Differential Pressure Flow Switch Across a Pump

What is the intended control function of this flow switch?

Purely for indication of flow to prevent dry running and if the pump fails to let the BMS know so it can start the pump changeover sequence

The flow switch should work OK. I would only verify that the output is readable by whatever device is controlling the shutoff switch. Typical LTHW system operate at a max pressure of something like 2-3 bar at most. I'm curios as of why the original switch went all the way up to 11 bar.

Should work fine (160 kPa = 1.6 bar).

What's the switch for?

Flow indication if the pumps running.....

I thought the same but what will be the pressure on the other side of the pump I have indication from the BMS that the pipe work is 1.6bar on the flow side, can we assume there will be a vacuum pulled or negative pressure on the other ???

<...there will be a vacuum pulled or negative pressure on the other ???...>

It depends upon a criterion known as the Net Positive Suction Head [NPSH] of the system. It needs local assessment, as it cannot be seen from here. Too hard a suck from the pump causes cavitation, a fluid/system phenomenon capable of wrecking a pump in short order.

Make sure that the replacement sensor, whatever its selected range, can withstand the maximum system pressure downstream of the pump. This pressure is the sum of:

• the pump-only closed-delivery-valve pressure (which can be found in the pump manufacturer's performance information sheet) and
• the upstream pressure at the pump suction connection.

Cheers for that I am now pretty confident that it should be ok

Line pressure is well under the rating of the Sensor I was just worried that it wouldn't switch and indicate flow..... very embarrassing

You could go for a thermo mass technology instead of a DP. There's a number of manufacturers making thermo mass flow switches. I suggest you pick one which is able to perform flowmeter function with 4-20 mA output being a SSR relay device (switch) indeed (I'd look at http://kayden.com/). Thus, one gauge would meet both of your targets and eliminate tentative headake with impulse line clogging (if any), process fluid density changes, leaks etc.

I'd pay atention for pressure limits while selecting thermo mass switch, despite it would install upstream of the pump normally.

RS-485 Modbus communication might be of help as well.

Juri.

Thanks for the input but way over complicated and I can imagine that a thermo mass flow switch with mod bus would be buckets of cash more then the £113 we are forking out..... keep it simple like for like this one has lasted 7 years so its not like a repeat problem......

If there are two small diameter tubes running to the switch; one from either side of the pump than I would check to see if they are clear, especially if the tubes are a dissimilar metal from where they are attached. Plugged tubes will mimic a failed switch.

yeah the impulse lines are cleared, we ran water out of both sides the diaphragm seems to be all gunked up or maybe the spring ???

you can get the contact to close if you push really!!!! hard on it from both side but the pump doesn't have enough pressure to make it close, could possibly rebuild it but better to have a functioning unit, then one that is likely to seize up again ........

If your loop is critical, you should think of redundancy or better technology. DP switch with its diaphragm and mechanical moving parts is being weared out by your process in a very agressive manner, you may not see it - vacuum issues, sticky dirt and sediment layer impact on diaphragm, vibration/cavitation, impulse line clogging etc. 133 GBP are peanuts compare to pump repair costs or its replacement. Also, count for losses if your process is down because of that pump failure. Third is a headake of your own to handle all that stupid work. However, building HVAC systems are not critical for people to survive as in general. Just chill them down a bit .

In case of thermo mass technology the dirt sticking is questionable, as the switch heats up the sensor and process fluid velocity helps to keep the sensor clean. The company I pointed out makes 3 types of switches and the cheapest one is some 300 USD List, while you always have to pay extra for things like advanced communcation etc (and save on cabling again). There's nothing to wear, non-coating, adds a flow rate across your pump. I would try to weigh all above up and note down "+" and "-"