7 replies to this topic

[Che#pet]

Hi All,

 

I have a query which many of us(Junior process engineers) are still confused with:

 

First of all this is a general query and is not a project specific one.

 

I have a system with a vessel upstream of a level control valve operated at 140 psig and a vessel downstream of the same control valve operated at 80 psig. Flow through the system is 150000 kg/h(153 m3/h) of water.This means that the piping and other components including the control valve shall be designed in such a way that the maximum pressure drop through the system shall not exceed the pressure differential between the two vessels(i.e 140-80=60 psi). So the question is

 

" As the % travel of the control valve is changed the pressure drop across the control valve will also change, but the overall system pressure differential remains the same (i.e 60 psi). Then what is the reason that the flow has to decrease or increase as the valve travel increase or decrease. I searched everywhere regarding this and all i got about this is something regarding the relationship between flowrate, resistence and pressure drop. Can you please clarify what is this relationship".

 

Regards

Che#pet

[PhilippM]

Dear Che#pet,

 

I would advise you to get good textbook on control valves, I can recommend the Fisher Control Valve Handbook, available here: https://www.emerson....-en-3661206.pdf

 

The most important value that describes the "resistance" of the control valve is the flow coefficient Cv = Q * (SG / dP)^1/2 with Q being the flow, SG the specific gravity and dP the pressure drop of the valve. This flow coefficient changes with the travel of the valve (if the valve is opened further, Cv increases), so for a given pressure drop over the valve the flow increases when the valve is opened.

 

Kind regards

Philipp

   

[Che#pet]

Dear PhilippM

Thanks for the reply. But what makes me confused are the terms "Resistance" and "Pressure drop". Can you make me understand the difference between them and their relationship with the flowrate.

Regards
Che#pet

Edited by Che#pet, 26 May 2019 - 09:18 PM.

[breizh]

hi,

Consider the Bernoulli equation ,  Resistance or Pressure drop are equivalent to Head loss , just pay attention to units .

 

Hi

Take a look also at the Darcy Weisbach formula .

 

I've attached a document to support.

 

Note : Let you purchase  Crane book : Flow of Fluids through valves , fittings and pipe  TP N 410 M , this is the bible.

 

Good luck

Breizh

 

 

[Chemitofreak]

Hi,

 

My two cents

 

Control valve is designed for Maximum Flow and Minimum Pressure drop. So, when you close the control valve, you add resistance i.e. pressure drop in the system and hence the flow through the system reduces.

 

Also, one thing you are forgetting that the frictional pressure drop for the piping will change with flowrate

 

Regards

Edited by Chemitofreak, 27 May 2019 - 05:31 AM.

[fallah]

 

Hi,

 

Actually a control valve is a variable area orifice. Hence considering the differential pressure across the mentioned control valve has been fixed, the only thing causes the flow through it being changed is decreasing or increasing the valve's opening cross section following which the passing flow will be reduced or increased respectively.

 

In fact, lower orifice area will create more resistance to flow than the higher.

[Che#pet]

Dear Breizh

Thanks for the reference.

Dear chemitofreak

From your response what I understand is that pressure drop and resistance are the same. But I found out a relation saying
Flow=pressure drop/resistance. Can you clarify your response based on this relation.

Dear Fallah

Thanks for the reply

Regards
Che#pet

[Chemitofreak]

Dear chemitofreak

From your response what I understand is that pressure drop and resistance are the same. But I found out a relation saying
Flow=pressure drop/resistance. Can you clarify your response based on this relation.
 

 

Can you give the reference of your equation? In which book did you find this?

 

In simple layman's language, more the resistance in the system, more will be the pressure drop.