Manometer requirements. How the rules interpret the application of the limit value mark on the scales of measuring instruments How the scale of the pressure gauge is selected

Choice of gauge scale.

Need to know:

1 Instrument scales according to GOST

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

To solve the problem, we have the formula Рshk=3/2Рrab.

For example: Given: Rrab \u003d 36kgf / cm 2. Determine Rshk?

Solution: Rshk \u003d 3 36/2 \u003d 54 kgf / cm 2.

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

Thus: Рshk=60

Manometer 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 - the installation of a pressure gauge is prohibited.

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

Pressure gauge maintenance rules.

According to the technical instructions, land on "O"

Departmental inspection once every 6 months.

State verification - 1 time in 12 months.

Remove and install pressure gauges only with a wrench.

In case of pressure pulsation, measures must be taken:

· at a small pulsation the compensator is welded;

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

There are three types of pressure:

1. Barometric (atmospheric) - Rb;

2. Manometric (excessive) - Rm;

3. Absolute Ra \u003d Rb + Rm.

Temperature measuring instruments

Classification

· Liquid thermometers;

· Manometric thermometers;

· resistance thermocouples;

· Thermoelectric converters.

Temperature units:

1. System units - K (Kelvin); (T)

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

3. OK° = -273.15°C

Conversion of non-system units to system ones

T \u003d t + 273.15

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

Manometric thermometers consist of:

2 - thermal bulb;

· one - capillary tube;

· 6 - sensitive element.

The principle of operation 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 - TPG; 2 gas - TPG, 3 vapor-liquid TPP. Measuring range -160 - +750С 0

Resistance thermocouples.

The operation of the device is based on the change in the resistance of the conductor due to temperature changes. Measurement range from -260 to +1100 о С.

The resistance thermal converter is installed in place. Works with a secondary device:

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

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. Secondary device THK - thermocouple chromel - kopel. ТХА - thermal converter chromel - alumel. The measurement range is from -100 "to +2200 ° C.

Each vessel and separate 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 stop valves.

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

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

On the pressure gauge scale, the owner of the vessel must put a red line indicating the working pressure in the vessel. Instead of a red line, it is allowed to attach a metal plate to the pressure gauge case, painted red and tightly adjacent to the pressure gauge glass.

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

The nominal diameter of the case of pressure gauges installed at a height of up to 2 m from the level of the observation site for them 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 level of the site is not allowed.

Between the pressure gauge and the vessel, a three-way valve or a device replacing it should be installed, which allows periodic checking of the pressure gauge using a control one.

If necessary, 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 environment or harmful substances of the 1st and 2nd hazard classes 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 within the time limits established by the Rules by removing it from the vessel, the installation of a three-way valve or a device replacing it 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 brand with a mark on the verification;

verification period has expired;

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

The glass is broken or damaged, which may affect the correctness of its readings.

Verification of pressure gauges with their sealing or branding should 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, recording the results in the log of control checks. In the absence of a control pressure gauge, it is allowed to carry out an additional check with a tested working pressure gauge that has the same scale and accuracy class with the tested pressure gauge.

On the scale of instruments intended for measuring pressure, a red line must be applied without fail. What does she mean? For what purposes is it established?

On the territory of our country, there are many regulatory documents that regulate the rules for the operation of pipelines, tanks, etc. And, in almost every document, it is indicated that a red stripe must be applied on the pressure gauge scale. Its purpose is to indicate the limit values of the measured parameter. Instead of drawing a line on the scale, it is permissible to use other marking methods, for example, a red metal flag. This is necessary in order to be able to observe the controlled parameter from afar.

In accordance with safety regulations in the oil and gas industry, it is expressly stated that pressure gauges located at a height of more than two meters must be marked with such a mark.

The technical pressure gauge in its design is classified as a tubular-spring mechanism. Structurally, it consists of:

corps;
riser;
hollow curved tube;
arrow(s);
sector with applied teeth;
gears;
springs.

The key part is the tube. Its lower end is connected to the hollow part of the riser. The upper end of the tube is sealed and can move, while it transmits movement to the sector installed on the riser, and at the end of this mechanism, a gear is installed with an arrow attached to it. After turning on the pressure gauge to the tank or pipeline on which the pressure will be measured. The pressure, which is concentrated inside the pressure gauge, through the mechanism described, tries to straighten the tube. The movement of the tube, as a result, leads to the movement of the arrow. After all this, the arrow shows the measured pressure.

How to use a technical manometer

Service of the technical pressure gauge consists of several simple operations. In particular, this is a check of its performance, removal of information from the measuring scale, pressure supply, zeroing. If the liquid in the device is contaminated, then it must be changed, otherwise, this will lead to distortion of the measurements. When carrying out maintenance, it is necessary to check the presence of a sufficient amount of working fluid. If its level is insufficient, it must be topped up, guided by the requirements of the operating instructions for the measuring device.

All pressure measuring devices must be leveled to measure. Otherwise, the readings will vary.

Most inclined instruments have a built-in device for leveling the pressure gauge. The device can be rotated until the bubble in the level takes the correct position at the zero mark.

Measuring pressure range

In practice, the following types of pressure are divided: absolute, barometric, excess, vacuum.
Absolute is a measure of pressure measured relative to full vacuum. This indicator cannot be lower than zero.
Barometric is atmospheric pressure. Its level is influenced by the height above zero (sea level). At this altitude, it is generally accepted that the pressure is 760 mm R.S. for pressure gauges, this value is zero.
Gauge pressure is a measurement between absolute and bromometric pressure. This is especially true when the absolute pressure is in relation to the barometric.

Vacuum is a value that shows the difference between absolute and barometric pressure when the barometric pressure is exceeded.

That is, the vacuum pressure cannot exceed the barometric pressure. In other words, instruments for measuring vacuum measure its discharge.

No modern building is complete without a heating system. And for its stable and safe operation, precise control of the coolant pressure is required. If the pressure is stable within the hydraulic curve, the heating system is working normally. However, when it increases, there is a risk of pipeline rupture.

A decrease in pressure can also lead to such negative consequences as, for example, the formation of cavitation, that is, air bubbles form in the pipeline, which, in turn, can cause corrosion. Therefore, it is essential to maintain normal pressure, and thanks to the pressure gauge, this becomes possible. In addition to heating systems, such devices are used in a wide variety of areas.

Description and purpose of the pressure gauge

A manometer is a device that measures the level of pressure. There are types of pressure gauges that are used in a variety of industries, and, of course, a different pressure gauge is designed for each of them. For example, you can take a barometer - a device designed to measure the pressure of the atmosphere. They are widely used in mechanical engineering, agriculture, construction, industry and other fields.

These devices measure pressure, and this concept is loose, at least, and this quantity 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 this surface. Almost any technological process is accompanied by this value.

Types of pressure:

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

The 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 feature, they are divided into:

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

According to the principle of action, they are divided into:

By appointment, there are such types of manometers as:

Device and principle of operation

The pressure gauge device may have a different design depending on the type and purpose. So, for example, a device that measures the pressure of water has a fairly simple and understandable design. It consists of a body and a scale with a dial that displays the value. The body has a built-in tubular spring or a membrane with a holder, a trippy-sector mechanism and an elastic element. The device operates on the principle of pressure equalization 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 with an arrow.

Liquid manometers consist of a long tube that is filled with liquid. There is a movable plug in the tube with liquid, which is affected by the working medium; the force of pressure should be measured depending on the movement of the liquid level. Pressure gauges can be designed to measure the difference, 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 indicator changes, the piston will move and actuate the arrow, which shows the pressure value.

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

Pirani pressure gauge named after Marcello Pirani, who first designed the device. Unlike thermal conductors, it consists of metal wiring, which also heats up during the passage of current through it and cools under the influence of the working medium, namely gas. When the gas pressure decreases, the cooling effect also decreases, and the temperature of the wiring increases. The magnitude is measured by measuring the voltage in the wire while current is flowing through it.

Ionization are the most sensitive devices that are used to calculate low pressures. As the name of the device implies, its principle of operation is based on the measurement of ions, which are formed when electrons act 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, a different type of McLeod pressure gauge is used for clarification. Refinement occurs by comparing the indicators of the ionization manometer with the readings of the McLeod device.

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

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

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

Color marking

Pressure gauges that measure gas pressure have colored cases, they are specially painted in different colors. There are several basic colors that are used to color the hull. As, for example, pressure gauges that measure oxygen pressure have a blue body with the symbol O2, ammonia pressure gauges have a yellow body, acetylene - white, hydrogen - dark green, chlorine - gray. Instruments that measure the pressure of combustible gases are painted red, and non-combustible - black.

Benefits of using

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

The use of such devices in such areas as medicine, chemical industry, mechanical and automotive industry, 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 in accordance with the requirements of GOST, which refers to the permissible error, expressed as a percentage of the measurement range.

There are 6 accuracy classes: 0.4; 0.6; one; 1.5; 2.5; 4. For each type of pressure gauge, they also differ. 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 - 0.05 and 0.2 and so on.

The accuracy class is inversely proportional to the diameter of the scale of the instrument 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 meet CL, which is indicated on the scale of the device.

The error value can be calculated. For this, two indicators are used: the accuracy class or KL and the 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 by the formula accuracy class and measuring 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 view, there is an additional error. If ideal conditions or natural quantities are used to calculate the first type, which 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.

0.6; 1.0; 1.6; 2.5; 4.0

6 10 16 25 40

60 100 160 250 400

600 1000 1600 kgf/cm 2

Ticket number 7

Pressure gauges with a single-coil tubular spring OBM

OBM-100; OBM-160 - general purpose pressure gauges;

100, 160 - body diameter in mm.

These devices are the most common. Their advantages: simplicity of the device; reliability in work; compactness; large measuring range; low cost.

The principle of operation is based on balancing the measured pressure with the force of elastic deformation of the spring.

Under the action of pressure, the cross section of the tube tends to take a round shape, as a result of which the tube unfolds by an amount proportional to the pressure. When the pressure is reduced to atmospheric, the tube returns to its original shape.

The sensitive element (SE) of the pressure gauge is a single-turn tubular spring, which is a tube bent around the circumference with an oval-shaped cross section. The tubular spring is made of bronze, brass or steel, depending on the purpose of the device and the measurement limits.

One end of the tube is soldered into a holder with a fitting, which is designed to connect the pressure gauge to a pressure source.

The other end of the tube is free, hermetically closed.

A rod is attached to the free end of the tubular spring. The other end of the rod is connected to the shank of the gear sector. The shank of the toothed sector has a slot (scene) along which the end of the rod can be moved when adjusting the device.

The toothed sector is held on the axis and engages with a small gear called a pinion. It is rigidly mounted on the axis of the arrow.

To eliminate the "dead motion" of the arrow, caused by the presence of backlash in the connections, the pressure gauge is equipped with an elastic hair of a spiral shape, made of phosphor bronze. The inner end of the hair is attached to the axis of the arrow, and the outer end to the fixed part of the device.

Under the action of pressure inside the tube, its free end moves and pulls the rod along with it. At the same time, the toothed sector and the tribka rotate, on the axis of which the arrow is mounted. The end of the arrow shows the value of the measured pressure on the scale of the device.

Rice. 2.4 Spring gauge:

1 - nipple;

2 - holder;

3 - (body) board;

5 - gear (tribe);

6 - spring;

7- Bourdon tube;

8- sealed end;

9 - gear sector;

10 - arrow;

Depending on the purpose, pressure gauges have the following markings:

MTP, MVTP - vibration-resistant;

SV - ultrahigh pressure;

MTI, VTI - accurate measurements (accuracy class 0.6; 1.0);

MO, VO - exemplary (class 0.4);

MT, MOSH, OBM - technical.

Ticket number 9

Selection of pressure gauges according to the permitted

working pressure

The scale of the manometer must be marked with a red line corresponding to the permitted working pressure.

The red line is placed on 2/3

manometer scales.

Rice. 2.5. red line gauge

Ticket number 10

If you need to select a pressure gauge according to the permitted pressure P razr, then

and select the nearest higher value from the gauge series.

Example:

Select the scale of the pressure gauge, if R razr = 10 kgf / cm 2

There is no such scale, so the scale is selected from 0 to 16 kgf / cm 2 .

Ticket number 11

Electrocontact pressure gauges

They have an electrical contact device that is triggered when a predetermined pressure is reached and sends a pulse to signaling devices.

The manometer is supplied with two control arrows with contacts. The control arrows are set to "max" and "min" pressure, and the arrow of the device, which carries the contacts, moving, gives a signal if the pressure has reached the values set by the control arrows.

EKM - electrocontact pressure gauge (used to signal parameters in explosion-proof rooms);

EKV - electrocontact vacuum gauge;

VE-16rb - electrocontact pressure gauge, explosion-proof design.

Ticket number 12

Pressure control circuit

Some conditions for the designation of instrumentation and automation: