Pressure drop of control valve 1

[sujins]

Please anyone who know suggest me with below information ;

1. What is the difference meaning between the pressure drop and differential pressure of control valve?
2. When making a hydraulic calculation example for transfer liquid with pump,how can we know the suitable pressure drop of control valve to gain the good control? I mean it should be estimated first and included in the pressure loss in pipe or it can be calculated from different pressure of performance curve and system curve of pump at rated flow rate.
3. The suitable pressure drop of control valve mentioned in 2, is it come from rated flow, maximum flow or ?
4. When sizing of control valve, shall we use pressure drop or differential pressure?

 

[JimCasey]

1.Differential pressure is the same as pressure drop. P1 (inlet) -P2 (outlet = DP

2. A guideline is for the valve to take 1/3 of the system pressure drop in its wide-open position. If the pump puts out 100 psi, and the system pressure must be 40 psi, then the system pressure drop is 60, and the valve should drop at least 20.
3. Max flow. At min flow the system drop goes away (proportional to Q^2) so essentially ALL the DP is taken by the valve.
4. Sometimes.DP and Pressure drop are the same but they are not the ONLY motive force to consider. For flow at greater than critical pressure drop (also known as choked flow) the sizing is done on the choke drop. This, by definition will be less than the actual drop, but the flowrate becomes independent of downstream pressure when the DP exceeds critical. The term "Choked Flow" tends to scare people. It's just a point where you use a different equation. Open the valve more and you get more flow. Increase inlet pressure and you get more flow. You just won't get any more flow by lowering the outlet (downstream) pressure.

 

[sujins]

Jimcasey! Thanks for your answer.

One question still in my mind is that the differential pressure or pressure drop of control valve will be a constant value for minimum , normal, maximum flow or it will depend on the flow rate. I mean when at low flow rate the pressure loss in pipe is low and control valve will absorb the different pressure between performance curve & system curve. And when the flow is higher, the pressure loss will be more and control valve will absorb lower, in order to obtain the constant pressure of production.(Due to discharge pressure of pump and required downstream pressure or pressure of production are constant.)

Please advise me.

 

[JimCasey]

It depends on the system.

If you have an infinite source discharging into a gently radiused nozzle, a short pipe, the valve, another short pipe, another gently radiused nozzle, and an infinite receptacle, then the pressure drop is the same at all flowrates.

In the real world, when the valve is at maximum flow, the system loss is greatest (fittings, heat exchangers, strainers, line loss, etc.), and the pump is run out on its curve. So Max Flow Delta-P is lowest. On the other end, at low flow there is almost no system loss, and the pump is backed up on its curve, so delta-P is greatest.

When you do the math, you will find that for a 10:1 flow range, your valve may need a rangeability of 20:1.

I have been selling valves for a LONG time, and 99 out of 40 design engineers either do not know this or do not take it into consideration.

 

[sujins]

Jimcasey!

1)I have heard that it should not operated the pump with the flow rate at a point where performance and sytem curve intersected (short circuit). Is it right? Then at that flow rate, the delta-P is zero, can we use it to find the maximum controllable flow of control valve? Do it have a rule of thumb for this estimation? My question is that how can we find that mentioned flow rate? If it have ,please suggest.

2)You stated above about the range 10:1, do you mean the term of turndown and for rangeablity 20:1, do you mean the ratio of maximum controllable / minimum controllable flow? If I misunderstood your point, please give me some explanation.

Many thanks in advance for your answer.

 

[JimCasey]

>>1)I have heard that it should not operated the pump with the flow rate at a point where performance and sytem curve intersected (short circuit). Is it right? <<
Look at it a different way. The system ALWAYS operates at a point where the pump curve intersects with the system curve. The valve resistance is part of the system curve. With the control valve wide open, you MAY be in a runout condition that is not good for the pump. It depends on how much resistance is inherently in the system.

Then at that flow rate, the delta-P is zero,
>>No, the delta-P is NOT zero. There is ALWAYS some loss through any component.<<

>>can we use it to find the maximum controllable flow of control valve? Do it have a rule of thumb for this estimation? <<
Most A&E firms want the valve to operate at no greater than 80% of travel at maximum specified flow. Look at your system at maximum flow. Add up all the losses. Subtract thet from the pump curve at the same flowrate. The remainder is valve delta-P.


>>My question is that how can we find that mentioned flow rate? If it have ,please suggest.<<
Usually, the flowrate is a design parameter. You start eith that and determine the other variables.

2)You stated above about the range 10:1, do you mean the term of turndown and for rangeablity 20:1, do you mean the ratio of maximum controllable / minimum controllable flow? If I misunderstood your point, please give me some explanation.

I used 10:1 for an example. 10:1 in my example was a flow range. 20:1 was a possible Cv ratio (rangeability) that you may need to achieve with the valve to get the 10:1 flow range. 20:1 is also just an example pulled from the air. THe actual Cv ratio will be different for every system. At low flow rates the flow is small and the differential is high, so the required Cv is small. At high flow rates the Flow is large and the DP is small, so the Cv required is high. The Cv changes more than the flowrate.

 

[sujins]

Thanks Jimcasey,

1.Anyway, for the min.,normal and maximum flow rate for sizing the control valve, can i use 75%, 100% and 110% of normal respectively? Any suggestion!!

2.Is there any limit for the pressure drop of control valve? Do we have to consider for the maximum pressure drop in order to prevent the cavitation and flashing of liquid in the control valve?

3.If the pressure difference between the pump curve and system curve at the maximum flow (design) is higher than the maximum allowable drop in above item 2, can we trim the impeller to adjust the performance curve of pump down to receive the head pressure of max.allow drop + system pressure?

4. Do we have the limitation for trimming the impeller from item 3?

Thanks again.

 

[JimCasey]

I got this out of a manufacturer's engineering handbook:

Critical pressure drop occurs if DP>=Fl^2*(P1-Ff*Pv)

Ff=0.96-0.28(Pv/Pc)^.5

DP is the pressure drop through the valve
Fl is the liquid pressure recovery factor for the valve. Fl for Globe valves tend to be at about 0.9 or a little more. Look up the published value for the valve you are using.
Pv is vapor pressure (absolute units) of your fluid at the system temperature.
Pc is the critical pressure of the fluid.

If the pressure drop in your system exceeds critical, you can do what is necessary to change variables to prevent DP from exceeding DPcrit. These include: Using a valve with a higher Fl(2) using 2 or more valves in series to split the pressure drop(3) reducing the DP by trimming the pump impeller (consult your pump manufacturer)(3) Raising the system inlet and outlet pressure at the valve (placing the valve in the bottom of a U adds .4335 psi head per foot)(4) reducing the vapor pressure of the fluid by relocating heat sources or cooling the fluid.