Throttling

[Blobajob88]

Hi,

Can someone please assist? If I have a pump with a pressure gauge, followed by a diaphragm valve on the discharge side and I start to throttle the flow with the diaphragm valve, why does the pressure start to increase and therefore flow decrease? If you narrow a pipe, wouldn't Bernoulli's principle apply i.e. the flow stays the same and the water would just go quicker through the restriction. it seems it applies up to a point.
Regards

Ben

 

[TheTick]

Look at any pressure vs flow graph for any centrifugal pump. Flow decreases as pressure increases. You are increasing pressure as a result of increasing the restriction on the outflow.

Yes, velocity likely increased through the valve, but with less flow.

 

[LittleInch]

Because of friction which bernouillis principle assumes is zero.

Sadly it isn't.

The valve closure is creating losses which need to be accounted for in the pressure balance equation.




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[Blobajob88]

Hi,

Yes, I know the relationship between pressure and flow for a centrifugal pump but this isn't quite what I mean. Bernoulli's equation teaches us that you don't increase the pressure as a result of increasing the restriction on the outflow, it states that the pressure will reduce and velocity will increase until it has overcome the restriction, at which one point it will return to the initial conditions minus some frictional losses. When I'm closing the diaphragm valve nothing seems to happen to the pressure in the gauge until it is almost closed at which point the pressure ramps up.

 

[LittleInch]

You also need to look at valve CV versus percent opening.

Also as you note it is "minus some frictional losses". In there lies your answer.

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

 

[Blobajob88]

Thanks but does that mean the bigger the frictional loss in the restriction, the bigger the pressure on the inlet to the restriction like:

Bernoulli stuff 1 + frictional loss 1 (before restriction) = Bernoulli stuff 2 + frictional loss 2 (after restriciton) - if frictional loss stays the same on both sides then something else has to give to make sure they are equal i.e. say P1 in the Bernoulli stuff before restriction part of the equation....to put it elegantly?

 

[LittleInch]

It's actually

"Bernoulli stuff 1 = Bernoulli stuff 2 + frictional loss "

https://www.itacanet.org/fluid-mech...-systems-friction-and-the-bernoulli-equation/

Also as soon as you get turbulence in the flow, bernouilli doesn't work too well.

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

 

[Compositepro]

Bernoulli has nothing to do with what you are observing. You are only measuring the effect of a downstream restriction on the outlet pressure of the pump. This only depends on the pump curve.

Bernoulli's equation is simply expressing the princple of conservation of energy as fluid moves from one point to another (without friction). In this case it is not even useful to describe the flow inside the valve because the flow will be turbulent and energy is being converted to heat. So there is no equating kinetic energy to potential energy in this case.

 

[handleman]

You are confusing volume flow with flow velocity. Yes, the flow velocity increases through the restriction, but because the AREA is so much smaller, the volume flow is lower.

 

[rb1957]

actually I thought he was trying to say volume flow is constant (for an incompressible fluid) … and confusing us with Bernoulli. Ok, conservation of momentum may be a result of Bernoulli but I remember it as a thing on it's own.

if the area changes, then velocity changes inversely … if area reduces, velocity increases (and static pressure reduces … that's Bernoulli).

now if you're taping the pressure upstream of the restriction you'll see the pressure increase (as the fluid tries to get through the constriction) as flow is now limited by the constriction.

another day in paradise, or is paradise one day closer ?