impeller (m/s)
where n-impeller speed, rpm; D1 and D2 - internal and external diameters of the impeller, m, w- angular speed of rotation of the impeller rad/s
When the impeller moves, fluid particles move along the blades. Rotating together with the impeller, they acquire peripheral speed, and moving along the blades - relative.
The absolute velocity v of fluid movement is equal to the geometric sum of its components: relative velocity w and district u, i.e. v = w + and.
The relationship between the velocities of fluid particles is expressed by a parallelogram or triangles of velocities, which makes it possible to give an idea of the radial and circumferential components of the absolute velocity.
Radial component
district component
where a is the angle between the absolute and circumferential speeds (at the inlet of the impeller a1 and at the outlet a2).
The angle b between the relative and peripheral speeds characterizes the outline of the pump blades.
We investigate the change for 1 since the moment of momentum Mass of the liquid t = rQ, where r is the density of the liquid; Q- pump supply.
Using the theorem of mechanics about the change in the moments of momentum in relation to the movement of fluid in the channel of the impeller, we derive the basic equation of a centrifugal pump, which will allow us to determine the head (or pressure) developed by the pump. This theorem states: the change in time of the main moment of the momentum of a system of material points relative to some axis is equal to the sum of the moments of all forces acting on this system.
The moment of momentum of the liquid relative to the axis of the impeller in the inlet section
Moment of momentum at the outlet of the impeller
where r1 and r2 - the distance from the wheel axis to the input V1 and output V2 velocity vectors, respectively.
According to the definition of the moment of the system, we can write:
Since, according to Fig.
Groups external forces- gravity, pressure forces in the calculated sections (inlet-outlet) and from the side of the impeller and the friction force of the fluid on the streamlined surfaces of the impeller blades - act on the mass of fluid filling the interblade channels of the impeller.
The moment of gravity relative to the axis of rotation is always zero, since the shoulder of these forces is equal to zero. The moment of pressure forces in the design sections for the same reason is also equal to zero. If friction forces are neglected, then the moment of friction forces is zero. Then the moment of all external forces about the axis of rotation of the wheel is reduced to the moment Mk the dynamic impact of the impeller on the liquid flowing through it, i.e.
Work Mk times the relative velocity is equal to the product of the flow rate and the theoretical pressure PT, created by the pump, i.e., equal to the power transmitted to the liquid by the impeller. Hence,
This equation can be represented as
Dividing both parts into Q, we get
Considering that the pressure H = P/(pg) and substituting this value we get
If we neglect the forces of friction, then we can obtain dependencies called basic equations of a vane pump. These equations reflect the dependence of the theoretical pressure or head on the main parameters of the impeller. The portable speeds at the inlet to the axial pump and at the outlet of it | are the same, so the equation takes the form
In most pumps, the liquid enters the impeller almost radially and, consequently, the velocity V1 » 0. Taking into account the above
or 
The theoretical pressure and head developed by the pump is the greater, the greater the circumferential speed on the outer circumference of the impeller, i.e., the greater its diameter, speed and angle b2, i.e. the “steeper” the impeller blades are.
The actual pressure and head developed by the pump are less than the theoretical ones, since the actual operating conditions of the pump differ from the ideal ones adopted when deriving the equation. The pressure developed by the pump decreases mainly due to the fact that with a finite number of impeller blades, not all fluid particles are deflected uniformly, as a result of which the absolute speed decreases. In addition, part of the energy is spent on overcoming hydraulic resistance. The influence of a finite number of blades is taken into account by introducing a correction factor k(characterizing a decrease in the circumferential velocity component V2u), a decrease in pressure due to hydraulic losses - by introducing a hydraulic efficiency hr. With these corrections, the total pressure
and full force 
Coefficient value hr depends on the design of the pump, its dimensions and the quality of the internal surfaces of the flow part of the impeller. Usually value hr is 0.8...0.95. Meaning k with the number of blades from 6 to 10, a2 = 8...14 0 and V2u = 1.5...4 m/s, it ranges from 0.75 to 0.9.
When the impeller of a centrifugal pump rotates, the liquid located between the blades, due to the developed centrifugal force, is ejected through the spiral chamber into the pressure pipeline. The outgoing liquid releases the space it occupies in the channels on the inner circumference of the impeller, so a vacuum is formed at the impeller inlet, and excess pressure is formed at the periphery. Under the influence of the atmospheric pressure difference in the receiving tank and reduced pressure at the inlet to the impeller, the liquid enters the interblade channels of the impeller through the suction pipeline.
A centrifugal pump can only work if its internal cavity is filled with a pumped liquid not lower than the pump axis, so the pumping unit is equipped with a device for filling the pump.
Question number 39. Operating characteristic centrifugal pump H-0.
The characteristic of a centrifugal pump, or external and operating characteristics, is the graphical dependence of the main indicators of the pump, such as head, power and efficiency, on the flow, and the cavitation characteristic is the graph of the dependence of pressure, flow and efficiency on the excess suction head N.
All pump parameters are interconnected, and changing one of them inevitably entails changing others. If, at a constant rotor speed, the pump flow is increased, then the pressure created by it will decrease. When operating conditions change, the pump efficiency also changes: for certain specific values of flow and head, the pump efficiency will be maximum, and for all other modes of its operation, the pump operates with the worst efficiency. Note that the efficiency is strongly affected by the speed coefficient .
The characteristics of centrifugal pumps clearly show the efficiency of their operation in various modes and allow you to accurately select the most economical pump for given working conditions.
The performance of the pump differs from the theoretical one due to hydraulic losses and variable hydraulic efficiency.
The head loss in the impeller consists of friction losses in the impeller channels, impact losses due to deviations of the speed at the impeller inlet from the tangential direction in the blade, etc.
As can be seen from fig. b, all dependencies are built on the same graph on the appropriate scales, and the flow Q the pump is plotted along the abscissa, and the pressure H, vacuum height, power and efficiency are plotted along the ordinate.
To determine the required pump parameters from the performance curve, proceed as follows. According to a given pump flow Qo found on the curve Q -H point C, from which a horizontal line is drawn to the intersection with the scale H, where the head corresponding to the given flow rate is found. To determine the power and efficiency of the pump, horizontal straight lines are drawn from points BUT and AT and on scales N and h and thus find the corresponding values no and ho.
Pump performance has several distinct points and areas. The starting point of the characteristic corresponds to zero flow of the pump Q=0, which is observed when the pump is operating with a closed valve on the discharge pipeline. As can be seen from fig. a, the centrifugal pump in this case develops some pressure and consumes power, which is spent on mechanical losses and heating of water in the pump.
The operating mode of the pump, corresponding to the maximum efficiency, is called optimal. The main goal of selecting pumps is to ensure their operation in the optimal mode, given that the efficiency curve has a flat character in the zone of the optimal point, however, in practice, the working part of the pump characteristic is used (the zone corresponding to approximately 0.9hmax, within which the selection and operation of pumps is allowed ).
Cavitation characteristics necessary to assess the cavitation properties of pumps and right choice suction height. To build the cavitation characteristics of the pump, it is subjected to cavitation tests on special stands.
Within certain limits of the change in the excess pressure at the suction Hin.ex. values Q, H and h remain unchanged. At some values of Hvs.izb, noise and crackling appear during pump operation, which characterize the onset of local cavitation. With a further decrease in the Nvs.ex. value Q, N and h begin to gradually decrease, the cavitation noise increases and eventually the pump fails. Accurately determine the moment of the beginning of the cavitation effect on Q, N and h is not possible, therefore it is conditionally taken as the minimum excess suction height Hvs.ex min, then its value at which the pump flow drops by 1% of its initial value.
Very often, the Hvac curve is also applied to the performance of pumps - Q, which gives the values of the permissible vacuum suction height depending on the pump flow.
Question number 40. Velocity triangles. Recalculation and modeling of parameters.
Fig.2.1. Fluid movement in the impellerIn the interblade channels of the impeller, fluid particles participate in a complex movement:
portable - together with the impeller;
relative - in relation to the walls of the interscapular channels;
absolute - resulting in relation to the above movements.
The absolute velocity vector of a particle can be represented by the sum of the portable (circumferential) velocity and the relative velocity.
The relative velocity of the particle at any point of the blade profile is tangent to it. The absolute speed is divided into the circumferential V iu and meridian (consumable) V i m components, which are determined by the following formulas
where i= 1.2. Index "1" - corresponds to the parameters of the fluid at the inlet to the impeller, and "2" - at the outlet of it.
2.1. Basic equation of turbomachines
(Turbine Euler equation)
The basic equation of turbomachines connects the geometric and kinematic characteristics of the impeller with the head it develops. When it is derived, it is assumed that the trajectory of fluid particles in the interblade channels repeats the outline of the blade profile, i.e. for the impeller, an assumption is made about the infinity of the number of infinitely thin blades located on it (the sign of this will be the symbol as an index).
The conclusion is based on the equation of moments of momentum in the steady motion of a fluid in uniformly rotating channels, according to which the change per unit time of the moment of momentum of the fluid L located in the channel is equal to the moment of external forces acting on it:
The external forces acting on the liquid in the channel include the forces with which the walls of the channel act on the liquid, pressure forces, friction forces, and gravity forces. The analysis shows that the resultant forces of pressure on the inner and outer generatrix of the wheel pass through the axis of rotation and do not create a moment. Due to the symmetry of the impeller, the forces of gravity are balanced, and the friction forces acting on the peripheral surfaces of rotation are small. Based on the above, it is assumed that the moment is created only by forces arising from the interaction of the walls of the working channels with the liquid in them.
This moment of external forces is related to the hydraulic power of the pump N r and angular velocity of rotation the following ratio:
Substituting the found values into the law of change of the angular momentum in time, we obtain the Euler equation:
. (2.1)
The Euler equation relates the theoretical head of the pump to the fluid flow rates, which depend on the pump flow, the angular velocity of the impeller, as well as its geometric characteristics.
The flow at the inlet to the impeller is created by the device preceding it (supply). Therefore, the moment of speed (twist) is determined by the design of the supply. The supply devices of many pumps do not swirl the flow and the moment of velocity at the inlet is zero. In this case, the theoretical head is determined by the following equation:
where is the circumferential speed at the periphery of the wheel.
Given that
where n- frequency of rotation, rpm;
and the projection of the absolute speed at the output of the wheel on the circumferential speed, as follows from the speed triangle (see Fig. 2.1), is determined by the expression
the equation for the theoretical head will take the form:
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where b 2 - width of the impeller channel at the outlet.
Theoretical head with a finite number of blades H m is less than , which is taken into account by introducing a correction factor into the Euler equation
From the consideration of triangles of velocities (Fig. 2.1), based on the cosine theorem, we can write
Taking into account the given dependencies, the Euler equation can be transformed to the form:
where is the pressure created by the action centrifugal forces in the stream;
The pressure created by changing the relative speed in the channel of the impeller;
The head created by changing the absolute speed in the channel of the impeller.
The value - is called the static part of the pressure, and - the dynamic part of the pressure.
In order to reduce losses in the pump, it is desirable that the static part of the head predominates, and at the expense of the centrifugal component.
Question number 41. The operation of a centrifugal pump on a given pipeline.
The totality of the pump, receiving and pressure tanks, pipelines connecting the above elements, control and shut-off valves, as well as control and measuring equipment, constitutes a pumping unit. To move liquid through pipelines from a receiving tank to a pressure tank, it is necessary to expend energy on:
lifting liquid to a height H g, equal to the level difference in the tanks (this value is called the geometric head pumping unit);
overcoming the pressure difference in them p n and p n;
overcoming the total hydraulic losses h in the suction and pressure pipelines.
Thus, the energy required to move a unit weight of liquid from a receiving tank to a pressure tank through pipelines, or required installation pressure is defined by the expression:
The characteristic of a pumping unit is the dependence of the required pressure on the flow rate of the liquid. Geometric head H g, pressure p n and p does not depend on consumption. Hydraulic losses are a function of flow and depend on the driving mode. In the laminar mode, the characteristic of the pipeline is depicted by a straight line, with turbulent movement in rough pipes, pressure losses, and therefore the characteristic has the form of a parabola.
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Question number 42. Parallel and series operation of centrifugal pumps.
Parallel operation of pumps is called simultaneous supply of pumped liquid by several pumps to a common pressure manifold. The need for parallel operation of several identical or different pumps arises in cases where it is impossible to provide the required water flow by supplying one pump. In addition, since water consumption in the city is uneven by the hours of the day and by the seasons of the year, the supply of the pumping station can be controlled by the number of simultaneously operating pumps.
When designing the joint operation of centrifugal pumps, one must know their characteristics well; pumps should be selected taking into account the characteristics of the pipeline.
Centrifugal pumps can work in parallel, provided that the pressure developed is equal.
If one of the pumps has a head less than the others, then it can be connected to parallel operation only in the field of recommended operation. With an increase in pressure in the system, this pump can take part in the work, but its efficiency will decrease. When the maximum head is reached, the pump flow will be equal to 0. A further increase in the pressure in the system will close the check valve and turn off the pump from operation. Therefore, for parallel operation, pumps of the same type with equal or slightly different heads and flows should be selected.
Various schemes of parallel operation of pumps are used quite often for water supply and pumping Wastewater, where it is advisable to combine the supply from several pumps or stations into a common manifold. The calculation of the operating mode according to such schemes can be done analytically or graphically. In the practice of designing pumping stations, the graphical method is most widely used.
With parallel operation of pumps in the network, the following layout options for the "pumps - network" system are possible:
several pumps with the same characteristics work in the system;
the system has several pumps with different characteristics;
the pumps are connected to a common pipeline at a close distance from each other, i.e. the pressure loss from the pump to the pressure conduit is considered equal for all installed pumps, or the pumps are at a sufficiently large distance from each other, i.e. the difference in pressure loss from pump before connecting to a common pressure pipe must be taken into account.
Parallel operation of several pumps with the same characteristics. When constructing the characteristics of several parallel pumps on a common pressure pipeline, the pump deliveries are summed up at equal pressures.
If pumps with a flat characteristic Q - H are installed at the pumping station and they are located asymmetrically with respect to the pressure pipeline, then in order to determine more accurate operating points for each pump during parallel operation, it is necessary to build the reduced characteristics Q - R ", for which they build the characteristics of the suction and pressure pipelines within the pumping station and subtract the ordinates of the obtained characteristics from the ordinates of the characteristics of the corresponding pumps.
Parallel operation of pumps located at different pumping stations. In water supply systems with several power sources, a scheme is used to supply water by several pumping stations to common collectors. In this case, it is necessary to calculate the system of parallel operating pumps located at different pumping stations.
Similar schemes are often used when pumping wastewater from individual sewerage areas into the pressure pipeline of another sewage pumping station. Such schemes can significantly reduce the length of pressure pipelines and reduce capital costs.
To calculate the system, it is necessary to determine the characteristics of the parallel operation of the pumps installed at each station. This calculation is made in the same way as for pumps operating in parallel, installed at a close distance from each other. Then, the reduced characteristics are built to the exit point of pressure conduits from the pumping station.
Sequential operation of pumps is called, in which one pump (stage I) supplies the pumped liquid to the suction pipe (sometimes to the suction pipeline) of another pump (stage II), and the latter supplies it to the pressure conduit
In the conditions of design and construction of pumping stations, sequential operation of pumps is used in cases where liquid is supplied through pipes for very long distances or to a greater height. In some cases, liquid can only be pumped by sequentially operating pumps. So, for example, at pumping stations pumping sludge, at the time of starting the working pump, it is required to create a pressure that exceeds the pressure developed by the pump, and which can be created by sequential operation of two pumps. A series connection is also used in cases where it is necessary to increase the pressure at a constant (or almost constant) flow rate, which cannot be done with one pump.
Consider the case of sequential operation of two identical centrifugal pumps installed side by side.
The pressure of one pump is insufficient even to lift water to a geometric height #r. When a second pump of the same type with the same characteristic is connected, it turns out that the pumps develop a pressure sufficient to raise water to a height h and overcome the resistance in the pipeline at a given flow.
The regime point of operation of series-connected pumps is determined by the point K, obtained by the intersection of the total characteristic Q - # 1 + c with the characteristic of the pipeline Q - # tr.
If the pumps are installed in series at one station, then when constructing the pipeline characteristic, it is necessary to take into account the losses in the section from the pump discharge pipe / to the pump suction pipe // and to amend the characteristic Q - #c. Ignoring losses in the connecting section is unacceptable, since usually the diameters of the fittings and the pipeline connecting the pumps are taken equal to the diameter of the suction pipe of the pump //. Due to the high velocity of the fluid, the head loss in this area is relatively large. For the same reason, it is necessary to strive for the maximum simplification of the connecting pipeline, avoiding turns as much as possible. It should be noted that series connection of pumps is usually less economical than the use of a single pump.
Two pumps connected in series are driven as follows. With closed valves 1 and 2 turn on the pump /. After the pump / develops a pressure equal to the pressure when the valve is closed, open the valve / and start the pump //. When the pump // develops a head equal to the head 2#o, the valve 2 is opened.
For sequential operation of the pumps, Special attention choice of pumps, since not all of them can be used for consistent operation in terms of casing strength. These conditions are specified in the technical passport of the pump. Usually series connection of pumps is allowed no more than two stages.
Series-connected pumps can be located in the same machine room, significantly reducing operating costs and capital investments in the construction of the station building, but in this case it is necessary to install reinforcement of increased strength and perform more massive pipe supports and stops. Therefore, sometimes it is more expedient to place pumps at a distance from each other when transporting water over a long distance.
The operation of each pump is characterized by a number of interrelated quantities, such as: productivity, head, speed, efficiency, power requirement.
Pumping units are often connections of centrifugal pumps with asynchronous three-phase alternating current electric motors, which do not allow adjusting the number of their revolutions.
A change in the speed of a centrifugal pump can take place, for example, when it is driven by an internal combustion engine or by means of a belt drive, with the possibility of changing the diameter of the pulley. DC motors allow you to change the number of revolutions, but have very limited applications.
The operation of the pump at a certain speed is characterized by a well-defined QH curve, which graphically expresses the relationship between the performance and pressure developed by the pump. In addition, as follows from the above:
.
The latter represents the parabola equation with the parameter:
The operation of the pump is also characterized by an efficiency curve depending on Q and a curve of required power depending on Q. As will be seen from the following, Q and H for a given n are set in conjunction with the operation of the network.
The total height of the overcome pressure consists of a static (geometric) part and a dynamic part - resistance in pipelines, which changes with a change in the amount of pumped liquid.
If we build in rectangular coordinates the geometric height of the rise H (parallel to the abscissa axis), at each point of this line we plot vertically (Fig. 24) segments equal to the losses in the pipeline (network) when the corresponding amounts of liquid are supplied, then we get a parabolic curve characterizing the work pipeline (network). The pump must provide the pressure necessary to pass a certain flow in the network.
When superimposing the QH curve of the pump and the curve characterizing the operation of the network, the point B of the intersection of these curves will determine the maximum flow of this pump when working in a given pipeline (network). Lower productivity can be obtained by partial repayment of excess pressure on the valve; so, for example, if it is desirable to obtain a performance of Q 2, then the required pressure should be H 2 ", and the pressure developed by the pump is H 2, therefore, part of the pressure equal to H 2 - H 2 "should be extinguished when the valve is partially covered required to reduce Q 1 to Q 2 . If you want to get a performance greater than Q 1 for example Q 3, it is necessary to develop a pump pressure H 3 ", and the pump at this performance develops a pressure H 3 In FIG. 25 shows a diagram of the parallel operation of two pumps, and Fig. 26 - characteristics of the pump with parallel connection of the wheels (double, triple pump). The total consumption Q is equal to the sum of the consumption of all wheels; the pressure developed by the pump varies within the same limits as the pressure developed by each wheel (abscissas are added at the same ordinates). When several pumps operate in the same pipeline (network) (parallel operation), the determination of the operating points B is of particular importance. Given that when two pumps are running, i.e., with doubled amounts of water, and when three pumps are running, i.e., with tripled amounts of water, losses will increase by about 4 times (2 2) in the first case and by about 9 times (Z 2) in the second, we artificially rebuild the loss curves for the case of operation of two and three pumps (Fig. 27), for which we plot loss segments from the geometric head line for the corresponding capacities, 4 times (for two pumps) and 9 times ( with three pumps) are greater than with one pump. The sequential operation of two pumps is shown in Fig. 37. The idea of sequential operation of centrifugal pumps is to some extent reflected in the type of multi-wheel pump. In FIG. 38 shows the characteristics of pumps with one, two and three identical wheels. The ordinates increase with the number of wheels, the abscissas are the same. The idea of sequential operation is reflected in some designs of units that develop very high pressures. The multi-chamber pump with a capacity of 3000 l/min and a head of 728 m, shown in Fig. 39 appears to be divided into two parts connected in series, driven by a common motor; water, after leaving the pressure fitting of the first part of the unit, enters the suction fitting of the second part and leaves the pressure fitting of this part of the unit with a pressure equal to the sum of the pressures due to the operation of the first and second parts of the pump. The arrangement of pumps driven by separate engines is called sequential when the pressure pipe coming from the first is connected to the suction fitting of the second; in this case, the pressures developed by both pumps are summed up (minus the losses in the pipeline connecting them). Serial connection of pumps is carried out if it is desired to increase the pressure of water supplied to any separate zone (if it is necessary to have sewer pumps with a significant pressure, sometimes they design the installation of two pumps in series; the operation of sewer pumps becomes more complicated). Question number 43. Selection of a centrifugal pump. Description of the pump Piston pumps are a type of positive displacement pumping units where liquid is moved by displacers, pushing it out of static working chambers. The working chamber of a piston pump is a closed space that communicates in turn with the inlet/outlet of the pump. The displacer is the working body of the pumping unit, which displaces the substance. Piston pumps impart energy to the pumped liquid, converting it from the mechanical energy of the engine, i.e. This type of pump gives energy to the fluid to be moved so that it can overcome phenomena such as resistance, inertia and static height inside the pipeline. There are various classifications of piston pumps, which take into account the design features of piston pumping units, the features of the functioning of the units, the type of liquid with which the pump works, the speed indicator of the working body and the generated working pressure. Varieties and types of piston pumps So, piston pumps can have a manual and mechanical drive. Pumps with a mechanical drive are divided, in turn, into two types: According to the type of working body that ensures the displacement of liquid, piston pumps are: According to the mode of action, piston pumps are of the following types: Piston pumps are classified by location (horizontal and vertical) and number of cylinders (equipped with one, two, three or more cylinders). According to the number of pistons, pumps with one, two or more pistons are distinguished. In addition, pumps with large pistons (diameter over 150 mm), medium pistons (diameter from 50 to 150 mm) and small pistons (diameter less than 50 mm) are distinguished according to the flow rate. In accordance with how fast the working body is, three types of pumps are distinguished: low-speed piston pumps (from 40 to 80 double strokes per minute), medium-speed piston pumps (50-80) and high-speed piston pumps (150-350). This type of pump is used for pumping cold water (ordinary pump), hot water (hot pump), for working with acidic substances (acid pump), slurry (mud pump), etc. The principle of operation of a double-acting piston pump According to the level of working pressure, pumps that create high, medium and low pressure are distinguished. Piston pumps can be direct-acting or shaft-operated according to the way the main link moves. In direct-acting pumps, the main link performs reciprocating movements, while in shaft units (for example, cam) the drive link rotates. Lobe and submersible pumps are widely used. So, the cam pump has one cylinder, in which the working element is activated by means of a cam, and returns to its original position by means of a spring. This type of pump delivers liquid unevenly, but is compact in design. In cam pumps, the cylinders are arranged radially, and the axes intersect at a common center. The shoes reduce the contact pressure between the displacer and the cam. Lobe pumps are capable of pumping high pressure and are therefore used in hydraulic lines, when pumping fluid in hydraulic presses, and as fuel pumps in diesel engines. Submersible pumps are very compact and are used for well work. Main components / parts of piston pumps Piston pumps are a type of volumetric units, where displacers push liquid out of static working chambers. The main components of a piston pump are such elements as its working chamber and displacer. The working chamber of volumetric pumping equipment is a closed space, which in turn communicates with the input / output of the pumping unit. The displacer is a working body that displaces the substance from the working chambers of the unit. The principle of operation of piston pumps In a single-acting piston pump, the displacer is connected by means of a rod to a crank mechanism and thus reciprocates inside the cylinder. When the piston moves to the right, a void is created in the working chamber. As a result, the valve sucks the working fluid into the chamber through the pipeline. When the piston reverses (to the left), the discharge valve is in the open position, the suction valve, respectively, in the closed position. So, the liquid is injected into the pressure pipeline. In order to increase the performance index of piston pumps, they are often made in double, triple, etc. The pistons in such pumps are activated by a single crankshaft. The principle of operation of the three-piston pump Benefits of piston pumps The advantages of piston pumps over other types of pumping units are: Piston pumps deliver fluid intermittently and are larger than, for example, centrifugal pumps. They are complex in design, but at the same time they can create large pressures. This type of pumps is used to work with clean liquids, because. they are equipped with valves. Impurities in the working fluid can lead to failure. Applications in the industry of piston pumps Piston pumps are actively used in water supply systems, households, in the food and chemical industries, in the production of equipment for spraying various materials. A variety of piston pumps, such as, for example, a diaphragm unit, is used as part of an internal combustion engine as a fuel supply system, as well as for working with building mixtures and other substances that contain impurities. The submersible pump is widely used when working on wells. The mud pump is used for pumping clay solutions. The figure shows the basic arrangement of a single-acting pump driven by machines that perform rotational motion, for example, from an electric motor. The piston pump consists of a working chamber 1, inside which there are suction V (k) and discharge H (k) valves; cylinder -5, piston-3, reciprocating inside the cylinder; suction 2 and pressure 6 nozzles. Rod 4, slider 7 and connecting rod 8 serve to convert the rotational motion of the crank 9 into reciprocating motion of the piston. Depending on the purpose, operating conditions and design features, piston pumps are classified as follows: By type of action By method of actuation By design of the working body By purpose 1) pumps simple action; 2) pumps double action. Double-acting pumps have working chambers 1 and 2 on both sides of the cylinder, each of which has discharge 3 and 4 and suction 5 and 6 valves. Therefore, both during the stroke of the piston 10, driven by the rod 12, in the cylinder 11 to the left and to the right, suction and discharge take place simultaneously. For example, when the piston moves to the right, the suction valve 5 is open in chamber 1 and liquid is sucked in, and the discharge valve 4 is open in chamber 2, the liquid is supplied to the pressure pipeline. Thus, in one stroke of the piston (movement to the right and left), almost twice the volume of liquid is pumped compared to single-acting pumps. 3)built pumps. They consist of three single-acting cylinders, the pistons of which are mounted on a common crankshaft, and the cranks are located at an angle of 120 ° to each other. Thus, for every third of the revolution of the shaft, one portion of water is sucked in and given out, which results in more uniform work; 4)
double double acting pumps. The pump consists of two double-acting pumps with common suction and discharge nozzles; 5)
differential pumps. In a differential pump, the fluid is supplied more evenly, in two steps; for the piston stroke 2 to the left, part of the liquid enters the right cavity of the cylinder 1, and for the piston stroke to the right, it is supplied to the pipeline in the presence of only two valves 4 - suction and 5 - discharge, instead of four. On fig. shown: 8 - suction air cap, 6 - discharge air cap, 7 - discharge pipe, 8 - stem. The dimensions of the differential pump are almost the same as the simple ones. The stem 8 of the differential pump is made with a cross-sectional area equal to half the area of the piston; then equal volumes are given for each move. 1) drive, operating from a separately located engine connected to the pump by a crank mechanism or other transmission; 2) steam - direct acting; they have pump pistons 1 and 3 and steam cylinder 2 have a common rod 4 3) manual, manually operated. These pumps of the BKF type have found wide application. 1) actually piston, in which a disk piston moves in a bored cylinder. O-rings or cuffs are used as a piston seal; 2) plunger (rock), in which the working body is a plunger in the form of a hollow glass, which moves in a sealing gland without touching the inner walls of the cylinder. In operation, these pumps are easier, since they do not have replaceable piston rings, cuffs, etc.; in fig. a diagram of such a pump is given, where 1 is a rolling pin; 2 - cylinder; 3 - stuffing box; 4 - discharge air cap; 5 - suction air chamber; V(k) and H(k) - suction and discharge valves; 3) diaphragm, in which the working body is a flexible diaphragm made of rubberized fabric or leather; 4) deep-water pumps with a through piston. 1) water; 2) sewer; 3) acidic and alkaline; 4) oil, etc. The principle of operation of a water jet pump or hydraulic elevator is based on the transfer of kinetic energy by the working fluid of the pumped fluid. The working (auxiliary) fluid has a large energy reserve compared to the energy reserve of the pumped fluid. Pumping occurs due to the action of one fluid flow with a large energy reserve, on another without any intermediate mechanisms. The hydraulic elevator installation consists of an auxiliary (feed) pump 1, a supply pipeline 2, a hydraulic elevator 3, a suction pipeline 4, a pressure pipeline 5. Water under high pressure passes through the converging nozzle of the hydraulic elevator 3. Due to a sharp increase in the speed at the constriction of the hydraulic elevator nozzle, the pressure p in the mixing chamber drops and becomes less than atmospheric pressure. Atmospheric pressure liquid from the tank For pumping liquids, a piston pump has been used for many years. This design has become very widespread, as it works on the principle of fluid displacement due to pressure transfer. The principle of operation of the piston pump of modern implementations is much more complicated in comparison with the first models, due to which reliability and efficiency are significantly increased. Let us consider the features of such a mechanism in more detail. Considering the principle of operation of a piston pump, it should be borne in mind that the first design appeared many decades ago. The scheme of work has the following features: The simplest principle of operation determines long-term and stable operation. It should be borne in mind that the flow created by such a device can move at different speeds. Too large volume of the working chamber leads to the fact that the flow will move in jumps. In order to exclude the appearance of such an effect, a device with several pistons is installed. The plunger pump has a relatively simple design. Among the features, we note the following points: The reciprocating motion is transmitted from the electric motor through a special mechanism that converts the rotation. Modern versions are compact, they can be installed for outdoor or indoor operation. In addition, in the manufacture of the case, a metal is used that has a high protection against environmental influences. The device of the two-sided model has a fairly large number of features: Despite a significant increase in the efficiency of the piston pump, its design is quite simple. In this case, each stroke involves the suction and expulsion of fluid. This significantly increases the value of efficiency. On sale there are a variety of versions of piston pumps. Classification is carried out according to the following criteria: On sale there is a double-acting piston pump, as well as a version with one and three, several pistons. As previously noted, by increasing the number of moving elements, the possibility of a pulsating flow movement is excluded. As for the number of cycles, there are single-acting and double-acting models, as well as differential models. Classification can also be carried out according to the following criteria: Piston pumps are produced by a variety of companies. The quality may depend on the type of materials used, brand popularity and the purpose of a particular model. The liquid pump can be used for a wide variety of applications. The created design is characterized by high versatility. However, the presence of a moving element and the use of sealing rings when creating a piston determines the impossibility of using a piston pump for pumping a large volume of liquids. Considering the scope, we note the following points: In the manufacture of the structure, a variety of materials can be used, which determine the scope. The piston liquid pump is characterized by a fairly large number of advantages and disadvantages. The advantages include: There are also several serious drawbacks. An example is low productivity. Such models are less suitable for pumping large amounts of liquid. In addition, the design is not suitable for continuous operation, as the active elements wear out quickly and lose their performance characteristics.
Piston pumps by type of action
Piston pumps by actuation method:
According to the design of the working body:
By appointment:
Water jet pumps
Principle of operation
Device
Varieties
Applications
Advantages and disadvantages