Pressure gauges. Pressure units
Manometers are designed to measure pressure, vacuum. Pressure gauges installed on the HP, TP (pipelines), devices show excess pressure. To obtain absolute pressure, it is necessary to add 1 (atmospheric pressure) in kgf / cm2 to the number of excess pressure taken from the pressure gauge.
Pressure gauges installed in gas supply systems are divided into:
· Liquid;
· Spring;
· Electrocontact;
· Pressure vacuum meters.
Pressure vacuum meters, designed to measure not only Rizb, but also to measure rarefaction, i.e. pressure is less than atmospheric.
Liquid manometers. They are designed to measure small pressures.
The zero mark of the scale is in the middle. One end of the tube freely communicates with the atmosphere. The second - through a rubber hose is connected to the measured medium HP. The tube up to the "0" mark is filled with water (tinted); alcohol, antifreeze, etc. can be used, but it is necessary to correct for density, i.e. bring its density to the density of water.
To take readings fromU-shaped liquid manometer, it is necessary to add the level decrease in one knee with its increase in the other.
Spring gauges. They are designed to measure all pressures. The spring pressure gauge consists of a round box - a body in which there is a bent brass tube of oval section. One end of the tube is sealed, and the other is connected to the measured medium through a three-way valve. The sealed end of the tube (Bourdon), through the lever, is connected to the gear sector, coupled with the 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 manometer;
2. Case size (100, 160mm);
3. Release date;
4. Pressure gauge accuracy class;
5. Accuracy expressed in %;
6. Pressure gauge scale units (MPa, kgf/cm, bar, kPa, Pa);
7. Limit of pressure measurement by this manometer;
8. Type (MTP, OBM, MO, etc.).
Electrocontact pressure gauges. This is a variation of the conventional spring pressure gauge. (EKM).
In addition to the black indicating arrow, EKM has one or more light contact arrows. Voltage is applied to the ECM through a special device.
EKM work in the system of automation, safety and regulation.
EKM are installed on the drums of boilers, in front of the burners of the boilers for pressure control, on the burner strictly according to the project.
Gauge faults:
· There is no stigma or seal of the sovereign.
· The state verification of the pressure gauge has expired.
· The glass is broken, the body is dented, the glass is dirty.
· There may be gas leaks through a leaky Bourdon tube on the pressure gauge.
· When landing on "0" the arrow does not sit on the zero mark.
· When checking the working pressure gauge, the control readings do not match.
The serviceability and correctness of the pressure gauge readings is checked at the following times:
1. Once a year - state verification in the laboratory of the state verification.
2. At least once per shift - landing on "0".
3. At least 1 time in 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.
The term “manometer” is used in the text, the name “manometer” is general. This concept also refers to vacuum gauges and mano-vacuum gauges. This material is not related to digital devices.
Pressure gauges are devices that are widely used in industry and housing and communal services. At enterprises in the production process, it becomes necessary to control the pressure of liquids, steam and gas. Depending on the specialization of the enterprise, there is a need to measure various media. Manometers have been developed for this purpose. for various purposes. The difference between the devices is due to the medium being measured and the conditions in which the measurement is made. Pressure gauges differ in design, size, connection thread, units of measurement and possible measurement range, accuracy class, as well as the material of manufacture, on which the possibility of using the device in aggressive environments depends. The choice of a device that does not correspond to the tasks performed, entails the failure of the device earlier than the expected life, measurement errors, or overpayment for unused device functions.
Classification of pressure gauges depending on the criteria
Depending on the area of application.
Technical pressure gauges of standard design - used to determine the excess and vacuum pressure of non-aggressive, non-crystallizing media: liquids, steam and gas.
Technical special - this type of pressure gauge is used to measure specific media (for example, aggressive) or under special conditions (high vibration or temperature, etc.).
Special devices:
Ammonia as well as corrosion resistant pressure gauges in their design they have parts and mechanisms made of stainless steel and alloys that are resistant to aggressive environments, as a result of which this type of device can be used for work where interaction with an aggressive environment is provided.
Vibration resistant pressure gauges can be used in conditions of exposure to vibration exceeding 4-5 times the frequency of vibration allowed for operation of a conventional pressure gauge.
The main distinguishing feature of vibration-resistant pressure gauges is the presence of a special damping device, which is located in front of the pressure gauge. This device helps to reduce pressure pulsations.
Some types of vibration resistant pressure gauges can be filled with damping fluid. Vibration resistance is achieved thanks to the vibration-absorbing substance, which is glycerin.
Accurate pressure gauges used in government sectors. mertological control, in heat supply, water supply, energy, mechanical engineering, etc. In addition, they are used as a standard for verification and calibration of instruments for measuring pressure in compliance with the requirements for compliance with the accuracy classes of the instrument used as a sample and the instrument under test.
Railway gauges used to measure the excess vacuum pressure of media that are non-aggressive with respect to copper alloys in systems and installations of rolling stock and to measure the pressure of freons in refrigerators in refrigerated cars.
Cases of pressure gauges, depending on the field of application, are painted in the appropriate colors. Ammonia - in yellow, for hydrogen - in dark green, for combustible flammable gases - in red, for oxygen - blue, for non-combustible gases - black.
Electrocontact manometers. The peculiarity of electrocontact pressure gauges is that they are devices with an electrocontact group. Designed to measure the pressure of non-aggressive, non-crystallizing media (steam, gas, including oxygen), as well as closing and opening electrical circuits when a certain pressure limit is reached. The electrocontact mechanism allows to carry out adjustment of the changed environment.
Possible versions of the contact groups of electrocontact pressure gauges, according to GOST 2405-88:
III – two NC contacts: blue indicator on the left (min), red indicator on the right (max);
IV - two closing contacts: the left pointer is red (min), the right one is blue (max);
V – left NC contact (min); right closing contact (max) - color of pointers - blue;
VI – left closing contact (min); right NC contact (max) – pointer color – red.
Option V is generally accepted by enterprises as a standard. If the type of execution is not specified, as a rule, it will be option V. In any case, you can identify the type of contact group depending on the color of the pointers.
Depending on the purpose and field of application, electrocontact (signaling) pressure gauges are general industrial and explosion-proof.
The type of explosion-proof device (its level of explosion protection) must correspond to the conditions of increased danger of the facility.
Pressure units. Graduation of scales of manometers.
Pressure gauge scales are calibrated in one of the following units: kgf/cm2, bar, kPa, MPa, provided that the instrument has one scale. For dual scale pressure gauges, the first is graduated in the above units of measurement, the second in psi - pounds-force per square inch. Psi is a non-systemic unit used in the USA.
In table. 1 shows the ratio of units of measurement relative to each other.
Tab. 1. Ratio of units of pressure.
The type of pressure gauges with a scale in units of kPa are instruments designed to measure low pressures substances in the gaseous state. In their design, a membrane box serves as a sensitive element. In contrast, pressure gauges for measuring high pressure have a sensitive element - a curved or spiral tube.
Range of measured pressures.
There are the following types of pressure: absolute, barometric, gauge, vacuum.
Absolute - the value of pressure measured relative to absolute vacuum. The indicator cannot be negative.
Barometric - atmospheric pressure. It is affected by altitude, humidity and air temperature. At zero altitude above sea level, the barometric pressure is taken equal to 760 mm Hg.
For technical pressure gauges, this value is assumed to be zero. This means that the measurement results are independent of the barometric pressure.
Gauge pressure is a value showing the difference between absolute and barometric pressure. This is relevant when the absolute pressure exceeds the barometric pressure.
Vacuum - a value showing the difference between absolute and barometric pressure, in conditions of excess barometric pressure relative to absolute. Therefore, the vacuum pressure cannot be higher than the barometric pressure.
Based on the foregoing, it becomes obvious that vacuum gauges measure vacuum. Pressure vacuum gauges cover the area of vacuum and overpressure.
The function of manometers is to determine the excess pressure.
As a result of standardization of the ranges of measured pressures, their correspondence to a certain range of values was accepted (Table 2).
Tab. 2. Standard range of values for scale graduation.
Accuracy class of manometers.
Under the accuracy class of the device is meant the permissible error, which is expressed as a percentage of the maximum value of the pressure gauge scale. The accuracy of the device is higher, the lower the error. The accuracy class is indicated on the instrument scale. Pressure gauges of the same type can be with different accuracy classes.
Gauge case diameter.
The most common pressure gauge case diameters are 40, 50, 60, 63, 100, 150, 160, 250 mm. But there are devices with other body sizes. For example, vibration-resistant pressure gauges manufactured by UAM, type D8008-V-U2, an analogue of DA8008-Vuf produced by Fiztekh, have a diameter of 110 mm.
Manometer design.
A fitting is used to connect the device to the system. The location of the fitting can be of two types - radial (lower) and axial (rear). The location of the axial fitting is central or offset from the center. The design of many types of pressure gauges provides exclusively a radial fitting. For example, electrocontact manometers.
The fitting thread size corresponds to the body diameter. Pressure gauges with diameters - 40, 50, 60, 63 mm have a thread M10x1.0-6g, M12x1.5-8g, G1 / 8-B, R1 / 8, G1 / 4-B, R1 / 4. Gauges with diameter bigger size are made with M20x1.5-8g or G1/2-B thread. European standards apply in addition to the above types of threads, conical - 1/8 NPT, 1/4 NPT, 1/2 NPT. V industrial environment Depending on the tasks and types of measured media, specific connections are used. Gauges operating at high and ultra-high pressure levels are characterized by an internal tapered thread or a variant of the cylindrical thread.
Depending on the type of equipment, it is necessary to indicate the required type of thread when ordering the device. This will help to avoid additional unforeseen costs that will entail the replacement of installation fittings.
The design of the pressure gauge body is also selected according to the method and place of installation. For open highways, the design of the devices does not provide for additional fasteners. Instruments installed in cabinets or control panels require a front and rear flange.
Depending on the execution, the following types are distinguished:
- with radial union without flange;
- with radial fitting with rear flange;
- with axial fitting with front flange;
- with axial union, no flange.
Degree of protection of manometers of standard execution – IP40. Special pressure gauges, according to the conditions of their use, are manufactured with degrees of protection IP50, IP53, IP54 and IP65.
In order to prevent unauthorized opening of the pressure gauge, the device must be sealed. To do this, an eyelet is made on the body, equipped with a screw with a hole in the head to establish a seal.
Protection against high temperatures and pressure drops.
The measurement error of the pressure gauge depends on the influence of temperature environment and temperature of the measured medium.
For most devices, the temperature measurement range is not more than + 60 ° C, maximum + 80 ° C. Instruments of some manufacturers have the ability to measure pressure at high temperatures of the measured medium up to +150°C, or even 300°C.
For standard version pressure gauges, operation in such conditions is possible only if there is a siphon outlet (cooler) through which the pressure gauge is connected to the system.
This is a special tube, of a special shape, at the ends of which there is a thread for connecting to the line and connecting a pressure gauge. The siphon branch creates a branch in which the measured medium does not circulate. Due to this, the temperature at the connection point of the device is much lower than in the main line.
In addition, the durability of the manometer is affected by sudden changes in the measured pressure and water hammer. In order to reduce the influence of these factors, damping devices are used. The damper is installed in front of the instrument as a separate device, or mounted in the channel of the pressure gauge holder.
If there is no need to constantly monitor the pressure in the system, you can install a pressure gauge through a push-button valve. This allows you to connect the device to the line only for the time the tap button is pressed. This will protect the instrument without the need for a damper device.
In this article we will try to consider in detail all issues related to pressure gauges, their choice and their operation. Also, together with pressure gauges, we will consider vacuum gauges and combined pressure and vacuum gauges. All recommendations for these devices are the same, so in the text we will only mention pressure gauges.
1. What is a pressure gauge, vacuum gauge and combined pressure and vacuum gauge?
2. What are pressure gauges?
3. What parameters are important when choosing a pressure gauge?
4. Conversion of pressure units of manometers.
5. How to install pressure gauges?
6. How to operate pressure gauges?
7. How are pressure gauges calibrated?
8. Which pressure gauge is better to buy?
9. What is important to pay attention to when buying a pressure gauge?
1. What is a pressure gauge, vacuum gauge and combined pressure and vacuum gauge?
Technical manometer.
A manometer is a device designed to measure the excess pressure of a working medium by means of deformation of a tubular spring (Bourdon tube).
Technical vacuum gauge.
A vacuum gauge is a device designed to measure the vacuum of the working medium by means of the deformation of a tubular spring. Standard scale for a vacuum gauge from -1..0 atm. The scale on the vacuum gauge is always negative, because the pressure is measured below atmospheric pressure.
Manovacuummeter technical.
A vacuum gauge is a device designed to measure the overpressure and rarefaction of the working medium by means of the deformation of a tubular spring.
The above is simple:
- if there is only positive pressure on the scale of the device, then this is a manometer.
- if there is only negative pressure on the scale of the device, then this is a vacuum gauge.
- if there is both negative and positive pressure on the scale of the device, then this is a manometer.
In industry and housing and communal services, pressure gauges with a Bourdon tube spring are most widely used. This is due to the simplicity of design and relatively low cost.
Manometer "from the inside".
2. What are pressure gauges?
Technical pressure gauges are the most common devices for measuring the pressure of water, air, gases, which are widely used in the housing and communal services and industry. If you do not have any specific requirements for the device, then you should definitely consider technical pressure gauges.
Technical manometer TM610R.
Boiler pressure gauges are technical pressure gauges with a body diameter of 250mm. These pressure gauges are used when installed at high altitudes or in hard-to-reach places, which allows you to take readings from a long distance.
Boiler pressure gauge TM810R.
Vibration-resistant manometers - devices for measuring pressure in conditions of increased vibration on a pipeline or installation. These devices are widely used in pumping stations, compressors, cars, ships and trains.
Vibration-resistant pressure gauge TM-320R.
Corrosion-resistant manometers - devices made entirely of stainless steel and designed to work with aggressive media.
Corrosion-resistant pressure gauge TM621R.
Welding pressure gauges - devices designed to control pressure on oxygen and acetylene reducers, propane cylinders Welding pressure gauges are oxygen (body color blue), acetylene (body color white or gray) and propane (body color red). On the dial of each device in a circle is the type of medium.
Pressure gauges for precise measurements (exemplary pressure gauges) - devices with a low accuracy class of 0.6 or 0.4 are used for pressure testing of gas pipelines, verification of technical pressure gauges, as well as for measuring the pressure of process lines that require increased measurement accuracy.
Sample manometer.
Ammonia manometers - instruments for measuring pressure in cold supply systems. These devices are made on the basis of corrosion-resistant pressure gauges with a modified dial.
Ammonia pressure gauge.
Automobile pressure gauges - devices for measuring air pressure in tires. These devices can be purchased at automotive stores or service centers.
Digital electronic pressure gauges - there are two varieties: in a monoblock case and a set of a pressure transducer and an electronic unit for indicating and adjusting parameters. These devices are used for precise pressure measurement and in process automation systems.
Electrocontact pressure gauges are technical pressure gauges with an electrocontact attachment designed for switching contacts in automation systems.
The fundamental difference of these devices from the whole variety of pressure gauges is the presence of the option of the pressure gauge design. To date, these devices are available in six versions.
3. What parameters are important when choosing a pressure gauge?
In this section, we will look at all the parameters that you need to consider when buying a pressure gauge. This is very helpful information for buyers who do not have the exact brand of the device or have a brand, but these devices cannot be bought and you need to correctly select analogues.The measuring range is the most important parameter.
Standard range of pressure gauges:
0-1, 0-1.6, 0-2.5, 0-4, 0-6, 0-10, 0-16, 0-25, 0-40, 0-60, 0-100, 0-160, 0- 250, 0-400, 0-600, 0-1000 kgf/cm2=bar=atm=0.1Mpa=100kPa
Standard range of pressures for pressure and vacuum gauges:
-1..+0.6, -1..+1.5, -1..+3, -1..+5, -1..+9, -1..+15, -1..+24 kgf/ cm2=bar=atm=0.1MPa=100kPa
Standard range of pressure gauges:
-1..0 kgf/cm2=bar=atm=0.1Mpa=100kPa.
If you do not know which scale to buy, then the choice of range is quite simple, the main thing is to operating pressure fell into the range from 1/3 to 2/3 of the measurement scale. For example, you usually have a water pressure of 5.5 atm in a pipe. For stable operation, you need to choose a device with a scale of 0-10 atm, because the pressure of 5.5 atm falls in the range from 1/3 to 2/3 of the scale of 3.3 atm and 6.6 atm, respectively. Many people wonder - what happens if the operating pressure is less than 1/3 of the scale or more than 2/3 of the measurement scale? If the measured pressure is less than 1/3 of the scale, then the pressure measurement error will increase sharply. If the measured pressure is more than 2/3 of the scale, then the mechanism of the device will operate in overload mode and may fail before the warranty period.
The accuracy class is the permissible percentage of the measurement error from the measurement scale.
Standard range of accuracy classes for pressure gauges: 4, 2.5, 1.5, 1, 0.6, 0.4, 0.25, 0.15.
How to calculate the error of the pressure gauge yourself? Let's say you have a 10 atm pressure gauge with an accuracy class of 1.5.
This means that the permissible error of the pressure gauge is 1.5% of the measurement scale, i.e. 0.15 atm. If the error of the device is greater, then the device must be changed. To understand without special equipment a serviceable device or not, according to our experience, is unrealistic.
Only an organization that has a calibration plant with a reference pressure gauge with an accuracy class four times less than the accuracy class of the problematic pressure gauge can make a decision about the discrepancy of the accuracy class. Two instruments are installed in line with pressure and the two readings are compared.
Gauge diameter is an important parameter for gauges in a round case. Standard range of diameters for manometers: 40, 50, 63, 80, 100, 150, 160, 250 mm.
The location of the fitting - there are two varieties: radial, in which the fitting exits the pressure gauge from below, and end (rear, axial) in which the connecting fitting is located behind the back of the device.
Connecting thread - two threads found the greatest distribution on pressure gauges: metric and pipe. Standard range of threads for pressure gauges: M10x1, M12x1.5, M20x1.5, G1/8, G1/4, G1/2. Almost all imported pressure gauges use pipe threads. Metric thread is used mainly on domestic devices.
The calibration interval is the period when it is necessary to recalibrate the instrument. All new devices come with initial factory verification, which is confirmed by the presence of the verifier's stamp on the device dial and the corresponding mark in the passport. At the moment, primary verification is for 1 year or 2 years. If the pressure gauge is used for personal purposes and verification is not critical, then choose any device. If the pressure gauge is installed at a departmental facility (heating station, boiler house, factory, etc.), then after the expiration of the initial verification period, it is necessary to recalibrate the pressure gauge at the CSM (center for standardization and metrology) of your city or at any organization that has a license for verification and necessary equipment. For those who are constantly faced with the verification of pressure gauges, it is not a secret that very often re-verification costs more or is comparable to the cost of a new device, as well as handing over the device for verification costs money even if the device does not pass re-verification and repair of the device with subsequent verification may be added to the price. .
Based on this, we have two recommendations:
- buy devices with primary verification for 2 years, because saving 50-100 rubles on the purchase of a device with a verification period of 1 year can lead to expenses of 200-300 rubles and unnecessary "running around" in a year.
- before making a decision on recalibration of instruments, calculate the costs of recalibration - in most cases it is much more profitable to buy new instruments. What you need to calculate - the cost of verification, several trips to the verification. If there are hydraulic shocks in the system, pulsation of the medium (proximity of pumps), vibration of the pipeline, then after 2 years of operation, usually 50% of the devices do not pass re-verification, and you have to pay for it, because verification work was carried out.
Operating conditions - if the device will work on a viscous or aggressive medium, as well as when using the device in difficult conditions - vibration, pulsation, high (more than + 100С) and low temperatures (less than -40С), then it is necessary to choose a specialized pressure gauge.
4. Conversion of pressure units of manometers.
When buying a pressure gauge, it often becomes necessary to measure pressure in non-standard units. Our work experience says that if we are talking about a small number of devices (less than 100 pieces), then the factories will not alter anything on their scales and will have to convert units of measurement on their own.1kgf/cm2=10.000kgf/m2=1bar=1atm=0.1Mpa=100kPa=100.000Pa=10.000mm.water column=750mm. rt. st.= 1000mbar
5. How to install pressure gauges?
To install the pressure gauge on the pipe, three-way valves and needle valves are used. Pressure gauges are protected by damper blocks, sampling loops and diaphragm seals.A three-way valve for a pressure gauge is a three-way ball or plug valve designed to connect a pressure gauge to a pipeline or any other equipment. It is allowed to install a two-way valve with the possibility of manual pressure relief from the pressure gauge when it is turned off. The use of standard ball valves is not recommended, because after the valve is closed, the pressure gauge mechanism is under residual pressure of the medium, which can lead to its premature failure. To date, this is the most common type for connecting pressure gauges at pressures up to 25 kgf / cm2. For high pressures, needle valves are recommended. When buying a three-way valve, you must make sure that the threads on the pressure gauge match the threads on the valve.
A needle valve is a control valve with the possibility of a smooth supply of the working medium, in which the shut-off element is made in the form of a cone. Needle valves are widely used for connecting various instrumentation and control equipment to equipment with high pressures. When purchasing needle valves, make sure that the threads on the pressure gauge match the threads on the valve.
damper block is protective device, which is installed in front of the pressure gauge and designed to dampen the pulsations of the working medium. In this case, pulsation means a sharp and frequent change in the pressure of the working medium. The main "organizers" of pulsations in the pipeline are powerful pumps without soft starters and the widespread installation of ball valves and butterfly valves, the rapid opening of which leads to water hammer.
damper block.
Loop selection devices (Perkins tube) are steel tubes that are designed to dampen the temperature in front of pressure gauges. The decrease in the temperature of the medium entering the pressure gauge occurs due to the "stagnation" of the medium in the loop. These devices are recommended to be installed at a working environment temperature of more than 80C. There are two types of selective devices: straight and angled. Direct selection devices are installed on horizontal sections of pipelines, and angular ones are designed for installation on vertical pipelines. Before buying, you must make sure that the threads on the tube match the threads on the three-way valve or pressure gauge.
Choice devices (straight and angular).
Membrane media separators are a protective device for a pressure gauge designed to protect the mechanism of the device from the ingress of aggressive, crystallizing and abrasive media into it. When choosing a diaphragm seal, it is necessary to pay attention to the coincidence of the threads on the pressure gauge and the separator.
Membrane separator RM.
When installing pressure gauges, there are several requirements that must be met:
- installation work with a pressure gauge must be carried out in the absence of pressure in the pipeline
- the manometer is installed with a vertical arrangement of the dial
- the pressure gauge is rotated by the fitting using wrench
- it is forbidden to apply force to the manometer case
6. How to operate pressure gauges?
When operating pressure gauges, it is necessary to follow the recommendations and physical parameters (medium temperature and allowable pressure) prescribed in the instrument's passport. The most important requirement for operation is a smooth supply of pressure to the pressure gauge. If the device is selected correctly and is operated without violations, then there are usually no problems.Consider the cases in which the operation of the pressure gauge is not allowed:
- when pressure is applied to the device, the arrow does not move
- the instrument glass is damaged or broken
- the arrow of the device moves in jumps
- after depressurizing the instrument, the pointer does not return to zero
- measurement error exceeds the allowable value
7. How are pressure gauges calibrated?
The pressure gauge is a means of measuring pressure and is subject to mandatory verification. Calibration of pressure gauges can be divided into two types:- primary verification is a verification that is performed by the manufacturer before the sale of the device and is confirmed by the presence of the verifier's mark on the glass or the pressure gauge body, as well as the corresponding mark in the device's passport. Primary verification without any problems is recognized by the controlling organizations and the device can be operated until the end of this period.
Recalibration of the pressure gauge is a verification of the device, which is performed after the expiration of the period for the primary calibration of the pressure gauge. Before rechecking the pressure gauge, you need to make sure that the device is in good order, because in the event of a device malfunction, you will receive a beautiful notification for money comparable to the cost of the device that the device is not working and needs to be repaired or thrown away. The pressure gauge is recalibrated at the CSM (center for standardization and metrology) of your city or at any organization that has a license for verification and the necessary equipment.
8. Which pressure gauge is better to buy?
To date, there are about 10 Russian instrument manufacturers, 2 Belarusian manufacturers and a myriad of foreign instrument manufacturers on the market. Consider the features of each device.Russian factories - the most optimal choice to buy gauges. Many will ask - why? Everything is quite simple - Russian pressure gauges are significantly cheaper than imported ones with comparable quality, the initial verification period is 2 years, unlike Belarusian ones, the entire line of instruments is produced from technical to corrosion-resistant.
Belarusian factories are rather cheap devices, but they have 3 significant drawbacks:
- primary verification for 1 year, which turns their cheapness into a "myth" and "running around" with rechecking.
- a simplified mechanism that does not work for a long time under serious loads.
- plastic glass instead of instrument glass also introduces difficulties in operation and reliability of the device.
Foreign manometers - our many years of experience in the sale of instruments shows that the point in buying is similar to purchasing a Russian instrument, but only 2-3 times more expensive. All explanations of foreign appliance sellers about unique quality, super technologies, etc. are a common ploy to explain to the client why he overpays so steeply. If the operating conditions are difficult, you just need to buy a specialized device instead of a technical one and it will work without problems. If you are tormented by doubts and you have the opportunity to disassemble two similar Russian and imported pressure gauges with a screwdriver, then you are unlikely to be lucky to find a few differences.
The exception is highly specialized devices with non-standard scales and parameters that are not produced in Russia.
9. What is important to pay attention to when buying a pressure gauge?
- the pressure gauge must be new. Many sellers of devices under the word new understand that the pressure gauge has not been in operation. But the pressure gauge can be 15 years old, and they will tell you that it is new. Specify the year of manufacture of the device or you may be in for an unpleasant surprise in the form of buying illiquid assets.- there should be a mark on the primary verification on the pressure gauge or in the passport. There are sellers of illiquid assets who erase the stigma of a trustee so that they cannot be accused of selling old appliances.
- verification on a pressure gauge should be 2 years, if you buy a device with primary verification for 1 year - in a year the savings will disappear and unnecessary difficulties will begin.
- the pressure gauge must have a passport and a valid certificate for measuring instruments.
- if the device is new and verified for 2 years, choose the cheapest option.
- pay attention to the measurement range, scale diameter, fitting location type, thread type and device version - if you buy the wrong device, then replacing it may be difficult, because if the device has non-standard parameters and is made for you, then most likely it will have to be left as a keepsake.
- you can search for reviews about pressure gauges on the Internet, but most of them are custom-made and it is better to rely on the advice of people who have experience in the actual operation of devices.
- pressure gauges should be bought in an organization that inspires confidence in you, because the sale of illiquid assets from the times of the USSR still exists and then it will be quite difficult to return old devices or exchange them for normal devices.
In this article, we tried to consider the most popular questions about the whole variety of pressure gauges. If you want other questions to be considered or you do not agree with some answers - write to us and we will try to expand the article based on your experience. In the letter, do not forget to indicate your data, place, conditions and region of installation.
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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 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.
The term "pressure gauge" used in the text is generic and, in addition to directly manometers, also includes vacuum gauges and combined pressure and vacuum gauges. This material does not cover digital devices.
Pressure gauges are one of the most common instruments in industry and housing and communal services. For more than a hundred years they have been serving people reliably. The needs of production initiated the development of pressure gauges for various purposes, differing in size, design, connecting thread, ranges and units of measurement, accuracy class. The wrong choice of devices leads to their premature failure, insufficient measurement accuracy or overpayment for excessive functionality.
Pressure gauges can be classified according to the following criteria.
- By area of application.
1.1. Technical pressure gauges of standard design - designed to measure the excess and vacuum pressure of non-aggressive, non-crystallizing liquids, steam and gas.
1.2. Technical special - pressure gauges for working with specific media or in specific conditions. The following pressure gauges are special:
Oxygen;
Acetylene;
Ammonia;
Corrosion resistant;
Vibration resistant;
Ship;
Railway;
Manometers for the food industry.
Oxygen pressure gauges are not structurally different from technical pressure gauges, but during the production process they are additionally cleaned from oils, since when oxygen comes into contact with oils, ignition or explosion can occur. The designation O 2 is applied to the scale.
Acetylene pressure gauges are made without the use of copper and its alloys. This is due to the fact that the interaction of copper and acetylene produces explosive acetylene copper. Acetylene pressure gauges are marked with the symbols C 2 H 2.
Ammonia and corrosion-resistant pressure gauges have mechanisms made of stainless steel and alloys that are not subject to corrosion when interacting with aggressive media.
The design of vibration-resistant pressure gauges provides operability when exposed to vibration in a frequency range approximately 4-5 times higher than the permissible vibration frequency of standard technical pressure gauges.
Some types of vibration resistant pressure gauges can be filled with damping fluid. Glycerin (operating temperature range from -20 to +60 o C) or PMS-300 liquid (operating temperature range from -40 to +60 o C) is used as a damping liquid.
Pressure gauges for the food industry do not have direct contact with the measured medium and are separated from it by a diaphragm seal. The supra-membrane space is filled with a special liquid, which transfers force to the pressure gauge mechanism.
Pressure gauge cases are usually painted in the color corresponding to the application: ammonia - yellow, acetylene - white, for hydrogen - dark green, for combustible gases, such as propane - red, for oxygen - blue, for non-combustible gases - into black.
2. Electrocontact (signaling) manometers.
Electrocontact (signaling) pressure gauges incorporate contact groups for connecting external electrical circuits. They are used to maintain pressure in technological installations in a given range.
Contact groups of electrocontact (signaling) pressure gauges according to GOST 2405-88 can have one of four versions:
III - two break contacts: left pointer (min) - blue, right (max) - red;
IV - two closing contacts: left pointer (min) - red, right (max) - blue;
V - left NC contact (min); right closing contact (max) - both pointers are blue;
VI - left closing contact (min); right NC contact (max) - both pointers are red.
Most Russian factories accept the V version as standard. That is, if the application does not indicate the design of the electrocontact pressure gauge, then the customer is almost guaranteed to receive a device with contact groups of this design. In the absence of a passport, you can determine the design of the contact groups by the color of the pointers.
Electroconiact (signaling) pressure gauges are divided into general industrial and explosion-proof. The order of explosion-proof pressure gauges must be approached very carefully, so that the type of explosion protection of the device corresponds to the object of increased danger.
3. Pressure units.
Graduation of manometer scales is carried out in one of the units: kgf / cm 2, bar, kPa, MPa. However, it is not uncommon to find pressure gauges with a double scale. The first scale is graduated in one of the units listed above, the second in psi - pounds-force per square inch. This unit is non-systemic and is used mainly in the United States. In table. 1 shows the ratio of these units to each other.
Tab. 1. Ratio of units of pressure
|
Pa |
kPa |
MPa |
kgf / cm 2 |
bar |
|
|
Pa |
10 -3 |
10 -6 |
10,197*10 -6 |
10 -5 |
|
|
kPa |
10 3 |
10 -3 |
10,197*10 -3 |
10 -2 |
|
|
MPa |
10 6 |
10 3 |
10,1972 |
||
|
kgf / cm 2 |
98066,5 |
98,0665 |
0,980665 |
0,980665 |
|
|
bar |
10 5 |
1,0197 |
|||
|
6894,76 |
6,8948 |
6,8948*10 −3 |
70,3069*10 −3 |
68,9476*10 −3 |
Instruments calibrated in kPa are called pressure gauges for measuring low gas pressures. A membrane box is used as a sensitive element, while a curved or spiral tube is used in pressure gauges for high pressures.
4. Range of measured pressures.
In physics, there are several types of pressure: absolute, barometric, excess, vacuum. Absolute pressure is the pressure measured relative to absolute vacuum. Absolute pressure cannot be negative.
Barometric is atmospheric pressure, which depends on altitude, temperature and humidity. At zero meters above sea level, it is taken equal to 760 mm Hg. In technical pressure gauges, this value is taken as zero, that is, the barometric pressure value does not affect the measurement results.
Gauge pressure is the difference between absolute pressure and barometric pressure, provided that the absolute pressure is greater than the barometric pressure.
Vacuum is the difference between absolute pressure and barometric pressure when the absolute pressure is less than the barometric pressure. Therefore, the vacuum pressure cannot be greater than the barometric pressure.
Based on this, it becomes clear that vacuum gauges measure vacuum. Pressure vacuum gauges cover the area of vacuum and overpressure. Manometers measure excess pressure. There is another class of instruments called differential pressure gauges. Differential pressure gauges are connected to two points of the same system and show the pressure drop of gaseous or liquid substances.
The ranges of measured pressures are standardized and taken equal to a certain range of values, which are given in Table. 2.
Tab. 2. Standard range of values for scale graduation.
|
Device type |
Ranges of measured pressures, kgf / cm 2 |
|
vacuum gauges |
1…0 |
|
Pressure vacuum meters |
1…0,6; 1,5; 3; 5; 9; 15; 24 |
|
Pressure gauges |
0…0,6; 1; 1,6; 2,5; 4; 6; 10; 16; 25; 40; 60; 100; 160; 250; 400; 600; 1000; 1600 |
|
0…2500; 4000; 6000; 10000
|
5. Accuracy class of manometers
Accuracy class - the permissible error of the device, expressed as a percentage of the maximum value of the scale of this device. The accuracy class is applied by manufacturers to the scale. The smaller this value, the more accurate the device. The same type of pressure gauge may have a different accuracy class. For example, the Manotom plant produces devices with an accuracy class of 1.5 as standard, and on request it can manufacture similar devices with an accuracy class of 1.0. In table. 3 shows data on accuracy classes in relation to various types pressure gauges.
Tab. 3. Accuracy class of pressure gauges from Russian manufacturers.
|
Device type |
Accuracy class |
|
Exemplary pressure gauges |
0,15; 0,25; 0,4 |
|
Accurate pressure gauges |
0,4; 0,6; 1,0 |
|
Technical pressure gauges |
1,0; 1,5; 2,5; 4 |
|
Ultra high pressure gauges |
For imported instruments, the value of the accuracy class may differ slightly from Russian counterparts. For example, European technical pressure gauges may have an accuracy class of 1.6.
The smaller the diameter of the body of the device, the lower its accuracy class.
6. Case diameter
Most often, pressure gauges are made in cases with the following diameters: 40, 50, 60, 63, 100, 150, 160, 250 mm. But you can find devices with other body sizes. For example, vibration-resistant pressure gauges manufactured by Fiztekh of the type DM8008-Vuf (DA8008-Vuf, DV8008-Vuf) are manufactured in cases with a diameter of 110 mm, and a smaller version of this device, DM8008-Vuf (DA8008-Vuf, DV8008-Vuf) Execution 1, has diameter 70 mm.
Pressure gauges with a body of 250 mm are often called boiler gauges. They do not have special designs and are used at thermal power facilities and allow the operator to control the pressure at several nearby plants from the operator's workplace.
7. Design of pressure gauges
A fitting is used to connect the pressure gauge to the system. There are radial (lower) location of the fitting and axial (rear). The axial fitting can be centrally located or off-center. Many types of pressure gauges effect design features do not have a version with an axial fitting. For example, signaling (electrocontact) pressure gauges are made only with a radial fitting, since an electrical connector is located on the back side.
The size of the thread on the fitting depends on the body diameter. Pressure gauges with diameters - 40, 50, 60, 63 mm are manufactured with M10x1.0-6g, M12x1.5-8g, G1/8-B, R1/8, G1/4-B, R1/4 threads. On larger pressure gauges, M20x1.5-8g or G1 / 2-B is used. European standards provide for the use of not only the above types of threads, but also conical ones - 1/8 NPT, 1/4 NPT, 1/2 NPT. In addition, specific connections are used in industry. Pressure gauges measuring high and ultra-high pressures may have an internal conical or cylindrical thread.
The design of the pressure gauge body depends on the installation method and location. Devices installed openly on highways, as a rule, do not have additional fasteners. When installed in cabinets, control panels, pressure gauges with a front or rear flange are used. The following versions of manometers can be distinguished:
With radial fitting without flange;
With radial fitting with rear flange;
With axial union with front flange;
With axial union without flange.
Gauges of standard execution, as a rule, have degree of protection IP40. Special pressure gauges, depending on the application, can be manufactured with degrees of protection IP50, IP53, IP54 and IP65.
In some cases, pressure gauges must be sealed in order to exclude the possibility of unauthorized opening of the devices. To this end, some manufacturers make an eyelet on the body and complete it with a screw with a hole in the head, allowing you to install a seal.
8. Protection against high temperatures and pressure drops
The temperature has a serious influence on the measurement error and the service life of pressure gauges. This factor affects the internal elements of the structure in contact with the measured medium, and externally through the ambient temperature.
Most manometers should be operated at ambient and measured medium temperatures not exceeding +60 ° C, maximum +80 ° C. Some manufacturers manufacture instruments designed for the temperature of the measured medium up to +150 ° C and even +300 ° C. However, measurements at high temperatures can be produced with standard gauges. To do this, the pressure gauge must be connected to the system via a siphon outlet (cooler). The siphon branch is a tube of a special form. At the ends of the outlet there is a thread for connecting to the main line and attaching a pressure gauge. The siphon branch forms a branch in which there is no circulation of the measured medium. As a result, at the point where the pressure gauge is connected, the temperature may differ by several times from the temperature in the main line.
Another factor affecting the durability of pressure gauges is sudden pressure drops or water hammer. To reduce the influence of these factors, dampers are used. The damper can be made as a separate device installed in front of the pressure gauge or mounted in the inner channel of the instrument holder.
You can protect the pressure gauge in another way. In cases where there is no need to constantly monitor the pressure in the system, the pressure gauge can be installed through a push-button valve. Thus, the device will be connected to the controlled line only for the time during which the tap button is pressed.
