Safety valve analysis and process flow

The safety valve is a kind of valve for safety protection. The opening and closing part of the safety valve is normally closed under the action of external force. When the medium pressure in the equipment or pipeline rises and exceeds the specified value, it is automatically opened and prevented by discharging the medium outside the system. The pressure of the medium in the pipeline or equipment exceeds the specified value. The safety valve is an automatic valve, which is mainly used in boilers, pressure vessels and pipelines. The control pressure does not exceed the specified value and plays an important role in personal safety and equipment operation.

The safety valve (relief valve, also known as the relief valve) is automatically opened and closed according to the working pressure of the pressure system (operating temperature [1]), and is generally installed on the equipment or pipeline of the closed system to protect the safety of the system. When the pressure or temperature in the equipment or pipeline exceeds the set pressure of the safety valve, it will automatically open the pressure relief or reduce the temperature to ensure that the medium pressure (temperature) in the equipment and pipeline is below the set pressure (temperature) and protect the equipment and pipeline to work normally. Prevent accidents and reduce losses. The safety valve plays a safety role in the system. When the system pressure exceeds the specified value, the safety valve opens, and part of the gas/fluid in the system is discharged into the atmosphere/outside of the pipeline, so that the system pressure does not exceed the allowable value, so as to ensure that the system does not cause accidents due to high pressure. There are two main types of safety valve structures: spring type and lever type. Spring type refers to the valve and valve seat seal spring force. Leverage is based on the leverage and weight of the force. With the need for large capacity, there is also a pulse-type safety valve, also called a pilot-type safety valve, composed of a main safety valve and an auxiliary valve. When the medium pressure in the pipeline exceeds the specified pressure value, the auxiliary valve is opened first, the medium enters the main safety valve along the conduit, and the main safety valve is opened, so that the increased medium pressure is reduced.

The discharge of the safety valve is determined by the valve seat diameter and the opening height of the valve disc. It can also be divided into two types: the micro opening height is the valve seat inner diameter (1/15) to (1/20), and the full activation type. Yes (1/3) to (1/4).

In addition, with the different requirements for use, there are closed and non-closed. The closed type, ie the discharged medium, is not leaked and is discharged along the specified outlet. It is generally used for toxic and corrosive media. The non-enclosed type is generally used for non-toxic or non-corrosive media. According to its overall structure and loading mechanism can be divided into heavy hammer lever type, spring type and pulse type three.

1. Heavy-duty lever type safety valve

The weighted lever type safety valve utilizes a weight and a lever to balance the force acting on the valve flap. According to the principle of leverage, it can use the weight of a smaller hammer to obtain a larger force through the increase of the lever, and adjust the opening pressure of the safety valve by moving the position of the weight (or changing the weight of the weight).

The heavy-lever lever type safety valve has a simple structure, is easy to adjust and is relatively accurate, and the applied load is not greatly increased due to the rise of the valve flap. The utility model is suitable for high temperature occasions and has been widely used in the past. It is used on boilers and higher pressure vessels. However, the heavy-lever lever type safety valve has a relatively heavy structure, the loading mechanism is easy to vibrate, and often causes leakage due to vibration; the pressure of its return seat is low, and it is not easy to close and keep tight after opening.

2. Spring lift valve

A spring-activated safety valve uses the force of a compression spring to balance the force acting on the valve flap. The amount of compression of the coil spring can be adjusted by turning the adjustment nut above it. With this structure, the opening (setting) pressure of the safety valve can be corrected as required. The spring-activated safety valve has a compact and lightweight structure, a high sensitivity, an unrestricted mounting position, and can be used on a mobile pressure vessel because of its low sensitivity to vibration. The disadvantage of this type of safety valve is that the applied load changes with the opening of the valve. That is, as the valve plate rises, the compression of the spring increases and the force acting on the valve flap also increases. This is detrimental to the quick opening of the safety valve. In addition, the spring on the valve will reduce the spring force due to long-term exposure to high temperatures. When used on a container with a high temperature, the heat insulation or heat dissipation of the spring is often considered, thereby complicating the structure.

3. Pulse safety valve

The pulse-type safety valve is composed of a main valve and a secondary valve, and the main valve is driven by the pulse action of the auxiliary valve. Its structure is complex, and it is usually only applicable to boilers and pressure vessels with large safety relief.

Among the above three types of safety valves, spring-loaded safety valves are commonly used.

According to the different media discharge methods, safety valves can be divided into three types: fully enclosed, semi-closed, and open.

1. Closed safety valve

When the fully-enclosed safety valve is exhausted, all the gas is discharged through the exhaust pipe, and the medium cannot leak outside and outside. It is mainly used for toxic media. Gas container.

2. Semi-closed safety valve

Some of the gas discharged from the semi-closed safety valve passes through the exhaust pipe, and some of it leaks from the gap between the valve cap and the valve stem. It is mostly used in containers where the medium is a gas that does not pollute the environment.

3. Open safety valve

The valve cover of the open safety valve is open, so that the spring chamber is open to the atmosphere. This helps to reduce the temperature of the spring. It is mainly used for containers with steam as the medium and high temperature gas that does not pollute the atmosphere.

According to the ratio of the large height of valve opening to the diameter of the safety valve flow path, the safety valve can be divided into two types: spring micro-enclosed high-pressure safety valve and spring full-open safety valve.

1. Spring micro-opening closed high pressure safety valve 　　

The opening height of the micro-relief safety valve is less than 1/4 of the diameter of the flow passage, which is usually 1/40 to 1/20 of the diameter of the flow passage. The actuation process of the micro-relief valve is proportional, and it is mainly used in liquid applications, and it is sometimes used in gas applications where the emissions are small.

2. Spring full-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 flow passage. The full-open safety valve discharge area is the small cross-sectional area of ​​the valve seat throat. The action process is a two-stage action, which must be achieved by means of a lift mechanism. The full-open safety valve is mainly used in gas medium applications.

3. Kai-type safety valve

The opening height is between the start-up mode and the full-open mode. That is, it can be made into two stages, or it can be made into a proportional one.

Classification according to the principle of action, can be divided into direct-acting safety valve and non-direct-acting safety valve.

1. Direct acting safety valve

The direct-acting safety valve is opened under the direct action of the working medium, ie, the mechanical load applied by the loading mechanism to the valve flap is overcome by the action of the working medium pressure to open the valve. The safety valve has the advantages of simple structure, rapid operation, and good reliability. However, due to structural loading, its load is limited and cannot be used in high pressure, large diameter applications.

2. Non-direct acting safety valve

This kind of safety valve can be divided into pilot safety valve and safety valve with power auxiliary device.

The pilot-operated safety valve is driven or controlled by the medium discharged from the pilot valve. The pilot valve itself is a direct-acting safety valve, and other types of valves are sometimes used. Pilot operated safety valves are suitable for high pressure, large diameter applications. The main valve of the pilot-operated safety valve can also be designed to seal on the working medium, or it can exert a much greater mechanical load on the valve flap than the direct-acting safety valve and thus has a good sealing performance. At the same time, its actions are rarely affected by back pressure. The disadvantage of this type of safety valve is that its reliability is related to the main valve and the pilot valve. The action is not as fast and reliable as the direct acting safety valve, and the structure is more complicated.

A safety valve with power assist is a force assist device that forces the safety valve to open below normal opening pressure. This type of safety valve is suitable for applications where the opening pressure is close to the working pressure, or where the safety valve needs to be opened periodically to inspect or blow off the sticky, frozen medium. At the same time, it also provides a means to forcefully open the safety valve in an emergency.

According to the pressure can adjust the classification, can be divided into fixed non-adjustable safety valve and adjustable safety valve

1. The pressure value of the fixed non-adjustable safety valve has been set at the factory and cannot be changed during use. It is commonly used in systems such as central air conditioners, boilers, and solar energy, such as the S10 series safety valves.

2. Adjustable safety valve takeoff pressure can be arbitrarily set in a certain range depending on the user's different requirements. It is often used in occasions where the system protection pressure needs to be changed frequently, such as the S10 series safety valve, but the price is generally higher.

1. The normal temperature safety valve is generally a safety valve that is only installed on the HVAC, air conditioning or water system and is resistant to 110°C, such as the 0480 safety valve.

2. High-temperature safety valve refers to a safety valve that is specially used in the solar system and mold temperature system and is resistant to temperature 180°C, such as the 1831 series safety valve.

1 Before the safety valve is delivered from the factory, the opening pressure should be adjusted to the user's required setting value. If the user proposes a spring working pressure level, the factory should be adjusted according to the lower limit of the pressure level.

2 Before the user installs the safety valve on the protected equipment or before installation, it must be re-adjusted at the installation site to ensure that the pressure setting value of the safety valve meets the requirements.

3 Within the spring working pressure range specified on the nameplate, the opening pressure can be adjusted by turning the adjusting screw to change the spring compression amount.

4 Before rotating the adjusting screw, the pressure at the inlet of the valve should be reduced to less than 90% of the opening pressure to prevent the valve disc from being rotated when the adjustment screw is rotated, so as to damage the sealing surface.

5 To ensure that the opening pressure value is accurate, the media conditions during the adjustment, such as the type of medium and temperature, should be as close as possible to the actual operating conditions. Changes in the type of media, especially when the state of media accumulation is different (for example, from liquid phase to gas phase), the opening pressure often changes. As the operating temperature increases, the cracking pressure generally decreases. Therefore, when used at high temperatures for adjustment at room temperature, the setting pressure at room temperature should be slightly higher than the required opening pressure. The degree to which it is high is related to the choice of valve structure and material, and should be based on the manufacturer's instructions.

6 When a conventional safety valve is used to fix additional back pressure, when the opening pressure is adjusted after inspection (at this time the back pressure is atmospheric pressure), the setting value shall be the required opening pressure value minus the additional back pressure value.

(1) To adjust the valve discharge pressure and return seat pressure, the valve must reach the full opening height operation test. Therefore, it can only be performed after a large-capacity test device or after the safety valve is installed on the protected equipment. The adjustment method differs depending on the valve structure.

2 For the structure with recoil plate and seat adjustment ring, use the seat adjustment ring to adjust. Unscrew the adjusting ring fixing screw, and extend a tool such as a thin iron bar from the exposed screw hole to move the adjusting ring's teeth and turn the adjusting ring to the left or right. When the adjustment ring is rotated counterclockwise to the left, its position is increased, and the discharge pressure and the return pressure will be reduced. Conversely, when the adjusting ring is rotated clockwise to the right, its position is lowered, and both the discharge pressure and the return pressure are increased. Each adjustment, adjustment: The rotation of the circle should not be too large (general rotation number can be). After each adjustment, the fixing screw should be screwed in so that its end is located in the groove between the two teeth of the adjusting ring, which not only prevents the adjusting ring from rotating but also does not generate radial pressure on the adjusting ring. For safety reasons, before the adjustment ring is toggled, the inlet pressure of the safety valve should be properly reduced (generally lower than 90% of the opening pressure) to prevent the valve from being suddenly opened during the adjustment and causing an accident.

3 For the structure with the upper and lower adjustment rings (one for the guide sleeve and the other for the valve seat), the adjustment is more complicated. The seat adjustment ring is used to change the size of the passage between the valve flap and the adjusting ring, thereby changing the degree of accumulation of pressure between the valve flap and the adjusting ring when the valve is initially opened. When the valve seat adjustment ring is raised, the degree of pressure build-up increases, so that the phase of valve opening is reduced and the sudden rapid opening is reached quickly. Therefore, raising the seat adjustment ring can reduce the discharge pressure. It should be noted that the seat adjustment ring must not be raised too close to the valve flap. In that case, leakage at the sealing surface may cause the valve to suddenly open abruptly, but since the medium pressure is not enough to hold the valve flap in the open position, the valve flap closes and the valve jumps. Seat Adjustment: "The ring is mainly used to reduce the valve ratio, open the stage and adjust the discharge pressure, but also affect the return pressure.

The upper adjusting ring is used to change the angle of the flow medium after it is reflected on the lower side of the valve flap, so as to change the size of the fluid acting force, so as to adjust the return seat pressure. When the upper adjusting ring is raised, the folding angle is reduced and the fluid force is reduced accordingly, thereby increasing the pressure of the return seat. Conversely, when the upper adjustment ring is lowered, the return pressure decreases. Of course, when the upper adjusting ring changes the pressure of the return seat, it also affects the discharge pressure. That is, raising the upper adjusting ring increases the discharge pressure, lowering the upper adjusting ring reduces the discharge pressure, but the effect is not as good as Seat pressure is as obvious.

After the safety valve is adjusted, it should be sealed to prevent any changes to the adjusted condition. When refitting the safety valve, the positions of the adjusting screw and the adjusting ring should be noted before disassembling the valve to facilitate the adjustment work after trimming. After re-adjustment, it should be sealed again.

This is mainly caused by the spring bending valve stem, the valve disc mounting position being incorrect or being stuck. Should be re-assembled.

Under the normal operating pressure of the device, leakage beyond the allowable level occurs between the disc and the sealing surface of the valve seat. The reasons are: There is dirt between the valve flap and the sealing surface of the valve seat. Use a lifting wrench to open the valve a few times to flush away the dirt; the sealing surface is damaged. Should be based on the degree of damage, grinding or grinding after grinding method to repair; valve stem bending, tilting or deflection of the lever and fulcrum, so that the spool and valve disc misalignment. Should be reassembled or replaced; spring elasticity reduced or lost elasticity. Replace the spring and readjust the opening pressure.

The reason for this situation is that constant pressure is not allowed. The amount of spring compression or the position of the weight should be readjusted; the valve flap sticks to the valve seat. The safety valve shall be manually deflated or drained periodically; the leverage of the lever-type safety valve is jammed or the hammer is moved. The weight position should be readjusted and the lever should move freely.

This is mainly due to the fact that the selected safety valve displacement is less than the safety discharge of the equipment, and the appropriate safety valve should be selected again; the stem centerline is not correct or the spring is rusted, so that the valve disc cannot be opened to the proper height, and the valve stem should be reassembled. Or replace the spring; exhaust pipe cut enough, should be taken to meet the safety discharge area of ​​the exhaust pipe.

This is mainly due to the high spring stiffness. Should use a spring with appropriate stiffness; improper adjustment of the adjustment ring will make the pressure in the seat back too high. The position of the adjusting ring should be readjusted; the discharge pipe resistance is too large, resulting in excessive discharge back pressure. The discharge pipe resistance should be reduced.

The main reason is that the constant pressure is not accurate; the elasticity of the spring is reduced. Tighten the adjusting screw or replace the spring properly.

　　 1、Steam boiler safety valve, generally choose the open full-open type spring safety valve 0490 series;

　　 2, the use of liquid medium safety valve, the general selection of micro-open spring safety valve 0485 series;

　　 3, air or other gas medium safety valve, generally selected closed full spring spring safety valve;

　　 4. Safety valves for liquefied petroleum gas vehicle tankers or liquefied petroleum gas railway tankers. Generally, full-entry type internal safety valves are selected;

　　 5, the safety valve for the outlet of the production well, the pilot valve is generally used;

　　 6. High-pressure bypass safety valve of steam power generation equipment, generally used pilot safety valve with double functions of safety and control

　　 7. If a regular opening test is required for the safety valve, a safety valve with a lifting wrench shall be used. When the medium pressure reaches more than 75% of the opening pressure, the valve flap can be lifted slightly from the valve seat with a lifting wrench to check the opening of the safety valve;

　　 8, if the medium temperature is high, in order to reduce the temperature of the spring chamber, generally when the use of closed safety valve temperature exceeds 300 °C and open safety valve use temperature exceeds 350 °C, should use a radiator with a safety valve;

　　 9. If the back pressure at the outlet of the safety valve is changed, and its variation exceeds 10% of the opening pressure, a bellows safety valve shall be used;

　　 10. If the medium is corrosive, bellows safety valves should be used to prevent failure of important parts due to media corrosion.

　　 11. Safety valve installation and maintenance should pay attention to the following issues Construction and installation points 1) The installation position, height, and direction of import and export must meet the design requirements. Note that the direction of medium flow should be consistent with the arrow direction of the valve body, and the connection should be firm. close. 2) Before the valve is installed, a visual inspection must be performed. The nameplate of the valve shall comply with the current standard "General Valve Mark" GB 12220. For valves with a working pressure greater than 1.0 MPa and a shut-off effect on the main pipe, strength and tightness tests shall be carried out before installation. In the strength test, the test pressure is 1.5 times the nominal pressure, and the duration is not less than 5 minutes. The valve housing and the packing should have no leakage as qualified. For tightness test, the test pressure is 1.1 times the nominal pressure; the duration of the test meets the requirements of GB 50243. 1. All safety valves should be installed vertically.

　　 12. There should be no resistance at the outlet of the safety valve to avoid pressure.

　　 13. The safety valve should be specially tested before installation and checked for leak tightness.

　　 14. The safety valve in use should be checked regularly.

Nominal pressure: Indicates the high allowable pressure of the safety valve at room temperature. The safety valve used for high temperature equipment should not consider the reduction of allowable stress of the material under high temperature. The safety valve is designed and manufactured according to the nominal pressure standard.

Opening pressure: also called rated pressure, refers to the inlet pressure of the safety valve disc when it starts to rise under operating conditions. Under this pressure, there is a measurable opening height, and the medium can be continuously discharged by visual or auditory dry support. .

Discharge pressure: The inlet pressure at which the disc reaches the specified opening height. The upper limit of discharge pressure must comply with the requirements of relevant standards or specifications.

Over pressure: The difference between the discharge pressure and the opening pressure is usually expressed as a percentage of the opening pressure.

Reseat pressure: After discharge, the valve flap comes into contact with the valve seat, ie the inlet pressure when the opening height becomes zero.

Opening and closing pressure difference: The difference between the opening pressure and the return pressure, usually expressed as a percentage of the return pressure and the opening pressure . The pressure difference between the two is only expressed when the opening pressure is very low .

Back pressure: The pressure at the outlet of the safety valve.

Rated discharge pressure: The standard specifies the upper limit of discharge pressure.

Seal test pressure: The inlet pressure at which the seal test is performed, at which the leakage rate through the sealing surface of the closure is measured.

Opening height: The actual lift of the disc from the closed position.

Flow path area: It refers to the small cross-sectional area of ​​the flow path between the inlet side of the valve disc and the sealing surface of the closing part, and is used to calculate the theoretical displacement without any resistance.

Flow channel diameter: corresponds to the diameter used for the flow channel area.

Curtain Area: The cylindrical or conical surface area of ​​the channel formed between its sealing surfaces when the valve flap is above the valve seat.

Emission area: The small cross-sectional area of ​​the fluid passage when the valve is discharged. For a full-inspiration safety valve, the discharge area is equal to the flow area; for a slightly-opened safety valve, the discharge area is equal to the area of ​​the screen.

Theoretical Displacement: The calculated displacement of an ideal nozzle equal to the cross-sectional area of ​​the flow path and the area of ​​the safety valve flow path.

Displacement coefficient: The ratio of actual displacement to theoretical displacement.

Rated displacement coefficient: The product of displacement coefficient and reduction coefficient (take 0.9).

Rated displacement: refers to the part of the actual displacement that is allowed as a reference for the safety valve.

Equivalent calculation displacement: the calculated displacement of the safety valve when the pressure, temperature, medium properties and other conditions are the same as the applicable conditions of the rated displacement.

Frequency jump: The safety valve disc moves back and forth rapidly and abnormally, and the valve flap contacts the valve seat during the movement.

Flutter: The safety valve disc moves back and forth rapidly and abnormally, and the valve does not contact the valve seat during movement.

When setting up the safety valve, please note the following points:

( 1 ) The safety valve should be installed in the gas phase when there are gas and liquid two-phase materials in the container.

( 2 ) When the safety valve is used to release flammable liquid, the outlet of the safety valve should be connected with the accident tank. When the discharged material is a high-temperature flammable material, its receiving container should have corresponding protective facilities.

( 3 ) The general safety valve can be vented on site. The vent opening should be more than 1 meter (m) above the operator and should not face an open flame location within 15 meters (m), emit sparks, and high temperature equipment. The vents for the safety valves of indoor equipment and containers shall be led out of the roof and shall be more than 2 meters (m) above the roof.

( 4 ) When the safety valve inlet has a block valve, the block valve shall be in the normally open state and shall be sealed to prevent mistakes.

Nominal diameter : DN 4-25 mm

Nominal pressure : PN 25MPa

Applicable medium : oxygen, nitrogen, argon, air, etc.

Applicable temperature : -40 °C-+80 °C

Body : Cast Steel / Stainless Steel

Seat : Copper

Valve top, stem : stainless steel

Spring : Spring steel