How to choose a cut-off valve
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In practice, I’ve encountered a situation where, when the shut-off valve is closed, the chemical medium in the pipeline causes a “water hammer” effect, resulting in a sudden increase in pipeline pressure. After analysis, it was found that its shut-off valve closes too quickly; we therefore sought advice from Haiyou. Are there cut-off valves of the “slow-close type”?
Yes, there is a type of shut-off valve called a “slow-close type”. This cut-off valve is designed with the issue of water hammer in mind, allowing it to effectively reduce the pressure shock caused by water hammer. When selecting a cut-off valve, you can consider the following points: 1. Consider the properties of the fluid: Choose the appropriate type of cut-off valve based on the characteristics of the chemical medium being handled (such as flow rate, pressure, temperature, etc.). 2. Consider working conditions: Choose a cut-off valve with appropriate materials and structure based on your working environment and requirements, to ensure it can function properly and is durable. 3. Consider the valve operation method: Choose a shut-off valve with manual, electric, or pneumatic operation based on your requirements. At the same time, the requirements for operational sensitivity and shutdown speed must also be taken into account. 4. Consider special requirements: If your shut-off valve needs to meet specific process or environmental requirements, such as corrosion resistance, you can choose a shut-off valve with the corresponding special functions. In summary, when selecting a shut-off valve, it is necessary to take into account the properties of the medium, operating conditions, and any special requirements, in order to ensure that a suitable shut-off valve is chosen one that can address the issue of water hammer. .
The switching speed should be determined in accordance with process requirements and safety standards; it cannot be chosen solely based on the risk of water hammer. If a valve with a slower opening and closing speed is indeed needed, the flow control valve on the pneumatic actuator can be adjusted to reduce the opening and closing speed of the existing valve. Or choose a quick-cut valve with a water hammer protector. Such as zero-friction spherical emergency shut-off valves.
1. There are time requirements for opening and closing of pneumatic shut-off valves. If it’s necessary to slow down this process, the rapid shut-off is typically achieved through spring reset. A flow control valve can be installed at the exhaust port of the solenoid valve to reduce the exhaust speed, thereby slowing down the valve closure rate.
1Q86: How to roughly calculate the response time of a cut-off valve? A: According to 10.3.5.5 in SHT3005, the total travel time of the valve starting from the control signal should not exceed 3 times the size of the valve body in inches (in seconds), and it should not be less than the size of the valve body in inches (in seconds)； The maximum travel time of the emergency shut-off valve (the time from the valve’s normal operating position to the safe position required by the interlock) should not exceed 10 seconds. The full-stroke closing and opening times of the on-off valve shall meet the requirements for safety under load and process operation. Section 5.4.2.8 of SHT3184 should specify appropriately the rated full-stroke time for pneumatic on-off valves. This specification must take into account the requirements of storage and transportation processes; at the same time, the stroke time should not be too short, as this could lead to \"water hammer\" or vibrations in the pipes, resulting in damage to the valves and pipes or a reduced lifespan. The rated full-stroke time should not be less than 10s × valve diameter in mm/100. Communications with the manufacturer also confirmed the above points; therefore, based on experience, it can generally be assumed that for each additional size increment (in inches) in the diameter, the single stroke time increases by 1 second, regardless of whether there is a load or not. For example, DN50 requires 3 seconds, DN80 requires 4 seconds, and DN100 requires 5 seconds. Generally, control valves do not have any requirements regarding response time. However, for anti-surge control valves, the following applies: As specified in 10.1.2.5 b) of SHT3005, when the output signal of the anti-surge control valve reaches 4 mA or when the solenoid valve loses power, the time it takes for the valve to go from fully closed to fully open should not exceed 2 seconds ； When the output signal reaches 20 mA or the solenoid is energized, the time it takes for the valve to go from fully open to fully closed should not exceed 5 seconds. To meet this requirement, the manufacturer of the control valve should equip it with accessories such as boosters and quick-release valves. (Refer to the rapid operation of the control valve in the accident involving a surge control valve failure in an ethylene cracking gas compressor during the Spring Festival.) To the best of my knowledge, damage to fire protection pipelines in industrial park tank areas has occurred due to valves opening rapidly.
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