Control Valve Cv

Is there a serial number that the original manufacturer might be able to use to look up this information from its "as shipped" records?

I do not think we could do that.

The original manufacturer might be able to advise from a scaled photograph of the components.

It is possible to calculate Cv but it requires knowing the flow as a function or pressure across the valve. For instance, if I want to test a servo valve I would connect the pump to the P port, the A port to the B port and the T port to the tank. Simple so far. I would then increase the pump flow until the pressure drop from P to T is 70 bar. Servo valves are rated at 70 bar. On off valves are usually rated at 10 bar of pressure drop. Now you measure the flow through the valve at the rated pressure drop. The rest is easy because use you solve for Cv in the formula Q=Cv*√(∆p/2). The reason ∆p is divided by two is that the pressure drop is across both the A and B ports and the Cv value is for the flow through each port. Note, the units for Cv are odd and have a square root term. Also, there is usually some control of the oil viscosity when doing the test. I don't have those numbers available. The hard, expensive, part is getting the test stand that can pump the volume of oil to test the bigger valves. Imaging trying to test a 500 lpm valve.

Easier said than done though!!

Hi there,

You would be able to calculate an aproximate valve Cv. You would not be able to calculate it with 100% accuracy.

If you use the ANSI/ISA-75.01.01-2002 (IEC 60534-2-1 Mod) standard - there is quite a bit more data you need.

Fd - Valve style modifier

Fl - Liquid pressure recovery factor

Xt - Pressure differential ratio factor at full rated travel

These values are typical, only the manufacturer would be able to give you the actual values. Without these actual values - your Cv calculation will not be spot on.

Of course there are other things that must be taken into account, pipe friction factor, turbulant flow, Reynolds number, compressibility, sound prediction etc etc.

If you are just looking for the basic/approximate Cv, then the previously mentioned formula should suffice.

I would reccomend getting a copy of the above mentioned standard as it will explain in quite a bit more detail about valve sizing. Another great resource is the Fischer Control valve Handbook. It is a free download and is available at: http://www2.emersonprocess.com/en-us/brands/fisher/Documentation/Pages/Documentation.aspx

Regards,

Craig