Turbulence in cold water inlet pipe when recirculation pump is running

[Jack Benson]

Hello,

The diagram below is a simplified version of a product we are developing.

I cannot get into my lab until September, and I am trying to work out if the flow meter will detect turbulence in the pipe when the recirculation pipe is running (when there is no flow through the system)

the Flow meter is a simple turbine

I can add another one way valve before the flow meter if necessary but I am trying to avoid additional parts unless necessary.

When the flow meter detects flow, it stops the recirculation pump from running.

We need a one way valve at the start of the system as shown so I cannot just move the flow meter behind the existing one way valve.

thank-you

 

[LittleInch]

There may be some swirl, but just set the trigger flow level to something other than zero.

You may need to play around a bit to prevent the recirculation pump starting and stopping to often.

But what fkie is the recirculation compared to full flow? 10%? Then why turn it off?

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[Jack Benson]

Thank-you.

that is very sensible

I will ask the PCB designer to make it a configurable parameter what the flow rate should be before the system considers the tap to be open.

That way I can test to determine what we can consider turbulence from the recirculation pump

 

[3DDave]

Even without the pump there will be some back-flow as water is heated and expands with the tap closed.

Moving the flow meter directly to the line to the tap would eliminate the effect.

 

[Jack Benson]

Dave,

the next 100 machines I have to build need the flow meter positioned as shown as we have ordered flow meters with temperature sensors in the same brass piece (to measure cold water temp).

in future machines I will do as you suggest.

thank-you

 

[LittleInch]

If you have space between the meter and the tee you could install some flow straighteners or vanes to reduce swirl?

Also turbine meters need min 5 to 10D upstream from a valve or tee to reduce swirl and give false readings.

Or fit flow straighteners or even flow discs.

How big is this pipe?

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Also: If you get a response it's polite to respond to it.

 

[Jack Benson]

the pipe is only 10mm internal diameter. Flow rate is 6 litres per minute

In the short term i will add another one way valve before the flow meter.

In the future i will move the flow meter to after where there might be turbulence in the water.

thank-you all for your help

 

[Joe591]

swop the position of the flowmeter and the non return valve that i circled in the picture and make sure there isbat least 3-4 piplpe diameters worth of distance between the flowmeter and the non return valve that has now been placed downstream from it. you can always double check that 3-4 diameter value with the supplier but i dont think im too far off.

 

[georgeverghese]

Minimum unobstructed straight length distance on feed and exit line for turbine meter will depend on whether this meter is for coarse or high accuracy readout. Ask the meter vendor or a field instrumentation engineer. A check valve is, if I'm not mistaken, an obstruction since it induces flow swirl. For coarse measurement, min distance requirement is much less.

 

[Jack Benson]

Joe591 - thank-you for your post.

one of the parameters i have been given is that there needs to be a non-return valve where our device connects to the incoming water.

the outgoing CTO told me that this is regulations in France, but I have no idea if its true.

If it is not necessary then your solution is perfect



georgeverghese - again thank-you for your comment. i will check with the flow meter supplier.

This is what we are using:

https://www.chnsensor.com/2ProductsShow.asp?id=2241&BigClassID=108&SmallClassid=8#topa

 

[LittleInch]

Nearly all public water suppliers require non return valves or air break tanks on public water supplies so there is no possibility of reverse flow contaminating the public potable water supply.

Usually they want two...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.