In fluid piping systems, regulating valves are control units and their investment is responsible for 30% to 50% of the piping engineering cost.
The main functions of valves are to open and close, throttle, regulate flow, isolate equipment and piping systems, prevent medium and regular backflow, and exhaust pressure.
Valves are also the most complex components in pipelines, generally assembled from multiple parts with high technical content.
With the rapid development of the petrochemical industry, the media in petrochemical production equipment are mainly toxic, flammable, explosive and highly corrosive, and the operating conditions are complex and severe, with high operating temperatures and pressures and long startup cycles.
Once a valve fails, it can cause medium leakage, polluting the environment and causing economic losses, and in serious cases, it can cause equipment production to shut down or even cause a catastrophic accident.
Therefore, in pipeline design, scientific and reasonable choice of valves can not only reduce equipment construction cost but also ensure safe operation.
This article mainly introduces the selection methods of various commonly used valves, such as gate valves, globe valves, butterfly valves, plug valves, ball valves and diaphragm control valves.
1. Key points for valve selection
1. Clearly define the purpose of the valve on the equipment or device.
Determine the working conditions of the valve, such as the properties of the applicable medium, working pressure, working temperature and operation control method.
2. Correctly select the valve type.
The correct selection of the valve type is a prerequisite for the designer to have a full understanding of the entire production process and operating conditions.
When selecting the valve type, the designer must first understand the structural characteristics and performance of each valve type.
3. Determine the valve end connection.
In threaded connections, flange connections and welding end connections, the first two are most commonly used. Threaded connections are mainly used for valves with nominal diameters below 50 mm.
If the diameter is too large, the installation and sealing of the connecting part will be very difficult. Flange-connected valves are relatively convenient to install and disassemble, but they are heavier and more expensive than thread-connected valves.
Therefore, they are suitable for connecting pipelines of various diameters and pressures.
Welded connections are more reliable than flanged connections under heavier loads, but are more difficult to disassemble and install.
Therefore, their use is limited to occasions where they can operate reliably for a long period of time or where operating conditions are severe and temperatures are high.
4. Selection of valve materials.
When selecting materials for the valve body, internal parts and sealing surface, in addition to considering the physical properties (temperature, pressure) and chemical properties (corrosion) of the working medium, the cleanliness of the medium (presence of solid particles) also must be taken into account.
Furthermore, the relevant regulations of the user's country and department should be consulted.
Choosing the correct and reasonable material for the valve can achieve the most economical service life and best performance.
The valve body material is selected in the order cast iron-carbon steel-stainless steel, and the sealing ring material is selected in the order rubber-copper-alloy steel-F4.
5. Others
Additionally, the flow rate and pressure level of the fluid flowing through the valve must be determined, and appropriate valves must be selected using available data such as valve product catalogs and samples.
2. Introduction to commonly used valves.
There are many complex types and varieties of valves, including gate valves, globe valves, throttle valves, butterfly valves, plug valves, ball valves, electric valves, diaphragm valves, check valves, safety valves, pressure reducing valves, air bleed valves. steam and emergency shut-off valves. Commonly used valves are gate valves, globe valves, butterfly valves, plug valves, butterfly valves, ball valves, check valves and diaphragm valves.
1. Gate valve
A gate valve is a valve that can open or close a fluid passage by driving the closing member (valve plate) up and down along the sealing surface of the valve seat with a valve stem.
Gate valves have better sealing performance and lower fluid resistance than globe valves. They are easier to open and close and have a certain regulatory performance.
They are one of the most commonly used shut-off valves.
The disadvantages of gate valves are large size, complex structure compared with globe valves, and the sealing surface is prone to wear and difficult to repair, so they are generally not suitable for throttling.
According to the position of the thread on the valve stem, gate valves can be divided into two types: rising stem and non-rising stem.
According to the structural characteristics of the gate plate, they can be divided into wedge type and parallel type.
2. Globe valve
A globe valve is a downward shut-off valve. The closing member (valve disc) is driven by a valve stem to move up and down along the axis of the valve seat (sealing surface).
Compared with gate valves, globe valves have better regulating performance, worse sealing performance, simple structure, easy manufacturing and maintenance, higher fluid resistance and cheaper price.
They are a commonly used shut-off valve and are generally used in medium and small diameter pipelines.
3. Ball valve
The closing member of a ball valve is a sphere with a circular hole, which rotates with the valve stem to achieve opening and closing of the valve.
Ball valves have simple structure, quick opening and closing, convenient operation, small size, light weight, fewer components, low fluid resistance, good sealing performance and easy maintenance.
4. Throttle valve
The structure of a butterfly valve is basically the same as a globe valve, except for the butterfly disc, which has different shapes and characteristics and has a smaller diameter than the valve seat.
It is not suitable to have a very large diameter, as the increase in average flow due to the smaller opening height can accelerate erosion against the throttle disc.
Butterfly valves have small dimensions, light weight, good regulation performance, but low regulation accuracy.
5. Plug valve
A plug valve has a plug with a through hole as a closing element. The plug rotates with the valve stem to open and close the valve.
Plug valves have simple structure, quick opening and closing, easy operation, low fluid resistance, fewer components and light weight. Plug valves can be direct, three-way or four-way.
Straight plug valves are used to cut off the medium, and three- and four-way plug valves are used to change the direction of the medium or to distribute it.
6. Butterfly valve
A butterfly valve uses a butterfly plate to rotate 90 degrees within the valve body to complete the opening and closing action. Butterfly valves are small in size, light in weight, simple in structure and only have a few components.
They can be opened and closed quickly by rotating 90 degrees and are easy to operate.
When the butterfly plate is fully open, the thickness of the plate is the only resistance that the medium flows through the valve body, so the pressure drop generated by the valve is small and the valve has excellent flow control characteristics.
Butterfly valves come in two seal types: soft elastic seal and hard metal seal.
For elastic sealing valves, the sealing ring can be embedded in the valve body or fixed on the circumference of the butterfly plate, which has good sealing performance and can be used for throttling, as well as for medium vacuum pipelines and corrosive media.
Metal-sealed valves generally have a longer service life than elastic-sealed valves, but complete sealing is difficult to achieve. They are generally used in situations where flow rate and pressure drop vary greatly and require good throttling performance.
Metal seal can adapt to higher operating temperatures, while elastic seal has the defect of being temperature limited.
7. Check valve
A check valve is a valve that can automatically prevent fluid from returning. The closing member of a check valve opens under the action of fluid pressure, allowing fluid to flow from the inlet side to the outlet side.
When the pressure on the inlet side is lower than that on the outlet side, the closing element closes automatically under the action of factors such as the fluid pressure difference and its own weight, to prevent fluid backflow.
Check valves can be divided into lift-type check valves and swing-type check valves according to their structural shapes.
Lift-type check valves have better sealing performance but greater fluid resistance than swing-type check valves.
For the suction port of a pump suction tube, a foot valve is recommended, which serves to fill the pump inlet tube with water before pumping and to keep the inlet tube and pump body filled with water. water after stopping the pump to prepare for the next start.
Foot valves are generally only installed on vertical pipelines at the pump inlet and the medium flows from bottom to top.
8. Diaphragm valve
The closing member of a diaphragm valve is a rubber diaphragm, which is clamped between the valve body and the valve cover.
The protruding part of the diaphragm is fixed to the valve stem, and the valve body is covered with rubber. Since the medium does not enter the inner cavity of the valve cover, the valve stem does not need a stuffing box.
Diaphragm valves have simple structure, good sealing performance, easy maintenance and low fluid resistance. Diaphragm valves can be divided into weir type, direct type, right angle type and direct flow type.
3. Common guidelines for valve selection
1. Selection Guidelines for Gate Valves
In general, gate valves should be the first choice. Gate valves are suitable not only for media such as steam and oil, but also for media containing solid particles and having high viscosity. They are also suitable for valves used in ventilation and low vacuum systems.
For media with solid particles, the gate valve body must have one or two bleed holes.
For low temperature media, special low temperature gate valves should be selected.
2. Selection Guidelines for Globe Valves
Globe valves are suitable for pipelines or devices with high temperature and pressure media where fluid resistance requirements are not strictly required, such as steam pipelines with DN < 200 mm.
Small valves such as needle valves, instrument valves, sampling valves, and pressure gauge valves may also use globe valves.
Globe valves can be used for flow or pressure regulation, but require less regulation accuracy, and when the pipeline diameter is relatively small, globe valves or butterfly valves are preferred.
For highly toxic media, gate valves with bellows seals are preferred; however, gate valves are not suitable for media with high viscosity or media containing particles that tend to settle. They are also not suitable for valves used for ventilation or low vacuum systems.
3. Selection Guidelines for Ball Valves
Ball valves are suitable for low temperature, high pressure and high viscosity media.
Most ball valves can be used in media with suspended solid particles, and can also be used for powder and granular media according to the requirements of the sealing material.
Full bore ball valves are not suitable for flow regulation and control, but are suitable for situations requiring rapid on-off action and are easy to implement emergency shut-offs in accidents.
Ball valves are recommended for pipelines with strict sealing performance, wear, contraction channels, rapid opening and closing actions, high pressure closing (large pressure difference), low noise, gassing phenomenon, low operating torque and low resistance to fluids.
Ball valves are suitable for light structures, low pressure closure and corrosive media. Ball valves are also the ideal valve for cryogenic and low-temperature media, and for pipelines and devices with low-temperature media, low-temperature ball valves with additional valve covers should be selected.
When selecting floating ball valves, the valve seat material must withstand the load of the ball and working medium.
Large diameter ball valves require greater force during operation, and ball valves with DN≥200mm must be equipped with helical gear transmission. Fixed ball valves are suitable for larger diameters and higher pressures.
Furthermore, ball valves used to handle highly toxic materials and combustible media must have fire-resistant and anti-static structures.
4. Selection Guidelines for Butterfly Valves
Regulating valves are suitable for situations with lower fluid temperatures but higher pressures, and for locations that require flow and pressure regulation.
However, they are not suitable for media with high viscosity or containing solid particles and should not be used as shut-off valves.
5. Selection Guidelines for Plug Valves
Plug valves are suitable for situations requiring rapid opening and closing actions, but are generally unsuitable for steam and higher temperature media.
They are suitable for media with lower temperatures and higher viscosity, and are also suitable for media with suspended particles.
6. Selection Guidelines for Butterfly Valves
Butterfly valves are suitable for larger diameters (such as DN﹥600mm) and shorter structural lengths, as well as situations that require flow regulation with rapid opening and closing.
They are generally used for water, oil, compressed air and other media with temperatures ≤ 80°C and pressures ≤ 1.0MPa.
Due to the greater pressure loss compared to gate and ball valves, butterfly valves are suitable for piping systems with less stringent pressure loss requirements.
7. Selection Guidelines for Check Valves
Check valves are generally suitable for clean media and should not be used for media with solid particles or high viscosity.
For sizes ≤40mm, lift check valves are recommended (only permitted for installation in horizontal pipelines).
For DN=50~400mm, swing check valves are recommended (they can be installed on horizontal and vertical pipelines, but for vertical pipelines, the medium must flow from bottom to top).
For DN≥450mm, damping check valves are recommended. Double flap check valves can also be used for DN=100~400mm. Swing check valves can be designed with high working pressure, with PN up to 42MPa.
They can be suitable for any working medium and working temperature range depending on the body material and sealing components.
The media may include water, steam, gas, corrosive media, oil, pharmaceuticals, etc., and the working temperature range may be between -196°C and 800°C.
8. Selection Guidelines for Diaphragm Valves
Diaphragm valves are suitable for oil, water, acidic medium and medium containing suspended solids with working temperature below 200℃ and pressure below 1.0 MPa.
They are not suitable for organic solvents and strong oxidizing media. For grinding particulate media, weir-type diaphragm valves should be selected and the flow characteristics table should be consulted when selecting the weir-type diaphragm valve.
For viscous fluids, cement paste and sedimentary media, direct diaphragm valves should be selected. Except for specific requirements, diaphragm valves must not be used in vacuum piping or vacuum equipment.
4. Pressure test methods for various valves
In general, industrial valves are not subjected to stress testing during use, but valves that have been repaired or valves with corroded or damaged bodies and covers must be subjected to stress testing.
For safety valves, their setting pressure, readjustment pressure and other tests must comply with their relevant instructions and regulations.
Resistance and sealing tests must be carried out during valve installation. Low pressure valves should be randomly inspected at 20%, and if they fail, a 100% inspection should be performed.
Medium and high pressure valves must be 100% inspected. Commonly used 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:
1. Pressure test method for ball valves
The resistance test of pneumatic ball valves must be carried out with the ball semi-open.
①Sealing test for floating ball valves:
Place the valve in a semi-open state, introduce the test medium at one end and close the other end.
Rotate the ball several times and check the sealing performance of the stuffing box and gasket when the valve is closed without any leakage.
Then introduce the test medium from the other end and repeat the above test.
②Sealing test for fixed ball valves:
Spin the ball several times without load before testing. Fixed ball valves must be in the closed state.
Introduce the test medium from one end to the specified value, and check the sealing performance of the inlet end with a pressure gauge.
The accuracy of the pressure gauge should be 0.5 to 1 level and the range should be 1.5 times the test pressure.
If there is no pressure drop within the specified time, it is qualified. Introduce the test medium from the other end and repeat the above test.
Then place the valve half-open, close both ends and fill the cavity with medium.
Check the sealing performance of the stuffing box and gasket under the test pressure without any leakage.
③Three-way ball valves must undergo a tightness test in each position.
2. Pressure test method for check valves
Test status: For lifting check valves, the valve flap axis is in a position perpendicular to the horizontal; for swing check valves, the axis of the channel and the axis of the valve flap are approximately parallel to the horizontal line.
During the resistance test, introduce the test medium from the inlet end to the specified value and close the other end. The valve body and cover must be free of leaks to be qualified.
During the seal test, introduce the test medium from the outlet end and check the sealing surface, stuffing box and gasket at the inlet end. There must be no leakage to be qualified.
3. Pressure test method for pressure reducing valves
① Pressure reducing valve resistance test is generally carried out after single-piece testing and assembly, and can also be carried out after assembly.
The duration of the resistance test is 1 minute for DN<50mm, more than 2 minutes for DN 65-150mm and more than 3 minutes for DN>150mm.
After welding the bellows and components, an air pressure resistance test must be performed at 1.5 times the highest pressure used with the pressure reducing valve.
② During the sealing test, it must be carried out according to the actual working environment.
When testing with air or water, the test pressure should be 1.1 times the rated pressure.
When testing with steam, the maximum working pressure permitted at the working temperature must be used.
The difference between inlet pressure and outlet pressure should not be less than 0.2 MPa.
The testing method is as follows:
After adjusting the inlet pressure, gradually adjust the valve regulating screw to make the outlet pressure change sensitively and continuously within the maximum and minimum value range without stagnation or card resistance.
For steam pressure reducing valves, after adjusting the inlet pressure, close the shut-off valve after closing the valve.
The outlet pressure is the highest and lowest value. Within 2 minutes, the increase in outlet pressure shall meet the requirements specified in Table 4.176-22.
At the same time, the volume of post-valve piping must meet the requirements specified in Table 4.18 to be qualified. For water and air pressure reducing valves, when the inlet pressure is adjusted and the outlet pressure is zero, the pressure reducing valve must be closed for a tightness test. No leakage within 2 minutes is qualified.
4. Pressure test method for butterfly valves
The resistance test for pneumatic butterfly valves is the same as for globe valves.
The sealing performance test of butterfly valves should introduce the test medium from the end where the medium flows.
The butterfly plate must be open and the other end closed. Then, the pressure must be injected at the specified value.
After checking that there is no leak in the stuffing box and other sealing locations, close the butterfly plate, open the other end of the valve and check that there is no leak in the butterfly plate sealing location.
Butterfly valves used to regulate flow may not require a sealing performance test.
5. Pressure test method for plug valves
During strength testing of plug valves, introduce the medium from one end, close the remaining passages, and rotate the plug to each working position until it is fully open for testing. The valve body must not leak to be qualified.
During the sealing performance test, the direct plug valve must maintain the same pressure in the chamber and passage. The plug must be rotated to the closed position and inspection must be carried out from the other end.
Then rotate the plug 180 degrees and repeat the test above. The three-way or four-way plug valve must maintain the same pressure at one end of the chamber and the passage.
Turn the plug to the closed position one by one and simultaneously inspect the other end.
Before plug valve testing, a layer of non-acidic thin lubricating oil can be coated on the sealing surface. There must be no leakage and expanded water droplets within the specified time to be qualified.
Plug valve test time can be shorter and generally follows the nominal diameter requirements, which is 1 to 3 minutes.
For gas plug valves, the air seal performance test must be carried out at 1.25 times the working pressure.
6. Pressure test method for diaphragm valves
During resistance testing of diaphragm valves, introduce medium from one end, open the valve disc, and close the other end. After the test pressure is increased to the specified value, the valve body and cover must be free of leaks to be qualified.
Then reduce the pressure to the sealing performance test pressure, close the valve disc and inspect from the other end. There must be no leakage to be qualified.
7. Pressure test method for globe valves and butterfly valves
The resistance test of globe valves and butterfly valves is generally carried out by placing the assembled valve on the pressure test frame, opening the valve disc, and introducing the medium at the specified value.
Check whether the valve body and cover are sweating or leaking. Single-piece tests can also be performed. Only globe valves require a sealing performance test.
When testing globe valves, the valve stem must be in a vertical position and the valve disc must be open.
The medium should be introduced from one end below the valve disc to the specified value, and the stuffing box and gasket should be inspected.
After passing the test, close the valve disc and inspect the other end for leaks. If strength and sealing performance tests are required, the strength test must be carried out first.
Then reduce the pressure to the sealing performance test pressure, inspect the stuffing box and gasket, close the valve disc, and inspect the outlet end for any leaks.
8. Pressure test method for gate valves
The resistance test for gate valves is the same as for globe valves. There are two methods for testing the sealing performance of gate valves:
① Open the gate and increase the pressure inside the valve to the specified value.
Then close the gate and immediately remove the gate valve. Check the seal on both sides of the gate for leakage, or directly inject the test medium into the valve cover plug to the specified value and inspect the sealing surfaces on both sides of the gate.
This method is called the intermediate pressure method, but it is not suitable for seal testing of gate valves with a nominal diameter of less than DN32mm.
② The other method is to open the gate and increase the test pressure inside the valve to the specified value.
Then close the gate and open one end of the blind plate to check whether there is a leak at the sealing face. Repeat the above test several times until you pass.
The sealing performance test of pneumatic gate valves must be carried out on the stuffing box and gaskets before the gate sealing performance test.
9. Pressure test method for safety valves
① The resistance test of safety valves is the same as other valves and is tested with water.
When testing the bottom of the valve body, introduce pressure from the inlet end and seal the sealing surface. When testing the top of the valve body and valve cover, apply pressure to the outlet end and seal the other end.
The valve body and cover must not leak within the specified time to be qualified.
② Sealing performance test and pressure adjustment test generally use the following media: saturated steam for steam safety valves, air for ammonia or other gases, and water or other non-corrosive liquids for liquid safety valves.
Nitrogen is commonly used as a test medium for safety valves in important positions.
The seal test is carried out with the test pressure being the nominal pressure value and must be repeated at least twice. There must be no leakage within the specified time to be qualified.
Leak detection methods include using butter to fix thin paper on the outlet flange, and the bulges on the paper are leaks, and using butter to fix a thin plastic plate or other plates on the bottom of the outlet flange , and the inspection is carried out after filling with water and the absence of bubbles indicates that there is no leak.
Pressure adjustment and readjustment pressure test of safety valves must be carried out at least 3 times and be qualified according to the specified requirements.
Other performance tests for safety valves can be found in Safety Valve Performance Test Method GB/T12242–1989.
Valve Selection Summary
Based on the above analysis, gate valves should generally be the preferred option.
Globe valves are suitable for pipelines with low fluid resistance requirements as well as for high temperature and high pressure media in pipelines or devices.
They should not be used for media with high viscosity or that contain particles, nor for air release valves or valves in low vacuum systems.
Ball valves are suitable for low temperature, high pressure and high viscosity media.
They are generally used in pipelines with strict sealing performance, wear, narrow passages, rapid opening and closing, high pressure difference, low noise, gasification, small operating torque and low fluid resistance.
Butterfly valves are suitable for occasions with low temperature and high pressure, not for media with high viscosity or containing solid particles, and not for shut-off valves.
Plug valves are suitable for occasions that require quick opening and closing. They are generally not suitable for steam and high-temperature media, but are suitable for media with low temperature and high viscosity, as well as media with suspended particles.
Butterfly valves are generally used for water, oil and compressed air media with a temperature of ≤80 ℃ and a pressure of ≤1.0 MPa. Due to the relatively large pressure loss compared to gate valves and ball valves, butterfly valves are suitable for piping systems with less stringent pressure loss requirements.
Check valves are generally suitable for clean media and should not be used for media containing solid particles or those with high viscosity.
Diaphragm valves are suitable for oil, water, acidic media and media containing suspended matter with working temperature less than 200 ℃ and pressure less than 1.0 MPa. They are not suitable for organic solvents or strongly oxidizing media.
In pipeline systems in industries such as petroleum and chemicals, valve applications, operating frequencies and service environments vary greatly. Controlling or eliminating small leaks is important and critical. Proper valve selection can reduce construction costs and ensure safe production.
Fixed ball valves control the opening and closing of the valve by rotating the ball inside the valve. There is a hole in the middle of the ball, which can rotate 90 degrees.
The diameter of the through hole is equal to or smaller than the diameter of the pipe. When the ball rotates 90 degrees, the inlet and outlet faces of the pipeline are both spherical surfaces, thus closing the valve and cutting off the fluid.
When the ball valve rotates 90 degrees, the inlet and outlet faces of the pipeline are spherical orifice surfaces, and the fluid passes through the valve. The fixed ball valve can be rotated to different angles to control the size of the fluid flow.
Fixed ball valves are commonly used in general pipelines, such as for transporting water, oil, steam and other fluids.
Globe valves, also known as gate valves, can completely seal the valve seat outlet by applying pressure through rotation of the valve stem, thus preventing fluid flow.
Globe valves are commonly used in pipelines for corrosive gases and liquids, such as natural gas, liquefied gas and sulfuric acid.
Gate valves work like a gate and control fluid flow by rotating the valve stem to move the gate valve up and down. The sealing rings on both sides of the gate valve can completely seal the entire section.
Gate valves can only be fully opened or fully closed and cannot be used as flow control valves. Gate valves are primarily used as shut-off devices in pipelines for water supply, sewage, ships and other applications.
The swing check valve is opened by fluid pressure and closed by gravity when the fluid pressure in the valve inlet and outlet pipes is balanced. Its main function is to prevent fluid return and belongs to automatic valves. It is mainly used in pipelines for petroleum, chemical, pharmaceutical and other industries.
Butterfly valves, also known as flip plate valves, can rotate 90 degrees, and the rotation of the valve stem drives the disc to change the angle of the disc, thereby controlling the flow of fluid. They can be used to disconnect, connect and regulate fluid flow in pipes. Butterfly valves are commonly used in water supply, gas supply and other pipelines as flow control and shutoff devices.
Regulating valves, also known as control valves, are used to control the size of fluid flow. When the regulating part of the valve receives the control signal, the valve stem will automatically control the opening and closing degree of the valve based on the signal, thereby achieving regulation of fluid flow and pressure. Regulating valves are commonly used in piping for heating, gas supply, petrochemical and other applications.
Overflow valve
The role of overflow valves and pressure reducing valves
Overflow valves are used to prevent system overload and ensure safety, while pressure reducing valves decrease system pressure, ensuring the system does not become overloaded. It can be said that overflow valves are passive while pressure reducing valves are active.
Here are some important differences between the two valves:
The pressure at the outlet is kept constant by the pressure reducing valve, while the overflow valve maintains the pressure at the inlet.
When not in use, the inlet and outlet of the pressure reducing valve are interconnected, while the inlet and outlet of the overflow valve are not.
When not in use, the pressure reducing valve orifice is open, while the overflow valve is normally closed.
The difference between overflow valves and pressure reducing valves.
The overflow valve is a pressure control valve that mainly controls the system pressure and also acts as an unloading device.
1. The pressure reducing valve is mainly used to reduce the pressure in a certain branch of the hydraulic system, so that the pressure of the branch is lower and stable than the pressure of the main oil circuit. Within the set pressure range, the pressure reducing valve, as well as the overflow valve, is closed.
However, as the system pressure increases and reaches the pressure set by the pressure reducing valve, the pressure reducing valve will open and some of the oil will return to the tank through it, causing the oil in the tank to heat up. This branch of oil pressure will no longer increase. It plays a role in reducing and stabilizing the pressure of this branch.
In contrast, the overflow valve is different. It is installed at the pump outlet to ensure the overall system pressure stability and prevent overpressure. Therefore, it has the function of safety, pressure regulation and stabilization.
2. The overflow valve is generally connected in parallel at the system branch to regulate pressure, stabilize pressure and reduce pressure, while the pressure reducing valve is generally connected in series at a certain system branch to reduce pressure and keep the pressure on this branch.
The overflow valve is normally closed and only works when the system is overpressured, while the pressure reducing valve is normally open and reduces pressure through a narrow passage.
The function of the overflow valve is pressure regulation, overflow and overload protection. The pressure reducing valve reduces pressure and reduces pressure in a certain part of the hydraulic system.
Their purposes are different, so they cannot be substituted for each other. The overflow valve controls the inlet pressure, while the pressure reducing valve controls the outlet pressure.
here are some examples:
Let's say you have a hydraulic system that includes an overflow valve. If the hydraulic pump's output flow exceeds a certain level, it will overflow through the overflow valve.
This will reduce the flow entering the system, which will stabilize the system pressure. The overflow valve is used to control this stable pressure.
Now let's consider a pressure reducing valve. There are two types of pressure reducing valves: the fixed differential pressure reducing valve and the fixed value pressure reducing valve. The first maintains a constant differential pressure between the inlet and outlet of the valve.
For example, if you set the value to 10 and the inlet pressure is x, the outlet pressure of the pressure reducing valve will be x-10. The latter maintains a constant outlet pressure.
For example, if you set the value of the fixed value pressure reducing valve to 20, and the inlet pressure is greater than 20, then the outlet pressure of the pressure reducing valve will always be 20. Got it?
Pilot operated safety valve is a new safety valve structure mainly used in the fields of petroleum, natural gas, chemicals, electricity, metallurgy and city gas. It is the best overpressure protection device for equipment, vessels or pipelines under pressure.
The main advantage of the piloted safety valve is that the direct action of the spring is replaced by the indirect action of the pilot valve, which improves the sensitivity of the action.
In addition, the main valve adopts a double-sealed valve seat sleeve piston structure, which has high action precision, good repeatability, fast closing, no leakage, and can handle high backpressure discharge.
It has a long service life, stable and reliable operation. The pilot-operated safety valve can also be calibrated online.
Even after repeated opening and flushing, it can still restart and close automatically, making operation and maintenance easier.
