What is a safety valve?

What is a safety valve?

Safety valve is a special branch of valve family. Its particularity is that safety valve only plays the role of switch unlike other valves. More importantly, it plays a role in protecting the safety of equipment.
Safety valve is in normal closed state under the action of external force. When the medium pressure in the equipment or pipeline rises above the prescribed value, it is a special valve that prevents the medium pressure in the pipeline or equipment from exceeding the prescribed value by discharging the medium outside the system.
Safety valves belong to automatic valves. They are mainly used in boilers, pressure vessels and pipes. The control pressure does not exceed the prescribed value, which plays an important role in protecting personal safety and equipment operation. Safety valves must undergo pressure tests before they can be used.

20190420215915 32190 - What is a safety valve?

Technical Indicators Of Safety Valves Standard Classification Of Safety Valve
Classification Of Safety Valves Working Principle Of Safety Valve
Importance Of Safety Valves Operation Method Of Safety Valve
Lead Seals For Safety Valves Cautions For Safety Valve Installation
Maintenance Of Safety Valve Selection Principle Of Safety Valve

With the rapid development of China’s economic construction, there are more and more equipment projects with pressure control. In view of the need of equipment pressure relief, safety valve plays a vital role in the process of protecting equipment. As of 2013, more than 700 manufacturers and traders of various types of safety valves have made more than 200 daily inquiries, and the supply and demand market of safety valves has obviously warmed up.
Safety valves are widely used in steam boilers, liquefied petroleum gas trucks or liquefied petroleum gas railway tankers, oil wells, High-pressure Bypass of steam power generation equipment, pressure pipes, pressure vessels, etc.

Technical indicators of safety valves

Code ofconnectiontype
Connection form
Code
Connection form
Code
Internal thread
One
Clip on
Seven
External thread
Two
Clamp
Eight
flange
Four
Card set
Nine
welding
Six
Structure of code
structural style
Code
Spring
close
With the heat sink
Full opening
Zero
Weiqi
One
Full opening
Two
With a wrench
Full opening
Four
Not closed
Double spring weiqi
Three
Full opening
Eight
Weiqi
Seven
With control mechanism
Full opening
Six
lever
Simple lever
Full opening
Two
Angular weiqi
Five
Double lever
Full opening
Four
pilot type
Nine
The valve seat sealing surface material
The valve seat sealing surface material
Code
The valve seat sealing surface material
Code
Copper alloy
T
Nitriding steel
D
rubber
X
Boronized steel
P
Nylon plastic
N
Hard alloy
Y
Acid resistant alloy steel or stainless steel
H
The valve body machining
W
Tin based bearing alloy (Babbitt)
B
Note: when valve seat And the valve sealing surface material do not at the same time, with low hardness material code.
Nominal pressure code
nominal pressure The code represented by Arabia digital, its value is MPa (MPa) 10 times the nominal pressure value of the units.
When the working temperature and labeling Working pressure When the working pressure must use the P logo and the word P in the lower right corner of the additional medium The highest temperature number, the number is an integer in 10 except the numerical medium maximum temperature. Such as the working temperature of 540, the working pressure is 10MPa valve The code for the P54100.
Body material code
body material
Code
body material
Code
Grey cast iron
H
carbon steel
C
Nodular cast iron
Q
chrome molybdenum steel
I
Ci Ni Ti
P
Vanadium chromium molybdenum alloy steel
V

Note: carbon steel body PN = 1.6MPa and PN = 2.5MPa gray cast iron body, omitted the code.

Standard classification of safety valve

Safety valve related standards (Table 1) is the basis for the design of safety valve. With the development of economic globalization and China’s demand for cooperation, the introduction of a large number of large sets of foreign Petroleum and chemical equipment, design, many domestic safety valve manufacturing enterprises and units mainly adopts API or ASME standard safety valve manufacture, selection and use. In 2010, the National Standardization Technical Committee according to the standard of API, the safety valve selection of the design, manufacture and installation standards, effective promotion of the domestic safety valve standards with international standards.
Safety valve related standards (Table 1)
Standard code
Standard name
Domestic standard
GB / T 12241
General requirements for safety valve
GB / T 12242
Performance test code – pressure relief devices
GB / T 12243
Spring loaded safety valve
GB / T 24920
Steel pressure relief valve in petrochemical industry
GB / T 24921
The petrochemical industry with pressure relief valve sizing, selection and installation
International Standard
ISO 4126-1
The first part: General requirements for safety valve
ASME boiler and pressure vessel code 1
Rules for construction of power boiler
ASME boiler and pressure vessel code 8
Pressure vessel
ASME PTC 25
Pressure relief device performance test specification
The first part of the RP520 API
The pressure release device in the refinery to determine the size and the installation size determination and installation
The second part API RP520
The pressure release device in the refinery to determine the size and the installation of the installation
API RP 526
Steel flanged end pressure relief valve
API STD 527
The pressure relief valve seat seal
API RP 576
Check pressure relief device
JIS B8210
Spring safety valve for steam and gas
The first part of the 6759 BS
Technical specification for safety valves for steam and hot water
The fourth part BS 6759
Safety valve and safety valve spring
DIN 3320
Close the valve safety valve safety valve

Classification of safety valves

There are two main types of safety valve structure: spring type and lever type. Spring type refers to the action of spring on the sealing of disc and seat. Leverage is the action of lever and hammer. With the need of large capacity, there is another kind of pulse safety valve, also known as pilot safety valve, which is composed of main safety valve and auxiliary valve. When the medium pressure in the pipeline exceeds the prescribed pressure value, the auxiliary valve opens first, and the medium enters the main safety valve along the conduit, and the main safety valve opens, which reduces the increased medium pressure.
The discharge of safety valve depends on the seat diameter and the opening height of the disc. It can also be divided into two types: the micro-opening height is (1/20) ~ (1/40) of the seat inner diameter, and the full-opening height is (1/3) ~ (1/4).

In addition, with the use of different requirements, there are closed and non-closed. The enclosed discharge medium is not leaked, all along the prescribed outlet discharge, generally used for toxic and corrosive media. Unenclosed type is generally used in non-toxic or non-corrosive media.

By structure

According to its overall structure and different loading mechanism, it can be divided into three types: hammer lever type, spring type and pulse type.
1. Heavy Hammer Leverage Safety Valve
Heavy hammer lever relief valve balances the force acting on the valve disc by using a heavy hammer and lever. According to the principle of lever, it can use the weight with smaller weight to gain greater force through the increase of lever, and adjust the opening pressure of safety valve by moving the position of the weight (or changing the weight of the weight).
The weight hammer lever relief valve is simple in structure, easy to adjust and more accurate. The load added will not increase greatly because of the increase of the valve disc. It is suitable for higher temperature situations. It was used widely in the past, especially in boilers and pressure vessels with higher temperature. However, the structure of the lever relief valve with heavy hammer is bulky, and the loading mechanism is easy to vibrate, and often leaks due to vibration; its backing pressure is low, and it is not easy to close and keep tight after opening.
2. Spring slightly open safety valve

20190420220108 67477 - What is a safety valve?

Spring micro-open safety valve uses the force of compression spring to balance the force acting on the valve disc. The compression of the coil spring can be adjusted by turning the adjusting nut on it. With this structure, the opening (setting) pressure of the safety valve can be adjusted according to the need. The spring slightly open safety valve has the advantages of compact structure, high sensitivity, unrestricted installation position, and low sensitivity to vibration, so it can be used in mobile pressure vessels. The disadvantage of this safety valve is that the load will change with the opening of the valve, that is, with the increase of the valve disc, the compression of the spring will increase, and the force acting on the valve disc will also increase. This is detrimental to the quick opening of the safety valve. In addition, the spring on the valve will be affected by high temperature for a long time and reduce the elastic force. When used in higher temperature containers, it is often necessary to consider the heat insulation or heat dissipation of springs, which makes the structure more complex.

3. Pulse relief valve
Pulse relief valve is composed of main valve and auxiliary valve. Pulse action of auxiliary valve drives the action of main valve. Its structure is complex. It is usually only suitable for boilers and pressure vessels with large safety discharge.
Among the above three types of safety valves, spring type safety valves are commonly used.

By medium

According to the different ways of medium discharge, safety valves can be divided into three types: fully closed, semi-closed and open.
1. Fully enclosed safety valve
When fully enclosed relief valve exhausts, all the gas is discharged through the exhaust pipe, so the medium can not leak out. It is mainly used for toxic medium. Containers for flammable gases.
2. Semi-closed safety valve
The gas discharged by the semi-closed safety valve is partly through the exhaust pipe, and partly leaks out from the gap between the valve cover and the valve stem. It is mostly used for containers where the medium is non-polluting gas.
3. Open safety valve
The valve cover of the open safety valve is open to connect the spring chamber with the atmosphere, which is conducive to reducing the temperature of the spring. It is mainly suitable for vessels with steam medium and high temperature gas which does not pollute the atmosphere.

Open by disc

According to the ratio of the maximum height of disc opening to the diameter of the flow passage of the safety valve, the safety valve can be divided into two types: spring micro-open closed high-pressure safety valve and spring full-open safety valve.
1. Closed high-pressure safety valve with spring slightly opened
The opening height of the micro-open safety valve is less than 1/4 of the diameter of the runner, usually 1/40-1/20 of the diameter of the runner. The action process of the micro-open safety valve is proportional, which is mainly used in liquid occasions and sometimes in gas occasions with small discharge.
2. Spring Fully Open Safety Valve
The opening height of the full-open safety valve is greater than or equal to 1/4 of the diameter of the runner. The discharge area of the full-open relief valve is the minimum cross-section area of the throat of the seat. Its action process belongs to two-stage action mode, which can only be fully opened by means of a lifting mechanism. Full-open safety valve is mainly used in the case of gas medium.
3. Open-in relief valve
The opening height is between micro-opening and full-opening. It can be made into two-stage action or proportional action.

According to the principle of action

According to the principle of action, it can be divided into direct-acting safety valve and non-direct-acting safety valve.
1. Direct acting safety valve
Direct acting relief valve is opened under the direct action of working medium, that is to say, relies on the action of working medium pressure to overcome the mechanical load imposed on the valve disc by the loading mechanism, so that the valve can be opened. The safety valve has the advantages of simple structure, quick action and good reliability. However, due to the structural loading, its load size is limited, and can not be used in high-pressure, large-caliber occasions.
2. Non-direct acting safety valve
This kind of safety valve can be divided into pilot safety valve and power assistant safety valve.
Pilot safety valve is driven or controlled by the medium discharged from the pilot valve. The pilot valve itself is a direct-acting safety valve, sometimes other forms of valves are also used. Pilot safety valve is suitable for high pressure and large caliber occasions. The main valve of pilot relief valve can also be designed to be sealed by working medium, or it can impose a much larger mechanical load on the disc than the direct acting relief valve, so it has good sealing performance. At the same time, its movements are seldom affected by back pressure. The disadvantage of this kind of safety valve is that its reliability is related to the main valve and guide valve. Its action is not as fast and reliable as that of direct acting safety valve, and its structure is more complex.
The safety valve with power assistant device relies on a power assistant device under normal opening pressure.

According to the principle of action

According to the principle of action, it can be divided into direct-acting safety valve and non-direct-acting safety valve.
1. Direct acting safety valve
Direct acting relief valve is opened under the direct action of working medium, that is to say, relies on the action of working medium pressure to overcome the mechanical load imposed on the valve disc by the loading mechanism, so that the valve can be opened. The safety valve has the advantages of simple structure, quick action and good reliability. However, due to the structural loading, its load size is limited, and can not be used in high-pressure, large-caliber occasions.
2. Non-direct acting safety valve
This kind of safety valve can be divided into pilot safety valve and power assistant safety valve.
Pilot safety valve is driven or controlled by the medium discharged from the pilot valve. The pilot valve itself is a direct-acting safety valve, sometimes other forms of valves are also used. Pilot safety valve is suitable for high pressure and large caliber occasions. The main valve of pilot relief valve can also be designed to be sealed by working medium, or it can impose a much larger mechanical load on the disc than the direct acting relief valve, so it has good sealing performance. At the same time, its movements are seldom affected by back pressure. The disadvantage of this kind of safety valve is that its reliability is related to the main valve and guide valve. Its action is not as fast and reliable as that of direct acting safety valve, and its structure is more complex.
The safety valve with power assistant device is forced to open under normal opening pressure by means of a power assistant device. This safety valve is suitable for situations where the opening pressure is very close to the working pressure, or where the safety valve needs to be opened regularly for inspection or blowing out the sticky and frozen medium. At the same time, it also provides a means of forcibly opening the safety valve in an emergency.

Pressure adjustment points

Whether the pressure can be adjusted or not can be classified into fixed and adjustable safety valves.
1. The pressure value of the fixed and non-adjustable safety valve has been set out in the factory and can not be changed in use.
Active, commonly used in central air conditioning, boiler wall furnace, solar energy and other systems, such as the S10 series safety valve.
2. The take-off pressure of adjustable safety valve can be set arbitrarily in a certain range according to the different needs of users. It is often used in situations where the system protection pressure needs to change frequently, such as S10 series safety valve, but the price is generally higher.

According to working temperature

  • 1. Normal temperature safety valves are generally installed only on HVAC, air conditioning or water systems, such as 0480 safety valves.
  • 2. High temperature safety valve refers to the safety valve which is specially used in solar energy system and model temperature machine system, such as 1831 series safety valve.

Working Principle of Safety Valve

When the steam pressure under the relief valve disc exceeds the spring pressure, the valve disc is opened. After the top of the disc is opened, the exhaust steam acts on the disc clamping ring due to the rebound of the lower regulating ring, which makes the valve open quickly.
As the valve disc moves up, the steam impinges on the upper regulating ring, which makes the exhaust direction tend to be vertical downward. The reaction force generated by the exhaust pushes the valve disc upward, and keeps the valve disc in a certain range of pressure at a sufficient lifting height. With the opening of the safety valve, the steam is continuously discharged, and the steam pressure in the system gradually decreases. At this time, the spring force will overcome the steam pressure on the valve disc and the reaction force of exhaust steam, thus closing the safety valve.
Safety valve is an automatic pressure relief protection device opened by imported static pressure. Pressure safety valve is one of the most important safety accessories of pressure vessel. Its function is that when the pressure in the vessel exceeds a certain value, the safety valve automatically opens the valve depending on the pressure of the medium itself, and the safety valve expels a certain amount of medium rapidly.

Importance of Safety Valves

In order to prevent major accidents, adding safety valve is a common safety method, which plays a vital role in the safety of enterprises. Safety valve itself is damaged due to the limitation of service life and environment in the use process, so it is necessary to find problems in time and reduce the possibility of damage.
At present, nuclear power plants are only used in nuclear islands, and the main steam system has not been used yet, but the main steam pilot safety valve has a good record in the use of nuclear power plants abroad. Because the structure of the main steam pilot safety valve is completely different from that of the nuclear island pilot safety valve, the function principle and application of the main steam pilot safety valve are introduced.

Safety release capability of safety valve

Safety release capability of safety valve, namely displacement, is one of the most important indexes to measure the performance of safety valve. Under the guidance of the patent technology of “On-line Functional Safety Valve and Its Detection System”, the displacement detection problem and related flow mechanism of the dynamic opening process of the safety valve are studied.
The main research contents include: According to the flow characteristics of nozzle and the basic fluid control equation of ideal gas isentropic flow, the calculation formula of safety valve opening is deduced. The system summarizes various factors affecting the discharge capacity of safety valve. The interaction mechanism between safety valve overpressure and open flow field parameters, especially the change of aerodynamic parameters, is proposed.
In the experimental study, displacement measurement and other related parameters were tested in the field using the spool detection sensor and flange detection sensor. Comparing the displacement calculated by two-dimensional and three-dimensional numerical simulation with the experimental results, the research shows that the simulation and test results are consistent, which shows that the displacement detection and measurement using special sensors has good maneuverability. As long as the numerical simulation of displacement calculation method is appropriate, the discharge capacity of safety valve can be predicted more accurately.

Analysis of the function of safety valve and matters needing attention

The function of safety valve is to prevent the pressure of medium in pipeline or equipment from exceeding the prescribed value, so as to achieve the purpose of safety protection. Safety valve is a safety protection valve whose opening and closing parts are usually closed under external force. When the medium pressure in the equipment or pipeline rises, when the value exceeds the prescribed value, it opens automatically and prevents it by discharging the medium from outside the system. The pressure of the medium in the pipeline or equipment exceeds the prescribed value. Safety valve is an automatic valve, mainly used for boilers, pressure vessels and pipes. Control pressure does not exceed the prescribed value, which plays an important role in protecting personal safety and equipment operation.
According to the disc opening height of safety valve, it can be divided into micro-open safety valve and full-open safety valve. The opening stroke height of the micro-open safety valve is less than 0.05 d0 (minimum discharge throat diameter) and the opening height of the full-open safety valve is less than 0.25 d0 (minimum discharge throat diameter).
According to the structure, safety valves are divided into vertical hammer safety valve, lever safety valve, spring safety valve and pilot safety valve (pulse safety valve). According to the structure of the valve body, it can be divided into closed type. There are two types of safety valve and non-closed safety valve.
Closed safety valve means that the excluded medium will not leak. All are discharged along the outlet to a designated location, usually for toxic and corrosive media. For air and steam safety valves, closed safety valves are usually used. The selection of safety valve products should be determined according to the actual sealing pressure.
For spring relief valves, springs with multiple working pressure levels within the nominal pressure (PN) range should pay attention to the sealing pressure of the valve body in addition to the type, name, medium and temperature of the relief valve. Otherwise, supply with maximum sealing pressure.

Operation Method of Safety Valve
Adjustment of opening pressure:

  • (1) Before the safety valve leaves the factory, the opening pressure should be adjusted step by step to the setting value required by the user. If the user puts forward the spring working pressure level, the lower limit value of the pressure level should be adjusted to leave the factory.
  • (2) Before installing the safety valve on the protected equipment or before installing it, the user must readjust it at the installation site to ensure that the setting pressure of the safety valve meets the requirements.
  • (3) Within the working pressure level of the spring indicated in the nameplate, the opening pressure can be adjusted by rotating the adjusting screw to change the compression of the spring.
  • (4) Before rotating the adjusting screw, the inlet pressure of the valve should be reduced to less than 90% of the opening pressure to prevent the disc from being driven to rotate when rotating the adjusting screw, so as to damage the sealing surface.

In order to ensure that the opening pressure is accurate, the medium conditions, such as the type of medium, temperature and so on, should be adjusted as close as possible to the actual operating conditions. When the type of medium changes, especially when the accumulation state of the medium is different (e.g. from liquid to gas), the opening pressure often changes. When the working temperature rises, the opening pressure generally decreases. Therefore, when it is adjusted at room temperature and used at high temperature, the setting pressure value at room temperature should be slightly higher than the required opening pressure value. To what extent is related to the valve structure and material selection, it should be based on the manufacturer’s instructions.
Conventional safety valves are used to fix additional back pressure. When the opening pressure is adjusted after inspection (when the back pressure is atmospheric pressure), the setting value should be the required opening pressure value minus the additional back pressure value.

Adjustment of discharge and backup pressure:

To adjust the discharge pressure and return pressure of the valve, it is necessary to carry out the action test of the valve reaching the full open height. Therefore, it is only possible to carry out the test on a large capacity test device or after the safety valve is installed on the protected equipment. The adjustment method varies according to the valve structure.
For the structure with recoil disc and seat adjusting ring, the seat adjusting ring is used to adjust. By unscrewing the fixing screw of the adjusting ring and extending a thin iron rod from the exposed screw hole, the gear teeth on the adjusting ring can be moved to turn the adjusting ring left and right. When the adjusting ring rotates counterclockwise to the left, the position of the adjusting ring rises, and the discharge pressure and the return pressure will be reduced. On the contrary, when the adjusting ring rotates clockwise to the right, the position of the adjusting ring decreases, and the discharge pressure and the return pressure will increase. When adjusting each time, adjust: the rotating range of the circle should not be too large (the number of teeth can be rotated in general). After each adjustment, the fixed screw should be screwed up so that the end of the screw is located in the groove between the two teeth of the adjusting ring, which can prevent the rotating of the adjusting ring and generate no radial pressure on the adjusting ring. In order to be safe, the inlet pressure of the safety valve should be reduced appropriately (generally less than 90% of the opening pressure) before the adjusting ring is moved to prevent the sudden opening of the valve during the adjusting process and cause accidents.
(3) For the structure with upper and lower adjusting rings (one adjusting ring on the guide sleeve and one adjusting ring on the valve seat), the adjustment is more complicated. The seat regulating ring is used to change the size of the passage between the disc and the regulating ring, thereby changing the degree of pressure accumulation in the chamber between the disc and the regulating ring when the valve is initially opened. When the seat regulating ring is raised, the degree of pressure accumulation increases, so that the proportional opening stage of the valve decreases and achieves sudden and rapid opening. Therefore, raising the seat regulating ring can reduce the discharge pressure. It should be noted that the seat adjustment ring should not rise too close to the disc. In that case, leakage at the sealing surface may cause the valve to suddenly open prematurely, but because the medium pressure is not enough at this time to keep the disc in the open position, the disc then closes, so the valve occurs frequent jumps. Seat adjustment: The ring is mainly used to reduce the valve proportion, open the stage and adjust the discharge pressure, but also has an impact on the return pressure.
The adjusting ring is used to change the angle of the flow medium bending after reflection on the lower side of the valve disc, thereby changing the magnitude of the fluid force, so as to adjust the return pressure. When the adjusting ring is raised and raised, the bending angle decreases, and the fluid force decreases accordingly, so that the return pressure increases. On the contrary, when the adjusting ring is lowered, the backing pressure decreases. Of course, while changing the backup pressure, the upward adjustment ring also affects the discharge pressure, that is, the upward adjustment ring increases the discharge pressure, and the lower adjustment ring reduces the discharge pressure, but its impact is not as obvious as the backup pressure.

Lead seals for safety valves:

When the safety valve is adjusted, it should be sealed with lead to prevent any change in the adjusted condition. When repairing the relief valve, the position of adjusting screw and adjusting ring should be noted before removing the valve so as to facilitate the adjustment after repairing. Lead seal should be applied again after readjustment.

Common failures of safety valves

Disc does not return after discharge: This is mainly caused by spring bending stem, incorrect installation position of disc or stuck. It should be reassembled.
Leakage: Under the normal working pressure of the equipment, leakage occurs between the disc and the sealing surface of the valve seat beyond the allowable level. The reasons are: there is dirt between the valve disc and the sealing surface of the valve seat. The lifting wrench can be used to open the valve several times to wash away dirt; the sealing surface is damaged. It should be repaired by grinding or grinding after turning according to the degree of damage; the valve stem is bent, inclined or the lever and fulcrum are deflected, so that the spool and disc are dislocated. It should be reassembled or replaced; spring elasticity is reduced or lost. Measures such as replacing spring and readjusting opening pressure should be taken.
Do not open when required pressure: the reason for this is the inaccuracy of constant pressure. The spring compression or weight position should be readjusted; the disc adheres to the seat. Manual ventilation or water discharge tests should be conducted regularly for the relief valve; the lever of the lever relief valve is jammed or the weight is moved. The position of the hammer should be readjusted and the lever should move freely.
Pressure continues to rise after exhaust: this is mainly because the selected relief valve displacement is less than the equipment’s safe discharge, the appropriate relief valve should be re-selected; the valve stem midline is not correct or the spring is rusty, so that the disc can not open to its proper height, the valve stem should be reassembled or the spring should be replaced; the exhaust pipe is not enough, and the exhaust pipe should be adopted in line with the safe discharge area.
Disc frequency jump or vibration: mainly due to the spring stiffness is too large. The spring with proper stiffness should be used instead, and the adjusting ring should be adjusted inappropriately to make the return pressure too high. The position of adjusting ring should be readjusted; excessive resistance of discharge pipeline results in excessive back pressure of discharge. The resistance of discharge pipe should be reduced.
The main reasons are inaccuracy of constant pressure and decline of spring aging elasticity. Adjusting screw or replacing spring should be properly tightened.

Cautions for Safety Valve Installation

20190420221515 65798 - What is a safety valve?

  • (1) The newly installed safety valve shall be accompanied by a certificate of product qualification. Before installation, it shall be re-checked, sealed with lead and checked with safety valve.
  • (2) Safety valves shall be installed vertically and installed at the gas-phase interface of the container or pipeline.
  • (3) There should be no resistance at the exit of the safety valve to avoid back pressure. If the discharge pipe is installed, its inner diameter should be larger than the outlet diameter of the safety valve. The outlet of the safety valve should be frozen-proof. For containers containing flammable or toxic or highly toxic media, the discharge pipe should go directly to the outdoor safe place or facilities for proper treatment. No valve should be installed in the discharge pipe.
  • (4) No valves shall be installed between pressure equipment and safety valves. For containers filled with flammable, explosive, toxic or viscous media, shutoff valves can be installed for easy replacement and cleaning. Their structure and size should not hinder the normal operation of safety valves. In normal operation, the globe valve must be fully open and sealed with lead.
  • (5) For pressure vessels with flammable, explosive or toxic media, the medium discharged by the safety valve must have safety devices and recovery systems. The installation of lever relief valve must keep the vertical position, and spring relief valve should also be installed vertically to avoid affecting its action. When installing, attention should also be paid to coordination, coaxiality of parts and uniform stress of bolts. The cross-sectional area of the short pipe connecting the safety valve and the boiler pressure vessel shall not be less than the flow cross-sectional area of the safety valve. The whole safety valve shall be installed on a nozzle at the same time, and the cross-sectional area of the nozzle shall not be less than 1.25 times of the total flow cross-sectional area of the safety valve.
  • (6) When there are gas and liquid two-phase materials in the container, the safety valve should be installed in the gas phase part.
  • (7) When the safety valve is used to discharge flammable liquid, the outlet of the safety valve shall be connected with the accident tank. When the discharged material is high temperature combustible, the receiving container should have corresponding protective facilities.
  • (8) General relief valves can be vented in situ. The vent should be higher than 1 m (m) of the operator and should not be directed towards the open flame location, spark emission location and high temperature equipment within 15 m (m). The safety valve vent of indoor equipment and containers should lead out the roof and be over 2 meters (m) above the roof.
  • (9) When there is a partition valve at the entrance of the relief valve, the partition valve should be in a normal open state and be sealed with lead to avoid mistakes.
  • (10) The cross-sectional area of the connecting pipe and the through-hole of the pipe fittings between the safety valve and the boiler or pressure vessel shall not be less than the import cross-sectional area of the safety valve; if several safety valves share an import pipe, the flow cross-sectional area of the import pipe shall not be less than the sum of the import cross-sectional area of the safety valve.
  • (11) In general, no cut-off valve shall be installed between the safety valve and the drum or header of the boiler, or the steam outlet pipe shall be used. It is generally not appropriate to install cut-off valves or other lead-out pipes between safety valves and pressure vessels; for pressure vessels with extremely high, high and moderate toxicity, flammable, corrosive, viscous or precious media, cut-off can be installed between safety valves and pressure vessels only with the approval of the technical person in charge of the pressure vessel of the user unit and the formulation of reliable preventive measures. Valves. During normal operation of pressure vessel, globe valve must be kept fully open, sealed or locked with lead. The structure and diameter of the stop valve shall not hinder the safe discharge of the safety valve.
  • (12) Spring relief valves with threaded connections shall be connected with short threaded pipes, while welding connections shall be used between short pipes and simplified bodies and headers.
  • (13) Safety valves must be equipped with discharge pipes. The discharge pipe should avoid twists and turns as far as possible to minimize resistance. The exhaust pipe should go straight to the safe place and have enough cross-section area to ensure smooth exhaust. For safety valves that can interact to produce chemical reactions, one discharge pipe cannot be shared; when safety valves are installed on equipment with corrosive and flammable gases, measures should be taken to prevent corrosion or fire and explosion; when the equipment equipped with safety valves is toxic medium and the steam density of the medium is greater than air density, the medium and steam discharged from safety valves should be introduced. To the closed system, and from the closed system recovery to production use.
  • (14) The relief valve discharge pipe should be fixed so as not to cause excessive additional stress or vibration of the relief valve.
  • (15) Safety valves installed in the open air shall be equipped with reliable measures to prevent the water content in the medium of the valve from freezing and affecting the discharge of the safety valve when the temperature is below 0 degrees.
  • (16) When the medium crystallization temperature of the safety valve is higher than the lowest ambient temperature, the safety valve must be equipped with an insulating jacket and installed with insulating and purging steam to prevent the medium crystallization from clogging the safety valve and affect the normal operation performance of the safety valve. The inlet and outlet pipes of safety valves must also be designed with steam insulation jackets or with additional insulation steam accompanying pipes to prevent medium crystallization from clogging the pipes.
  • (17) Safety valves should be equipped with devices to prevent the self-movement of weights and guides to limit the deviation of levers. Spring safety valves should be equipped with lifting handles and devices to prevent the screw from being unscrewed.
  • (18) It shall be installed vertically at the highest position of the cooker and header. Between the safety valve and the drum or header, there shall be no outlet pipe and valve for taking steam.
  • (19) For boilers with rated steam pressure less than or equal to 3.82 MPa, the throat diameter of safety valve shall not be less than 25 mm; for boilers with rated steam pressure greater than 3.82 MPa, the throat diameter of safety valve shall not be less than 20 mm.
  • (20) The safety valve of the pressure vessel should be installed directly at the highest position of the pressure vessel body. Safety valves for liquefied gas tanks must be installed in the gas phase. Generally, the short pipe can be connected with the container, and the diameter of the short pipe of the safety valve should be no less than the diameter of the safety valve.
  • (21) Boilers with rated evaporation greater than 0.5 t/h shall be equipped with at least two safety valves; boilers with rated evaporation less than or equal to 0.5 t/h shall be equipped with at least one safety valve. Safety valves must be installed at the outlet of separable economizer and steam superheater.
  • (22) There should be no resistance at the exit of the safety valve to avoid back pressure. If the discharge pipe is installed, its inner diameter should be larger than the outlet diameter of the safety valve. The outlet of the safety valve should be frozen-proof. For containers containing flammable or toxic or highly toxic media, the discharge pipe should go directly to the outdoor safe place or facilities for proper treatment. No valve shall be installed in the discharge pipe.
  • 23) For corrosive media, the safety valve should be installed in combination with the blasting disc.
  • 24) For highly toxic media, the type of safety valve with good sealing should be selected.
  • 25) For high temperature medium relief valve, high temperature has great influence on spring, so spring relief valve should be avoided.
  • 26) For important safety valves, or safety valves vulnerable to fire, spray protection should be provided.
  • 27) For safety valves for spherical tanks, double safety valves should be installed, and the pressure relief capacity of any of them can meet the requirements of safety pressure relief for spherical tanks.
  • 28) The safety valve should be equipped with electrostatic cross-line.
  • 29) In order to prevent the safety valve from opening and closing repeatedly, causing flutter and damaging the valve, it is necessary to reduce the pressure drop of the inlet pipe of the safety valve, that is, to increase the diameter of the inlet pipe and shorten the inlet pipe section.

Installation position and requirements of safety valve

20190420221734 83833 - What is a safety valve?

  • 1) Vertical upward installation.
  • 2) Installation location, as close as possible to the protected equipment or pipeline.
  • 3) Installed in easy to repair and adjust, there is enough space around.
  • 4) The safety valve of the pressure vessel is installed in the gas phase space above the liquid level of the vessel body, or in the pipeline where the connecting point is located in the gas phase space of the pressure vessel.
  • 5) For containers and equipment containing flammable, toxic and viscous media, globe valves may be installed in front of safety valves, but the flow area of globe valves shall not be less than the minimum flow area of safety valves, and lead seals shall be added to ensure that the globe valves are fully open and normally open.
  • 6) Safety valve should be installed in reverse with globe valve to reduce the influence of self-gravity and avoid stress fatigue and exhaust vibration fatigue.
  • 7) For safety valves that may be blocked or corroded by materials, explosive discs are installed in front of the entrance, and check valves are installed between the safety valve and the explosive discs, and blockage prevention measures such as back purge, heat tracing or heat preservation are adopted on the entrance pipeline.
  • 8) The safety valve installed on the pipeline should be set in the place where the fluid pressure is relatively stable and there is a certain distance from the source of fluctuation, and can not be installed at the dead corner of the horizontal pipeline.
  • 9) For pipes, heat exchangers or pressure vessels with liquid medium, when the valve closes, thermal expansion may occur, causing high pressure constraints, the safety valve can be installed horizontally, and the liquid can be discharged directly downward.
  • 10) For emergency air discharge, the nozzle should be flat, not sharp and without burrs to prevent electrostatic discharge.
  • 11) Safety valve installation point, can not make the safety valve bear excessive back pressure value, should be within the prescribed allowable range.
  • 12) The body of the safety valve should be supported smoothly.
  • 13) The medium is a container of extremely hazardous or highly flammable and explosive medium. The outlet of the safety valve should be directed to the safe place and handled properly. When two or more safety valves share one discharge pipe, the cross-sectional area of the discharge pipe should not be less than the sum of the cross-sectional area of all the outlets of the safety valve. However, oxygen or combustible gas and other two gases that can react with each other can not share one discharge pipe.

Maintenance of Safety Valve

Strengthen daily maintenance and maintenance, keep clean, prevent corrosion and blockage of grease scale and dirt; check lead seals regularly to prevent others from freely moving the weight of lever-type safety valve or twisting the adjusting screw of spring-type safety valve; in order to prevent the valve and seat from sticking to each other, formulate regular manual (or hand-lifting) discharge system according to the actual situation of pressure equipment, such as steam boiler drum safety valve I. Generally, it should be discharged artificially once a day, and the discharge pressure should be above 80% of the prescribed maximum working pressure. If leakage is found, it should be repaired or replaced in time. It is strictly forbidden to use the method of increasing load (such as lever relief valve shifting the weight out or overtightening the adjustment screw of spring relief valve) to eliminate leakage.

20190420221948 85687 - What is a safety valve?

Safety valves are inspected regularly at least once a year. The contents of inspection include dynamic examination and decomposition examination. The steps of dynamic inspection include assembling, boosting (up to working pressure), holding pressure (no leakage in 3-5 minutes), boosting action (immediate action under opening pressure), buckling return and re-holding pressure (no leakage at working pressure for 3 minutes after return).

  1. Safety valve should have a device to prevent the weight from moving by itself and a guide to limit the deviation of lever. Spring safety valve should have a lifting handle and a device to prevent the screw from screwing randomly.
  2. It should be installed vertically at the highest position of the cooker and header. Between the safety valve and the drum or header, there shall be no outlet pipe and valve for taking steam.
  3. The cross-sectional area of the connecting pipe between the safety valve and the boiler shall be no less than the cross-sectional area of the inlet of the safety valve. If several safety valves are installed on a short tube directly connected to the pot drum, the cross-section area of the short tube should not be less than 1.25 times the exhaust area of all safety valves.
  4. For boilers with rated steam pressure less than or equal to 3.82 MPa, the throat diameter of safety valve shall not be less than 25 mm; for boilers with rated steam pressure greater than 3.82 MPa, the throat diameter of safety valve shall not be less than 20 mm.
  5. Safety valves should generally be equipped with exhaust pipes, which should go straight to the safe place and have enough cross-section area to ensure smooth exhaust. The bottom of the relief valve exhaust pipe should be equipped with a drain pipe connected to the safe place. Valves are not allowed to be installed on the exhaust pipe and the drain pipe.
  6. The safety valve of pressure vessel should be installed directly at the highest position of pressure vessel body. Safety valves for liquefied gas tanks must be installed in the gas phase. Generally, the short pipe can be connected with the container, and the diameter of the short pipe of the safety valve should be no less than the diameter of the safety valve.
  7. Valves should not be installed between safety valves and containers. For flammable, explosive or viscous media containers, globe valves can be installed in order to facilitate the cleaning or replacement of safety valves. The globe valves must be fully opened and sealed with lead during normal operation to avoid chaos.
  8. Boilers with rated evaporation greater than 0.5 t/h shall be equipped with at least two safety valves; boilers with rated evaporation less than or equal to 0.5 t/h shall be equipped with at least one safety valve. Safety valves must be installed at the outlet of separable economizer and steam superheater.
  9. For pressure vessels with flammable, explosive or toxic media, the medium discharged by the safety valve must have safety devices and recovery systems. The installation of lever relief valve must keep the vertical position, and spring relief valve should also be installed vertically to avoid affecting its action. When installing, attention should also be paid to coordination, coaxiality of parts and uniform stress of bolts.
  10. There should be no resistance at the exit of the safety valve to avoid back pressure. If the discharge pipe is installed, its inner diameter should be larger than the outlet diameter of the safety valve. The outlet of the safety valve should be frozen-proof. For containers containing flammable or toxic or highly toxic media, the discharge pipe should go directly to the outdoor safe place or facilities for proper treatment. No valve should be installed in the discharge pipe.
  11. The newly installed safety valve shall be accompanied by a product qualification certificate. Before installation, it shall be re-checked, sealed with lead and checked with safety valve.
  12. No valves shall be installed between pressure equipment and safety valves. For containers filled with flammable, explosive, toxic or viscous media, cut-off valves can be installed for easy replacement and cleaning. Their structure and size should not hinder the normal operation of safety valves. In normal operation, the globe valve must be fully open and sealed with lead.

Safety valves belong to automatic valves. They are mainly used in boilers, pressure vessels and pipes. The control pressure does not exceed the prescribed value, which plays an important role in protecting personal safety and equipment operation.

Selection Principle of Safety Valve

  1. Steam Boiler Safety Valves, generally open full-open spring safety valves 0490 series;
  2. Safety valves for liquid media are generally of the 0485 series of micro-open spring safety valves.
  3. Safety valves for air or other gaseous media are usually full-open spring safety valves.
  4. Safety valves for liquefied petroleum gas trucks or liquefied petroleum gas Railway Tankers generally use full-open internal safety valves.
  5. Pilot safety valve is generally used for the exit of oil production wells.
  6. High-pressure bypass safety valves for steam power generation equipment are generally selected as pilot safety valves with dual functions of safety and control.
  7. If regular opening test is required, the safety valve with lifting wrench should be selected. When the medium pressure reaches 75% of the opening pressure, the lifting wrench can be used to lift the disc slightly from the seat to check the flexibility of the safety valve opening.
  8. If the medium temperature is high, in order to reduce the temperature of the spring chamber, the safety valve with radiator should be selected when the temperature of the closed safety valve exceeds 300 ~C and that of the open safety valve exceeds 350 ~C.
  9. If the back pressure at the outlet of the relief valve is variable and the change exceeds 10% of the opening pressure, the bellows relief valve should be selected.
  10. If the medium is corrosive, the bellows safety valve should be selected to prevent the failure of important parts due to the corrosion of the medium.
  11. Safety valve installation and maintenance should pay attention to the following matters: construction, installation points 1), installation location, height, import and export direction must meet the design requirements, pay attention to the direction of medium flow should be consistent with the direction of arrow marked by the valve body, and the connection should be firm and close. 2) Appearance inspection must be carried out before the valve is installed. The nameplate of the valve should conform to the current national standard “General Valve Mark” GB 12220. For valves with working pressure greater than 1.0 MPa and cutting off effect on main pipe, strength and tightness test should be carried out before installation, and the valves can be used only after they are qualified. In the strength test, the test pressure is 1.5 times of the nominal pressure, and the duration is not less than 5 minutes. The valve shell and packing should be qualified without leakage. The test pressure is 1.1 times of nominal pressure in tightness test, and the test duration meets the requirements of GB 50243. 1. All kinds of safety valves should be installed vertically.
  12. There should be no resistance at the exit of safety valve to avoid pressure.
  13. Safety valve shall be specially tested before installation and its sealing shall be checked.
  14. Safety valves in use should be inspected regularly.

Source: China Safety Valves Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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Summary
what is a safety valve - What is a safety valve?
Article Name
What is a safety valve?
Description
Safety valve is a special branch of valve family. Its particularity is that safety valve only plays the role of switch unlike other valves. More importantly, it plays a role in protecting the safety of equipment.
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www.steeljrv.com
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