Why are small bore valves (2" and under) usually spec'ed out at a higher ASME pressure class rating? 4

[tenacity]

Why are small bore valves (2" and under) usually spec'ed out at a higher ASME pressure class rating than the other components in the system?

For example, based on the piping design table, for a 600# Pressure Class System, 2" and under valves are spec'ed out to be ASME 1500# pressure class, socket weld ends, whereas 2 1/2" & larger valves are specified to be 600# Class, butt welded ends. The design pressure is 1410 psig and the design temperature is 150 degrees F. It is surprising that the small bore valves are required to be two pressure classes higher (1500# class) in this 600# class system. Does anybody know why? I have seen this as a standard somewhat in the industry (e.g., Bechtel's PDTs and Sargent & Lundy's PDTs), but don't know why? Some engineers are challenging/arguing that the higher pressure class is not required since the 600# small bore valves still meet the pressure and temperature requirements. I know there has to be sound reasons why higher pressure classes are spec'ed out for small bore valves, beyond just pressure and temperature considerations (internal stresses, Quality Control of the process, I don't know, I am just guessing here). Logic is telling that the small bore valves (and components) are likely the weakest links in the system.

(By the way, for this 600# pressure class system, the design pressure is 1410 psig and the design temperature is 150 degrees F. PDT also spec'ed out fittings 2" and smaller to be 6000# rating, socket welded ends).

 

[mk3223]

Small size of forged valves are made per API-602, including pressure ratings of 800# and 1500#. I think that, from the manufacturing point of view, it can be economical to have higher rating valves to cover a wide pressure range of application.

 

[KevinNZ]

6000# or 3000# fittings are spec'ed to match pipe thickness as much to give pressure rating. The pipe wall thickness may have a high allowance for corrosion or robustness and the fittings will also.

 

[moltenmetal]

Never assume that any specification is developed for "sound reasons". They are often just a standardization exercise which then devolves into armour for someone's backside, generally with someone else paying the bill for the wasted money which results.

If you want forged gate globe and check valves, you start at 800#. Bumping up to 1500# makes no sense when 800# are available and adequate for the job, which they are if your design pressure is 600# flange class or below. If you can tolerate cast, you can get below 800# and save some money.

Going to 6000# socket weld fittings is silly when 3000# are available and more than adequate. Someone likely applied an inappropriately large corrosion allowance to small bore pipe, which took the wall thickness requirement up to a high level, and then chose fittings to match that wall thickness. SW fittings have an enormous corrosion allowance already inherent in their construction.

 

[davefitz]

A higher pressure rated valve or fitting on small bore pipe is better able to withstand mechanical forces imposed from the pipeline. Normally pipe systems smaller than 2" NPS are not deliberately "engineered" to prevent expansion forces from exceeding desing limits and are field routed "by eye".

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[GD2]

As davefitz has stated small bore pipings are designed for structural strength than pressure design. It’s an accepted practice in piping design.

GDD
Canada

 

[tenacity]

Thank you mk3223, KevinNZ, moltenmetal, davefitz, and GD2 for taking the time to respond to this thread. I appreciate all the good information. davefitz and GD2, is there an ASME standard that discusses the approach that small bore components (valves, fittings) are more designed for structural strength than pressure design? I feel it is somewhat unwise to deviate from the piping design table and install 600# class valve instead of the 1500# class it calls for. Thanks.

 

[LittleInch]

Why is it done this way?

Because that's what the last project did and we just copied the piping spec.

There is no design code reference.

It all comes down to cost versus risk plus a sprinkling of experience.

At 2" and below, the cost difference for valves, flanges etc is not big, at least in the scheme of a large plant. These valves and flanges are thicker and have a higher mechanical strength. Over time this has proved to be cost efficient because breakage of a small bore valve in service / some operator using it as a hand / foot hold causes a costly shutdown.

At 2" and below the ASME spec for class 900 is actually the same as class 1500, at least for flange sizes, so there is no cost difference between the two, but not sure if this applies to valves.

So, IMHO, there is no requirement to use smaller valves at a higher class than the rest of the system, but you need to look at each location to see if increased structural strength is required at certain locations. That is difficult until you've done the piping design by which point the cost of making changes and having some connections class 600 and others class 1500 starts to become higher than just making them all class 1500....

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

 

[tenacity]

Thank you LittleInch. These valves have continued to fail over a period of time in the four locations. It appears that the pipes they are welded to are overstressing these valves and if the 1500# class valves were indeed used, then the increased structural strength would hopefully keep these valves from continuously leaking over time.

 

[moltenmetal]

If pipe is excessively stressed to the point of making valves fail, making the valves stronger is NOT the right way to solve it.

 

[GD2]

Tenacity,
There could be more reasons for valve failure. Small bore valves with elastomer seals are prone to failure due to welding heat. Check what seal/seats are used and what temp it can stand. Many customers project standard specifies small bore valves to weld first then assemble the trim. Many don’t care and leave it to the construction folks who find it economical and easy to weld the valves without disassembly.
GDD
Canada

 

[moltenmetal]

These are 800# gates, globes and checks- if you're melting seals out of them, you're doing something wrong.

 

[tenacity]

Thanks again 'moltenmetal' and 'GD2' are your comments. They are very much appreciated. The stresses in the pipes that are welded to the valves were checked and to our surprise, the stresses were low. The balls valves have polymer seat (PEEK) material. So, the following consensus has been reached:
- Three element ball valve design (two flanges and one center body) may not have been the best choice in the past due to possible (perhaps due to applied loads or moments transmitted through the valve body.
-PEEK material has been know to lose its elastic rebound over time; also this condition has a potential of having debris in the closing path of the ball orifice. (the system is a slurry like system with suspended solids).
-The replacement ball valves will be a single piece body style.
-The replacement ball valves will have metal seats.
- Possibly also looking at the way the system is operated and flushed of debris.

Based on this, perhaps a 600# class valve with the right style is the answer, even though the PDT called for a 1500# class valve. Thanks again.

 

[moltenmetal]

OK, we finally have enough information to help you!

These are BALL VALVES. Small ball valves generally do not have # ratings (a class rating related in archaic terms to steam), but rather WOG pressure ratings for their bodies (WOG standing for "water, oil or gas" and the rating being a room temperature maximum pressure in psig). The actual P/T ratings of the valves in service depend on what seats and body seals and packing are chosen.

Ball valves need a means to be maintained, which requires a means of disassembly. There is really no such thing as a "1-pc body"- what you mean is that the valve will be either end entry (reduced port if it is socket welding) or top entry, i.e. with a substantial top body flange and seal.

Whatever problem you're encountering, it is doubtful- extremely so in my opinion- that it was caused by the use of a 3 pc ball valve design. That's the STANDARD design for a field-maintainable ball valve 2" NPS and below which has welded end connections, since welding requires disassembly. The 3 pc design gives the best access to permit maintenance, which is going to still be required (to replace seats or other parts when the valve starts to leak)even IF you were to choose metal seats.

Metal seated valves are generally not bidirectional sealing, and they also generally are only class IV shutoff- they leak when closed, even when new. They also tend to be very expensive, hence are used only where the metal seats are essential. Even if the vendor claims a higher shut-off class, after a few cycles it will be a lot worse than it was once the lubrication is gone. Resilient seats (PEEK is at least somewhat resilient) give tighter sealing as long as the conditions are suitable for their mechanical survival. Depending on the nature of your slurry, metal seats might be the right tool for the job, or not.

Best of luck.