Control Valves' Pressure Rating 1

[Steve010]

Is there a technical reason why most companies specify a minimum 300 rating for control valves below certain sizes ?

Is there any code that prohibits the use of 150 rating for 1 inch control valves ?

 

[LittleInch]

Not heard of that one before, but nothing surprises me. I can only think that the extra meat in the flange and body is more important when you're winding it open or closed a lot, but AFAIK, there is no code requirement and it's not something I've come across.

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

 

[pennpiper]

Steve,
There was no Code involvement in this issue at all.
You wrote:"why [do] most companies specify a minimum 300 rating"

There are Companies who are Control Valve "makers", there are Companies who are Control Valve "specifiers" and there are Companies who are Control Valve "users".

Now which of these are you talking about?

Based on my experience, the CV "makers" saw little potential demand for Class 150 Control Valves. Therefore they did not want to make a wide range of sizes and then be stuck with them gathering dust on the shelf.
So they made their starting point the Class 300 valve and the "specifiers" and "users" adjusted as required. It has worked just fine for hundreds of years.



Sometimes its possible to do all the right things and still get bad results

 

[Steve010]

I mean engineering companies that purchase control valves for using them in oil and gas plants. Most of them state in their control valves' specs that the minimum rating for size below 6" (some state 2") shall be 300#, and that for larger sizes 150# is accepted. I found many control valves' manufacturers that make small valve sizes with 150#. So is there anything that i should worry about for using a 1" control valve (for continueous throttling or on/off snap action control), provided that the 150# covers my design press. and temp. ? Any worries?

 

[Steve010]

Did anybody use such small control valve sizes with rating 150# ?

 

[MickMc]

Steve010,

I think pennpiper has nailed it and there is no code issue I am aware and like a lot of things logic is buried in the past and if you want Class 150 you will get it if you pay the premium as the makers don't tend to produce. I have not seen Class 150 used in smaller sizes only large bore and even then only occasionally but there is no reason not to use as long as actuator can be fitted.

 

[Steve010]

Why did API Recommended Practice 553 (Refinery Control Valves) state that valves for vapor depressurizing should be minimum 2" rating 300# for mechanical integrity?
isn't the rating determined solely by the design pressure and temperature ?
What is the relation between size and rating and such service ?

 

[davefitz]

I think steve010 is probably correct. There are other mechanical ( piping, actuator, valve wrench) loads imposed on the valves besides fluid pressure. In particular, may specs for piping systems do not require engineered piping drawings for line sizes below ( 3 " NPS) so the field run piping can impose high loads for those instances where the piping was run "by eye".

"Whom the gods would destroy, they first make mad "

 

[1gibson]

Are the 150# valves available "off the shelf" and are they significantly cheaper than 300#? If so, will both of those things remain true in the future?

 

[Piper1962]

A wise old piping owl once explained to me that the reason for the selection of 300# (minimum)rated flanges at smaller sizes on control valves (and PSV's) was that almost invariably the piping reduces down a size or two at such valves. The combination of large bore (stiff) piping and small low rated 150# flanges makes the flange prone to leakage due mechanical loadings transmitted into the flange from the stiff and heavy connecting piping. It also makes the flange difficult to get through piping stress analysis for the same reason. Hence the practice of 'beefing up' the flanges on such valves.