limit switch vs flow switch

Charbel · Jan 26, 2015

dear,

I am working on a design of pumping station consisting of three (3) pumps two duty and one standby.

I was wondering what is better to detect no flow against a closed valve, is it limit switch on the check valve downstream of each pump or flow switch on each pump discharge?

If I chose proximity switch I feel on the long run it is better since it doesn't have any moving parts like the flow switch or a mechanical limit switch
all it needs is periodical maintenance to clean it.

I really appreciate any thought on this

regards,

charbel

pino2003fr · Jan 26, 2015

ill use limit switch on check valve.

dginbuffalo · Jan 26, 2015

I would second the limit switch on the check valve. Like you said, no moving parts and probably less issues in the long run.

amberman · Jan 26, 2015

Charbel said:
dear,

I am working on a design of pumping station consisting of three (3) pumps two duty and one standby.

I was wondering what is better to detect no flow against a closed valve, is it limit switch on the check valve downstream of each pump or flow switch on each pump discharge?

If I chose proximity switch I feel on the long run it is better since it doesn't have any moving parts like the flow switch or a mechanical limit switch
all it needs is periodical maintenance to clean it.

I really appreciate any thought on this

regards,

charbel

You can get flow switches with no moving parts.

I'd have thought both were necessary (in fact I'd go for open and closed switched for the valve) but as ever it depends what sort of budget you're on.

PLCTech42 · Jan 26, 2015

I prefer the flow switch because it helps protect against other obstructions in the line that could cause a no flow situation.

Sherlock4me · Jan 26, 2015

In the stations I've built and or serviced...Generally, since the pump control valve was part of the control system, a limit switch on that was integral and a flow switch as well protects against no flow conditions for any number of reasons. budget not withstanding

Giarc · Jan 26, 2015

For me it would depend on the severity of failure. If it's basically just used as a status bit and you're just going to use it for data and basic indication I'd use a limitswitch. If it's a higher impact and important to your process I'd put both so you can put alarms in place.

Tom Jenkins · Jan 26, 2015

You don't mention whether you are pumping clean water or sewage. It matters.

I generally try to avoid anything inserted into the flow stream for sewage - it will attract rags and other debris, causing a switch failure at best and plugging the line at worst. In that case a limit switch on the check valve arm is better. Bear in mind that at low flows the check valve won't open completely unless the sizing is done right.

For potable water either will work. I'd avoid the paddle type flow switches as they are prone to mechanical failure. There are thermal type flow switches that seem to do well and only insert minimally into the line.

danny.michael · Jan 26, 2015

You can also use a motor load monitor. I prefer the ones from Load Controls.

Mickey · Jan 26, 2015

Never trust a check valve.

Charbel · Jan 28, 2015

Dear,

thank you for your feedback, they are very helpful.
What about using pressure switch downstream of each pump to detect against a closed valve?

Tom Jenkins · Jan 28, 2015

That will work, but you have to pick a pressure setting that is above the maximum system pressure plus local valve and fitting losses, yet is within the pressure capability of the pump at minimum flow.

Charbel · Jan 28, 2015

Thanks, do you think it is reliable to be used as hardwired interlocked. When blockage is detected downstream of each pump, the pumps will stop

Tom Jenkins · Jan 28, 2015

Yes, it will be reliable. The high pressure setpoint should be at around the shutoff head of the pump(s). If this is a sewage application consider using an isolation diaphragm to minimize clogging of the switch port.

limit switch vs flow switch

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