What is a pilot-operated safety valve

What is a pilot-operated safety valve?

Pilot-operated safety valve is a non-direct load type safety valve. It consists of a main valve and a pilot valve (also known as a “secondary valve”), which is driven by the medium discharged from the pilot valve and is also known as a “pulse safety valve” because the pressure in the system is pulsed. From a separate point of view, the pilot valve itself is also a direct-load safety valve. When the medium turn force reaches the opening pressure of the pilot valve, the pilot valve opens first and the discharged medium enters the main valve through the bypass tube. Pilot-operated safety valve is a kind of safety valve that relies on the discharge medium from the pilot valve to drive or control. The safety valve controlled by the pilot valve, its fixed pressure is controlled by the pilot valve, the action is basically independent of the back pressure. The safety valve with pilot valve is also divided into two types of fast-opening (full opening) and regulating (gradual opening).

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Classification of pilot operated safety valves

Pilot-operated safety valves are classified by structural form into pendant hammer type, lever type, spring type and pilot type (pulse type).
According to the valve body structure to divide, can be divided into closed and unclosed type. Closed safety valve that the excluded medium does not leak out, all along the outlet to the designated location, generally used in toxic and corrosive media. For air and steam safety valves, more than the use of non-closed safety valve. For the selection of safety valve products, should be determined by the actual sealing pressure. For spring type safety valve, in a nominal pressure (PN) range, with several working pressure levels of the spring, when ordering in addition to the safety valve model, name, medium and temperature, should also specify the valve body sealing pressure, otherwise, according to the maximum sealing pressure supply.

Features of pilot-operated safety valves

Pilot-operated safety valves are suitable for high pressure and large diameter applications. Pilot-operated safety valve has good sealing performance, at the same time, its action is rarely affected by the back pressure. The disadvantage of this type of safety valve is that its reliability is related to the main valve and pilot valve, the action is not as fast and reliable as the direct acting safety valve, and the structure is more complex.

The working principle of pilot operated safety valve

When the protected system is in normal operating condition. Pilot valve cavity slide valve in the open state, the system pressure from the main valve inlet through the lower conduit, pilot valve cavity and upper conduit into the main valve flap (piston) above the gas chamber, the gas pressure above the chamber is equal to the system pressure of the main valve inlet. The sealing surface of the pilot valve is sealed to ensure that the gas in the chamber above the main valve cannot be discharged through the pilot valve relief port. Since the piston area is larger than the valve sealing surface area, the system pressure produces a downward force on the main valve flap, which keeps the main valve in a closed and sealed state.
When the system pressure rises to the set pressure, the gas at the sealing surface of the pilot valve will lift the spool of the pilot valve upward, so that the sealing surface is in the open state, while the slide valve in the pilot valve cavity moves upward to close the over-gas passage of the pilot valve cavity. The medium in the chamber above the main valve flap is discharged through the open sealing surface of the pilot valve, so that the pressure above the main valve flap is reduced. The main valve flap is pushed open by the inlet pressure and the system is depressurized.
When the system pressure is reduced to a certain value, the pilot valve spool is seated downward under the action of the spring, the sealing surface of the pilot valve is closed, the slide valve of the pilot valve cavity is opened, the valve cavity is open, the system pressure is again transmitted through the pilot valve cavity into the gas chamber above the main valve flap, and push the main valve flap closed.

Installation of pilot-operated safety valve precautions

All kinds of safety valves should be installed vertically.
There should be no resistance at the outlet of the safety valve to avoid the phenomenon of pressure.
Safety valves should be specially tested and checked for tightness before installation.
The safety valve in use should be inspected periodically.

Installation requirements for pilot operated safety valves

  • (1) Rated evaporation greater than 0.5t / h boiler, at least two safety valves installed: rated evaporation less than or equal to 0.5t / h boiler, at least one safety valve. Separable coal conservator outlet, steam superheater outlet must be installed at the safety valve.
  • (2) Safety valve should be installed vertically in the pot business, the highest position of the set box. In the safety valve and the pot or between the set box, shall not be equipped with the export pipe and valve to take the steam.
  • (3) Pilot-operated safety valve to prevent the weight from moving on its own device and limit the lever to cross the guide frame, spring-loaded safety valve to have a lifting handle and prevent the random unscrewing of the adjusting screw device.
  • (4) For rated steam pressure less than or equal to 3.82MPa boiler, safety valve throat diameter should not be less than 25mm: for rated steam pressure greater than 3.82MPa boiler, safety valve throat diameter should not be less than 20mm.
  • (5) Safety valve and boiler connection tube, its cross-sectional area should not be less than the cross-sectional area of the safety valve inlet. If several safety valves are installed together in a short pipe directly connected to the pot, the short pipe pathway cross-sectional area should not be less than 1.25 times the area of all safety valve steam discharge.
  • (6) Safety valve should generally be installed with a vapor exhaust pipe, the exhaust pipe should go straight to the safety location, and have sufficient cross-sectional area to ensure the smooth flow of exhaust. The bottom of the safety valve exhaust pipe should be installed with a safe place to receive the drainage pipe, in the exhaust pipe and drainage pipe are not allowed to install the valve.

The price of pilot operated safety valve.

Pilot-operated safety valve regardless of the size of the diameter, the pilot valve used in a certain pressure range is the same size. Therefore, large-diameter, high-pressure pilot operated safety valve cost less than the spring safety valve, but in low-pressure small-diameter, the price of pilot operated safety valve will be more than the same specifications of the spring safety valve.

The above comparison shows that although the direct load spring type safety valve is a commonly used safety valve for pressure vessels, but its performance is not as good as the pilot operated safety valve. The use of pilot-operated safety valves, pressure vessels and safety valves related to pressure and maximum allowable working pressure of the discussion involved in some of the contradictions can basically be resolved, that is, the use of pilot-operated safety valves can be designed from the process system level to solve some chemical equipment design professional in the hot hand problem. However, it should be ensured that the use of non-direct load type safety valve, must do even if the system valve failure, the main valve can still open and discharge its rated discharge volume under the specified opening pressure.

Source: China Pilot-operated Safety Valve Manufacturer – Yaang Pipe Industry Co., Limited (www.sfutube.com)

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