Pressure test method and precautions for valves
In chemical production, as long as there is fluid, the valve will be used. The importance of the valve in the chemical industry is self-evident. So today, Yikong Jun will discuss with you the pressure test methods and precautions for common valves.
Commonly used media for valve pressure test are water, oil, air, steam, nitrogen, etc. The pressure test methods for various industrial valves with pneumatic valves are as follows:
1 Shut-off valve and throttle valve
For the strength test of the stop valve and the throttle valve, usually put the assembled valve in the pressure test frame, open the valve flap, inject the medium to the specified value, and check whether the valve body and the valve cover are sweating and leaking. The strength test can also be carried out in one piece. The seal test is only used as a shut-off valve. During the test, the valve stem of the shut-off valve is in a vertical state, the valve flap is opened, the medium is introduced from the lower end of the valve flap to the specified value, and the packing and the gasket are inspected; after being qualified, the valve flap is closed, and the other end is opened to check for leakage. If both the valve strength and the tightness test are to be done, the strength test may be performed first, then the pressure is reduced to the specified value of the seal test, and the packing and the gasket are inspected; then the valve flap is closed, and the outlet end is opened to check whether the sealing surface leaks.
The strength test of the gate valve is the same as that of the shut-off valve. There are two methods for sealing the gate valve.
- The shutter is opened to raise the pressure inside the valve to the specified value; then close the gate, immediately take out the gate valve, check whether there is leakage at the seal on both sides of the gate or directly inject the test medium into the plug on the valve cover to the specified value Check the seals on both sides of the gate. The above method is called intermediate pressure test. This method is not suitable for sealing tests on gate valves with nominal diameters below DN32mm.
- Another method is to open the shutter and raise the test pressure of the valve to the specified value; then close the shutter, open the blind plate at one end, and check whether the sealing surface leaks. Then go back and repeat the above test until it is qualified.
The seal test of the pneumatic gate valve packing and gasket shall be carried out before the seal sealing test.
The strength test of the pneumatic ball valve should be carried out with the ball half open.
- Floating ball valve tightness test: the valve is in a half-open state, one end is introduced into the test medium, and the other end is closed; the ball is rotated several times, the closed end is opened when the valve is closed, and the sealing performance of the packing and the gasket is checked. There must be no leakage. The test medium was then introduced from the other end and the above test was repeated.
- Fixed ball valve tightness test: Before the test, the ball is idling several times, the fixed ball valve is closed, the test medium is introduced from one end to the specified value; the pressure at the inlet is used to check the sealing performance of the lead end, and the accuracy of the pressure gauge is 0.5. ~1, the range is 1.6 times the test pressure. In the specified time, no depressurization is acceptable; then the test medium is introduced from the other end, and the above test is repeated. Then, the valve is in a half-open state, the ends are closed, the inner cavity is filled with the medium, and the packing and the gasket are inspected under test pressure, and there is no leakage.
- Three-way ball valves should be tested for leaks at various locations.
- When the plug valve is tested for strength, the medium is introduced from one end, the remaining passages are closed, and the plug is rotated to the fully open working positions for testing. The valve body is not found to be qualified.
- In the sealing test, the straight-type cock should keep the pressure in the cavity equal to the passage pressure, rotate the plug to the closed position, check from the other end, and then rotate the plug 180° to repeat the above test; three-way or four-way plug valve The pressure inside the chamber should be kept equal to the end of the passage. The plug should be rotated to the closed position in turn, the pressure is introduced from the right angle end, and the inspection is performed simultaneously from the other end.
Before the plug valve test bench, a non-acid thin lubricating oil is allowed to be applied on the sealing surface, and no leaks and enlarged water droplets are found to be qualified within the specified time. The test time of the plug valve can be shorter, generally 1 to 3 minutes according to the nominal diameter.
The plug valve for gas should be tested for air tightness at 1.25 times the working pressure.
The strength test of the pneumatic butterfly valve is the same as that of the shut-off valve. The sealing performance test of the butterfly valve shall be introduced into the test medium from the end of the medium flow. The disc shall be opened, the other end closed, and the injection pressure shall be the specified value. After checking the packing and other seals without leakage, close the disc and open the other end to check the butterfly. No leakage at the plate seal is acceptable. As a butterfly valve for regulating the flow rate, the sealing performance test may not be performed.
The diaphragm valve strength test introduces the medium from either end, opens the valve flap, and the other end is closed. After the test pressure rises to the specified value, the valve body and the valve cover are not leaked. Then, the pressure is reduced to the sealing test pressure, the valve flap is closed, and the other end is opened for inspection, and no leakage is acceptable.
Check valve test state: the lift check valve disc axis is in a position perpendicular to the horizontal; the swing check valve passage axis and the flap axis are in a position approximately parallel to the horizontal line.
In the strength test, the test medium is introduced from the inlet end to the specified value, and the other end is closed, and the valve body and the valve cover are not leaked.
The sealing test was introduced into the test medium from the outlet end. At the inlet end, the sealing surface was inspected, and no leakage at the packing and gasket was qualified.
- The strength test of the safety valve is the same as other valves, and it is tested with water. When testing the lower part of the valve body, the pressure is introduced from the I=I end, and the sealing surface is closed. When the upper part of the valve body and the bonnet are tested, the pressure is introduced from the El end and the other ends are closed. The valve body and the valve cover have no leakage during the specified time.
- Sealing test and constant pressure test, the general medium used is: steam safety valve with saturated steam as test medium; ammonia or other gas valve with air as test medium; water and other non-corrosive liquid valves with water as test medium. For some important positions of safety valves, nitrogen is commonly used as the test medium.
The sealing test is carried out with the nominal pressure value as the test pressure, the number of which is not less than two, and no leakage is qualified within the specified time. There are two methods for leak detection: one is to seal the joints of the safety valve, and the thin paper is sealed on the El flange with butter, the tissue is bulged as a leak, and the bulge is not qualified; the second is to use a butter to thin The plastic plate or other plate is sealed on the lower part of the outlet flange, and the valve flap is filled with water to check that the water does not bubble. The number of times of the safety valve constant pressure and return pressure test is not less than 3 times, and it is qualified according to the regulations.
The strength test of the pressure reducing valve is generally assembled after a single piece of test, and can also be tested after assembly.
- Duration of strength test: 1 min for DN<50 mm; greater than 2 min for DN65~150 mm; greater than 3 min for DN>150 mm.
- After the bellows and the assembly were welded, the pressure test valve was applied 1.5 times higher than the high pressure of the Zui, and the strength test was performed with air.
The sealing test is carried out according to the actual working medium. When testing with air or water, the test is carried out at 1.1 times the nominal pressure; when using the steam test, it is carried out at a high working pressure allowed at the working temperature. The difference between the inlet pressure and the outlet pressure is not less than 0.2 MPa.
The test method is as follows: after the inlet pressure is set, the adjusting screw of the valve is gradually adjusted, so that the outlet pressure can be sensitively and continuously changed within the range of small value of zui and zui, and there is no stagnation or jamming phenomenon.
Valve pressure test precautions
- (1) Under normal circumstances, the valve is not tested for strength, but the valve body and bonnet or corroded valve body and bonnet shall be tested for strength after repair. For safety valves, the constant pressure and return pressure and other tests shall comply with the specifications and relevant regulations.
- (2) The valve installation shall be tested for strength and tightness. The low pressure valve is checked 20%. If it is not qualified, it should be 100% inspection; the medium and high pressure valves should be 100% inspection.
- (3) When testing, the valve should be installed in a direction that is easy to inspect.
- (4) Valves in the form of welded joints may be pressure tested with a tapered seal or O-ring seal when the test is not possible with a slab.
- (5) The valve air should be excluded as much as possible during the hydraulic test.
- (6) The pressure should be gradually increased during the test, and it is not allowed to pressurize sharply and suddenly.
- (7) The duration of the strength test and the seal test is generally 2~3min, and the important and special valves should last for 5min. The test time of the small-diameter valve can be correspondingly shorter, and the test time of the large-diameter valve can be correspondingly longer. During the test, the test time can be extended if in doubt. During the strength test, the body and bonnet are not allowed to sweat or leak. For the sealing test, the valve is generally only used once, and the safety valve, high pressure valve and other raw valves need to be carried out twice. During the test, a small amount of leakage is allowed for low-pressure, large-diameter unimportant valves and valves that are allowed to allow leakage; since general-purpose valves, power station valves, marine valves, and other valves have different requirements, leakage requirements are required. It should be implemented in accordance with relevant regulations.
- (8) The throttle valve shall not be tested for the tightness of the closure, but shall be tested for strength and sealability at the packing and gasket.
- (9) During the pressure test, the valve closing force only allows one person’s normal physical strength to close; it must not be applied by means of a lever or the like (except the torque wrench). When the diameter of the hand wheel is greater than 320 mm, the two are allowed to work together. shut down.
- (10) For valves with upper seals, the packing should be taken out for sealing test. After sealing the upper part, check for leaks. When testing with gas, check the water in the stuffing box. When using the packing tightness test, the upper seal is not allowed to be in a tight position.
- (11) Where a valve with a drive unit is tested for its tightness, the drive unit is used to close the valve to perform a tightness test. For manual drives, a seal test using a closed valve should also be performed.
- (12) After the strength test and the seal test, the bypass valve installed on the main valve shall be tested for strength and tightness in the main valve; when the main valve closing member is opened, it shall also be opened.
- (13) When testing the strength of cast iron valves, tap the valve body and bonnet with a copper hammer to check for leaks.
- (14) When the valve is tested, except that the plug valve is specified to allow the sealing surface to be oiled, other valves are not allowed to apply oil test on the sealing surface.
- (15) When the valve is pressure tested, the pressing force of the blind plate to the valve should not be too large, so as to avoid deformation of the valve and affect the test effect (the cast iron valve will be damaged if it is pressed too tightly).
- (16) After the pressure test of the valve is completed, the water in the valve should be removed and cleaned in time, and the test record should be made.
Source: China Valve Manufacturer – Yaang Pipe Industry Co., Limited (www.metallicsteel.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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