May I ask about the sequence of the bypass valves for the control valve?
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As the title suggests, I would like to ask: in a control valve system where a bypass route includes a stop valve and a ball valve, should the ball valve be installed first and then the stop valve, in terms of the flow direction of the medium? Also, what are the sizes of these two valves? (Generally, the control valve is smaller in size than the main pipeline; so, should the valves match the size of the main pipeline or the size of the control valve?)
1: Follow the flow direction of the medium – install the ball valve first, followed by the globe valve. Reason: The ball valve only serves to close the flow; it has no function for regulation, whereas the globe valve can be used for regulation. 2: The bypass line must have the same diameter as the main pipeline. The selection of the control valve takes into account the sizes of the upstream and downstream sections; however, since the main function of a control valve is to regulate flow and pressure, its diameter is usually reduced to save costs.
Why are two valves needed for the bypass? Wouldn’t just one stop valve suffice?
We generally install only a shut-off valve in the bypass; when the control valve fails, manual adjustment is carried out through this bypass shut-off valve. I’ve never seen a bypass for a ball valve + globe valve; please advise
Additionally, there is a table in the piping manual that allows one to determine the size of the bypass based on the size of the control valve and the main pipe. Generally, one rank lower than the supervisor. The principle behind how to determine the specific bypass dimensions is unclear.
Reply to 3# dabing820225: Stop valves are used to regulate flow, while ball valves prevent internal leakage
This post was last edited by arpcd on 2011-4-29 05:02. Install the globe valve first and then the stop valve; isn’t it the same in reverse order, with the globe valve being used for opening and closing and the stop valve for adjustment? The sequence of the valves doesn’t seem to be directly related to their functions, right? Personally, I think that by installing the ball valve first and then the globe valve, the globe valve will only have to withstand the reverse backpressure behind the control valve; whereas if the globe valve is installed first and then the ball valve, the globe valve will have to endure the forward pressure of the fluid. Moreover, the pressure of the fluid behind the control valve is generally lower than the pressure before the valve, and this throttling effect serves as a form of protection for the globe valve. Actually, this ball valve is a bit redundant; if the concern is that a globe valve might leak due to poor quality, then as long as there is no external leakage, a small amount of internal leakage isn’t a big problem. After all, it’s a bypass valve that isn’t used very often. If quality issues with the valves are a concern, ball valves can also have problems. It doesn’t seem necessary to have two valves. I’ve never seen such a configuration before.
The ball valve should come first, followed by the globe valve; the valve sizes should be the same as those of the control valve
This post was last edited by airkai on 2011-4-29 at 12:34. The bypass sizes seen on site are generally smaller than those of the main pipe, and only one stop valve is installed. The reason why the control valve is smaller in size: 1. A smaller size results in a lower cost; 2. Reducing the diameter helps to minimize pressure drops, meeting the process requirements, and the force required to operate the control valve is also reduced.