The safety valve is one of the three indispensable safety accessories on the boiler. It plays an important role in the control of the internal pressure limit of the boiler and the safety protection of the boiler. When the pressure in the boiler rises above the specified value, the safety valve is automatically opened to relieve the pressure and discharge steam; when the pressure in the boiler drops to the specified value, the safety valve is automatically closed. Inappropriate selection of safety valves or failure to meet the requirements for use in the adjustment and inspection will easily cause overpressure and even explosion accidents in the boiler. Therefore, correct understanding and use of safety valves is an important part of boiler safety management.
1. Structure management
(1) Spring type safety valve: Spring type safety valve is mainly composed of valve seat, valve core, valve stem, spring, guide sleeve, adjusting screw, lifting handle, valve body and other components. It uses the pressure of the spring acting on the valve core to balance the supporting force of the steam pressure acting on the valve core. When the supporting force of the steam pressure acting on the valve core is greater than the pressure of the spring acting on the valve core, the spring is compressed, the valve core is lifted away from the valve seat, and the steam is discharged outward; when the supporting force is less than the elastic force, the spring stretches, Press the valve core down until it is tightly combined with the valve seat, and the steam stops draining outward.
The spring-loaded safety valve controls the exhaust pressure by adjusting the tightness of the spring. This kind of safety valve has a compact structure, light weight, and can be shaken without air leakage. It is currently the most commonly used safety valve on boilers. However, because the elasticity of the spring changes due to factors such as temperature and time, the reliability is poor, and the safety valve must be inspected regularly during use.
(2) Lever type safety valve: Lever type safety valve is composed of valve body, valve disc, valve seat, valve stem and heavy hammer. It uses the action of a lever and a heavy hammer to press the valve disc on the valve seat. When the pressure of the steam acting on the valve disc exceeds the pressure of the heavy hammer acting on the valve stem, the valve disc is lifted and the steam passes through the safety valve. When the steam pressure is discharged, the steam pressure decreases; when the steam pressure used on the valve flap is less than the weight pressure acting on the valve stem, the valve flap is re-pressed on the valve seat to keep the boiler running. The opening pressure of the lever type safety valve is controlled by adjusting the distance between the weight and the fulcrum. After the opening pressure of the safety valve is determined, the lever principle can be used to calculate the distance of the heavy hammer fulcrum to determine the position of the heavy hammer. The lever-type safety valve has simple structure, convenient adjustment, movement recording, and is more accurate and reliable. However, the heavy hammer is relatively heavy, the exhaust capacity of a single safety valve is limited, and the requirements for installation are relatively strict.
2. Safety valve technical requirements
The selection of safety valve should meet the requirements of use. First of all, we must understand the specifications and models of the safety valve. When using and identifying, pay particular attention to whether the sealing surface material, valve body material and nominal pressure meet the requirements, as well as issues such as sealing and adhesion. Generally used for steam safety valves. The valve body material is carbon steel, and the valve seat material is stainless steel.
The nominal pressure of the safety valve requires a reference temperature of 120°C, and that of the steel valve body is 200°C. When the boiler safety valve medium exceeds 200℃, the maximum allowable working pressure is lower than the nominal pressure. Therefore, for boilers with saturated steam pressure exceeding 1.47MPa and superheaters, safety valves should be selected according to the operating temperature.
The number of safety valves. For boilers with a rated evaporation capacity> 0.5t/h, at least two safety valves (excluding economizer safety valves) shall be installed. For boilers with an evaporation capacity of ≤0.5t/h, at least one safety valve should be installed. Safety valves must be installed at the outlet (or inlet) of the separable economizer and at the outlet of the steam superheater.
The safety valve should be installed perpendicular to the ground, and as far as possible in the highest position of the drum and header. Between the safety valve and the drum or between the safety valve and the header, no gas outlet pipe and valve for taking steam shall be installed.
Check the opening pressure of the safety valve. The opening pressure of the safety valve should be adjusted and verified according to the value specified in the table. For boilers using only one valve, the opening pressure of the safety valve should be adjusted according to the lower value in Table 1.2-2; for boilers with superheaters, the safety valve on the superheater should be adjusted for lower pressures; for boilers with a separable economizer , The opening pressure of the safety valve on the economizer is 1.1 times the working pressure at the installation site. For boilers with economizers and superheaters, the safety valve opening sequence is as follows: first the safety valve of the superheater, then on the drum, and finally on the economizer, in order to ensure that the components with high working temperature have good safety performance the rules.
The lever type safety valve must have a device to prevent the heavy hammer from moving by itself and a guide frame to restrict the lever from going out. The spring-loaded safety valve must have a lifting handle and a device to prevent the adjustment screw from being unscrewed.
Connection of safety valve. If several safety valves are jointly installed on a short pipe directly connected to the drum, the cross-sectional area of the short pipe should not be less than 1.25 times the cross-sectional area of all safety valves. For boilers with working pressure ≤ 3.82Mpa, the inner diameter of the safety valve seat should not be less than 25mm.
The safety valve should generally be equipped with an exhaust pipe. The exhaust pipe should be as direct as possible to the outdoors and have enough cross-sectional area to ensure smooth exhaust. The bottom of the safety valve exhaust pipe should be equipped with a drain pipe connected to a safe place. Valves are not allowed to be installed on the exhaust pipe and the drain pipe. The safety valve of the economizer should be installed with a drain pipe and lead to a safe place. It is not allowed to install any valve on the drain pipe.
Do deflation or water release tests on a regular basis. In order to prevent the disc and valve seat of the safety valve from sticking, the safety valve should be periodically vented or drained manually or automatically.
Precautions for management and use. After the safety valve is checked, it should be locked or sealed. It is strictly forbidden to add heavy objects, move the heavy hammer, block the valve clack, etc., to prevent arbitrarily increasing the opening pressure of the safety valve or causing the safety valve to fail.