different Cv definition

[sini4s]

What is difference between Nominated Cv, rated Cv & Calculated Cv.

My understanding goes like this:-
Nominated Cv:The valve CV
Calculated Cv: the Cv based on the process parameters calculation

 

[bcd]

Hello,

There is only one Cv. It is the flow coefficient as defined in the ISA standard.

 

[TD2K]

I haven't seen nominated Cv used so I can't help you on that.

Rated Cv to me is likely to be the valve's full open Cv. Calculated Cv is the Cv (or Cvs) for the various cases used to size the valve. That can be minimum Cv requirement, normal Cv and maximum Cv. I like to define/consider all three cases to allow the instrumentation engineer to understand the full range of the operating requirements.

However, those terms may be dependent on the company in question. I would try to look at their use (say on a valve's data sheet) and see if their meaning can be inferred.

 

[Drexl]

Yes there seem to be some different ideas on how to complicate this. I would agree that the calculated Kv/Cv is what you calculate based on the process given as a min/norm/max. The actual value for the valve fully open is given as Kvs.

 

[psafety]

No Kv is 86.5% of Cv, 100% Cv is the maximum the valve will pass, we used to size for valve operation around 70%, now the norm is more like 40% requiring much larger safeties (if) the control valve is the limiting basis.

 

[gerhardl]

Seems that none of the above have found any standardized references to the different 'additional' Cv's, and somewhat different opinions are aired. Any technical reference to be used as measurements needs a clear and concise mathematical definition, with cross-reference to the definition for common use. I can understand that sini4 is searching for this.

As known Kv and Cv are defined as the flow through the valve per time unit with fully open valve at a given pressure drop at with all figure units exact specified and dimensioned, European units (Kv) and US units (Cv) respectivly. Hence the difference between Cv and Kv if you have to convert the one to the other for the same valve. (The capacity or 'flow-through' is of course the same, just defined in other units). Ergo: exactly as bcd has stated.

Now, the question about flow through for other openings: yes, both the necessity and definitons for this exists. I believe I have, however, only seen this as defined from manufacturors manuals(?). Can anyone help sini4 (and myself) with clarification for this?

I am not sure theese other definition exists in any standards, but as mentioned above: when to be used clear cross-references or full explanation for definition and measurement control must be referred to.

 

[4bee]

sini4s, gerhardl,

Acc. to IEC 60534-2-4:
Flow coefficient: A basic coefficient used to state the flow capacity of a control valve under specified conditions.
Rated flow coefficient: the value of the flow coefficient at the rated travel.
Rated travel: the displacement of the closure member from the closed position to the designated full open position.

Thus, rated Cv is Cv attained at full open position.

In my opinion, calculated Cv is a Cv value calculated at particular conditions (P1, P2, flow rate, specific gravity etc.). Conditions can give you min Cv, max Cv or a Cv value somewhere between those two. So when selecting a valve min and max Cv are of importance.

As regards nominated Cv: I have never heard of such term/definition, however I expect this to be a definition widely used in some local group of specialists (in particular country or company).

Regards.