Control Valve Minimum Pressure Rating

[Steve010]

Apart from companies'/owners' standards, is their any technical reason (valve cracking, mechanical integrity ..... etc.) or any obligatory code or standard that prohibits the use of control valve rating 150# for raised face flanged control valves (Globe Style) smaller than 2 inches in oil and gas applications?

 

[LittleInch]

Steve010,

If you're going to double post -

http://www.eng-tips.com/viewthread.cfm?qid=386167

at least reference the previous one in your post.

did you not like the answers you got in that post?

Codes and standards are not normally in the business of prohibiting one size of something that is allowed in a different size.

AFAK there is no technical or code reason behind this, simply economics and practicality

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

 

[Steve010]

Sorry, I was thinking to ask the piping/mechanical engineers as well.
what do you mean by practicality ? I found that most vendors manufacture them (so it seams that nothing inhibits their use).
I am concerned if there may be any mechanical concern on control valves (globe valves) with small sizes, like cracking or mechanical integrity ... etc. in fuel gas applications.

The valve is already present with a flange and body rating 150# (size 1 inch) but i am thinking to bring another one (with 300#) because of the sentence i find in most owner companies' specifications " control valves with sizes below 2" should have a minimum rating of 300#"

Have you ever used one?

 

[BigInch]

Generally I do not use anything (pipe included) less than 2", unless there is a very good reason only a smaller size will work. Vibrations, unaccounted for loadings, accidental impacts, deflection control between supports ... all must be considered. I have worked with numerous company standards that recommend the same, and in addition require XXS wall for any use of 2" pipe. Using less is generally considered false economy in industrial scale operations. If you have a small domestic water system, that's another thing.

 

[Steve010]

No, it is not domestic water system. It is an oil and gas plant (fuel gas blanketing service). The piping is 1". Should i use a 2" valve on a 1" pipe?
I have seen that most companies standards alow the use of control valves' with sizes not smaller than 1".

 

[BigInch]

Low stress situation, not main piping, not blowdown, well protected pipe, little possibility of excess or unknown loads, no vibration, sure, why not. I don't think I would think of using #300.

 

[Steve010]

You mean it is ok to use a 1" valve with flange and body rating 150#?

 

[BigInch]

If you have a good reason to use it, no reason not to, and you don't need more pressure rating than #150. Yes.

 

[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 ?

 

[BigInch]

Vibration, high forces from slugging, or cooling from vapor expansion could be a concern. Design temperature is only a guide from the process engineers. Did they say no slugging, or vibration? Is the relief stack exposed to high winds, seismic loads?

While RPs are fine as general guides, I do not think following their recommendations are mandatory in all circumstances.

 

[Steve010]

I am not a mechanical engineer. But isn't vibration an issue that is solved by good pipe supporting and not by size and increasing rating? Isn't rating only a measure of the pressure/ temperature the valve and pipes can withstand?

 

[BigInch]

Not totally. Larger size and wall thickness usually decreases vibration frequency and amplitude, for example.

 

[Steve010]

Biginch. Thank you very much that's a very good point that never crossed my mind. Yes frequency of vibrations depends on dimensions.
I know how to calculate the cooling effect from depress. Using the jt effect. But how can i decide whether vibrations are going to happen or not ?

 

[BigInch]

That's another science unto itself.
This might help,

http://turbolab.tamu.edu/proc/turboproc/T19/T19119-134.pdf

 

[Steve010]

Also the piping project spec states that globe valves for sizes from 2" to 4" flanged RF with rating 150# for that class. And that globe valves from 1/2" to 2" are socket weld rating 800#. Does that makes the 1" control valve with flanged RF end connections rating 150#, deviate from the piping spec? It seams that piping manual valves vendors manufacture such small sizes with SW 800# as standard. Any concerns?

 

[MickMc]

Steve010,

Although the Piping Spec stipulates globe valves at Class 150 & Class 800 these are manual valves and control valves are usually by Instrument Group and if Class 300 selected Piping will provide Class 300 flanges to mate. There have been previous posts on this topic of minimum Class 300 control valves and the general consensus I believe is that there is no technical reason just manufacturers preference.

 

[Steve010]

Something came across my mind. The valve body thickness does not take into account corrosion allowance. for example when piping specifies a corrosion allowance of 2 mm for the pipes this corrosion allowance is not taken into account by the valve manufacturer.
For Small valve sizes, the body wall thickness is small unlike larger valve sizes. For example as per ASME B16.34 the minimum body wall thickness for 1" valve with rating 150# is 3.9 mm.
Maybe the minimum rating of 300# is specified for small valve sizes is to cover this issue. Any idea ?

 

[BigInch]

For valves you would normally use a corrosion resistant trim. For small lengths inside a valve use of exotic materials is cost effective. Corrosion allowances mess up the mfgrs machining dimension tables.

 

[Steve010]

Yes, the trim is corrosion resistant 316 SS, but the body is carbon steel, so it will loose thickness by time, just like the pipes. Piping include corrosion allowance of 2 mm for the pipes, how is that taken into account for the valve? especially that small valves with rating 150# has a very small body wall thickness. For example as per ASME B16.34 the minimum body wall thickness for 1" valve with rating 150# is 3.9 mm. Does this has anything to do with specifying the small valve sizes with minimum rating 300#?

 

[Steve010]

After the corrosion allowance and according to the piping spec. the 1" pipe schedule is sch.160 which means that the pipe wall thickness is 6.35 mm. According to ASME B16.34 the minimum body wall thickness for 1" valve with rating 150# is 3.9 mm.
This means that the pipe wall thickness is greater than the valve body thickness ???