Heating system test. Determining the dynamic pressure in the duct

Lecture 2. Pressure loss in ducts

Lecture plan. Mass and volumetric air flows. Bernoulli's law. Pressure losses in horizontal and vertical air ducts: coefficient of hydraulic resistance, dynamic coefficient, Reynolds number. Loss of pressure in the outlets, local resistances, for the acceleration of the dust-air mixture. Loss of pressure in a high-pressure network. The power of the pneumatic conveying system.

2. Pneumatic parameters of air flow

2.1. Air flow parameters

Under the action of the fan, an air flow is created in the pipeline. Important parameters air flow are its speed, pressure, density, mass and volume flow of air. Air volume volumetric Q, m 3 /s, and mass M, kg/s, are interconnected as follows:

;
, (3)

where F- square cross section pipes, m 2;

v– air flow velocity in a given section, m/s;

ρ - air density, kg / m 3.

The pressure in the air flow is divided into static, dynamic and total.

static pressure R st It is customary to call the pressure of particles of moving air on each other and on the walls of the pipeline. Static pressure reflects the potential energy of the air flow in the section of the pipe in which it is measured.

dynamic pressure air flow R din, Pa, characterizes its kinetic energy in the pipe section where it is measured:

.

Full pressure the air flow determines all its energy and is equal to the sum of static and dynamic pressures measured in the same pipe section, Pa:

R = R st + R d .

Pressures can be measured either from absolute vacuum or relative to atmospheric pressure. If the pressure is measured from zero (absolute vacuum), then it is called absolute R. If pressure is measured relative to atmospheric pressure, then it will be relative pressure H.

H = H st + R d .

Atmospheric pressure is equal to the difference between the total pressures of absolute and relative

R atm = R – H.

Air pressure is measured by Pa (N / m 2), mm of water column or mm of mercury:

1 mm w.c. Art. = 9.81 Pa; 1 mmHg Art. = 133.322 Pa. Normal condition atmospheric air corresponds to the following conditions: pressure 101325 Pa (760 mm Hg) and temperature 273K.

Air density is the mass per unit volume of air. According to the Claiperon equation, the density of pure air at a temperature of 20ºС

kg / m 3.

where R– gas constant equal to 286.7 J/(kg  K) for air; T is the temperature on the Kelvin scale.

Bernoulli equation. By the condition of the continuity of the air flow, the air flow is constant for any section of the pipe. For sections 1, 2, and 3 (Fig. 6), this condition can be written as follows:

;

When the air pressure changes within the range of up to 5000 Pa, its density remains almost constant. Concerning

;

Q 1 \u003d Q 2 \u003d Q 3.

The change in air flow pressure along the length of the pipe obeys Bernoulli's law. For sections 1, 2, one can write

where  R 1,2 - pressure losses caused by flow resistance against the pipe walls in the section between sections 1 and 2, Pa.

With a decrease in the cross-sectional area 2 of the pipe, the air velocity in this section will increase, so that the volume flow remains unchanged. But with an increase v 2 the dynamic flow pressure will increase. In order for equality (5) to hold, static pressure should fall exactly as much as the dynamic pressure increases.

With an increase in the cross-sectional area, the dynamic pressure in the cross section will drop, and the static pressure will increase by exactly the same amount. The total pressure in the cross section remains unchanged.

2.2. Pressure loss in a horizontal duct

Friction pressure loss dust-air flow in a direct duct, taking into account the concentration of the mixture, is determined by the Darcy-Weisbach formula, Pa

, (6)

where l- length of the straight section of the pipeline, m;

 - coefficient of hydraulic resistance (friction);

d

R din- dynamic pressure calculated from the average air velocity and its density, Pa;

To– complex coefficient; for roads with frequent turns To= 1.4; for straight lines with a small amount turns
, where d– pipeline diameter, m;

To tm- coefficient taking into account the type of transported material, the values ​​​​of which are given below:

Hydraulic resistance coefficient  in engineering calculations are determined by the formula A.D. Altshulya

, (7)

where To uh- absolute equivalent surface roughness, K e = (0.0001 ... 0.00015) m;

d is the inner diameter of the pipe, m;

Re is the Reynolds number.

Reynolds number for air

, (8)

where v is the average air velocity in the pipe, m/s;

d– pipe diameter, m;

 - air density, kg / m 3;

 1 – coefficient of dynamic viscosity, Ns/m 2 ;

Dynamic coefficient value viscosities for air are found by the Millikan formula, Ns/m2

 1 = 17,11845  10 -6 + 49,3443  10 -9 t, (9)

where t– air temperature, С.

At t\u003d 16 С  1 \u003d 17.11845  10 -6 + 49.3443  10 -9 16 \u003d 17.910 -6.

2.3. Pressure loss in vertical duct

Pressure loss during the movement of the air mixture in a vertical pipeline, Pa:

, (10)

where  - air density,  \u003d 1.2 kg / m 3;

g \u003d 9.81 m / s 2;

h– lifting height of the transported material, m.

When calculating aspiration systems, in which the concentration of the air mixture   0.2 kg/kg value  R under only taken into account when  h 10 m. For inclined pipeline  h = l sin, where l is the length of the inclined section, m;  - the angle of inclination of the pipeline.

2.4. Pressure loss in outlets

Depending on the orientation of the outlet (rotation of the duct at a certain angle), two types of outlets are distinguished in space: vertical and horizontal.

Vertical outlets are denoted by the initial letters of words that answer questions according to the scheme: from which pipeline, where and to which pipeline the air mixture is directed. There are the following withdrawals:

- Г-ВВ - the transported material moves from the horizontal section upwards to the vertical section of the pipeline;

- G-NV - the same from the horizontal down to the vertical section;

- ВВ-Г - the same from vertical upwards to horizontal;

- VN-G - the same from vertical down to horizontal.

Horizontal outlets There are only one type G-G.

In the practice of engineering calculations, the pressure loss in the outlet of the network is found by the following formulas.

At the values ​​of consumption concentration   0.2 kg/kg

where
- the sum of the coefficients of local resistance of branch bends (Table 3) at R/ d= 2, where R- radius of turn of the axial line of the branch; d– pipeline diameter; dynamic airflow pressure.

At values ​​  0.2 kg/kg

where
- the sum of conditional coefficients that take into account the pressure loss for turning and dispersing the material behind the bend.

Values  about conv are found by the size of the tabular  t(Table 4) taking into account the coefficient for the angle of rotation To P

 about conv =  t To P . (13)

Correction factors To P take depending on the angle of rotation of the taps :

Table 3

Coefficients of local resistance of taps  about at R/ d = 2

To the question Static pressure is atmospheric pressure or what? given by the author Eating Bondarchuk the best answer is I urge everyone not to copy overly clever encyclopedia articles when people ask simple questions. Golem physics is not needed here.
The word "static" literally means - constant, unchanging in time.
When you pump a soccer ball, the pressure inside the pump is not static, but different every second. And when you pump up, inside the ball there is a constant air pressure - static. And atmospheric pressure is static in principle, although if you dig deeper, this is not so, it still changes slightly over the course of days and even hours. In short, there is nothing abstruse here. Static means permanent, and nothing else.
When you say hello to guys, rraz! Shock from hand to hand. Well, it happened to everyone. They say "static electricity". Correctly! A static charge (permanent) has accumulated in your body at this moment. When you touch another person, half of the charge passes to him in the form of a spark.
That's it, I won't load any more. In short, "static" = "permanent", for all occasions.
Comrades, if you do not know the answer to the question, and moreover, you have not studied physics at all, you do not need to copy articles from encyclopedias !!
just like you are wrong, you didn’t come to the first lesson and they didn’t ask you Bernoulli’s formulas, right? they began to chew on you what pressure, viscosity, formulas, etc., etc. are, but when you come and give you exactly as you said, a person is disgusted by this. What curiosity for learning if you don't understand the symbols in the same equation? It's easy to say to someone who has some sort of base, so you're completely wrong!

Answer from roast beef[newbie]
Atmospheric pressure contradicts the MKT of the structure of gases and refutes the existence of a chaotic movement of molecules, the result of which impacts is the pressure on the surfaces bordering on the gas. The pressure of gases is predetermined by the mutual repulsion of like molecules. The repulsion voltage is equal to the pressure. If we consider the column of the atmosphere as a solution of gases of 78% nitrogen and 21% oxygen and 1% others, then atmospheric pressure can be considered as the sum of the partial pressures of its components. The forces of mutual repulsion of molecules equalize the distances between like ones on isobars. Presumably, oxygen molecules do not have repulsive forces with others. So, from the assumption that like molecules repel with the same potential, this explains the equalization of gas concentrations in the atmosphere and in a closed vessel.

Answer from Huck Finn[guru]
Static pressure is that which is created under the influence of gravity. Water under its own weight presses on the walls of the system with a force proportional to the height to which it rises. From 10 meters this indicator is equal to 1 atmosphere. In statistical systems, flow blowers are not used, and the coolant circulates through pipes and radiators by gravity. These are open systems. Maximum pressure in open system heating is about 1.5 atmospheres. AT modern construction such methods are practically not used, even when installing autonomous circuits country houses. This is due to the fact that for such a circulation scheme it is necessary to use pipes with a large diameter. It's not aesthetically pleasing and expensive.
Pressure in closed system heating:
The dynamic pressure in the heating system can be adjusted
Dynamic pressure in a closed heating system is created by an artificial increase in the flow rate of the coolant using electric pump. For example, if we are talking about high-rise buildings, or large highways. Although, now even in private homes, pumps are used when installing heating.
Important! We are talking about excess pressure without taking atmospheric pressure into account.
Each of the heating systems has its own permissible tensile strength. In other words, can withstand different load. To find out what working pressure is in a closed heating system, it is necessary to add a dynamic one, pumped by pumps, to the static one created by a column of water. For correct operation system, the pressure gauge must be stable. A manometer is a mechanical device that measures the pressure with which water moves in a heating system. It consists of a spring, an arrow and a scale. Gauges are installed in key locations. Thanks to them, you can find out what the working pressure is in the heating system, as well as detect malfunctions in the pipeline during diagnostics (hydraulic tests).

Answer from capable[guru]
In order to pump liquid to a given height, the pump must overcome the static and dynamic pressure. Static pressure is the pressure due to the height of the liquid column in the pipeline, i.e. the height to which the pump must raise the liquid .. Dynamic pressure - the sum of the hydraulic resistances due to the hydraulic resistance of the pipeline wall itself (taking into account the roughness of the wall, pollution, etc.), and local resistances (pipeline bends, valves, gate valves, etc.). ).

Answer from Eurovision[guru]
Atmosphere pressure - hydrostatic pressure atmosphere on all objects in it and the earth's surface. Atmospheric pressure is created by the gravitational attraction of air to the Earth.
And static pressure - I did not meet the current concept. And jokingly, we can assume that this is due to the laws of electric forces and attraction of electricity.
Maybe this? -
Electrostatics is a branch of physics that studies the electrostatic field and electric charges.
Electrostatic (or Coulomb) repulsion occurs between like-charged bodies, and electrostatic attraction between oppositely charged bodies. The phenomenon of repulsion of like charges underlies the creation of an electroscope - a device for detecting electric charges.
Statics (from the Greek στατός, “immovable”):
A state of rest in any certain moment(book). For example: Describe a phenomenon in statics; (adj.) static.
A branch of mechanics that studies the conditions for the equilibrium of mechanical systems under the action of forces and moments applied to them.
So I have not seen the concept of static pressure.

Answer from Andrey Khalizov[guru]
Pressure (in physics) is the ratio of the force normal to the interaction surface between bodies to the area of ​​this surface or in the form of a formula: P = F / S.
Static (from the word Statics (from the Greek στατός, “immovable”, “constant”)) pressure is a constant in time (unchangeable) application of a force normal to the surface of interaction between bodies.
Atmospheric (barometric) pressure - the hydrostatic pressure of the atmosphere on all objects in it and the earth's surface. Atmospheric pressure is created by the gravitational attraction of air to the Earth. On the earth's surface, atmospheric pressure varies from place to place and over time. Atmospheric pressure decreases with height because it is created only by the overlying layer of the atmosphere. The dependence of pressure on height is described by the so-called.
That is, these are two different concepts.

Bernoulli's Law on Wikipedia
See the Wikipedia article about Bernoulli's Law

Kinetic energy of moving gas:

where m is the mass of the moving gas, kg;

s is the gas velocity, m/s.

(2)

where V is the volume of moving gas, m 3;

- density, kg / m 3.

Substitute (2) into (1), we get:

(3)

Let's find the energy of 1 m 3:

(4)

The total pressure is made up of and
.

total pressure in air flow is equal to the sum of static and dynamic pressure and represents the energy saturation of 1 m 3 of gas.

Scheme of experience for determining the total pressure

Pitot-Prandtl tube

(1)

(2)

Equation (3) shows the operation of the tube.

- pressure in column I;

- pressure in column II.

Equivalent hole

If you make a hole with a section F e through which the same amount of air will be supplied
, as well as through a pipeline with the same initial pressure h, then such an opening is called equivalent, i.e. passing through this equivalent orifice replaces all resistances in the conduit.

Find the size of the hole:

, (4)

where c is the gas flow rate.

Gas consumption:

(5)

From (2)
(6)

Approximately, because we do not take into account the coefficient of narrowing of the jet.

- this is a conditional resistance, which is convenient to enter into calculations when simplifying the real complex systems. Pressure losses in pipelines are defined as the sum of losses in individual places of the pipeline and are calculated on the basis of experimental data given in reference books.

Losses in the pipeline occur at turns, bends, with expansion and contraction of pipelines. Losses in an equal pipeline are also calculated according to reference data:

suction pipe

Fan housing

Discharge pipe

An equivalent orifice that replaces a real pipe with its resistance.

- speed in the suction pipeline;

is the outflow velocity through the equivalent orifice;

- the value of the pressure under which the gas moves in the suction pipe;

static and dynamic pressure in the outlet pipe;

- full pressure in the discharge pipe.

Through the equivalent hole gas leaks under pressure , knowing , we find .

Example

What is the motor power to drive the fan, if we know the previous data from 5.

Taking into account losses:

where - monometric coefficient of efficiency.

where
- theoretical pressure of the fan.

Derivation of fan equations.

Given:

To find:

Decision:

where
- mass of air;

- initial radius of the blade;

- final radius of the blade;

- air speed;

- tangential speed;

is the radial speed.

Divide by
:

;

Second mass:

,

;

Second work - the power given off by the fan:

.

Lecture No. 31.

The characteristic shape of the blades.

- circumferential speed;

With is the absolute velocity of the particle;

- relative speed.

,

.

Imagine our fan with inertia B.

Air enters the hole and is sprayed along the radius at a speed С r . but we have:

,

where AT– fan width;

r- radius.

.

Multiply by U:

.

Substitute
, we get:

.

Substitute the value
for radii
into the expression for our fan and get:

Theoretically, the fan pressure depends on the angles (*).

Let's replace through and substitute:

Divide the left and right sides into :

.

where BUT and AT are replacement coefficients.

Let's build the dependency:

Depending on the angles
the fan will change its character.

In the figure, the rule of signs coincides with the first figure.

If an angle is plotted from the tangent to the radius in the direction of rotation, then this angle is considered positive.

1) In the first position: - positive, - negative.

2) Blades II: - negative, - positive - becomes close to zero and usually less. This is a high pressure fan.

3) Blades III:
are equal to zero. B=0. Medium pressure fan.

Basic ratios for the fan.

,

where c is the air flow velocity.

.

Let's write this equation in relation to our fan.

.

Divide the left and right sides by n:

.

Then we get:

.

Then
.

When solving for this case, x=const, i.e. we'll get

Let's write:
.

Then:
then
- the first ratio of the fan (the performance of the fan is related to each other, as the number of revolutions of the fans).

Example:

- This is the second fan ratio (theoretical fan heads refer as the squares of the speed).

If we take the same example, then
.

But we have
.

Then we get the third relation if instead of
substitute
. We get the following:

- This is the third ratio (the power required to drive the fan refers to the cubes of the number of revolutions).

For the same example:

Fan calculation

Data for fan calculation:

Set:
- air consumption (m 3 /sec).

From design considerations, the number of blades is also selected - n,

- air density.

In the process of calculation are determined r 2 , d- diameter of the suction pipe,
.

The entire fan calculation is based on the fan equation.

scraper elevator

1) Resistance when loading the elevator:

G C- the weight running meter chains;

G G- weight per linear meter of cargo;

L is the length of the working branch;

f - coefficient of friction.

3) Resistance in the idle branch:

Total force:

.

where - efficiency taking into account the number of stars m;

- efficiency taking into account the number of stars n;

- efficiency taking into account the stiffness of the chain.

Conveyor drive power:

,

where - conveyor drive efficiency.

Bucket conveyors

He is bulky. They are mainly used on stationary machines.

Thrower-fan. It is applied on silo combines and on grain. Matter is subjected to specific action. Big expense power at increase. performance.

Canvas conveyors.

Applicable to conventional headers

1)
(D'Alembert's principle).

per particle of mass m weight force is acting mg, force of inertia
, friction force.

,

.

Need to find X, which is equal to the length at which you need to pick up speed from V 0 before V equal to the speed of the conveyor.

,

Expression 4 is remarkable in the following case:

At
,
.

At an angle
the particle can pick up the speed of the conveyor on the way L equal to infinity.

Bunker

There are several types of bunkers:

with screw discharge

vibration unloading

hopper with free flow of bulk medium is used on stationary machines

1. Bunker with auger unloading

Productivity of screw unloader:

.

scraper elevator conveyor;

distributing auger hopper;

lower unloading auger;

inclined unloading auger;

- fill factor;

n- the number of revolutions of the screw;

t- screw pitch;

- specific gravity of the material;

D- screw diameter.

2. Vibrobunker

vibrator;

1. unloading tray;

   flat springs, elastic elements;

a– amplitude of oscillations of the bunker;

With- center of gravity.

Advantages - freedom formation, simplicity of structural design are eliminated. The essence of the impact of vibration on a granular medium is pseudo-motion.

.

M– mass of the bunker;

X- its movement;

to 1 – coefficient taking into account speed resistance;

to 2 - the stiffness of the springs;

- circular frequency or speed of rotation of the vibrator shaft;

- the phase of the installation of loads in relation to the displacement of the bunker.

Let's find the amplitude of the bunker to 1 =0:

very little

,

- the frequency of natural oscillations of the bunker.

,

At this frequency, the material begins to flow. There is an outflow rate at which the bunker is unloaded in 50 sec.

diggers. Collection of straw and chaff.

1. Haulers are mounted and trailed, and they are single-chamber and two-chamber;

2. Straw choppers with collection or spreading of chopped straw;

3. Spreaders;

4. Straw presses for collecting straw. There are mounted and trailed.

Heating systems must be tested for pressure resistance

From this article you will learn what static and dynamic pressure of a heating system is, why it is needed and how it differs. The reasons for its increase and decrease and methods for their elimination will also be considered. In addition, we will talk about the pressure various systems heating and methods of this check.

Types of pressure in the heating system

There are two types:

• statistical;
• dynamic.

What is the static pressure of a heating system? This is what is created under the influence of gravity. Water under its own weight presses on the walls of the system with a force proportional to the height to which it rises. From 10 meters this indicator is equal to 1 atmosphere. In statistical systems, flow blowers are not used, and the coolant circulates through pipes and radiators by gravity. These are open systems. The maximum pressure in an open heating system is about 1.5 atmospheres. In modern construction, such methods are practically not used, even when installing autonomous contours of country houses. This is due to the fact that for such a circulation scheme it is necessary to use pipes with a large diameter. It's not aesthetically pleasing and expensive.

The dynamic pressure in the heating system can be adjusted

Dynamic pressure in a closed heating system is created by artificially increasing the flow rate of the coolant using an electric pump. For example, if we are talking about high-rise buildings, or large highways. Although, now even in private homes, pumps are used when installing heating.

Important! We are talking about excess pressure without taking atmospheric pressure into account.

Each of the heating systems has its own permissible tensile strength. In other words, it can withstand a different load. To find out what working pressure is in a closed heating system, it is necessary to add dynamic, pumped by pumps, to the static one created by a column of water. For the system to work properly, the pressure gauge readings must be stable. A manometer is a mechanical device that measures the force with which water moves in a heating system. It consists of a spring, an arrow and a scale. Gauges are installed in key locations. Thanks to them, you can find out what the working pressure is in the heating system, as well as identify malfunctions in the pipeline during diagnostics.

Pressure drops

To compensate for the drops, additional equipment is built into the circuit:

1. expansion tank;
2. emergency coolant release valve;
3. air outlets.

Air testing - test pressure heating systems are increased to 1.5 bar, then lowered to 1 bar and left for five minutes. In this case, the losses should not exceed 0.1 bar.

Testing with water - the pressure is increased to at least 2 bar. Perhaps more. Depends on working pressure. The maximum operating pressure of the heating system must be multiplied by 1.5. For five minutes, the loss should not exceed 0.2 bar.

panel

Cold hydrostatic testing - 15 minutes at 10 bar pressure, no more than 0.1 bar loss. Hot testing - raising the temperature in the circuit to 60 degrees for seven hours.

Tested with water, pumping 2.5 bar. Additionally, water heaters (3-4 bar) and pumping units are checked.

Heating network

The permissible pressure in the heating system is gradually increased to a level higher than the working one by 1.25, but not less than 16 bar.

Based on the test results, an act is drawn up, which is a document confirming the statements stated in it. performance characteristics. These include, in particular, the working pressure.

Question 21. Classification of pressure measuring instruments. The device of the electrocontact pressure gauge, methods of its verification.

In many technological processes, pressure is one of the main parameters that determine their course. These include: pressure in autoclaves and steaming chambers, air pressure in process pipelines, etc.

Determining the pressure value

Pressure is a quantity that characterizes the effect of force per unit area.

When determining the magnitude of pressure, it is customary to distinguish between absolute, atmospheric, excess and vacuum pressure.

Absolute pressure (p a ) - this is the pressure inside any system, under which there is a gas, vapor or liquid, measured from absolute zero.

Atmospheric pressure (p in ) created by the mass of the air column of the earth's atmosphere. It has a variable value depending on the height of the area above sea level, geographical latitude and meteorological conditions.

Overpressure is determined by the difference between absolute pressure (p a) and atmospheric pressure (p b):

r izb \u003d r a - r c.

Vacuum (vacuum) is the state of a gas in which its pressure is less than atmospheric pressure. Quantitatively, the vacuum pressure is determined by the difference between the atmospheric pressure and the absolute pressure inside the vacuum system:

p vak \u003d p in - p a

When measuring pressure in moving media, the concept of pressure is understood as static and dynamic pressure.

Static pressure (p st ) is the pressure depending on the potential energy of the gas or liquid medium; determined by static pressure. It can be excess or vacuum, in a particular case it can be equal to atmospheric.

Dynamic pressure (p d ) is the pressure due to the speed of the flow of a gas or liquid.

Total pressure (p P ) moving medium is composed of static (p st) and dynamic (p d) pressures:

r p \u003d r st + r d.

Pressure units

In the SI system of units, the unit of pressure is considered to be the action of a force of 1 H (newton) on an area of ​​1 m², i.e. 1 Pa (Pascal). Since this unit is very small, the kilopascal (kPa = 10 3 Pa) or megapascal (MPa = 10 6 Pa) is used for practical measurements.

In addition, the following pressure units are used in practice:

millimeter of water column (mm water column);

millimeter of mercury (mm Hg);

atmosphere;

kilogram force per square centimeter (kg s/cm²);

The relationship between these quantities is as follows:

1 Pa = 1 N/m²

1 kg s/cm² = 0.0981 MPa = 1 atm

1 mm w.c. Art. \u003d 9.81 Pa \u003d 10 -4 kg s / cm² \u003d 10 -4 atm

1 mmHg Art. = 133.332 Pa

1 bar = 100,000 Pa = 750 mmHg Art.

Physical explanation of some units of measure:

1 kg s / cm² is the pressure of a water column 10 m high;

1 mmHg Art. is the amount of pressure reduction for every 10m of elevation.

Pressure Measurement Methods

The widespread use of pressure, its differential and rarefaction in technological processes makes it necessary to apply a variety of methods and means for measuring and controlling pressure.

Methods for measuring pressure are based on comparing the forces of the measured pressure with the forces:

pressure of a liquid column (mercury, water) of the corresponding height;

developed during deformation of elastic elements (springs, membranes, manometric boxes, bellows and manometric tubes);

cargo weight;

elastic forces arising from the deformation of certain materials and causing electrical effects.

Classification of pressure measuring instruments

Classification according to the principle of action

In accordance with these methods, pressure measuring instruments can be divided, according to the principle of operation, into:

liquid;

deformation;

cargo piston;

electrical.

The most widely used in industry are deformation measuring instruments. The rest, for the most part, have found application in laboratory conditions as exemplary or research.

Classification depending on the measured value

Depending on the measured value, pressure measuring instruments are divided into:

pressure gauges - for measuring excess pressure (pressure above atmospheric pressure);

micromanometers (pressure meters) - for measuring small excess pressure(up to 40 kPa);

barometers - for measuring atmospheric pressure;

microvacuum meters (thrust gauges) - for measuring small vacuums (up to -40 kPa);

vacuum gauges - for measuring vacuum pressure;

pressure and vacuum gauges - for measuring excess and vacuum pressure;

pressure gauges - for measuring excess (up to 40 kPa) and vacuum pressure (up to -40 kPa);

absolute pressure gauges - for measuring pressure, measured from absolute zero;

differential pressure gauges - for measuring the difference (differential) pressures.

Liquid pressure measuring instruments

The action of liquid measuring instruments is based on the hydrostatic principle, in which the measured pressure is balanced by the pressure of the barrier (working) fluid column. The difference in levels depending on the density of the liquid is a measure of pressure.

U-shaped manometer- This is the simplest device for measuring pressure or pressure difference. It is a bent glass tube filled with a working fluid (mercury or water) and attached to a panel with a scale. One end of the tube is connected to the atmosphere, and the other is connected to the object where the pressure is measured.

The upper limit of measurement of two-pipe pressure gauges is 1 ... 10 kPa with a reduced measurement error of 0.2 ... 2%. The accuracy of pressure measurement by this tool will be determined by the accuracy of reading the value h (the value of the difference in the liquid level), the accuracy of determining the density working fluidρ and be independent of the cross section of the tube.

Liquid pressure measuring instruments are characterized by the absence of remote transmission of readings, small measurement limits and low strength. At the same time, due to their simplicity, low cost, and relatively high measurement accuracy, they are widely used in laboratories and less frequently in industry.

Deformation pressure measuring instruments

They are based on balancing the force created by the pressure or vacuum of the controlled medium on the sensitive element with the forces of elastic deformations of various types of elastic elements. This deformation in the form of linear or angular displacements is transmitted to a recording device (indicating or recording) or converted into an electrical (pneumatic) signal for remote transmission.

As sensitive elements, single-turn tubular springs, multi-turn tubular springs, elastic membranes, bellows and spring-bellows are used.

For the manufacture of membranes, bellows and tubular springs, bronze, brass, chromium-nickel alloys are used, which are characterized by sufficiently high elasticity, anti-corrosion, low dependence of parameters on temperature changes.

Membrane devices are used to measure low pressures (up to 40 kPa) of neutral gaseous media.

Bellows devices designed to measure excess and vacuum pressure of non-aggressive gases with measurement limits up to 40 kPa, up to 400 kPa (as pressure gauges), up to 100 kPa (as vacuum gauges), in the range of -100 ... + 300 kPa (as combined pressure and vacuum gauges).

Tubular spring devices are among the most common manometers, vacuum gauges and combined pressure and vacuum gauges.

A tubular spring is a thin-walled, bent in an arc of a circle, tube (single or multi-turn) with a sealed one end, which is made of copper alloys or stainless steel. When the pressure inside the tube increases or decreases, the spring unwinds or twists at a certain angle.

The pressure gauges of the considered type are produced for the upper measurement limits of 60 ... 160 kPa. Vacuum gauges are produced with a scale of 0…100kPa. Pressure vacuum gauges have measurement limits: from -100 kPa to + (60 kPa ... 2.4 MPa). Accuracy class for working pressure gauges 0.6 ... 4, for exemplary - 0.16; 0.25; 0.4.

Deadweight testers are used as devices for verification of mechanical control and exemplary pressure gauges of medium and high pressure. The pressure in them is determined by calibrated weights placed on the piston. As a working fluid, kerosene, transformer or Castor oil. The accuracy class of deadweight pressure gauges is 0.05 and 0.02%.

Electrical pressure gauges and vacuum gauges

The operation of devices in this group is based on the property of certain materials to change their electrical parameters under pressure.

Piezoelectric pressure gauges used for measuring pressure pulsating with a high frequency in mechanisms with permissible load on the sensitive element up to 8·10 3 GPa. The sensitive element in piezoelectric manometers, which converts mechanical stresses into electric current oscillations, are cylindrical or rectangular shape a few millimeters thick from quartz, barium titanate or PZT ceramics (lead zirconate titonate).

Strain Gauges have small dimensions, simple device, high precision and reliable operation. The upper limit of readings is 0.1 ... 40 MPa, accuracy class 0.6; 1 and 1.5. They are used in difficult production conditions.

As a sensitive element in strain gauges, strain gauges are used, the principle of operation of which is based on a change in resistance under the action of deformation.

The pressure in the gauge is measured by an unbalanced bridge circuit.

As a result of deformation of the membrane with a sapphire plate and strain gauges, an unbalance of the bridge occurs in the form of voltage, which is converted by an amplifier into an output signal proportional to the measured pressure.

Differential pressure gauges

Are applied to measurement of a difference (difference) of pressure of liquids and gases. They can be used to measure the flow of gases and liquids, the liquid level, as well as to measure small excess and vacuum pressures.

Diaphragm differential pressure gauges are non-jackal primary measuring devices designed to measure the pressure of non-aggressive media, converting the measured value into a unified analog DC signal 0 ... 5 mA.

Differential pressure gauges of the DM type are produced for limiting pressure drops of 1.6 ... 630 kPa.

Bellows differential pressure gauges are produced for limiting pressure drops of 1…4kPa, they are designed for maximum allowable operating overpressure of 25kPa.

The device of the electrocontact pressure gauge, methods for its verification

Electrocontact pressure gauge device

Figure - Schematic diagrams of electrocontact pressure gauges: a- single-contact for short circuit; b- single-contact opening; c - two-contact open-open; G– two-contact for short circuit–short circuit; d- two-contact opening-closing; e- two-contact for closing-opening; 1 - pointer arrow; 2 and 3 – electrical base contacts; 4 and 5 – zones of closed and open contacts, respectively; 6 and 7 – objects of influence

A typical diagram of the operation of an electrocontact pressure gauge can be illustrated in the figure ( a). With an increase in pressure and reaching a certain value, the index arrow 1 with electrical contact enters the zone 4 and closes with the base contact 2 electrical circuit of the device. Closing the circuit, in turn, leads to the commissioning of the object of influence 6.

In the opening circuit (Fig. . b) in the absence of pressure, the electrical contacts of the index arrow 1 and base contact 2 closed. Under voltage U in is electrical circuit device and object of influence. When the pressure rises and the pointer passes through the zone of closed contacts, the electrical circuit of the device breaks and, accordingly, the electrical signal directed to the object of influence is interrupted.

Most often in production conditions, pressure gauges with two-contact electrical circuits are used: one is used for sound or light indication, and the second is used to organize the functioning of systems of various types of control. Thus, the opening-closing circuit (Fig. d) allows one channel to open one electrical circuit when a certain pressure is reached and receive a signal of impact on the object 7 , and according to the second - using the base contact 3 close the open second electrical circuit.

Closing-opening circuit (Fig. . e) allows, with increasing pressure, one circuit to close, and the second - to open.

Two-contact circuits for closing-closing (Fig. G) and opening-opening (Fig. in) provide, when the pressure rises and reaches the same or different values, the closure of both electrical circuits or, accordingly, their opening.

The electrocontact part of the pressure gauge can be either integral, combined directly with the meter mechanism, or attached in the form of an electrocontact group mounted on the front of the device. Manufacturers traditionally use designs in which the rods of the electrocontact group were mounted on the axis of the tube. In some devices, as a rule, an electrocontact group is installed, connected to the sensitive element through the index arrow of the pressure gauge. Some manufacturers have mastered the electrocontact pressure gauge with microswitches, which are installed on the transmission mechanism of the meter.

Electrocontact pressure gauges are produced with mechanical contacts, contacts with magnetic preload, inductive pair, microswitches.

The electrocontact group with mechanical contacts is structurally the most simple. A base contact is fixed on the dielectric base, which is an additional arrow with an electrical contact fixed on it and connected to an electrical circuit. Another electrical circuit connector is connected to a contact that moves with an index arrow. Thus, with increasing pressure, the index arrow displaces the movable contact until it is connected to the second contact fixed on the additional arrow. Mechanical contacts, made in the form of petals or racks, are made of silver-nickel (Ar80Ni20), silver-palladium (Ag70Pd30), gold-silver (Au80Ag20), platinum-iridium (Pt75Ir25) alloys, etc.

Devices with mechanical contacts are designed for voltages up to 250 V and withstand a maximum breaking power of up to 10 W DC or up to 20 V×A AC. The small breaking power of the contacts ensures a sufficiently high actuation accuracy (up to 0.5% full value scales).

A stronger electrical connection is provided by contacts with magnetic preload. Their difference from mechanical ones is that small magnets are fixed on the reverse side of the contacts (with glue or screws), which enhances the strength of the mechanical connection. The maximum breaking power of contacts with magnetic preload is up to 30 W DC or up to 50 V×A AC and voltage up to 380 V. Due to the presence of magnets in the contact system, the accuracy class does not exceed 2.5.

ECG verification methods

Electrocontact pressure gauges, as well as pressure sensors, must be periodically verified.

Electrocontact pressure gauges in the field and laboratory conditions can be checked in three ways:

zero point verification: when the pressure is removed, the pointer should return to the “0” mark, the pointer shortfall should not exceed half the instrument error tolerance;

verification of the working point: a control pressure gauge is connected to the device under test and the readings of both devices are compared;

verification (calibration): verification of the device according to the procedure for verification (calibration) for of this type appliances.

Electrocontact pressure gauges and pressure switches are checked for accuracy of operation of signal contacts, the error of operation should not be higher than the passport one.

Verification procedure

Carry out maintenance of the pressure device:

Check the marking and safety of the seals;

The presence and strength of the fastening of the cover;

No broken ground wire;

The absence of dents and visible damage, dust and dirt on the case;

The strength of the sensor mounting (on-site work);

Integrity of cable insulation (on-site work);

Reliability of cable fastening in the water device (work at the place of operation);

Check tightening of fasteners (on-site work);

For contact devices, check the insulation resistance against the housing.

Assemble a circuit for contact pressure devices.

Slowly increasing the pressure at the inlet, take readings exemplary instrument with forward and reverse (pressure reduction) stroke. Reports should be made at 5 equally spaced points of the measurement range.

Check the accuracy of the contacts operation according to the settings.