Selecting the pressure gauge scale.

Need to know:

1 Instrument scales according to GOST

2 Requirements of the rules for pressure gauges (the optimal reading of the pressure gauge if the needle of the device at operating pressure is at 2/3 of the scale).

To solve the problem we have the formula Rshk=3/2Rrab.

For example: Given: Prab=36kgf/cm2. Determine Rshk?

Solution: Rshk = 3 36/2 = 54 kgf/cm 2.

We select the nearest scale according to GOST in the upward direction. This is 60 kgf/cm 2

Thus: Rshk=60

Pressure gauge installation requirement

1. The scale must be clearly visible.

2. The approach to the pressure gauge must be free.

3. Depending on the installation height of the pressure gauge, the diameter of the device is selected:

· up to 2 meters - diameter 100mm;

· from 2 to 3 meters - diameter 160mm;

· over 3 meters - installation of a pressure gauge is prohibited.

4. Each pressure gauge must have a shut-off device (3x running valve, valve or tap)

Pressure gauge maintenance rules.

According to technical instructions land on "O"

Departmental inspection once every 6 months.

State verification - once every 12 months.

Remove and install pressure gauges only using a wrench.

In case of pressure pulsation, the following measures must be taken:

· when the pulsation is low, a compensator is welded in;

· for large pulsations, a special device is used - an expander with two chokes.

There are three types of pressure:

1. Barometric (atmospheric) - RB;

2. Gauge (excessive) - Rm;

3. Absolute Ra = Рb + Рm.

Temperature measuring instruments

Classification

· Liquid thermometers;

· Manometric thermometers;

· Resistance thermal converters;

· Thermoelectric converters.

Temperature units:

1. System units – K (Kelvin); (T)

2. Non-system -C (Celsius) (t)

3. OK° = -273.15С°

Conversion of non-system units to system units

T = t+273.15

Liquid thermometers : accuracy class not lower than 1.5. Based on changes in the volume of liquid due to heating. Measuring range from -190 to +600 C. It is a closed glass tank connected to a capillary tube. Mercury, ethyl alcohol, and ether are used as liquids.

Manometric thermometers consist of:

· 2 - thermal cylinder;

· 1 - capillary tube;

· 6 - sensitive element.

The operating principle of the device is based on: on the dependence of the pressure of a liquid or vapor with a liquid in a closed system of constant volume on temperature.

There are: 1 liquid – TPZh; 2 gas - TPG, 3 vapor-liquid TPP. Measuring range -160 -+750С 0

Resistance thermal converters.

The operation of the device is based on changes in conductor resistance due to temperature changes. Measuring range from -260 to +1100 o C.

The resistance thermocouple is installed locally. Works with secondary device:

Connecting wires. Secondary device (does not work without a secondary device) Vj TSP - platinum resistance thermometer. TSM - copper resistance thermal sensor.

Thermoelectric converter. The operation of the device is based on the phenomenon of thermoelectric effect. In this case, when the temperature changes, the emf changes. Thermoelectric converter. Connecting wires. The secondary THC device is a Chromel-Copel thermal converter. THA - thermal converter chromel - alumel. Measurement range from -100" to +2200 o C.

Pressure gauges. Pressure units

Pressure gauges are designed to measure pressure and vacuum. Pressure gauges installed on GP, TP (pipelines), and devices indicate excess pressure. To obtain absolute pressure it is necessary to the number overpressure taken from the pressure gauge, add 1 (atmospheric pressure) in kgf/cm2.

Pressure gauges installed in gas supply systems are divided into:

· Liquid;

· Spring;

· Electric contact;

· Pressure and vacuum gauges.

Pressure and vacuum gauges, intended for measuring not only Rizb, but also for measuring rarefaction, i.e. pressure is less than atmospheric.

Liquid pressure gauges. They are designed to measure small pressures.

The zero mark of the scale is in the middle. One end of the tube communicates freely with the atmosphere. The second one is connected to the GP medium being measured through a rubber hose. The tube is filled with water (tinted) to the “0” mark; You can use alcohol, antifreeze, etc., but you need to make an adjustment for density, i.e. reduce its density to the density of water.

To take readings fromU-shaped liquid pressure gauge, it is necessary to add the decrease in the level in one elbow with its increase in the other.

Spring pressure gauges. They are designed to measure all pressures. A spring pressure gauge consists of a round box - a housing, in which there is a curved brass tube of oval cross-section. One end of the tube is sealed, and the other is connected through a three-way valve to the medium being measured. The sealed end of the tube (Bourdon) is connected through a lever to a gear sector associated with a gear, on the axis of which there is an arrow.

The pressure gauge has a scale (dial) on which the following data is printed:

1. GOST pressure gauge;

2. Case size (100, 160mm);

3. Issue date;

4. Pressure gauge accuracy class;

5. Error expressed in %;

6. Pressure gauge scale units (MPa, kgf/cm, bar, KPa, Pa);

7. The pressure measurement limit of this pressure gauge;

8. Type (MTP, OBM, MO, etc.).

Electric contact pressure gauges. This is a variation of the conventional spring pressure gauge. (ECM).

In addition to the black indicating arrow, the ECM has one or more light contact arrows. Voltage is supplied to the ECM through a special device.

ECMs operate in a system of automation, safety and regulation.

ECMs are installed on boiler drums, in front of boiler burners to control pressure, on the burner strictly according to the design.

Pressure gauge malfunctions:

· There is no stamp or government seal.

· The state verification of the pressure gauge is overdue.

· The glass is broken, the body is dented, the glass is dirty.

· There may be gas leaks through a leaky Bourdon tube of the pressure gauge.

· When landing at “0” the needle does not land at the zero mark.

· When checking the working pressure gauge, the readings do not coincide with the control one.

The serviceability and correctness of the pressure gauge readings is checked in the following periods:

1. Once a year - state verification in the laboratory of the state verifier.

2. At least once per shift – landing at “0”.

3. At least once every 2 months - check with a control pressure gauge.

The working position of the pressure gauge needle should be in the second third of the scale.

Pressure units

The basic SI unit of pressure is the pascal (Pa).

« One pascal - this is the pressure on a flat surface under the influence of a force that is directed perpendicularly and uniformly distributed to the surface and is equal to 1 Newton.”

In practice they use kilopa-rock (kPa) or megapascal (MPa), since the Pa unit is too small.

The pressure gauges currently in use also use the MKGSS system unit (meter, kilogram-force, second) kilogram-force per square meter () and non-system units of measurement for example kilogram-force per square centimeter ().

Another common unit of measurement is bar (1 bar = 10 Pa = 1.0197 kgf/cm). It is in the bars that the pressure gauges under study are calibrated.

The relationships between pressure units can be calculated using the formula:

P 1 =KЧP 2, (1.4 )

Where P 1 - pressure in the required units; P 2 - pressure in original units.

The values of coefficient K are given in Table 1.1.

Table 1.1.

Pressure gauges. Classification of pressure gauges

GOST 8.271-77 defines a pressure gauge as a device or measuring installation for determining the actual value of pressure or pressure difference.

Pressure gauges are classified according to the following characteristics:

the type of pressure for which the pressure gauge is designed;
operating principle of the pressure gauge;
purpose of the pressure gauge;
pressure gauge accuracy class;
characteristics of the measured environment;

Classifying pressure gauges according to the type of pressure measured, they can be divided into:

- measuring absolute pressure;
- measuring excess pressure;
- measuring discharge pressure, which are called vacuum gauges;

Most manufactured pressure gauges are designed to measure excess pressure. Their peculiarity is that when exposed to atmospheric pressure on the sensitive element, the instruments show “zero”.

There are also many variations of instruments united by the common name “pressure gauge”, for example pressure and vacuum gauges, pressure gauges, draft gauges, draft pressure gauges, difnanometers.

Pressure-vacuum meter- pressure gauge, with the ability to measure both excess pressure and rarefied gas pressure (vacuum).

Pressure meter- a pressure gauge that allows you to measure ultra-low values of excess pressure (up to 40 kPa).

Traction gauge- a vacuum gauge that allows you to measure small values of vacuum pressure (up to -40 kPa).

Diffnanometer- a device designed to measure the pressure difference at two points.

“Based on their operating principle, pressure gauges are classified into:

- liquid;
- deformation;
- deadweight piston;
- electric;

TO liquid include pressure gauges, the principle of operation of which is based on the pressure difference between the pressure of a liquid column. An example of such a pressure gauge is U-shaped pressure gauges. They consist of graduated communicating vessels in which the measured pressure can be determined by the level of liquid in one of the vessels.

Rice. 1.1. U-shaped liquid glass pressure-vacuum gauge:

1 -- U-shaped glass tube; 2 -- fastening brackets; 3 -- basis; 4 -- scale.

Strain gauges are based on the dependence of the degree of deformation of the sensing element on the pressure applied to this element. Basically, a tubular spring acts as a sensing element. We will learn more about them later.

Electrical pressure gauges operate on the basis of the dependence of the electrical parameters of the sensitive element of the converter on pressure.

IN deadweight pressure gauges a liquid is used as a working fluid, which creates pressure. This pressure is balanced by the mass of the piston and weights.

By the number of loads required for balance, we determine the pressure that the liquid creates.

Rice. 1.2. Schematic diagram deadweight pressure gauge:

1 --oil tank, 2 --pump, 3 --valves, 4, 5, b--inlet, drain and measuring column valves, respectively, 7 --measuring column, 8, 9 -- racks, 10, 11 -- rack valves, 12 --press.

According to their purpose, pressure gauges are divided into general technical and standard. General technical are intended for carrying out measurements during production activities. In general technical ones, vibration resistance to frequencies in the range of 10-55 Hz is structurally provided. They also provide resistance to external influences such as:

- ingress of external objects;
- temperature effects;
- water ingress;

« Reference manometric instruments are designed to store and transmit the size of pressure units to ensure uniformity, reliability and assurance high precision pressure measurements."

“According to the characteristics of the medium being measured, all pressure gauges are classified into:

general technical;
corrosion-resistant (acid-resistant);
vibration-resistant;
special;
oxygen;
gas."

General technical pressure gauge instruments are designed for measurements under normal conditions. Made from aluminum and copper alloys.

Corrosion resistant devices are made from chemically resistant materials such as steel of various grades. Also supplied with tempered laminated glass.

Special pressure gauges are designed to measure media different from normal conditions, for example for measuring the pressure of viscous substances or containing solid particles.

Vibration-resistant Pressure gauges are used in operating conditions where the vibration frequency exceeds 55 Hz. The internal volume of such pressure gauges is filled with a viscous liquid, such as glycerin or silicone. The housing in a vibration-resistant pressure gauge must be sealed and contain special rubber seals.

In gas Pressure gauges use a number of design solutions that should ensure safety in the event of a rupture of the sensitive element. A dividing partition is installed between the scale and sensitive element. The viewing window in such pressure gauges is multi-layered and reinforced. There is an unloading valve on the rear wall, which, in case of excess permissible pressure opens and relieves pressure. During production special attention pay attention to materials because many gases have specific properties.

“Oxygen pressure gauges are used to measure pressure in environments with an oxygen content of 23% or more.” Since when oxygen comes into contact with some organic substances and mineral oils, it detonates, there are strict requirements for cleanliness of oils. Structurally they do not differ from general technical pressure gauges.

Required marks on pressure gauges

The following must be marked on the pressure gauge dial:

1) Units of measurement;
2) Operating position of the device;
3) Accuracy class;
4) Name of the measured medium in the case of a special version of the device;

-trademark of the manufacturer;
- sign of the State Register;

Table 1.2 shows the main symbols on the dial of pressure gauges.

Table 1.2

Labels indicating resistance to external conditions should also be indicated.

Table 1.3

And the degree of protection from external influences is also indicated.

None modern building cannot do without a heating system. And for its stable and safe operation, precise control of coolant pressure is required. If the pressure within the hydraulic graph is stable, then the heating system is operating normally. However, when it increases, there is a risk of pipeline rupture.

A decrease in pressure can also lead to such negative consequences, such as the formation of cavitation, that is, air bubbles form in the pipeline, which, in turn, can cause corrosion. Therefore, maintaining normal pressure is extremely necessary, and thanks to the pressure gauge this becomes possible. Besides heating systems Such devices are used in a wide variety of fields.

Description and purpose of the pressure gauge

A pressure gauge is a device that measures pressure levels. There are types of pressure gauges that are used in a variety of industries, and, of course, each of them has its own pressure gauge. For example, you can take a barometer - a device designed to measure atmospheric pressure. They are widely used in mechanical engineering, in agriculture, in construction, industry and other areas.

These devices measure pressure, and this concept is flexible, at least, and this value also has its own varieties. To answer the question of what pressure the pressure gauge shows, it is worth considering this indicator as a whole. This is a quantity that determines the ratio of the force acting per unit area of a surface perpendicular to that surface. Almost any process accompanied by this value.

Types of pressure:

To measure each of the above types of indicators, there are certain types of pressure gauges.

Types of pressure gauges differ in two ways: by the type of indicator they measure and by the principle of operation.

According to the first sign, they are divided into:

They work on the principle of balancing the pressure difference with a certain force. Therefore, the design of pressure gauges is different, depending on how exactly this balancing occurs.

Based on their operating principle, they are divided into:

According to their intended purpose, there are such types of pressure gauges as:

Device and principle of operation

The pressure gauge device may have different design depending on the type and purpose. For example, a device that measures water pressure has a fairly simple and understandable design. It consists of a body and a scale with a dial that displays the value. The housing has a built-in tubular spring or membrane with a holder, a tripod-sector mechanism and an elastic element. The device operates on the principle of equalizing pressure due to the force of changing the shape (deformation) of the membrane or spring. And the deformation, in turn, sets in motion a sensitive elastic element, the action of which is displayed on the scale using an arrow.

Liquid pressure gauges consist of a long tube that is filled with liquid. In the tube with the liquid there is a movable plug, which is influenced by the working environment; the pressure force should be measured depending on the movement of the liquid level. Pressure gauges can be designed to measure differences; such devices consist of two tubes.

Piston - consist of a cylinder and a piston located inside. The working medium in which the pressure is measured acts on the piston and is balanced by a load of a certain size. When the reading changes, the piston moves and triggers a needle that shows the pressure value.

Thermal conductive consist of filaments that heat up when an electrical discharge is passed through them. The operating principle of such devices is based on a decrease in the thermal conductivity of gas with pressure.

Pirani pressure gauge named after Marcello Pirani, who first designed the device. Unlike thermal conductors, they consist of metal wiring, which also heats up when current passes through it and cools under the influence of the working medium, namely gas. As the gas pressure decreases, the cooling effect also decreases, and the wiring temperature increases. The quantity is measured by measuring the voltage in a wire while current passes through it.

Ionization are the most sensitive devices used to calculate low pressures. As the name of the device suggests, its operating principle is based on the measurement of ions that are formed under the influence of electrons on a gas. The number of ions depends on the density of the gas. However, ions have a very unstable nature, which directly depends on the working medium of gas or steam. Therefore, for clarification, another type of McLeod pressure gauge is used. Clarification occurs by comparing the readings of the ionization pressure gauge with the readings of McLeod's device.

There are two types of ionization devices: hot and cold cathode.

The first type was designed by Bayard Allert; it consists of electrodes that operate in triode mode, and the filament acts as the cathode. The most common type of hot cathode is an ion pressure gauge, in the design of which, in addition to a collector, filament and grid, a small ion collector is built in. Such devices are very vulnerable; they can easily lose calibration, depending on operating conditions. Therefore, the readings of these devices are always logarithmic.

The cold cathode also has its own varieties: an integrated magnetron and a Penning pressure gauge. Their main difference is the position of the anode and cathode. There is no filament in the design of these devices, so they require a voltage of up to 0.4 kW to operate. The use of such devices is not effective at low pressure levels. Because they may simply not earn money and not turn on. The principle of their operation is based on the generation of current, which is impossible in the complete absence of gas, especially for a Penning pressure gauge. Since the device only works in a certain magnetic field. It is necessary to create the desired trajectory of ion movement.

Marking by color

Pressure gauges that measure gas pressure have colored bodies; they are specially painted in various colors. There are several primary colors that are used to paint the body. Like, for example, pressure gauges that measure oxygen pressure have a housing blue color With symbol O2, ammonia pressure gauges have a body painted yellow, acetylene - white, hydrogen - dark green, chlorine - gray. Devices measuring the pressure of flammable gases are painted red, and non-flammable ones are painted black.

Benefits of use

First of all, it is worth noting the versatility of the pressure gauge, which lies in the ability to control the pressure and maintain it at a certain level. Secondly, the device allows you to obtain accurate indicators of the norm, as well as deviations from them. Thirdly, accessibility, almost anyone can afford to purchase this device. Fourthly, the device is capable of operating stably and uninterruptedly for a long time, and does not require special conditions or skills.

The use of such devices in areas such as medicine, the chemical industry, mechanical and automotive industries, maritime transport and others requiring precise pressure control greatly facilitates the work.

Instrument accuracy class

There are a lot of pressure gauges, and each type is assigned a certain accuracy class according to GOST requirements, which means the permissible error expressed as a percentage of the measurement range.

There are 6 accuracy classes: 0.4; 0.6; 1; 1.5; 2.5; 4. Each type of pressure gauge also differs. The above list refers to working pressure gauges. For spring devices, for example, the following indicators correspond to 0.16; 0.25 and 0.4. For piston engines - 0.05 and 0.2 and so on.

The accuracy class is inversely proportional to the diameter of the instrument scale and the type of instrument. That is, if the scale diameter is larger, then the accuracy and error of the pressure gauge decreases. The accuracy class is conventionally denoted by the following Latin letters KL; you can also find CL, which is indicated on the instrument scale.

The error value can be calculated. For this, two indicators are used: accuracy class or KL and measurement range. If the accuracy class (KL) is 4, then the measurement range will be 2.5 MPa (Megapascal), and the error will be 0.1 MPa. The product is calculated using the formula accuracy class and measurement range divided by 100. Since the error is expressed as a percentage, the result must be converted to a percentage by dividing by 100.

In addition to the main type, there is an additional error. If for the calculation of the first type we use ideal conditions or natural values that affect the design features of the device, then the second type directly depends on the conditions. For example, from temperature and vibration or other conditions.

Each vessel and independent cavities with different pressures must be equipped with direct-acting pressure gauges. The pressure gauge is installed on the vessel fitting or pipeline between the vessel and the shut-off valve.

Pressure gauges must have an accuracy class of at least: 2.5 - at a vessel operating pressure of up to 2.5 MPa (25 kgf/cm2), 1.5 - at a vessel operating pressure above 2.5 MPa (25 kgf/cm2).

The pressure gauge must be selected with a scale such that the limit for measuring working pressure is in the second third of the scale.

The owner of the vessel must mark the pressure gauge scale with a red line indicating the operating pressure in the vessel. Instead of the red line, it is allowed to attach a metal plate painted red to the pressure gauge body and tightly adjacent to the glass of the pressure gauge.

The pressure gauge must be installed so that its readings are clearly visible to operating personnel.

Nominal diameter the housing of pressure gauges installed at a height of up to 2 m from the level of the observation platform must be at least 100 mm, at a height of 2 to 3 m - at least 160 mm.

Installation of pressure gauges at a height of more than 3 m from the site level is not permitted.

A three-way valve or a device replacing it must be installed between the pressure gauge and the vessel, allowing periodic checking of the pressure gauge using a control valve.

In necessary cases, the pressure gauge, depending on the operating conditions and the properties of the medium in the vessel, must be equipped with either a siphon tube, or an oil buffer, or other devices that protect it from direct exposure to the medium and temperature and ensure its reliable operation.

On vessels operating under pressure above 2.5 MPa (25 kgf/cm2) or at an ambient temperature above 250°C, as well as with an explosive atmosphere or harmful substances For hazard classes 1 and 2 according to GOST 12.1.007-76, instead of a three-way valve, it is allowed to install a separate fitting with a shut-off device for connecting a second pressure gauge.

On stationary vessels, if it is possible to check the pressure gauge in established by the Rules terms by removing it from the vessel, installation of a three-way valve or a replacement device is optional.

Pressure gauges and pipelines connecting them to the vessel must be protected from freezing.

The pressure gauge is not allowed for use in cases where:

· there is no seal or stamp indicating verification;

· the verification period has expired;

· when it is turned off, the needle does not return to the zero scale reading by an amount exceeding half the permissible error for this device;

· the glass is broken or there is damage that may affect the accuracy of its readings.

Checking of pressure gauges with their sealing or branding must be carried out at least once every 12 months. In addition, at least once every 6 months, the owner of the vessel must carry out an additional check of the working pressure gauges with a control pressure gauge and record the results in a log. control checks. In the absence of a control pressure gauge, it is allowed to carry out an additional check with a proven working pressure gauge that has the same scale and accuracy class as the pressure gauge being tested.