Control Valve Sizing: deltaP increases/ decreases w/ increasing load? 2

[LiamC]

1) It is my understanding that if a control valve is installed downstream of a centrifugal pump, then the following will occur for minimum flow:
namely, the ACTUAL inlet pressure will be maximum and the ACTUAL pressure drop across the control valve will be maximum.

Similarily, the following would be true at maximum flow:
namely, the ACTUAL inlet pressure will be minimum and the ACTUAL pressure drop across the control valve will be minimum.

In other words, for increasing load (flow), the ACTUAL pressure drop across the valve is decreasing.

Question A:
Are the statements in (1) above correct?

Question B:
Will someone please explain to me why, in some situations,
the pressure drop across a control valve would INCREASE with INCREASING load?? What are these situations?

Question C:
When a process engineer fills in my valve spec sheet with the MAX deltaP occurring at MAX flow and the MIN deltaP occurring at MIN flow, how do I determine if this is the actual case or if indeed this is not correct and in actual fact the MAX deltaP occurs at MIN flow and the MIN deltaP occurring at MAX flow ????

Will some expert please please enlighten me. Thanking you profusely in anticipation.

 

[MortenA]

A) if you add "pump" to the statement the appropriate placeses then yes

B) When the valve is fully open then an increase in the flow for instance by turning on a second pump) would lead to an increase across the valve

Max/min flow for spec sheets is usually not related to a specific pump and the way i read what you write min could actually be larger than max because they are related to different flows

Best Regards

Morten

 

[ggordonstewart]

With a Centrifugal Pump, the maximum head is developed at the minimum (or no flow) rate and the maximum flow is acheived at the minimum output head. Hence, if you have a flow control valve controlling the flow rate with the downstream pressure constant, then the minimum flow would have the maximum pressure drop. Normally, you would choose a valve for the design flow rate and pressure drop at a valve opening of 50 to 70%. At the pump minimum flow condition, the valve could be almost closed. Even with the valve fully open, you may not reach the full pump potential flow rate. What in fact you actually reach is the pump flow rate at the downstream pressure plus the valve and piping pressure drop at the combined resulting head. You may have to do a couple of calculations to determine this rate. This rate must be at least the required process maximum rate. G. Gordon Stewart, P.Eng.
Gas & Oil Process Engineering Consultant

http://www.ggordonstewart.com/

ggstewar@telusplanet.net

 

[Sandeep1970]

What you have said is correct only in simple systems. Often in the reninaries that is not true. There may be scenarios where the control valve dp is higher for higher flow. This could be dictated by the downstream conditions. Keep in mind that you should never look at the control valve without looking at the whole system, "control valve is a tool to adjust the hydraulics to meet the system design". Therefore the thorough analysis of the system is required or trust the datasheet provided, person who has devaloped it may know more about the system. If you are in doubt always talk to the originator.

e.g. at high flow there may be a branch stream going to somewhere which is at lower pressure. Therefore you will have to increase the control valve dp. There may be lot of other reasons too. Therefore thorough review of the system and discussion with appropriate people is must.

 

dear,
gpm = K x sqr.root of P
when gpm increases p increases upstream of valve. The delta P of the valve is fixed though the flow rate has increase. Therefore you will have a higher pressure difference accross the valve. Keep in mind that there will be high pressure on downstream of the valve to due to high flow rate.
Asghar Khan

 

[saidsaz]

may be this will help,,,

for fixed restriction system ...dp will increase across as flow increases ...talking about typical liquid ..this will follow q=kSQRT(dp),,

example of fixed restriction system is pipe , fitting ,equipments like heat exchangers ..filters.....etc.

now for any system there will be available maximum pressure drop call it dpt that will enforce maximum possible flow (as per the relation highlighted above)...now

if you need to decrease flow through this system .,from relation hilighted above ,you have to decrease total available dpt ...how to do it ?

you install a (variable resistence device ...as opposed to fixed restriction component like pipe ,,exchanger ,,,)that will behave opposite to fixed resistance device following that relation i,e will take more dp for less flow and vice versa ...this is your control valve ..as the valve opens it will take less dp from available dpt ...and leave the balance (more dp ) will drive the higher flow in the system ..

so as long as the valve is opening or closing (variable restriction ) then flow will be inversible to dp unlike the fixed restriction components...(now you have restriction hydraulic dia changing every time ).

if valve is fully open or fixed at one position ( control valve on manual or open on/off valve )then its no longer avariable restriction device but a fixed restriction one like a fitting or any other equipment in the system ..and in that case if you manage to increase flow in the system ( increasing upstream or decreasing downstream pressure ) then flow as it increases will require higher dp across the valve ...


confused ?! hope not ....

regards