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NRV slam on pump stop

NRV slam on pump stop

(OP)
We have swing check valves (NRVs) in the vertical discharge from a pump. When the pump stops the NRVs slam very hard.

Not much room to move the valves.

How can we reduce the slam? Some ideas below, what would be best?

1. Change the valve type. Flapper, ball etc (faster acting)
2. Pump soft stop (opposite to a soft starter)
3. Header tank/accumulator
4. damper on swing check arm

RE: NRV slam on pump stop

Options 2 & 3 would appear expensive. Option 4 is adding complexity and could turn out to be more of an "experiment".
I would tend to go for the first option.

RE: NRV slam on pump stop

you need slower acting not faster acting -- what size valves are you discussing? Google 'slow acting non return valves'

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: NRV slam on pump stop

If you slow down the operation of the check valve, what would happen at the pump during the stop transient whe there is momentary reverse flow? Would the pump reverse spin or is there a reverse spin barring mechanism?

RE: NRV slam on pump stop

A second or two of reverse rotation (if it actually takes place) is far better than the NRV slamming shut and the possibility of water hammer.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: NRV slam on pump stop

This implies that the static head or static pressure of the system is very high compared to the frictional losses. Changing your valve type won't help, what you need is either some sort of damped closure ( expensive) or maybe actuate an isolation valve and close it before you turn the pump off. This is a variation on option 2. slowing the flow down over a period of a few seconds and allowing the check valve to close before getting hit by a pressure wave is a better long term solution IMO.

A slamming check valve will fairly quickly be an ex check valve.

Putting a swing check vertically is not helping either - any chance you can relocate to a horizontal system.

You can't get much better than this, but they are not cheap. https://mokveld.com/en/axial-check-valve

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

RE: NRV slam on pump stop

Impossible to answer your question without an idea of the pump size. Is this a 1-Inch pump or a 24-Inch pump? The recommendation for the 1-Inch pump problem may not be appropriate for the 24-Inch pump problem.

Quick closing check valves (silent checks) will work:

http://www.valmatic.com/silentcheck.html

http://www.valmatic.com/pdfs/DynamicCharacteristic...

The pump soft start will only be successful if you have reliable power. It will not work with power failure situations.

The solution depends on the size of your installation.

RE: NRV slam on pump stop

I think you need to be faster not slower in your check valve response.

In my opinion, swing check valves (NRVs) are notorious for their delay in checking up. There's a definate delay as the swing falls into its seat.

During this time your flow begins to accelerate in the reverse direction and when the seat closes you get the slam. You need a prompt (faster) closing check valve that doesn't need to wait for the swing check to fall onto the seat

I'd suggest a spring loaded nozzle check valve that has an internal spring to quickly close the valve before substantial back-flow velocity develops.

Jeff
Pipe Stress Analysis Engineer
www.xceed-eng.com

RE: NRV slam on pump stop

IMO, it's similar to a water hammer in a reverse flow as the pump was shot off. It may help to let the pump run at smaller flow rate, such as through the minimum flow bypass, or manually close the discharge valve slowly before turned off.

RE: NRV slam on pump stop

To add to this list of options would be a variant of option 2, which is a mechanical version of a electrical soft stop. This would be some sort of flywheel on the pump driver train that would add to the rotational inertia of the pump and so help to slow down the rate of deceleration during the stop event.

RE: NRV slam on pump stop

Any response KevinNZ?? Some good ideas here - would be nice to get some response.

Ta

LI

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

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