In general, strength tests of industrial valves are not taken during use, but the valve body and bonnet that are corroded or repaired should take strength tests. For safety valves, the constant pressure, return pressure and other tests should comply with the specifications and relevant regulations. Valve strength and seal tests should be performed on the valve hydraulic test bench before installation. 20% of low valves need to be checked and every valve should be checked if the valves in the previous spot check are not qualified. Medium and high-pressure valves should be checked 100%. Commonly used media for valve pressure test are water, oil, air, steam, nitrogen, etc.
Pressure test methods for gate valves
The strength test of gate valves is the same as that of globe valves. And there are two sealing test methods of gate valves.
- Open the shutter to raise the pressure inside the valve, then close the shutter, immediately take out the gate valve and check whether there is leakage at the seal on both sides. Or directly inject the test medium into the plug on the bonnet to the specified value then check the seals on both sides. The above method is not suitable for testing gate valves with a nominal diameter below 32 mm.
- The other method is to open the shutter and raise the test pressure to the specified value, then close the shutter, open the blind plate at one end, and check whether the sealing surface leaks. Next, repeat the above test until it is qualified. The seal test for pneumatic gate valve packing and gasket should be carried out before the sealing test for the shutter.
Pressure test methods for ball valves
The strength test of the pneumatic ball valve should be carried out when the valve is half open.
- Floating ball valve sealing test: When the valve is half open, import the medium from one side while the other side is closed. Rotate the ball a couple of times. When the valve is closed, open and check the side which previously is closed, and check the packing and sealing performance at the gasket, where must have no leakage. Next, check the sealing performance of the other side according to the above procedures.
- Trunnion ball valve sealing test: Before testing, let the zero-load ball rotate several times. The valve should be closed, then import the medium from one side to the specified value. Use the pressure gauge to inspect the sealing performance of the side. If there is no decompression in the specific time, the product is qualified. Follow the above procedures to check the other side. After that, let the valve be half open while two sides are both closed, fill it with the medium, inspect the packing and gasket under test pressure, make sure that there is no leakage.
- Every position of three-way ball valves should be tested.
Pressure test methods for check valves
The axis of lift check valve disc should be in a position perpendicular to the horizontal line while that of the swing check valve passage and that of the disc should be in a position approximately parallel to the horizontal line. In the strength test, the test medium should be imported from the inlet end to the specified value while the other end is closed, and the valve body and the valve cover are qualified if there is no leakage. For the sealing test, the medium should be imported from the outlet end. At the inlet end, the sealing surface is qualified if there is no leakage at the packing and gasket.
Pressure test methods for safety valves
The strength test of the safety valve is the same as other valves and is tested with water. When testing the lower part of the valve body, the pressure is imported from the inlet end, and the sealing surface is closed. When the upper part of the valve body and the bonnet are tested, the pressure is imported from the outlet end while the other ends are closed. The valve body and the valve cover are qualified if they have no leakage during the prescribed time.
The media generally used for sealing test and constant pressure test: saturated steam for safety valves used for steam; air for valves used for ammonia or other gases; water for valves used for water and other non-corrosive liquid. For safety valves used in some important positions, nitrogen is commonly used as the test medium.
The frequency of sealing tests that are carried out with the nominal pressure value as the test pressure should not be less than two, and the tested valve is qualified if there is no leakage within the prescribed time. There are two methods for leak detection: one is to seal the joints of the safety valve, and paste a piece of thin paper on the outlet flange with butter. The paper bulges if there is leakage, and the valve is qualified if the paper does not bulge; The second method is to paste a plastic plate or other kinds of plates on the lower part of the outlet flange, and the valve disc is filled with water to check that if there is no bubble. The number of times of the safety valve constant pressure and return pressure tests should be not less than 3 times, and the valve is qualified according to the regulations.
The diaphragm valve is different in structure from the general valve. It is a new type of valve. It is a special type of shut-off valve. Its opening and closing member is a diaphragm made of soft material. The inner cavity of the cover and the driving member are separated and are widely used in various fields. Commonly used diaphragm valves are rubber-lined diaphragm valves, fluorine-lined diaphragm valves, unlined diaphragm valves, and plastic diaphragm valves.
The diaphragm valve is provided with a flexible diaphragm or a combined diaphragm in the valve body and the valve cover, and the closing member is a compression device connected to the diaphragm. The valve seat can be a dome or a wall that is straight through the runner. The advantage of the diaphragm valve is that its operating mechanism is separated from the medium passage, which not only ensures the purity of the working medium, but also prevents the possibility of the medium in the pipeline from impacting the working parts of the operating mechanism. In addition, there is no need for any form of separate seal at the stem unless it is used as a safety feature in controlling hazardous media. In the diaphragm valve, since the working medium is only in contact with the diaphragm and the valve body, both of which can be used in a variety of different materials, the valve can ideally control a variety of working media, especially for chemically corrosive or suspended particles. medium. The operating temperature of the diaphragm valve is typically limited by the materials used in the diaphragm and body lining, and it operates over a temperature range of approximately -50 to 175 °C. The diaphragm valve is simple in structure and consists of only three main components: the valve body, the diaphragm and the valve head assembly. The valve is easy to disassemble and repair quickly, and the diaphragm can be replaced in the field and in a short time.
Pressure test method for diaphragm valve
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.
The pressure reducing valve is a valve that adjusts the inlet pressure to a certain required outlet pressure and relies on the energy of the medium itself to automatically maintain the outlet pressure. From the point of view of fluid mechanics, the pressure reducing valve is a throttling element whose local resistance can be changed, that is, by changing the throttling area, the flow velocity and the kinetic energy of the fluid are changed, resulting in different pressure loss, thereby achieving the purpose of decompression. Then, relying on the adjustment of the control and regulation system, the fluctuation of the post-valve pressure is balanced with the spring force, so that the post-valve pressure remains constant within a certain error range.
Pressure test method for pressure reducing valve
- The strength test of the pressure reducing valve is generally assembled after a single piece of test, or it can be assembled after assembly. Strength test duration: DN 150 mm greater than 3 min. After the bellows and the assembly are welded, the maximum pressure is 1.5 times after the pressure reducing valve is applied, and the strength test is performed with air. 50min lmin; dn65 ~ 150mm is greater than 2min; dn>
- 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, the highest working pressure allowed at the operating temperature is carried out. 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 maximum and minimum, and there is no stagnation or jamming phenomenon. For the steam pressure reducing valve, when the inlet pressure is adjusted, the valve is closed after the valve is closed, and the outlet pressure is the highest and lowest value. Within 2 minutes, the outlet pressure should be increased according to the provisions in Table 4.176-22, and the valve behind the valve The volume is qualified according to the provisions in Table 4.18; for the water and air pressure reducing valve, when the inlet pressure is set, when the outlet pressure is zero, the pressure reducing valve is closed for the leak test, and no leakage is qualified within 2 minutes.
The gate valve is a shutter of the opening and closing member. The direction of movement of the gate is perpendicular to the direction of the fluid. The gate valve can only be fully open and fully closed, and cannot be adjusted and throttled. The gate valve is sealed by the contact between the valve seat and the gate. Usually, the sealing surface will be welded with metal material to increase wear resistance, such as surfacing 1Cr13, STL6, stainless steel, and the like. The gate has a rigid gate and an elastic gate. According to the gate, the gate valve is divided into a rigid gate valve and a flexible gate valve.
Pressure test method for gate valve
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 and open the blind plate at one end to 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 butterfly valve, also known as the flap valve, is a simple regulating valve. The butterfly valve that can be used for the switching control of the low-pressure pipeline medium means that the closing member (valve or disc) is a disc that rotates around the valve shaft to achieve opening and closing. a kind of valve.
Valves can be used to control the flow of various types of fluids such as air, water, steam, various corrosive media, mud, oil, liquid metals, and radioactive media. Mainly cut and throttle on the pipeline. The butterfly valve opening and closing member is a disc-shaped butterfly plate that rotates around its own axis in the valve body to achieve the purpose of opening and closing or adjusting.
Pressure test method for butterfly valve
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 plug valve is a closing or plunger-shaped rotary valve that opens or closes by opening or closing the passage port on the valve plug by rotating 90 degrees.
Its valve plug can be cylindrical or conical in shape. In a cylindrical valve plug, the passage is generally rectangular; in a tapered valve plug, the passage is trapezoidal. These shapes make the structure of the plug valve light. It is most suitable for use as a cutting and closing medium and a shunt, but it can also be used for throttling depending on the applicable properties and the erosion resistance of the sealing surface.
Pressure test method for plug valve
- 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, 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 specified as 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.
Stop valve (Globe Valve)
The opening and closing member of the stop valve (Globe Valve) is a plug-shaped valve flap having a flat surface or a sea cone surface, and the valve flap linearly moves along the center line of the valve seat. The movement of the valve stem (common name: dark rod), also has a lifting rotary rod type that can be used to control various types of fluids such as air, water, steam, various corrosive media, mud, oil, liquid metal and radioactive media. flow. Therefore, this type of shut-off globe valve is well suited for shut-off or regulation and throttling. Due to the relatively short opening or closing stroke of the valve stem of this type of valve, and the very reliable cutting function, and because the change of the valve seat opening is proportional to the stroke of the valve flap, it is very suitable for the adjustment of the flow rate.
Pressure test method for globe 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.
Source: China Valves Manufacturer – Yaang Pipe Industry (www.steeljrv.com)