safety valve

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To ensure safe operation, certain valves are required to open automatically to release the medium when its pressure exceeds a specified value, thereby preventing damage to the equipment or pipelines； It can close automatically once the pressure returns to normal; a valve with this function is called a safety valve. The most common type is the spring-loaded safety valve, followed by pulse-type safety valves and lever-type (i.e., weight-type) safety valves. The working principle of a spring-loaded safety valve is that the force of the spring balances the normal pressure exerted by the medium on the valve core, thereby ensuring a tight seal between the sealing surfaces ； When the medium pressure is too high, the spring is compressed, causing the valve disc to move away from the valve seat and allowing the medium to flow out ； When the pressure returns to normal, the spring pushes the valve disc back against the valve seat, restoring a tight seal between them. A lever-type safety valve is an ancient type of valve that relies on levers and weights to balance the pressure on the valve core. The pressure level is adjusted by moving a weight on the lever. This type of valve is bulkier and slower than spring-loaded safety valves, but it is not affected by the heat of the medium due to the absence of a spring. It is currently mainly used in some small boilers with lower pressure. A pulse safety valve is a combination of a large safety valve (main valve) and a small safety valve (auxiliary valve); the pulsating action of the auxiliary valve drives the opening and closing of the main valve. Larger safety valves are less sensitive, while smaller ones are more sensitive. The fluid passing through the main valve is connected to a secondary valve; when the pressure becomes too high, the secondary valve opens, allowing the fluid to flow into a piston located below the main valve, thereby pushing the piston and opening the main valve. When the pressure drops, the secondary valve closes, and the pressure of the fluid beneath the piston in the main valve decreases, causing the valve core to descend and seal shut. This type of safety valve has a complex structure and is used only in cases where the diameter is very large. Safety valves are classified into slightly open type and fully open type based on the ratio of the valve disc’s opening height to the diameter of the valve seat. The lift height of the fully open safety valve disc is greater than 1/4 of the nozzle diameter, resulting in a high discharge capacity; it is suitable for use with gaseous and liquid media. The lift height of the slightly-opening type safety valve disc is 1/40 to 1/4 of the nozzle diameter; it has a low discharge volume and is suitable for liquid media. Structural safety valves can be divided into four types: ① closed and open types, with the closed type being used for flammable, explosive, or toxic media ； ②With and without a wrench; the wrench is used to check the flexibility of the valve disc ； ③With and without radiators; those with radiators are used when the medium temperature is above 300℃ ； ⑷There are those with a bellows and those without. Those with a bellows are of the balanced-type safety valve type, and they are used in situations where the medium is highly corrosive or where there are significant fluctuations in back pressure. Safety valves must be used in accordance with the following regulations: ① Safety valves that have passed calibration must be sealed with a lead seal. ②When there is a valve between the container or pipeline and the safety valve, that valve must be fully open and sealed with a lead seal. ③The set pressure value of the safety valve must not exceed the design pressure of the container. ④For a safety valve that activates during operation, it is necessary to analyze the reason for its activation, and if needed, the setting pressure should be adjusted again. ⑤The nameplate on the safety valve, as well as the pressure setting plate, must not be removed without permission. The safety valve should have markings indicating its installation location, and these markings must be followed during installation to avoid misuse. ⑥The safety valve must be calibrated once a year. Main interlock cause-and-effect table for hydrocracking unit (taking pre-furnace hydrogen mixing as an example)
Accident scenario: Reaction feed pump, hydraulic turbine, reaction furnace, hydrogen circulation pump, fresh hydrogen pump, emergency pressure relief, high-temperature liquid level control valve, hot high-pressure section, cold high-pressure section, fire.
Response: Stop, stop, stop; maintain, stop; start manually at 7 bar; monitor, monitor.
Overtemperature in hydroprocessing reactor: Stop, stop, stop; maximum limit; stop; start manually at 7 bar; monitor, monitor.
Overtemperature in hydrocracking reactor: Stop, stop, stop; maximum limit; stop; start manually at 21 bar; monitor, monitor; emergency pressure relief; stop; start at 7 bar; monitor, monitor.
Failure of hydrogen circulation pump: Stop; maintain, or stop; stop, stop, stop; maintain; start at 7 bar; monitor, monitor.
Failure of fresh hydrogen pump: Maintain, stop; maximum limit; stop; monitor, monitor.
Failure of feed pump: Stop, stop, stop; maximum limit; maintain; monitor, monitor.
Reaction furnace: Stop. Low liquid level in hot high-pressure section: Shut down. Low liquid level in cold high-pressure section: Shut down. High liquid level in hydrogen circulation pump inlet tank: Maintain, or stop; stop, stop, stop; maintain; start at 7 bar; monitor, monitor.
Low outlet flow rate of feed pump: Stop, stop, stop; maximum limit; maintain; monitor, monitor.
Power outage: Stop, stop, stop; maximum limit; stop. Manual operation: Manual, manual.
Stop supply of instrument air: Stop, stop, stop; maximum limit; stop. Manual operation: Manual, manual