Questions regarding the use of flow switches
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Recently, the company carried out renovations on an old workshop and installed several new pumps; the project engineer purchased some flow switches to protect these pumps. As per standard practice on site, the flow switch was installed behind the pump. During testing, it was found that the pump would not start because the flow switch could not detect any changes in flow rate. After some discussion, three solutions were proposed: the first is to connect the flow switch in front of the pump, and start the pump using the flow rate of the liquid as it flows naturally when the valve is opened ； Second is the level switch before the pump is replaced ； Another approach is to increase the PLC program settings to enable delayed pump shutdown. Later I thought about it – with a flow switch in place, the pump shouldn’t be able to start at all. That’s not right; there must be some problem somewhere. Since the distribution box connected to the pump is quite old, I suspect it’s a problem with the switching mechanism. I’m not from this field originally; I’ve come here to ask everyone for some advice
Protect the pump with a flow switch? What to protect? Stop the pump without flow? Or what? ?
The flow switch signal enters the electrical control circuit via relay contacts; since the switch signal is pulsed, self-locking is required in the electrical system.
What is the function of a flow switch? After the pump, a pressure transmitter is generally used to monitor the outlet pressure, while before the pump it serves to provide chain-lock protection linked to the liquid level in the tank
By stopping the pump automatically when there is no activity, the liquid level switch in that container can prevent this problem； Because without the pump running, there is no flow; and without flow, the pump stops – which is a contradiction in itself ； If a flow switch must be used to protect the pump, a timing approach can be adopted to handle startup. A bypass switch can be used to disconnect the contact in parallel with the flow switch; during startup, this bypass switch closes and the pump starts up, after which it opens after a certain period of time. It should be noted that when there is a pause, the control circuit is disconnected, the pump stops, and at the same time the bypass switch closes again.
We usually use flow switches to detect whether a pipe is blocked and to issue an alarm signal. As for what you mentioned regarding protecting the pump, an additional condition can be added: the flow switch signal should only be checked while the pump is in operation
The pump has no flow rate before it is started or when the steel is started; this condition is the same as that for the activation of the protection mechanism. Therefore, it makes no difference whether the flow meter is installed in front of or behind the pump. The interlock is disconnected as soon as the start button is pressed ； A delayed approach can be adopted, allowing the protection circuit to activate after a while, once the pump flow has been established, or temporary alternatives to the flow conditions can be used and then removed once flow is present.
A flow switch generally has two sets of output points: an upper limit and a lower limit. In your case, the flow switch is connected in series within the pump’s control circuit, and it is a normally open type of switch. Therefore, when there is no flow, this switch does not close, the pump’s control circuit is not completed, and as a result, the pump cannot start. As for whether it is connected in front of or behind the pump, it doesn’t matter; the key is to consider the process conditions to determine whether the flow switch should be connected in series or in parallel in the pump’s control circuit. Flow switches come in flow-closing and flow-opening types; it seems that you have chosen the flow-closing type, which closes only when a certain flow rate is reached. Connect this point in parallel to the control circuit; as for how to do this, it needs to be discussed in conjunction with the electrical engineering team.
If old devices all use distribution boxes, add a switch bypass to solve the issue； If it’s to start automatically, it’s better to use a delay relay.