What is a pressure relief valve?
What is a pressure relief valve?
The relief valve or pressure relief valve is a valve that can reduce the inlet pressure to a certain required outlet pressure through regulation and make the outlet pressure automatically maintain stable by relying on the energy of the medium itself. From the point of view of hydrodynamics, the pressure relief valve is a throttling element whose local resistance can be changed. That is to say, by changing the throttling area, the velocity and kinetic energy of the fluid can be changed, resulting in different pressure losses, so as to achieve the purpose of pressure reduction. Then, by the adjustment of the control and regulation system, the fluctuation of the pressure behind the valve is balanced with the spring force, and the pressure behind the valve is kept constant within a certain error range.
- Performance of pressure relief valve
- Principle of pressure relief valve
- Types of pressure relief valves
- Selection standard of pressure reducing valve
- Use skill of pressure reducing valve
- Precautions for installation of pressure reducing valve
- Why do steam control valves need to be used in process control
- Advantages of pressure reducing valve
- Type of steam pressure reducing valve
- The development of Chinese valve industry
- (1) pressure regulating range: it refers to the adjustable range of pressure relief valve output pressure P2, within which the specified accuracy is required. The pressure regulating range is mainly related to the rigidity of the pressure regulating spring.
- (2) pressure characteristic: it refers to the characteristic of output pressure fluctuation caused by input pressure fluctuation when flow G is constant. The smaller the output pressure fluctuation, the better the characteristics of the pressure relief valve. The output pressure must be lower than the input pressure – set value to basically not change with the input pressure.
- (3) flow characteristic: it refers to the stability of input pressure timing and output pressure changing with the change of output flow G. When the flow rate G changes, the smaller the change of the output pressure is, the better. Generally, the lower the output pressure is, the smaller its fluctuation with the change of output flow.
It can reduce the high liquid pressure in the front line of the valve to the level required by the rear line of the valve. The transmission medium here is mainly water. The pressure relief valve is widely used in high-rise buildings, areas with high water pressure in urban water supply network, mines and other occasions to ensure that all water points in the water supply system obtain appropriate service water pressure and flow. Since the leakage rate and waste degree of water are almost proportional to the water pressure of the water supply system, the pressure relief valve can improve the operation condition of the system and potentially save water. According to statistics, its water saving effect is about 30%.
- 1. Close the gate valve in front of the pressure relief valve, open the gate valve behind the pressure relief valve, and manufacture the downstream low-pressure environment;
- 2. Adjust the screw: turn anticlockwise to the highest position (relative to the lowest outlet pressure), and then close the gate valve behind the pressure relief valve;
- 3. Slowly open the gate valve in front of the pressure relief valve to the full open position;
- 4. Slowly turn the adjusting screw clockwise to adjust the outlet pressure to the required pressure (subject to the gauge pressure behind the valve); after adjustment, lock it
- Lock the nut and open the gate valve behind the pressure relief valve;
- 5. If the outlet pressure is higher than the set pressure during adjustment, readjustment must be started from the first step, i.e. it can only be adjusted from low pressure to high pressure.
Function principle: the pressure relief valve is to reduce the water pressure by the local resistance of the flow passage in the valve to the water flow. The range of the water pressure drop is automatically adjusted by the water pressure difference between the inlet and outlet of the diaphragm connecting the valve disc or both sides of the piston. The principle of constant pressure reduction is to control the water pressure ratio of floating piston in the valve body. The pressure reduction ratio at the inlet and outlet is inversely proportional to the piston area ratio at the inlet and outlet. This pressure relief valve works stably without vibration; there is no spring in the valve body, so there is no consideration of spring corrosion and metal fatigue failure; the sealing performance is good without leakage, so it not only reduces the pressure (when the flow is moving) but also reduces the static pressure (when the flow is 0); especially when the pressure is reduced, it does not affect the water flow.
Working principle: the pressure relief valve controls the opening of the opening parts in the valve body to regulate the flow of the medium and reduce the pressure of the medium. At the same time, it adjusts the opening of the opening parts with the help of the pressure behind the valve to keep the pressure behind the valve within a certain range and spray cooling water into the valve body or behind the valve to reduce the temperature of the medium. This valve is called the pressure reducing and temperature relief valve. The type selection of quick and easy automatic pressure relief valve is included. The characteristic of the valve is to keep the outlet pressure and temperature within a certain range when the inlet pressure is constantly changing.
1. Automatic regulation principle of combined pressure relief valve:
The combined pressure relief valve is a kind of pressure stabilizing valve which can also use water pressure to adjust itself under complex and changeable working conditions. When the inlet pressure and flow change, it can keep the outlet pressure and flow stable. It realizes self-control completely, debugging is simple and running is reliable.
2. Dual feedback switching principle of combined pressure relief valve:
The feedback system of the combined pressure relief valve is an independent system which controls the flow area (throttle cone opening) according to the change signal of the pressure at the outlet of the pressure relief valve. The pressure relief valve is equipped with a double feedback system which is standby for each other. The operation mode of system a is to stop system B when system a is enabled, so that the pressure relief valve can be maintained without shutdown.
3. Working principle of backflushing and blowdown of combined pressure relief valve:
The operation condition of hydropower station is complex, especially the quality of water quality is directly related to the safe operation of equipment. For hydropower stations with large sediment content, in addition to using stainless steel material and surfacing nickel base alloy to prevent abrasion at the overflow position of the pressure relief valve, the backwash blowdown device of the pressure relief valve can also effectively prevent the blocking of the feedback control system, so that the pressure relief valve can maintain a good working condition in the water quality with sediment and debris.
Combined pressure relief valve
The combined pressure relief valve is a pressure reducing device specially designed for the water supply system under complex working conditions, which is composed of the outlet pressure locking system, the feedback system and the main valve body.
Action relief valve
Simplest pressure relief valve, direct acting type, with flat diaphragm or bellows. Because it is an independent structure, there is no need to install an external sensor line downstream. It is the smallest and the most economical one of the three pressure relief valves, designed for low and medium flow. The accuracy of the direct acting pressure relief valve is usually + / – 10% of the downstream set point.
Piston type pressure relief valve
This type of pressure relief valve integrates two kinds of valves: pilot valve and main valve. The design of the pilot valve is similar to the direct acting pressure relief valve. The exhaust pressure from the pilot valve acts on the piston to open the main valve. If the main valve is too large to open directly, this design will use the inlet pressure to open the main valve. Therefore, this type of pressure relief valve has higher capacity and accuracy (+ / – 5%) under the same pipe size compared with the direct acting pressure relief valve. The same as the direct acting pressure relief valve is that the pressure inside the pressure relief valve senses the pressure, and the sensor line does not need to be installed outside.
Membrane pressure relief valve
In this type of pressure relief valve, the double diaphragm replaces the piston in the internal guide pressure relief valve. This increased diaphragm area opens a larger main valve, and at the same pipe size, its capacity is larger than the internal guided piston relief valve. In addition, the diaphragm is more sensitive to pressure changes with an accuracy of + / – 1%. The higher accuracy is due to the positioning of the downstream sensing line (outside of the valve), where there is less gas or liquid turbulence. The pressure relief valve is very flexible and can adopt different types of pilot valves (such as pressure valve, temperature valve, air loading valve, solenoid valve or several valves which are suitable for matching at the same time).
Direct acting pressure relief valve
The direct operated pressure relief valve is shown as the structure diagram of the direct operated pressure relief valve with overflow valve (hereinafter referred to as overflow pressure relief valve).
The pressure relief valve is reduced by the throttling effect of the air inlet, stabilized by the balance effect of the force on the diaphragm and the overflow effect of the overflow hole; the output pressure can be changed within a certain range by adjusting the spring. In order to prevent the pollution of the surrounding environment caused by a small amount of gas flowing out of the above relief valve, the relief valve without relief valve (i.e. ordinary relief valve) can be used.
Pilot-operated relief valves
When the output pressure of the internal pilot type pressure relief valve is high or its diameter is large, the pressure can be adjusted directly by the pressure regulating spring, so the spring stiffness will be too large. When the flow changes, the output pressure will fluctuate greatly, and the structure size of the valve will also increase. In order to overcome these shortcomings, the pilot type pressure relief valve can be used. The working principle of the pilot type pressure relief valve is basically the same as that of the direct acting type. The pressure regulating gas used in the pilot type pressure relief valve is supplied by a small direct acting pressure relief valve. If the small direct operated pressure relief valve is installed inside the valve body, it is called internal pilot type pressure relief valve; if the small direct operated pressure relief valve is installed outside the main valve body, it is called external pilot type pressure relief valve. Compared with the direct acting pressure relief valve, a nozzle flapper amplifying link consisting of nozzle 4, flapper 3, fixed orifice 9 and air chamber B is added. When the distance between the nozzle and the baffle changes slightly, the pressure in chamber B will change significantly, which will cause a large displacement of the diaphragm 10. To control the up and down movement of the valve core 6, the inlet valve port 8 will be opened or closed, and the sensitivity of the valve core control will be improved, that is, the stability accuracy will be improved.
There is also a small direct acting pressure relief valve outside the main valve body to control the main valve. This kind of valve is suitable for places with a diameter of more than 20 mm, a long distance (within 30 m), a high place, a dangerous place and difficult pressure regulation.
- Standard 1: in the given range of spring pressure level, the outlet pressure can be adjusted continuously between the maximum value and the minimum value without jamming and abnormal vibration;
- Standard 2: for the pressure relief valve with soft seal, there shall be no leakage within the specified time; for the pressure relief valve with metal seal, the leakage shall not be greater than 0.5% of the maximum flow;
- Standard 3: when the outlet flow changes, the deviation value of the outlet pressure of the direct acting type is not more than 20%, and that of the pilot type is not more than 10%;
- Standard 4: when the inlet pressure changes, the outlet pressure deviation of direct acting type shall not be greater than 10%, and that of pilot type shall not be greater than 5%;
- Standard 5: generally, the pressure behind the pressure relief valve should be less than 0.5 times of the pressure before the valve;
- Standard 6: the pressure relief valve has a wide range of applications. It can be used in equipment and pipelines of steam, compressed air, industrial gas, water, oil and many other liquid media. The amount of media flowing through the outlet of the pressure relief valve is generally expressed by mass flow or volume flow;
- Standard 7: bellows direct acting pressure relief valve is suitable for low pressure, medium and small caliber steam medium;
- Standard 8: membrane direct acting pressure relief valve is suitable for medium and low pressure, medium and small caliber air and water medium;
- Standard 9: pilot piston type pressure relief valve is applicable to steam, air and water media of various pressures, diameters and temperatures. If it is made of stainless acid resistant steel, it can be applied to various corrosive media;
- Standard 10: pilot bellows type pressure relief valve, suitable for low pressure, medium and small caliber steam, air and other media;
- Standard 11: pilot diaphragm type pressure relief valve, suitable for medium such as low pressure, medium pressure, small and medium caliber steam or water;
- Standard 12: the fluctuation of the inlet pressure of the pressure relief valve shall be controlled within 80% – 105% of the given value of the inlet pressure. If it exceeds this range, the performance in the early stage of pressure reducing will be affected;
- Standard 13: generally, the pressure behind the pressure relief valve shall be less than 0.5 times of the pressure before the valve;
- Standard 14: each spring of the pressure relief valve is only applicable within a certain range of outlet pressure, and the spring shall be replaced if it exceeds the range;
- Standard 15: in case of high working temperature of medium, pilot piston type pressure relief valve or pilot bellows type pressure relief valve is generally selected;
- Standard 16: when the medium is air or water (liquid), direct acting membrane type pressure relief valve or pilot membrane type pressure relief valve should be selected generally;
- Standard 17: when the medium is steam, pilot piston type or pilot bellows type pressure relief valve should be selected;
- Standard 18: for the convenience of operation, adjustment and maintenance, the pressure relief valve shall be generally installed on the horizontal pipeline.
When setting the driving pressure of the mechanical pump, the correct way is to make the driving pressure 1-1.5kg/cm2 (15-20psig) higher than the back pressure. This is true in most cases. But sometimes in order to achieve the required displacement of the pump, it is necessary to set the driving pressure slightly higher. When the driving pressure is high, the disadvantage is that it is easy to wear the intake valve. If the driving pressure is higher than the required value, it will cause serious wear to the intake valve and the intake valve has to be replaced in advance.
In this case, it is recommended to use a pressure relief valve or a pressure relief valve station. The pressure relief valve can not only maintain the driving pressure at the set pressure that can meet the pump displacement, but also protect the intake valve from wear. In addition, when the application space is narrow, it is recommended to use a pressure relief valve station with a steam header. In the process of pump water, the steam header supplies steam to the pump and provides a buffer zone for the pump. This buffer zone makes the operation of the whole device soft and reduces the wear of the pressure relief valve.
- 1. For the convenience of operation and maintenance, the valve is generally installed vertically on the horizontal pipeline.
- 2. The pressure relief valve must be installed in strict accordance with the arrow direction on the valve body to keep consistent with the flow direction of the fluid. If the water quality is not clean and contains some impurities, a filter must be installed at the upstream water inlet of the pressure relief valve.
- 3. In order to prevent overpressure behind the valve, a pressure relief valve shall be installed at least 4m away from the valve outlet.
- 4. The pressure relief valve plays a certain check role in the pipeline. In order to prevent the harm of water hammer, small expansion water tank can also be installed to prevent damage to the pipeline and valve. The filter must be installed in front of the inlet pipe of the pressure relief valve, and the expansion water tank must be installed behind the outlet pipe of the pressure relief valve!
- 5. If the pressure relief valve needs to be installed in the hot water system, you must install a check valve between the pressure relief valve and the expansion water tank. In this way, the expansion tank can not only absorb the volume of water increased by thermal expansion, but also prevent the influence of hot water return or pressure fluctuation on the pressure relief valve, so as to ensure the long-term normal operation of the pressure relief valve.
When heated, water molecules are constantly destroyed and combined. Heating makes the binding between molecules fragile, and eventually some molecules will break at high temperature, producing steam or dry steam. When some water molecules release their latent heat and form tiny water droplets, they form wet steam.
Steam is used in many industrial applications. The most common applications are process heating and turbine driven power generation. In addition, steam is also used for atomization, cleaning, moisturizing and humidification. However, in most cases, some parameter control is needed when using steam, and the steam control valve naturally becomes a part of process control engineering.
Technically, a valve is a device used in any system to control the flow of liquid or steam. The main purpose of steam control valve is to reduce the inlet steam pressure in the process. In addition to controlling the pressure, the steam control valve also controls the temperature.
The boiler usually works under high pressure, because low pressure operation will cause water to be carried in the steam. The specific volume of high-pressure steam is low, which helps to reduce the weight of pipeline. In fact, the distribution of steam is easier and cheaper due to the lower cost of pipes and insulation materials.
Industrial processes use steam at lower pressures. The reason is that low pressure steam has high latent heat, which greatly improves the energy efficiency. The steam pressure is related to the temperature, so the temperature can be adjusted automatically by controlling the steam pressure. Reducing steam pressure is also related to the safety requirements of the plant. The steam pressure can be controlled by the steam control valve.
In equipment where steam is used, it is usually produced at high pressure and provides heat to each steam user by local decompression. The pressure relief valve is usually used to minimize the diameter of the steam transmission pipeline and make the steam transmission more cost-effective.
By using the combination of start-up control valve, pressure sensor and controller, the pressure can be kept constant. However, the pressure relief valve has the advantage of fully automatic independent operation to control the pressure without any type of external power. It can make extremely fast response actions based on the immediate perception and adjustment of downstream pressure.
So that the heating process can use the steam under the condition of lower than the boiler pressure, so as to benefit it. As a result, pressure relief valves are often used to reduce the steam pressure to the set point required by the process.
In a pressure relief valve, the mechanism that automatically regulates the downstream pressure usually uses the balance of the force achieved between the steam pressure and the regulating spring. At present, this is the general concept adopted by almost all pressure relief valves. However, there are two different ways to implement this mechanism to control the valve opening:
No pilot, direct acting pressure relief valve: the regulating spring directly applies the setting force of the reduced pressure to the main valve.
Pilot type pressure relief valve: the regulating spring directly applies the pressure setting force to the pilot valve, which is smaller and different from the main valve.
The following is an overview of the characteristics of each type of PRV.
Direct acting pressure relief valve (without pilot)
For small loads that do not require very precise pressure control.
Advantages: small size, low price, easy to install.
Disadvantages: higher pressure drop than the pilot PRV (variation from set pressure).
Adjust the output steam pressure by directly acting on the balance force of the valve itself: compress the downward force generated by the regulating spring to resist the upward force from the secondary pressure acting on the bottom of the bellows or diaphragm.
As the pressure builds up downstream, it is transferred to the underside of the adjusting spring (usually against the bellows or diaphragm), where the force on this axis is balanced against the compression force of the spring. The spring pressure to open the valve is limited to counteract the change in downstream pressure while achieving sufficient spring sensitivity. The end result is a simple control of the pressure at a high flow rate through the valve orifice that will cause the pressure to drop.
Pilot-operated relief valves
For larger loads where very precise pressure control is necessary.
Advantages: compared with direct acting type, it can be used in a larger flow range with accurate pressure control and fast response to load change.
Disadvantages: large size and high price.
The output steam pressure is regulated in a similar way to the direct acting PRV, but indirectly through the pilot valve.
When the pilot valve is activated, a much higher opening flow is provided to the larger main valve than the pilot valve itself. Then, the pilot valve is balanced by the secondary pressure to control the opening flow to the main valve accordingly.
In a pilot operated pressure relief valve, the pilot valve is used to load the piston or diaphragm, thus increasing the downward force used to open the larger main valve. This increases the flow at a lower pressure compensation (depressurization). The opening and closing of the pilot valve is controlled by the force balance between the regulating spring and the secondary pressure, and the action mode is the same as that of the direct acting valve.
However, in the pilot PRV, this opening and closing of the pilot valve will purposefully transfer pressure to the main valve piston or diaphragm. Then, the pilot flow pressure creates a downward force, which is amplified on the piston or diaphragm surface, allowing the main valve to open more and the flow to a high level.
Since the use of piston or diaphragm will enlarge the downward force, the slight change of pilot valve opening will lead to the large change of flow through the main valve and downstream pressure. Therefore, in order to achieve rapid response within a large range of steam flow, the spring force on the regulating valve hardly needs to be changed. Compared with direct acting type, fast response and tight transfer pressure control are the main advantages of the valve.
It can be seen from the above characteristics that the function and application of the direct acting pressure relief valve without pilot is quite different from that of the pilot pressure relief valve. In short:
When the load is small, the direct acting pressure relief valve can be used, and the downstream pressure can be allowed to drop to a certain extent. They are usually used for light load conditions. The pilot type pressure relief valve can respond to different load conditions quickly, and maintain stable secondary pressure where precise pressure control is required. They are usually used for high load conditions.
Typical applications of steam service units:
For small load applications, such as sterilizers, unit heaters, humidifiers, and small process equipment, a simple direct acting PRV is usually used to reduce pressure.
If the flow rate is large, such as in a steam delivery line, the load may fluctuate significantly depending on the operating condition of the receiving equipment.
This load change and large capacity requires the use of pilot operated pressure relief valves to reduce pressure. In addition, the amount of steam used by some equipment may vary significantly from normal operation at startup.
Such a large change may also require the use of a pilot PRV to depressurize.
China’s valve industry chain is numerous, but it is not a powerful valve country. Generally speaking, China has entered the ranks of the world’s valve powers, but there is still a long gap between China and the valve powers in terms of product quality. There are still some problems in the industry, such as low concentration of production, low R & D capacity of high-end products, low level of manufacturing technology in the valve industry. The trade deficit between import and export continues to expand.
The next few years will be a period of high-speed shock in the valve industry. The direct consequence of this high-speed shock is to expand the trend of polarization in the valve cabinet brand camp. It is predicted that there will not be so many valve enterprises that can really survive in the market in the next few years. But this kind of high-speed shock in the valve industry will bring huge opportunities, and the result of the shock will make the market operation more rational.
The way of localization of high-end valves is very rough. Basic parts have become a short board restricting the development of China’s manufacturing industry to high-end. During the 12th Five Year Plan period, the government will continue to increase the localization of high-end equipment parts. Here we select several representative valves of key development industries mentioned in the implementation plan for the feasibility analysis of import substitution. From the analysis, we can see that the feasibility of valve import substitution in various sub industries is very different, high-end valves need more policy guidance and scientific research support.
Valve industry plays a very important role in the development of national economy as an important link in the preparation of manufacturing industry. Due to the gap between the domestic valve manufacturing industry and the international advanced level, many key valves with high parameters, high temperature, high pressure and high pound level have been imported. In order to promote the localization of valves, after the opinions on accelerating the revitalization of the equipment manufacturing industry issued by the State Council, the relevant departments of the state made the requirements for the localization of major equipment according to the requirements of the state , successively made a series of major deployments, led by the national development and Reform Commission, China Machinery Industry Federation and China General Machinery Industry Association, deployed and formulated the valve localization scheme for major equipment in related fields, and coordinated with relevant departments for many times. Now, the valve localization has formed a consensus in the domestic valve industry.
Actively adopt international standards for product design; absorb foreign excellent design structure; conduct product test and performance inspection in strict accordance with international standards; absorb foreign production process experience and attach importance to the research and promotion of new materials; make clear the technical parameters and working conditions of imported high parameter valve products, so as to speed up the localization process, promote the continuous updating of valve products, and comprehensively implement Now a way of valve localization.
With the acceleration of the reorganization of valve industry, the future industry will be the competition between the quality and safety of valve products and product brands. The products will develop towards the direction of high technology, high parameters, strong corrosion resistance and high service life. Only through continuous technological innovation, development of new products and technological transformation, can the technical level of products be gradually improved to meet the needs of domestic devices, and Valves be fully realized Localization. China’s valve manufacturing industry in the huge demand environment, will show a better development prospects.
Source: China Relief Valve Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.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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