Determining pressure drop for Valve Sizing 7

[Muud]

We need to install a control valve in our water filter test system. I see online a lot of formulas and calculators to determine the Cv. However, for it's calculation pressure drop is needed. Is it the max acceptable pressure drop of the valve or something else?

Secondly, the formula also requires the flow rate for the Cv calculation. Since we use different flow rates e.g. 5 m3/hr, 10 m3/hr and 15 m3/hr for different types of filters. So I believe, using the max flow rate for Cv calculation should accommodate the cases for the lower flow rates also.

Lastly, I can calculate the headloss in the system at a specific flow rate with a clean filter. As the filter gets loaded/blocked the pressure drop in the system also increases. Different types of fully blocked filters introduce additional pressure drop of 1-2.5 bar. So for pump sizing I should use the max drop of a blocked filter in addition to other head losses, correct?

 

[Latexman]

For the max. required test flow rate, 15 m3/hr, I'd size a CV to be about 75% open. Then check the min. required test flow rate, 5 m3/hr, that the CV is about 25% open. Select a CV and trim type so the test flow rate range of 5-15 m3/hr is 20-80% open, if possible. There may be some compromise, just stay away from the ends, 0-10 % open and 90-100% open, for the test flow range needed.

Good Luck,
Latexman

 

[LittleInch]

Agree with latexman.

You really need to come up with a few different scenarios or process cases and provide that data to the control valve supplier.

So min flow, max DP across the CV is one. Max flow, min DP across the CV is another to bound the system.

How this affect the pump only you can know as we don't know, with this control valve in the system, whether an extra 2.5 bar makes a big difference or not.

Maybe opening the control valve a bit more will be enough to compensate? That might be your worst case noted above.

It's rare for a filter to go above 1 bar DP in my experience though - tends to burst the filter bag / mesh at that point.

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

 

[1503-44]

Muud, If not obvious, Cv varies with the valve %Open.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[bimr]

Muud,

Typically one can assume for flow diagram purposed that the control valve will have 5 psi pressure drop. The actually pressure drop depends on how much the valve is open.

A blocked filter will have zero flow, so you don't want to size for that. Size the pump for the loaded filter.

 

[Muud]

Thanks all for your useful advice. Could you please also comment on my first question regarding the differential pressure used in Cv calculation? Forgive me for my naivety but from my understanding, it's just the pressure loss caused by the control valve that will of course vary with flow rate. So shouldn't this value be supplied by the valve manufacturer as they know what's the inherent drop caused by the valve at different flow rates?

I will take into account the scenarios as mentioned by @LittleInch and @Latexman.

Indeed the bag filters will burst but for melt blown cartridge type filters we need to test the stability at pressure > 2 bar.

@1503-44, I believe %Open is same as lift?

 

[katmar]

The rule of thumb for setting the differential across the control valve used to be 30% of the overall system pressure. This gave the valve sufficient pressure drop to work with and give stable control. Modern controllers may get away with less than that, but if you are not consuming a lot of power having a bit of extra pressure drop to work with adds flexibility.



Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[1503-44]

Lift in UK is an elevator. Ground floor is level 0.
Lift in oil production means the depth of the producing formation.
Lift in pumps can mean the differential head.
Lift in aerodynamics is up force of the wings.
Lift in mechanical engineering is a force opposite to gravity.
Lift to a traveler is a free ride.
Do you live in Australia? "Raise up lights" means "razor blades".
Lift for valves? Personally I've never heard of one, but if you think so, OK.

Many valve manufacturers have Cv vs. %Open curves and charts. Many do not and you will have to find charts typical for the type of valve you are considering.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[Muud]

Thanks for the heads-up on the various definitions of Lift

I saw the datasheet of the one valves I am considering and it mentions Cv vs lift. I guess that manufacturer uses lift instead of %open.

 

[1503-44]

That's it. Interesting. Not seen that before. Always something new to learn.

Maybe it refers to the % of raising of the valve stem in the actuator mechanism. ?? Who knows.

But that is apparently what you need. Since the values are normalized in % vs %, it shows characteristic values for all linear and eq% valve types as a % function of Cv at full open.



A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[LittleInch]

So shouldn't this value be supplied by the valve manufacturer as they know what's the inherent drop caused by the valve at different flow rates?

The valve supplier has three main variables to consider for a fixed fluid.

1) Flow rate
2) Pressure Drop
3) Valve CV

At the start when you ask him for a new valve he needs two out of those three. Normally the customer (you) supplies items one and two in two or more scenarios and then the supplier works out 3) within his options available for a particular valve or trim inside the valve..

Turndown in flow of more than 5:1 can be interesting to try and get in one valve as can large variance in DP.



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

 

[pierreick]

Hi,
To add to this thread :

https://www.instrumentationtoolbox.com/2015/11/rules-of-thumbs-for-sizing-control.html

Good luck
Pierre

 

[1503-44]

No. Many mfgrs will give you Cv[sub]100[/sub]. Not all. But it's usually not a problem, because Cv[sub]100[/sub] is a generally well known value for all standard valves types and of all diameters. All you need are the diameter-normalized tables. I think I can find my standard valve tables, if you can't google them up. If it's a non-standard valve, then the mfgr must supply a full table for that specific valve.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[danw2]

The pressure drop is the delta P in the diagram, the pressure drop the control valve needs to create over the flow range required.

 

[davefitz]

I would suggest reading the "ISA handbook of control valves" for a summary of methods to size or select control valves, for the size and type of valve.

The flow capacity of the valve in simple incompressible ( water) flow is a simple formula, where W=63.4*Cv*SQRT(DP/sv) in US units. Other formulas are given for other flow types, including compressible flow, compressible choked flow, and flashing hot fluids.

The different valve types have different behaviors in terms of noise, controllability, cost,actuator strength or type , and maintainability. Not every type of valve has the Cv vs % open listed in prior posts.

To pick the right size in the simplest case, list all of the expected process conditions, as the flowrate , inlet pressure , and outlet pressure may be different for each case. Calculate the Cv required for each , and the largest Cv may determine the size of the valve. It is also possible to oversize the valve body and install a reduced trim. It is also neccesary to define the fluid, max design pressure ( inlet and outlet) , allowable noise levels, connected piping material and size, and design code.

Other information needed from the vendor is max casualty flowrate ( to enable sizing of downstream safety valves), noise levels, stem actuator force ( to size the actuator) ,allowable nozzle loads and Cv vs % open curve ( to define controllability). You might also ask for a list of satisfied customers and spare parts availability.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick