Shut off with the front valve or the rear valve
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Should the shut-off valve be placed at the front or at the back? In device production, it is necessary to restrict the flow of various fluids to prevent excessive flow rates from affecting operations. Generally, it is necessary to regulate the flow before or after the control valve – what are the differences between these two approaches?
It’s different; especially in the high and low pressure reduction sections of hydrogenation, the backflow valve must not be adjusted. Adjusting the manual valve can easily cause pressure buildup in the low-pressure pipeline. Some design institutes use low-pressure valves right after the high and low pressure reduction sections. This is not advisable; the money isn’t that important, and it can lead to mistakes by those with less experience.
It works either way; for our devices with many control valves, we generally use upstream valves for retention
Personally, I don’t think there’s any difference, but I’ve never worked on high-pressure equipment like hydrogenation units.
Basically, the flow interruption by the front and rear valves is similar; it is mainly the control valve that serves to interrupt the flow, allowing for timely adjustments in emergency situations. When the internal leakage of the control valve is high, it is possible to consider using a manual valve for shutoff.
Normally, the front hand valve is used. But it doesn’t work; the valves at the front and back should be gate valves, while the valve on the bypass line should be a globe valve – that works!
It’s better to have a manual valve behind the control valve; the control valve is selected based on the flow capacity. The manual valve in front can alter the flow characteristics, and sometimes it’s also easier to take samples when there’s a manual valve behind.
There are several scenarios regarding the issue of isolation using a front valve or a back valve: 1. For valves that can be closed, such as sampling valves and drain valves, a back valve is used for isolation. The reason is that when the shut-off valve is damaged, the upstream valve is still in good condition and can be closed to allow for the replacement of the valve. 2. Control valves equipped with front and rear hand valves; this type of control valve is generally used for throttling ； 3. When a control valve equipped with front and rear hand valves is difficult to control due to a large pressure difference between the front and rear, the front and rear hand valves are used to shut off flow, ensuring that the control valve can be disconnected in critical situations.