Pilot-type safety valves maintain the closing force of the main valve and operate the main and pilot valves. Energy may come from the system medium, an external source of energy, or both. When the system pressure begins to exceed a set limit, the pilot valve either forces the main valve to open by either removing or reducing the closing force, or creates a force to open the main valve. When the system pressure drops, the pilot valve regenerates the closing force or eliminates the opening force. According to this opening and closing mode, a large closing force can be maintained before the safety valve is opened. Therefore, the pilot-type safety valve can maintain a high sealing performance even when the operating pressure approaches the setting pressure.
The size of the closing force of the pilot-operated safety valve immediately before opening may not be limited, but the pilot-operated safety valve of some structures may also be limited by choice. The so-called closing force is limited, which means that once the pilot valve fails, the system pressure can open the main valve within the allowable overpressure range. For pilot-operated safety valves with limited closing force, the mounting requirements are generally not as stringent as pilot-operated safety valves with infinite closing force. From the point of view of easy access to the operating medium, the most reliable one is the system medium itself. However, pilot valves must be designed to accommodate this type of media under operating conditions.
On the other hand, where it is difficult to use the system media itself, selecting an external operating medium may simplify the design of the pilot-operated safety valve. Such an operating medium usually has compressed air, liquid, or electric power.
Main valve operating principle: The main valve can be designed in two forms that are opened with its pilot valve being energized or losing energy. If you use the principle of open energy supply. When the accidental valve loses energy in the event of an accident, the main valve will remain closed. Therefore, using the system media as the operating medium is the safest in this case. However, some specifications also allow the use of external operating media as long as these operating media are supplied by several independent sources of energy, or as long as the pilot operated safety valve load is limited. If the disengagement opening principle is adopted, depending on the design structure of the pilot-operated safety valve, the failure of the pilot valve will cause the main valve to automatically open when the pressure is lower than or equal to the set pressure. Thus, in this case, the failsafe operation of the main valve will not be affected by the operating medium.
The operating principle of the pilot valve: The pilot valve can be designed to open (discharge) the main valve as it is energized or loses energy. If the energy release principle is used, the main valve will remain closed when energy is lost in the event of an accident. Therefore, the pilot type safety valve used in this case is the safest to use the system medium as the operating medium. However, some specifications also allow the use of external operating media as long as the operating media is supplied by several independent sources of energy, or as long as the pilot-type safety valve has a limited load. If the disabling principle is used, disabling will open the main valve. In this case, the failsafe operation of the pilot valve will not be affected by the operating medium.
a. The main valve operated by the system medium is allowed to have several different structural designs in accordance with different operating principles of the main valve. Pilot-type safety valve is a typical representative structure. The first two pilot-type safety valves operate on the principle of disabling energy, while the latter two pilot-type safety valves operate on the principle of energy supply opening.
The closing force of these pilot-operated safety valves is provided in three ways: first, by the pressure of the medium acting directly on the disc; second, by the spring; and third, by the piston acting on the pilot valve. Medium pressure available. The pilot piston of this latter type of pilot-operated safety valve must be equipped with an effective seal that can be sealed at the maximum line pressure when the pilot operated safety valve is closed.
Pilot operated safety valves operate on the principle of disengagement and their pistons must remain sealed at the maximum medium pressure of the system when the safety valve is closed.
b. The main valve operated by the external medium, the spring-loaded main purpose drive mechanism is powered by external medium. There are mainly compressed air or electrical energy.
Pilot operated safety valves operate on the principle of disabling energy. The drive mechanism consists of an air motor. Under normal operating conditions, the air motor exerts a downward thrust on the valve stem, together with the spring, the safety valve is kept closed and sealed. When the operating pressure reaches the setting pressure, the air motor generates a lifting force and opens the safety valve together with the medium pressure acting under the flap. Once the air motor energy is cut off, the safety valve will open like a direct acting safety valve.
The pilot-operated safety valve also operates according to the disengagement principle. Under normal operating pressure, the electromagnet energizes the valve stem and together with the main spring keeps the safety valve closed and sealed. When the operating pressure reaches the set pressure, the electromagnet loses power and the pilot-operated safety valve opens like a direct-acting safety valve.
Pilot operated safety valve operates on the principle of energy supply opening. In this case, the closing force of the pilot operated safety valve is only provided by the spring. And keep it sealed. When the operating pressure reaches the set pressure, the air-operated lift motor is energized. The resulting lifting force just reduces the closing force generated by the spring to the extent that the force of the media acting under the flap can open the safety valve. The pilot-type safety valve is designed such that when the hoist motor fails, the medium pressure can still open the valve within the allowable overpressure range.
c. The function of the main valve safety valve with dual functions of safety and control can also be combined with the function of the control valve. An example is the high pressure bypass safety valve for steam power generation equipment. Its main valve is equipped with a hydraulically-powered drive mechanism that operates on the principle of disabling energy. At the same time, the pilot valve on the hydraulic control line operates on the principle of dissipating energy.
Typical installation of safety valve. When the power plant is operating normally, the safety valve operates as an automatic high pressure bypass valve to supply steam to the reheat system. The reheat system is connected to the condenser through a low pressure bypass valve and protected by a separate relief valve that discharges directly to the atmosphere. If the pressure in the high pressure system suddenly rises due to the turbine unloading, the safety pilot disables the drive mechanism. Thus, the steam pressure quickly opens the main valve with the help of a spring that acts on the opening direction of the drive mechanism piston.
This article is organized by the Shanghai Five-Yue Safety Valve: Sharing knowledge, exchanging experiences, and learning from each other! This article is for reference only. To learn more about safety valve knowledge, call Shanghai Wuyue Pump & Valve Manufacturing Co., Ltd.
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