Hi there,

It is possible to convert your Cv to SCFM. You would have to calculate from the Cv backwards to get the correct constant you are using. This is the long way round. Why to you not take your existing flowrate that you calculated the Cv for and convert that to SCFM?

I would suggest though that you try to get hold of ANSI/ISA-75.01.01-2002(IEC 60534-2-1 Mod) Flow equations for sizing control valves.

To calculate the flowrate for you I would need much more data than just the valve Cv. You would need to know the max upstream pressure, the max pressure drop. The upstream line size, the downstream line size. The valve body size, reducer size if any, and then quite a bit of data on the medium. Compresability factor, viscoscity, temperature and that sort of thing.

There is quite a good freeware program out there called convert123 that can help you with most conversions. you can download it at www.pipeflow.co.uk

You might also want to take at the fischer control valve handbook - freeware. Fischer also have a free valve sizing package that you can download - but I only use this to check my calcualted Cv to see whether I might have made a mistake.

Regards,

Craig

In the semiconductor industry we use flow controllers for gas and liquids. I can not offer names or devices except to tell you that looking on line I am confident you cn find one that will provide the .24 you are looking to provide. The control is electrical, can withstand high to low pressures and will provide complete control as the manufacture has already done the work. Look for "semiconductor, gas flow controllers" Good luck

I was just wondering if there was a simple equation to do a quick estimate knowing the supply conditions to the valve as well as pressure drops. I understand all the varying conditions that need to be taken into account though as well. I just went ahead and measured the SCFM under various circumstances to gather the data. Thanks guys.

Hello AcesFull:

This is the site convertion factor is on this site:

http://www.mead-usa.com/reference/faq/valves.aspx

There is a Convertion Factor Table allowing you to:

======================================================

Convert Cv.....into SCFM.......|<<O>>|Convert SCFM into Cv.........

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Example: What is the output inSCFM of a value with a Cv of 0.48 when operated at 100 PSI?
0.48(Cv)/0.0177(Factor) = 27SCFM
To convert SCFM into Cv, simply reverse the process and multiply the SCFM times factor.

I know this is thw answer to your question.

A simple convertion table fromCV to SCFM, and, back to CV.

This will help you.............

Good luck...................

Hi all,

I think that you must be very carefull here. It is very hard to say that for a given Cv you will get X flow.

There are many things that you need to take into account. When sizing any control valve you will need to know quite a few things. First of all you need to know whether you want volumetric flow (as in this case) or mass flow.

You will need to know the upstream and downstream line size and whether there are reducers or not (mechanical data). You will then need a whole lot of process/chemical data to calculate amongst other things pipe friction factors, chocked/non-chocked turbulent flow. Whether the medium is compressible or not etc etc. These data will consist of upstream pressure, pressure drop, viscoscity, compressability, process medium, flow rate required, operating temperature @ flow, operating density, specific heats ratio, vapour/critical pressure etc etc etc.

You will usually be given the above mentioned data with min/norm/max @ flow and the min/norm/max flow requirement. From this you can work out the required Cv as well as other data needed to determine whether you would need to add accessories for noise reduction or to prevent flaashing/cavitation. You will actually calculate three Cv values i.e. min/norm/max so that you have sufficient capacity but also so that you do not oversize that valve. By oversizing the valve you will not use the control to its maximum. Either oversizing or undersizing can damage components of the valve and or the pipes due to excessive friction, cavitation and/or flashing or noise.

For the above mentioned reasons it is very difficuilt to say that your valve with xCv will give you the required flow. The easiest thing to do would be to contact the valve manufacturer and give him the above data and he would be able to tell you the flow that you will get or alternatelty do the calculations yourself or install a flow meter. The problem arises that you have different values for Fd(valve style modifier); Fl(liquid pressure recovery factor) Xt(pressure differential ratio factor). These constants change per valve type and per manufacturer.

If you want a really rough formula for a general approximation you have to take the definition of Cv into account: The Cv is the number of US Gallons of water the valve will pass with a differentail pressure of one psi. This applies to liquid. But you can convert if you know the SG to get a very rough approximation of flow. by using the follwing formula: Q=Cv(dP/SG)^0.5

Where

Q = volumetric flowrate in gpm

Cv = valve flow coefficient

dP = differential pressure accross the valve in psi

SG = specific gravity of the liquid

I must stress that this formula is not going to be spot on accurate but will be able to get you into the ball park. When sizing valves in the industry I work in this is not enough to give the accuracy of control that would be needed. As I stated in a previous post try to get hold of the ANSI/ISA standard (not free) or the fischer control valve handbook (free download). They will give you a much better understanding of the sizing and if you apply the formulas you will get a much more accurate answer.

I can see the need for all that info in a large system. This application however is about as simple as it gets. I have an input supply line connected directly to a filter regulator, which is then connected directly to the solenoid valve(control), and out to an air operated double diaphram pump, roughly 2 feet away connected by 3/8" poly tubing. There is not much here to consider as far as line sizes, pressure drops and all the other things you mentioned. Since it is simple, I was looking for just maybe a general "rule of thumb" type of conversion.

This is a current setup that a customer uses. With the components that are currently being used, the pump only operates at about 1/3rd its capacity. When looking at the performance chart for the pump, the output (in GPM), is determined by the supply pressure(psi) and volume(SCFM) applied. Since I have the information on the current components and I know the solenoid valve is the limiting factor based on calcs and physical testing, I need now to bring in a few sample valves for testing. I believe the Cv rating is one of many specifications to be considereing and if there was a general calculation I could use to match up a valve with the pump supply requirements, it would save me quite a bit of time.

Not sure all that made sense, but that's what I working with.

I have to agree with you, I was under the understanding that actually Cv is derived from empirical data.

Thanks, that's exactly what I was in search of. Unfortunately, I'm not at work right now to compare the calcs to the actual measured values. Hopefully they are close and I can rely on the conversion factor to help me in my search for components. Again , thanks.

Hello AcesFull:

I am not sure if your post was aimed at me.

You say you are not at work. Can you not email using WORD so the links will work? Just a thought.

Take care...............

Yes, my post was directed to you, I just hit the wrong reply button. What I meant is that since you gave me a general conversion table, I can now calculate a theoretical flow in SCFM for a given Cv. I can then compare that to the measured values I tested for in the lab at work, butsince I'm not going to be back at work for a few days, it will have to wait.

Hello AcesFull:

No problems regarding how or what you do at all. But, to work out certain values, as you know, the psi and really as many 'hard facts' as possible will all help confirm your result. And, it will be interesting to try another solenoid, and or check for leaks at the machine, then to the solenoid, then from there on. You should a known drop after the solenoid, and it depends if there is any line damage, as to whether there is a significant drop at the 'trigger end' (For want of a better phrase).

Good luck...............

Hello AcesFull:

No problem with what you say. Everyone needs a break?

It look a pretty useful conversion table. Good luck with it.

And...........................where's my GA!?........... Only joking, OK?

Take care..........