Generally, industrial valves are not subjected to strength tests when in use, but the valve body and valve cover after repair or the valve body and valve cover damaged by corrosion should be subjected to strength tests. For safety valves, the set pressure and return seat pressure and other tests should comply with the provisions of their instructions and relevant regulations. The valve should be tested for strength and sealing after installation. 20% of low-pressure valves are randomly inspected. If they are unqualified, 100% inspection should be carried out; medium and high-pressure valves should be inspected 100%. Common media for valve pressure testing include water, oil, air, steam, nitrogen, etc. The pressure test methods for various industrial valves including pneumatic valves are as follows:

Ball valve

Ball valve (ball valve), defined in the standard GB/T21465-2008 "Valve Terminology" as: a valve whose opening and closing member (ball) is driven by the valve stem and rotates around the axis of the ball valve. It can also be used for fluid regulation and control. The hard-sealed V-type ball valve has a strong shear force between its V-type ball core and the metal valve seat of hard alloy surfacing, which is particularly suitable for media containing fibers, tiny solid particles, etc. The multi-way ball valve can not only flexibly control the confluence, diversion, and flow direction switching of the medium on the pipeline, but also close any channel to connect the other two channels. This type of valve should generally be installed horizontally in the pipeline. Ball valves are divided into: pneumatic ball valves, electric ball valves, and manual ball valves according to the driving method.

Ball valve pressure test method

The strength test of pneumatic ball valves should be carried out when the ball is half-open.

① Floating ball valve sealing test: put the valve in a half-open state, introduce the test medium at one end, and close the other end; turn the ball several times, open the closed end when the valve is in a closed state, and check the sealing performance of the packing and gasket at the same time. There must be no leakage. Then introduce the test medium from the other end and repeat the above test.

②Fixed ball valve sealing test: Before the test, rotate the ball several times without load, the fixed ball valve is in the closed state, and the test medium is introduced from one end to the specified value; use a pressure gauge to check the sealing performance of the introduction end, and use a pressure gauge with an accuracy of 0.5 to 1 level and a range of 1.5 times the test pressure. Within the specified time, if there is no pressure drop, it is qualified; then introduce the test medium from the other end and repeat the above test. Then, the valve is in a half-open state, both ends are closed, the inner cavity is filled with medium, and the packing and gasket are checked under the test pressure. There must be no leakage.

③ The three-way ball valve should be tested for sealing at various positions.

Check valve

A check valve refers to a valve that automatically opens and closes the valve disc by relying on the flow of the medium itself to prevent the backflow of the medium. It is also called a check valve, a one-way valve, a reverse flow valve, and a back pressure valve. The check valve is an automatic valve, and its main function is to prevent the backflow of the medium, the reversal of the pump and the drive motor, and the discharge of the container medium.

Pressure test method of check valve

Test state of check valve: the axis of the valve disc of the lifting check valve is in a position perpendicular to the horizontal; the axis of the channel and the axis of the valve disc of the swing check valve are in a position approximately parallel to the horizontal line.

During the strength test, the test medium is introduced from the inlet end to the specified value, and the other end is closed. The valve body and the valve cover are qualified if there is no leakage.

For the sealing test, the test medium is introduced from the outlet end, and the sealing surface, packing and gasket are checked at the inlet end. It is qualified if there is no leakage.

Diaphragm valve

The structure of the diaphragm valve is very different from that of the general valve. It is a new type of valve and a special form of shut-off valve. Its opening and closing parts are a diaphragm made of soft material, which separates the inner cavity of the valve body from the inner cavity of the valve cover and the driving components. It is now widely used in various fields. Commonly used diaphragm valves include rubber-lined diaphragm valves, fluorine-lined diaphragm valves, unlined diaphragm valves, and plastic diaphragm valves.

The diaphragm valve is a valve body and a valve cover with a flexible diaphragm or a combination diaphragm, and its closing part is a compression device connected to the diaphragm. The valve seat can be a weir or a pipe wall of a straight-through flow channel. 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 impacting the working parts of the operating mechanism. In addition, no separate seal is required at the valve stem unless it is used as a safety facility in controlling harmful media. In the diaphragm valve, since the working medium contacts only the diaphragm and the valve body, both of which can be made of a variety of different materials, the valve can ideally control a variety of working media, especially for media with chemical corrosiveness or suspended particles. The operating temperature of the diaphragm valve is usually limited by the materials used for the diaphragm and the valve body lining, and its operating temperature range is about -50 to 175°C. The diaphragm valve has a simple 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 replacement of the diaphragm can be completed on site and in a short time.

Pressure test method of diaphragm valve

The strength test of diaphragm valve is to introduce medium from either end, open the valve disc, and close the other end. After the test pressure rises to the specified value, check if there is no leakage in the valve body and valve cover. Then reduce the pressure to the sealing test pressure, close the valve disc, open the other end for inspection, and pass if there is no leakage.

Pressure reducing valve

The pressure reducing valve is a valve that reduces the inlet pressure to a certain required outlet pressure through adjustment, and automatically maintains the outlet pressure stable by relying on the energy of the medium itself. From the perspective of fluid mechanics, the pressure reducing valve is a throttling element with variable local resistance, that is, by changing the throttling area, the flow rate and the kinetic energy of the fluid are changed, resulting in different pressure losses, thereby achieving the purpose of pressure reduction. Then, relying on the adjustment of the control and regulation system, the fluctuation of the pressure behind the valve is balanced with the spring force, so that the pressure behind the valve is kept constant within a certain error range.

Pressure test method of pressure reducing valve

① The strength test of the pressure reducing valve is generally assembled after a single piece test, or assembled and tested. Strength test duration: 1min for DN<50mm; more than 2min for DN65～150mm; more than 3min for DN>150mm. After the bellows and components are welded, the strength test is carried out with air at 1.5 times the highest pressure after the pressure reducing valve.

② 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 testing with steam, the test is carried out at the highest working pressure allowed at the working temperature. The difference between the inlet pressure and the outlet pressure is required to be not less than 0.2MPa. The test method is: after the inlet pressure is set, gradually adjust the adjusting screw of the valve so that the outlet pressure can change sensitively and continuously within the maximum and minimum value range, and there must be no stagnation or blocking. For steam pressure reducing valves, when the inlet pressure is adjusted away, the shut-off valve behind the valve is closed, and the outlet pressure is the highest and lowest value. Within 2 minutes, the rise of its outlet pressure should meet the requirements of Table 4.176-22. At the same time, the volume of the pipeline behind the valve meets the requirements of Table 4.18. For water and air pressure reducing valves, when the inlet pressure is adjusted and the outlet pressure is 0, the pressure reducing valve is closed for sealing test. No leakage within 2 minutes is qualified.

Gate valve

The gate valve is a gate of the opening and closing parts. The movement direction of the gate is perpendicular to the direction of the fluid. The gate valve can only be fully opened and fully closed, and cannot be adjusted or 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 materials to increase wear resistance, such as 1Cr13, STL6, stainless steel, etc. The gate plate has rigid gate plates and elastic gate plates. According to the different gate plates, the gate valve is divided into rigid gate valve and elastic gate valve.

Pressure test method for gate valves

The strength test of gate valves is the same as that of stop valves. There are two methods for sealing test of gate valves.

① Open the gate to raise the pressure in the valve to the specified value; then close the gate, take out the gate valve immediately, check whether there is leakage at the seals on both sides of the gate, or directly inject the test medium into the plug on the valve cover to the specified value, and check the seals on both sides of the gate. The above method is called intermediate pressure test. This method is not suitable for sealing test of gate valves with nominal diameter below DN32mm.

② Another method is to open the gate to raise the valve test pressure to the specified value; then close the gate, open the blind plate at one end, and check whether the sealing surface is leaking. Then turn the head back and repeat the above test until qualified.

The sealing test of the packing and gasket of the pneumatic gate valve should be carried out before the sealing test of the gate.

Butterfly valve

Butterfly valve, also known as flap valve, is a simple regulating valve that can be used for the switch control of low-pressure pipeline media. A butterfly valve refers to a valve whose closing part (valve disc or butterfly plate) is a disc, which rotates around the valve axis to achieve opening and closing.

Valves can be used to control the flow of various types of fluids such as air, water, steam, various corrosive media, mud, oil, liquid metal and radioactive media. It mainly plays a role of cutting off and throttling on the pipeline. The butterfly valve opening and closing part is a disc-shaped butterfly plate, which rotates around its own axis in the valve body to achieve the purpose of opening and closing or regulation.

Pressure test method of butterfly valve

The strength test of pneumatic butterfly valve is the same as that of stop valve. The sealing performance test of butterfly valve should introduce the test medium from the medium flow end, the butterfly plate should be opened, the other end should be closed, and the pressure should be injected to the specified value; after checking that there is no leakage in the packing and other seals, close the butterfly plate, open the other end, and check that there is no leakage in the butterfly plate seal to be qualified. The butterfly valve used for regulating flow does not need to be tested for sealing performance.

Plug valve

Plug valve is a rotary valve in the shape of a closing piece or a plunger. It is a kind of valve that can be opened or closed by rotating 90 degrees to connect or separate the channel opening on the valve plug with the channel opening on the valve body.

Its valve plug can be cylindrical or conical. In a cylindrical valve plug, the channel is generally rectangular; in a conical valve plug, the channel is trapezoidal. These shapes make the structure of the plug valve light. It is most suitable for cutting off and connecting media and diversion, but it can sometimes be used for throttling according to the applicable properties and the erosion resistance of the sealing surface.

Pressure test method of plug valve

① When the plug valve is tested for strength, the medium is introduced from one end, the rest of the passage is closed, and the plug is rotated to each fully open working position for testing. The valve body is qualified if no leakage is found.

② During the sealing test, the straight-through plug valve should keep the pressure in the cavity equal to that in the passage, rotate the plug to the closed position, check from the other end, and then rotate the plug 180° to repeat the above test; the three-way or four-way plug valve should keep the pressure in the cavity equal to that at one end of the passage, rotate the plug to the closed position in turn, introduce the pressure from the right-angle end, and check from the other ends at the same time.

Before the plug valve test, it is allowed to apply a layer of non-acidic thin lubricating oil on the sealing surface. If no leakage or enlarged water droplets are found within the specified time, it is qualified. 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 sealing at 1.25 times the working pressure.

Stop valve

The opening and closing parts of the stop valve (stop valve, globe valve) are plug-shaped valve discs, the sealing surface is flat or sea cone, and the valve disc moves linearly along the center line of the valve seat. The movement form of the valve stem (general name: dark stem) is also available in the lifting and rotating stem type, which can be used to control the flow of various types of fluids such as air, water, steam, various corrosive media, mud, oil, liquid metal and radioactive media. Therefore, this type of shut-off stop valve is very suitable for cutting, regulating and throttling. Since the valve stem opening or closing stroke of this type of valve is relatively short and has a very reliable cutting function, and since the change of the valve seat opening is proportional to the stroke of the valve disc, it is very suitable for regulating the flow.

Pressure test method for stop valves and throttle valves

For the strength test of stop valves and throttle valves, the assembled valves are usually placed in the pressure test stand, the valve disc is opened, the medium is injected to the specified value, and the valve body and valve cover are checked for sweating and leakage. Strength tests can also be performed on a single piece. The sealing test is only performed on stop valves. During the test, the valve stem of the stop valve is in a vertical state, the valve disc is opened, and the medium is introduced from the bottom end of the valve disc to the specified value, and the packing and gasket are checked; after passing the test, the valve disc is closed and the other end is opened to check for leakage. If both the valve strength and sealing tests are to be done, the strength test can be done first, and then the pressure can be reduced to the specified value of the sealing test, and the packing and gasket can be checked; then the valve disc is closed, and the outlet end is opened to check whether the sealing surface is leaking.

Safety valve

The safety valve is a special valve whose opening and closing parts are in a normally closed state under the action of external force. When the medium pressure in the equipment or pipeline exceeds the specified value, the medium is discharged to the outside of the system to prevent the medium pressure in the pipeline or equipment from exceeding the specified value. Safety valves belong to the automatic valve category, mainly used in boilers, pressure vessels and pipelines, and control the pressure not to exceed the specified value, which plays an important protective role in personal safety and equipment operation. Note that the safety valve must be pressure tested before it can be used.

Pressure test method of safety valve

① The strength test of the safety valve is the same as other valves, and water is used for testing. When testing the lower part of the valve body, the pressure is introduced from the inlet I=I end, and the sealing surface is closed; when testing the upper part of the valve body and the valve cover, the pressure is introduced from the outlet El end, and the other ends are closed. The valve body and valve cover are qualified if there is no leakage within the specified time.

② For the sealing test and constant pressure test, the media generally used are: saturated steam is used as the test medium for safety valves for steam; air is used as the test medium for valves for ammonia or other gases; water is used as the test medium for valves for water and other non-corrosive liquids. Nitrogen is often used as the test medium for safety valves in some important locations.