Cv vs. Travel or Characteristic for Manual Globe Valve?

[KernOily]

Hi guys. Any of you ever see a table of Cv's for manual globe valves vs. travel? I can find the Cv for the valve at 100% open easily enough, but have not thus far found anything for any manual globe valve for Cv vs. travel.

I am loathe to use a Cv for a Fisher et al. control valve as a Cv for a manual valve since I have no idea how the trims compare. That would be the last resort.

Failing a Cv vs. travel data table/curve, I can fake it with the Cv at full travel IF I know the valve characteristic (e.g. equal %, linear, etc.). But I can't find that either for any manual globe valve.

Any help, ideas, suggestions, crass insults, etc. kindly appreciated.

Thanks guys!
Pete

 

[MFJewell]

Why does it need to be a manual globe valve?

This has globe control valves (starts on page 24). I would suspect they would similar or the same.

Link

 

[KernOily]

Thanks for the reply and the link. It needs to be a manual globe valve because it's the manual bypass throttling valve around a control valve.

 

[Compositepro]

https://www.google.com/search?q=gat...ako8LbAhWCzlMKHY7lCSQQsAQIXQ&biw=1446&bih=767

 

[Danlap]

Hi KernOily,

Have done this exercise before, it is unlikely you will get what you want (Cv value vs stem travel).
Since you are not mentioning any brand, so I assume you are or about to use standard industrial globe valve.

Be cautious:
- typical class 150#, 300# and 600# will have this flat shape Plug configuration. Only higher Class 900#, 1500# and so on will have "pointier" Plug

By this design, with small opening / stem travel, surge of flow is to be expected. As per Compositepro google suggestion, IMO the characteristic will be considered as extreme Quick Opening (the inclining line will be closer to y axis, and usually no manufacturer draw this graphic)

- Check API RP615, snip below

By that respect no "industrial valve" manufacturer will guarantee control-ability of bypass valve for stem travel below 20%

Hence, no Cv will be provided or even measured in the first place for the whole range. And only at 100% opening

Even for same sizes, the Cv value can varies, depending on flow character (usually linear or equal percentage), its control-ability range (let say ones more accurate movement between 25%-40%, and the other range does not really matter) and its sensitivity.
See example for hand operated control valve MarkOne by Flowserve. For sure this is not cheap

Would suggest, check the control valve operation range. Let say 35-60%, and then check that 60% Cv value, let say 13. Then size the bypass valve at 100% opening with that Cv value, in this case can be 1" as per link provided by MFJwell. Don't use the 100% Cv value of control valve

Good luck,
Regards,
MR

https://nosuchvalve.com

All valves will last for years, except the ones that were poorly manufactured; are still wrongly operated and or were wrongly selected

http://www.eng-tips.com/faqs.cfm?&rat1=2&fid=376

 

[mk3223]

The global valves from the different Manufacturers may be changed in Cv values and the control characteristics. For the required accuracy of the process control, one can't just depend on turning hand wheel of the manual global valve to have the proper Cv. Instead, it may need to be used with the certain process instrumentation, such level, flow, or pressure indicators while operating these manual global valves.

 

[gerhardl]

Only by very low pressure difference and large opening will a standard globe valve be suitable as a bypass valve around a larger control valve. Cavitation may easily occur if the opening is small e.g. throttling is high.

With use of a regulating valve that are adapted to a wide range, selected for the actual needed pressure difference and flow, you will have a valve that will do the job, have defined regulation curve and stem travel/position indicator.

As I see it, your problem is to get a suitable valve solution, that will last a long time without trouble, in stead of make a guesstimated operation curve for a valve that probably will give you problems. A standard valve will also have a turbulent/undefined flow near closing position and a measurable bit upwards.

The answer 'the valve will only operate for a short time, and we need not select anything but a standard valve' is the wrong answer if you cannot prove that the solution is cavitation free under all circumstances,- and you probably can not.

 

[KernOily]

Guys thanks for the replies. Very much appreciated! Pete