4 replies to this topic

[oyakobi]

Hi,

 

I am confused between the pressure drop of the system (Driving force) and the pressure drop across a Valve.

 

 

-Is the pressure drop across a Valve added to the initial pressure drop of the entire system (source and destination) or is it shared. For example before adding a valve, the pressure at the inlet is 10 psi and at the outlet its 5 psi. When a Valve is added, does it create an extra pressure drop or what does it do?

 

- Pressure drop due to the Valve, is it more of a pressure loss due to friction and hence the flow rate is decreased? or is it the same as the pressure drop between the source and destination where it is the driving force for the flow? or a mix of both?

 

Not sure if I quiet got my message through.

 

Sincerely,

 

O.Yakobi

[Steve Hall]

Please review attached excerpt from my book. If your question isn't answered please formulate it again.

Attached Files

•  05 - Pumps - selected pages.pdf   618.65KB   169 downloads

[ChemEng01]

Hey

 

If you have flow going through a pipe you have a pressure drop due to friction. (The flow of fluid is losing its energy to the pipe walls (pressure drop)) .Friction increases with velocity.  Hence pressure drop increases with velocity. 

 

Everything you add to the system increases the friction. 

 

When we add stuff we give it an equivalent length. This means we give it an equivalent pipe length. 

 

So if we add a 90 degree bend in a pipe for example. In reality this will have an actual length of say 0.1m, however because of the frictional losses caused by having to turn that bend it could be equivalent to a 1m section of straight pipe of losses. 

 

This makes sense though when you think about it. Imagine a fluid flowing it a straight section of pipe. It suddenly gets to a bend and pretty much smashes into a dead end before turning. It loses energy smashing into that wall, hence loses more energy (pressure) than it would have if it was travelling straight. 

 

Same goes for a valve. The flow gets disturbed because it flows through a restriction. Parts of the flow are going to smash straight into parts of the restriction an lose energy (pressure).

 

So yes, everything you add to a system will add to the frictional pressure drop. 

 

cheers

Edited by JRudd, 26 September 2013 - 03:51 AM.

[JosvantWestende]

What will happen exactly will depend on your system, the valve itself and the driving force of the flow (usually pumps).

 

As Jrudd has mentioned: adding a valve will create more flow resistance to your system.

 

Normally this means that the flow will decrease (assuming that the driving force remains the same). To what extend this will happen depends on the flow resistance of the valve wrt that of the system (when adding a valve with equivalent lenght of say 50D to a system that is 50D long the effect will be stronger than adding this valve to a system that is 5000D long).

 

However, when the driving force is created by, e.g. Positive Displacement Pumps, (pumps which tend to have a given flowrate, independent on the flow resistance of the system) then the driving force will go up (so does the power consumption). In case a radial pump (or centrifugal pump) is used, the pump will keep its delivered pressure more or less constant, resulting in a decrease of flow (actually due to the pump performance characteristics, this will increase the pump delivered pressure a bit).

[peclpassic]

Oyakobi

For example, the pump flow is lower than rated flow during normal case, but engineer used rated flow to size the pump so the control valve located pump discharge line should be closed and consume more pressure drop to develop more head. The control valve pressure drop at normal flow is higher than the pressure drop at rated flow. While pipe dynamics head loss increase at higher flow rate (rated flow), control valve pressure drop decreases. At higher flow rate, the control valve has to open more and pass larger flow with less resistance.