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Method of measuring pressure and pressure difference. Rules and methods for installing pressure gauges. Selection of a pressure gauge and requirements for its installation and operation Installation of pressure gauges on pipelines

I was faced with the need to indicate to the welder where to insert select fittings for installing pressure gauges on the pipeline. The project provides for selection for pressure sensors, but with a cunning wording: “Determine the installation location.”

The first thing I remembered was maintaining the straightness of the pipeline sections (about 5D before, and 3D after sampling), then, based on work experience, I remembered ensuring subsequent maintenance of devices related to ease of access, the possibility of installing devices with other overall dimensions, for example MP -4 instead of MP-3, providing a headroom for installing the main valve, siphon tube and three-way valve, preventing moisture from entering the electrical contact pressure gauge and its power cable.

Requirements for the installation of pressure gauges are regulated by Chapter 6.4 of the Rules for the Construction and Safe Operation of Steam and Hot Water Boilers and Chapter 2.8 of the Rules for the Construction and Safe Operation of Hot Water and Steam Pipelines (PB 10-573-03)

6.4. Pressure gauges

6.4.1. Each steam boiler must be equipped with a pressure gauge indicating steam pressure.

On steam boilers with a steam output of more than 10 t/h and hot water boilers with a heat output of more than 21 GJ/h (5 Gcal/h), the installation of a recording pressure gauge is required.

The pressure gauge must be installed on the boiler drum, and if the boiler has a superheater, also behind the superheater, before the main valve.

On direct-flow boilers, the pressure gauge must be installed behind the superheater, in front of the shut-off valve.

Installation of a pressure gauge on steam superheaters of locomotive, locomotive, fire tube boilers and vertical type boilers is not required.

6.4.2. Each steam boiler must have a pressure gauge installed on the supply line in front of the body that regulates the water supply to the boiler.

If several boilers with a steam capacity of less than 2.5 t/h each are installed in the boiler room, it is allowed to install one pressure gauge on the common supply line.

6.4.3. When using a water supply network, instead of a second feed pump, a pressure gauge must be installed in the immediate vicinity of the boiler on this water supply network.

6.4.4. On a water-switched economizer, pressure gauges must be installed at the water inlet, up to the shut-off valve and safety valve, and at the water outlet, up to the shut-off valve and safety valve.

If there are pressure gauges on the common supply lines to the economizers, installing them at the water inlet to each economizer is not necessary.

6.4.5. On hot water boilers, pressure gauges are installed at the water inlet to the boiler and at the outlet of heated water from the boiler to the shut-off valve, on the suction and discharge lines of circulation pumps located at the same height, as well as on the boiler supply lines or heating network feed lines.

6.4.6. The accuracy class of pressure gauges must be no lower than:

a) 2.5 - at operating pressure up to 2.5 MPa (25 kgf/cm2);

b) 1.5 - at a working pressure of more than 2.5 to 14 MPa (25 to 140 kgf/cm2);

c) 1.0 - at a working pressure of more than 14 MPa (140 kgf/cm2).

6.4.7. The pressure gauge scale is selected based on the condition that at operating pressure the pressure gauge needle should be in the middle third of the scale.

6.4.8. A red line should be marked on the pressure gauge scale at the division level corresponding to the operating pressure for a given element, taking into account the additional pressure from the weight of the liquid column.

Instead of the red line, it is allowed to attach to the pressure gauge body a metal plate painted red and tightly adjacent to the pressure gauge glass.

6.4.9. The pressure gauge must be installed so that its readings are clearly visible to operating personnel, and its scale should be positioned vertically or tilted forward up to 30° to improve the visibility of the readings.

The nominal diameter of pressure gauges installed at a height of up to 2 m from the level of the pressure gauge observation platform must be at least 100 mm, at a height of 2 to 5 m - at least 160 mm, at a height of more than 5 m - at least 250 mm. When installing a pressure gauge at a height of more than 5 m, a reduced pressure gauge must be installed as a backup.

6.4.10. A three-way valve or other similar device must be installed in front of each pressure gauge to purge, check and turn off the pressure gauge; in front of the pressure gauge designed to measure steam pressure, in addition, there must be a siphon tube with a nominal diameter of at least 10 mm.

On boilers with a pressure of 4 MPa (40 kgf/cm2) and higher, instead of a three-way valve, valves should be installed that allow the pressure gauge to be disconnected from the boiler, to ensure its communication with the atmosphere and to purify the siphon tube.

6.4.11. Pressure gauges are not allowed for use in the following cases:

a) if there is no seal or stamp on the pressure gauge indicating that verification has been carried out;

b) if the period for checking the pressure gauge has expired;

c) if the pressure gauge needle, when it is turned off, does not return to the zero scale mark by an amount exceeding half the permissible error for a given pressure gauge;

d) if the glass is broken or there is other damage to the pressure gauge that may affect the accuracy of its readings.

The accuracy class of pressure gauges used in the gas industry must be at least 1.5

Well, it is imperative to take into account the requirements: SNiP 2.04.01-85 clause 12.16 states: (12.16. A check valve, valve and pressure gauge should be installed on the pressure line of each pump, and a valve and pressure gauge should be installed on the suction line.)

1. Schemes for measuring common media with pressure gauges installed “in situ”

Measured medium gas, liquid

2. Schemes for measuring common media with manometers installed “in situ”

Medium being measured HOT gases, liquids, steam

Installation diagram of a pressure gauge on a vertical pipeline

Installation diagram of a pressure gauge on a horizontal pipeline

1-pipe, 2-condensation tube, 3-3-way valve, 4-pressure gauge(pressure sensor)

3. Circuits for measuring aggressive media

4. a) aggressive liquid

1 - pipeline;

2 - shut-off valve;

3 - separation vessel;

4 - coupling;

5 - pressure gauge.

1 - pipeline;

2- three-way valve;

3- membrane separator;

4 - coupling;

5 - pressure gauge.

4. Circuits for measuring pulsating media


1 - pipeline; 2 - shut-off valve; 3 - three-way valve; 4 - throttle damper;

5 - spiral damper; 7 - ring-shaped damper; 8 - adapter connector;

9 - pressure gauge.

Flow meters

I. Measuring the flow of media with variable differential flow meters

General provisions

1. Flow measurement with variable differential flow meters is carried out using orifice devices (CDs), operating in sets with differential pressure gauges-flow meters. Standard control systems are chamber and disk diaphragms, nozzles, Venturi nozzles, etc. When controlled media flow through the control system, pressure differences are created on the latter (pressure differences before and after the control system), which are functions of the measured flow rates. The pressure differences created by the control system are supplied to the differential pressure gauges through two impulse pipes of the measuring pipework (ITP). Differential pressure gauges perceive pressure differences across the control system and convert them into appropriate output (usually electrical: natural or unified) signals. ITP impulse lines are thus an integral and quite important part of measurement circuits.

2. Both impulse pipes must be in the same temperature conditions, which must be ensured by their joint laying in close proximity to each other. The places where ITP is laid are called pipe routes. Pipe lines are laid, as a rule, along special supporting and fastening structures, over the shortest distances and with mandatory appropriate slopes for drainage and removal of accumulated liquids and gases.

3. The main problem of using variable differential flow meters is to “fight” the contamination of impulse lines (when measuring liquid flow) and the filling of impulse lines with liquids (when measuring gas flow). Gases and liquids that interfere with measurements, reduce the accuracy of measurements and lead to failure of instruments, firstly, randomly enter the impulse lines from pressure points, and secondly, are naturally released in the impulse pipes themselves due to the difference in temperature and density of the media in places of their selection and places of measurement.

4. ITP must ensure the possibility of their operation: checking, testing, purging, washing, filling with separation liquids, etc. both instruments and automation equipment, as well as the pipe lines themselves, periodic removal of accumulated liquids and gases from impulse lines without stopping the process equipment. For this purpose, ITPs are equipped with: shut-off valves (taps, valves, gate valves, etc.) - to disconnect impulse lines during repair and adjustment work, gas collectors (when measuring liquid flow) and moisture collectors (when measuring gas flow) - to collect and removal, respectively, of gases and liquids interfering with measurements, membrane separators, separation, condensation and equalization vessels, etc. Shut-off valves are installed immediately after the pressure tapping points on the control system, gas collectors - in places where gases accumulate, i.e. at the highest points of the ITP, moisture collectors - in places where liquids and condensate accumulate, i.e. at the lowest points of the ITP.

5. Pipe wiring of automation systems must have mechanical strength and tight connections (taking into account pressure, temperature, vibration, pulsation, the degree of aggressiveness of the measured media, as well as atmospheric and climatic influences). Pipe lines are made from appropriate materials (steel, aluminum and aluminum alloys, copper, brass, plastics, in some cases rubber, etc.). Changing the direction of the ITP should, as a rule, be done by bending the pipes (if necessary, corner connectors, adapters, splitters, etc. can be used). In some areas, compensators (turns, elbows) for thermal expansion of pipes can be used.

6. To reduce local resistance, full-bore shut-off, connecting and connecting devices must be installed on impulse lines. The diameters (flow sections) of the pipes must be optimal in terms of dynamic properties so that the signal transmission time is minimal. In this case, a certain ratio of lengths and diameters must be met, for example, length up to 45 m: water, air, dry gas - diameter 10 mm; wet gas – diameter 13 mm; contaminated environments – diameter 25 mm.

7. In practice, measurement schemes are used with the installation of differential pressure gauges both below and above the control unit, which makes significant differences in the operating conditions of the flow meters.

1. Schemes for measuring the flow of non-aggressive media with variable differential flow meters

  1. Before proceeding with installation, make sure that the device meets the requirements for its measuring range and design. The operating pressure reading should be in the middle third of the range.
  2. The device should be positioned in such a way that it is convenient to read its readings. The pressure gauge must be secured so that vibrations are minimal. If vibration loads exceed the permissible limit, use vibration-resistant instruments to avoid high measurement errors.
  3. Check the tightness of the connection.
  4. To ensure the possibility of replacing the device and monitoring the “zero”, a shut-off device should be installed between the pipeline or other pressure measurement point and the pressure gauge. A three-way valve can serve as such a device.
  5. Depending on the purpose of the device, it may be equipped with valves or shut-off valves.
  6. Place on the pipeline or technical. The equipment to which the pressure control device is connected is called a pressure tap or pulse.
  7. The route that connects the pressure gauge and the pressure tap is called the impulse line.
  8. Copper, solid-drawn steel, or PVC tubes are used as impulse lines. The material used to manufacture the tubes depends on the aggressiveness of the medium being measured, the pressure, as well as the fire and explosion hazard of the medium.
  9. Depending on the length of the route and the maximum operating pressure limits of the measured medium, the thickness and diameter of the impulse tubes are selected.
  10. To measure the pressure of the medium for the purpose of control, impulse lines must be laid strictly following the installation diagram of the facility automation, which indicates the full characteristics of the line (type of material used, wall thickness and cross-section). The diagram also shows the length of the route.
  11. The connection point for the pressure sampling device (pressure pulses) must be on a straight section of the pipeline and technical. equipment taking into account turns, bends, tees and elbows, since in the above areas, as a result of the centrifugal force of the measured medium flow, there is an additional measurement error.
  12. The effect of temperature on the accuracy of readings must be monitored. To do this, the pressure gauge is installed taking into account the influence of convection and thermal radiation, in order to prevent the temperature of the surrounding and measured medium from being higher or lower than permissible for the operation of the measuring device. To do this, pressure gauges and shut-off valves should be protected with water dead-end pipes or measuring lines of sufficient length.
  13. If there are media of high viscosity, aggressive, crystallizing, contaminated, or hot media, it is necessary to use membrane media separators to prevent them from entering the device. The internal space of the pressure gauge and separator is filled with a special working fluid, which transmits pressure from the separator membrane to the measuring device. The liquid is selected depending on the measuring range, compatibility with the medium being measured and taking into account the temperature.
  14. When measuring aggressive media (acids, alkalis), special separation vessels are used to protect the sensitive element of the device from exposure. They are filled with water, ethyl alcohol, glycerin or light mineral oils, etc.
    1. Sensing elements must be protected from overload.

If the pulsation of the measured medium exceeds the permissible norm or there is a possibility of water hammer, it is necessary to minimize their impact on the sensitive elements of the device.

  • Dampening of water hammer can be achieved by installing a throttle (reducing the cross-section of the pressure channel), or by installing an adjustable throttle device.
  • To minimize pressure pulsation of the measured medium at compensating stations, tech. equipment, pipelines, pumps, etc., a throttle must be installed in the pressure gauge fitting, which reduces the diameter of the inlet. This will prevent failure of the transmission mechanism of the devices.
  • If, in order to obtain more accurate results, the measuring range is selected smaller than the magnitude of short-term pressure surges, the sensing element must be protected from damage. This can be done by installing a special overload protection device. This device closes immediately in the event of a water hammer. If the pressure increases gradually, closure is also carried out gradually.
  • The closing amount is set depending on the nature of the pressure change in a certain time period.
  • Also, in the case of increased pulsation of the medium and hydraulic shocks, you can use special vibration-resistant pressure gauges, the design of which is designed to work with excess pressures.
  • Pressure gauge mounting.
    • If the connection to the pressure gauge does not provide sufficient mounting stability, you need to use additional fasteners on the wall or pipe, or provide capillary wiring for the device.
  • If it is necessary to dampen vibrations of the measuring system:
    • If installation does not solve the problem of minimizing shock and vibration, it is necessary to use specialized vibration-resistant pressure gauges with hydraulic filling.
    • When installing the pressure gauge, the dial must be oriented vertically. In case of deviations, pay attention to the position symbol on the dial.
    • To secure the pressure gauge in a position where the readings can be read as accurately as possible, you can use a union nut or a turnbuckle. It is not recommended to screw the device in and out by the housing. For this purpose, the connecting piece is provided with surfaces for a wrench.
    • At the junction of the pressure gauge with the pressure source, gaskets, fibers, washers made of leather, lead or soft copper should be used to seal.

    It is unacceptable to use tow and red lead for compaction!

    • In instruments that are used to measure oxygen pressure, gaskets should only be made of lead and copper.
    • In devices that are used to measure acetylene pressure, it is PROHIBITED to use gaskets made of copper and copper alloys with a copper content of more than 70%!
    • If the pressure gauge is located below the fittings for measuring pressure, immediately before connecting it is necessary to thoroughly flush the measuring line to prevent solids from entering the system.
    • Some devices have plugged holes to compensate for internal pressure. The hole is marked "closed" and "open". Usually the lever is in the “closed” position. Before checking, after installation and before starting work, the devices are filled with air, and accordingly the lever is moved to the “open” position.
    • During crimping, as well as purging containers or pipelines, the measuring device must not be subjected to a load such that the indicator exceeds the limit mark indicated on the dial. If this happens, you need to lock or dismantle the pressure gauge.
    • In case of dismantling, it is necessary to stop applying pressure to the measuring element. Or remove the voltage from the measuring line.
    • In pressure gauges that have a leaf spring, the tightening screws on the top and bottom flanges must not be removed.
    • In pressure gauges, after dismantling, residual measured media can have a negative impact on the environment. You need to take the necessary precautions for safety.
    • For devices in which sensitive elements are filled with water or a water mixture, protection against freezing must be provided.
    • The measuring line must be constructed or installed in such a way that tensile, thermal and vibration loads are absorbed.
    • When measuring gas pressure, provision must be made for drainage at the lowest point. If the medium being measured is liquid, provision must be made for deaeration at the highest point.
    • If the measured medium contains solid impurities, separators are used for this - cutting devices. During operation of the device, separators can be separated from the installation through shut-off valves in order to free them from impurities.

    • The error in measuring pressure depends on the instrumental errors of the measuring instruments, the operating conditions of the pressure gauges, and the methods of selecting pressure and transmitting it to the instruments. When choosing the measurement limits of a pressure gauge, they are guided by the values ​​of the measured pressure and the nature of its changes. With a stable measured pressure, its value should be 3/4 of the measuring range of the device, and in the case of variable pressure, 2/3. To eliminate the possibility of the formation of explosive and flammable mixtures, pressure gauges designed to measure the pressure of gases such as oxygen, hydrogen, ammonia are painted in blue, dark green, and yellow in accordance with the standard.

    Rules for installing pressure gauges at industrial facilities, pressure sampling and its transmission to devices using impulse lines are regulated by internal departmental standards that guide the installation of measuring devices. The main points of these guidance materials are discussed below.

    Indicating pressure gauges with remote transmission of readings are usually installed near pressure tapping points in a place convenient for maintenance. The exception is pressure gauges used for in-reactor monitoring and pressure monitoring in devices located at nuclear power plants in restricted access areas. Modern serial pressure transducers cannot be placed inside the core, therefore they are located at a considerable distance from the pressure sampling points, which leads to an increase in the inertia of instrument readings. It must be taken into account that the presence of a liquid column in the impulse line creates a systematic error in the readings, which will have a negative or positive sign depending on whether the pressure gauge is located above or below the pressure tapping point. The impulse lines of differential pressure gauges are long, the maximum value of which is 50 m.

    Pressure sampling is carried out using pipes connected to the pipeline or internal space of the object where the pressure is measured. In general, the tube should be flush with the inner wall so that the protruding part does not create flow retardation. When measuring pressure or pressure difference in liquid media, it is not recommended to take pressure from the lower and upper points of the pipeline, so that sludge and gases do not get into the impulse lines; in case of gas media, from the lower points of the pipeline, so that condensate does not get into the impulse lines .

    When measuring pressures and vacuums in gas ducts, air ducts, and dust pipes, there is often a need to smooth out pressure pulsations and separate suspended particles.

    Rice. 1. :

    1 - cyclone; 2 - dust pipe; 3 - metal wall; 4.5 - tubes; 6 - hole with plug

    In Fig. Figure 1 shows the installation of cyclone I on the pressure sampling line in the dust pipe 2, which has a metal wall 3. The dust-air mixture is supplied to the cyclone by tube 4 tangentially, pressure is taken to the device from the cyclone from its middle part by tube 5. In the cyclone, suspended particles are separated and periodically removed from it through hole 6. To smooth out pulsations, chokes are installed in front of the measuring device. The length of the lines from the pressure tapping point to the device must ensure cooling of the measured medium to ambient temperature. Using switch taps, one pressure meter or draft meter can be connected to several pressure or vacuum tapping points.



    Rice. 2. :

    1 - pressure gauge; 2 - three-way valve; 3 - shut-off valve; 4 - ring-bent tube

    Installation diagram of pressure gauge 1 on the pipeline shown in Fig. 2. To ensure the possibility of turning off the pressure gauge, purging the line and connecting the control pressure gauge, a three-way valve 2 is used; when measuring pressure above 10 MPa (100 kgf/cm2), as well as when monitoring the pressure of the radioactive coolant, an additional shut-off valve 3 is installed at the outlet of the pipeline. When measuring the pressure of media with temperatures above 70 °C, tube 4 is bent into a ring in which water is cooled and steam is condensed. At nuclear power plants, the purging of impulse lines of pressure gauges and differential pressure gauges working with radioactive media is carried out into a special drainage system.

    When measuring the pressure of aggressive, viscous and liquid metal media, membrane and liquid separators are used to protect pressure gauges and differential pressure gauges. The diagram of a pressure gauge with a membrane separator is shown in Fig. 3.



    Rice. 3. :

    1, 2 - aggressive and neutral environment

    1 - measured medium; 2 - separation vessel; 3 - line filled with neutral medium

    The aggressive medium is supplied under membrane 7, the lower part of which and the adjacent walls are covered with fluoroplastic. The space above the membrane 2 and the internal cavity of the pressure spring are carefully filled with silicone liquid. In order for the pressure above the membrane to correspond to what is being measured during the measurement process, it is necessary that the rigidity of the membrane be much less than the rigidity of the sensing element. When using liquid separators (Fig. 4), this limitation is absent.



    Rice. 4. Installation diagram of pressure gauges with separation vessels when the density of the measured medium is less than the neutral density (a) and greater (b):

    The neutral separating liquid filling part of the separating vessel 2, the measuring chamber of the device and the lines between them 3 must differ significantly in density from the measured medium 1 and not mix with it. In Fig. 4, and the density of the aggressive medium is less than the separation medium, and in Fig. 4, b - more.

    When measuring pressure differences, the differential pressure gauges must be connected in such a way that the medium filling the impulse lines does not create errors due to differences in densities or heights of liquid columns in them. The lines should not have horizontal sections, the minimum angle of inclination should be at least 5°. When measuring the pressure difference between water and steam, the measuring chambers of differential pressure gauges must first be filled with water.

    Not only the efficiency of the operation of technological objects, but in many cases also safety depends on the correct readings of pressure gauges; in this regard, pressure gauges and other pressure instruments are subject to periodic verification. For most instruments, the verification period is one year. If devices operate in conditions of high vibration and temperature, then this period can be shortened. Verification of instruments is carried out by representatives of metrological services.

    To carry out verification of working pressure devices, standard instruments and devices that reproduce pressure are used. For deadweight pressure gauges, these functions can be combined. When calibrating pressure gauges intended for measuring the pressure of chemically active gases, such as oxygen, deadweight pressure gauges filled with oil cannot be used.

    For certain types of equipment (pipelines, powerful steam and hot water boilers) there are strict standards for maximum permissible pressure. Exceeding these standards entails disruption of the production process, as well as a serious danger to workers and the environment. In order to comply with safety regulations, technical pressure gauges are installed on such equipment, regular checks of the readings of which make it possible to prevent emergency situations.

    Requirements for technical pressure gauges

    • high accuracy;
    • protection against temperature changes;
    • high pressure protection;
    • anti-corrosion protection.

    Information on how to install technical pressure gauges is contained in Chapter. 6.4 Rules for the operation and installation of various types of boilers and chap. 2.8 Rules for the operation and installation of pipelines (PB 10-573-03).
    To install a pressure gauge, a steam boiler pipe is usually used in front of the valve that regulates the water supply to the steam boiler. If there are several low-power boilers in the boiler room, fed from one pipe, then it is permissible to install one powerful pressure gauge in front of the entrance to the boilers. Water heating boilers are equipped with pressure gauges at the water inlet (before the valve) and at the outlet (before the safety valve), as well as on the suction and discharge units of circulation pumps.

    According to established standards, it should be done once a year. The verification process consists of a visual inspection of the device and comparison of its readings with the readings of the control pressure gauge. If the verification period has expired or there is no mark on the pressure gauge body indicating that its verification was carried out on time, it is prohibited to use the device.