Valve Corrosion Allowance

[Giuss]

Hi to all.
When a piping line specification requires 6mm of corrosion allowance, do you consider necessary to indicate this requirement also for valve description?
Considering that valve's body thickness (according to main standard: API, BS, ASME) is greater than the pipe wall thickness (including 6mm of c.a.)it would seem unuseful.
Anyway sometimes this is required by Client/Licensors in Oil and Gas Downstream Business.
Does anybody know why?
Furthermore, according to ASME B.16.34 (Ed.'96)ANNEX F, it's stated that the minimum wall thickness indicated is 2.54mm more than that calculated according a pressure based formula; which should i say? ASME B.16.34 considers as minimum a C.A. of 2,5 mm, so i could ask for plus 3.5 mm of C.A.?
Besides, if i compare minimum body thickness according to std such as API, BS, etc. they are much more greater than those indicated in ASME B.16.34, so if i specify a gate valve according to API 600 i don't think that requiring 6mm of c.a. on body valve is necessary.
I'd like to receive as many comments and approaches on this subject.
Thank you in advance

Giuss

 

[BValve]

Giuss,

Your statement about overall valve body thickness against standards and calculation is generally right since valve makers take into consideration other parameters such as tools, machining and standard parts to design and built their products.
However, you MUST advise corrosion allowance required by your piping system. Never forget that valves are not simple items to carry the fluid such as pipe, fittings and other bulk items. They have a function and contain moving parts. Some sealing designs/arrangements do not tolerate any corrosion or dimension variation. Also, if corrosion allowance is expected, valve maker shall consider the same in its design and this is on top of body wall thickness calculation.
To make it short, there are two types of sealing arrangements in valves, static sealing and dynamic sealing. Static sealings are easier to handle, while dynamic sealings shall take care of moving part positions combine with corrosion. Important to advise allowances!
In most of cases, response is in higher grade material selection and/or special coating/overlay/plating,...

Finally, since CRA shall be added to minimum wall thickness calculation, 2.5 mm CRA is generaly covered by a standard design due to other parameters, 6 mm is generally above standard design capacities.
Hence, highly risky to hide this information to valve makers!

 

[JimCasey]

The standards that govern valve shell thickness generally have corrosion allowance built in. Some parts of a valve have essentially NO corrosion allowance: as soon as the seating surfaces corrode ANY AMOUNT, the valve will leak. It can be dangerous to throw in a number for corrosion allowance that conflicts with accepted standards. If you ask for an additional 6mm corrosion allowance on a DN16 valve, vendors might decline to bid, or they might charge you for all new patterns/engineering/ special castings which all add up to mucho dinero.

"Anyway sometimes this is required by Client/Licensors in Oil and Gas Downstream Business.
Does anybody know why?"
If I were cynical I would point out that Architect/Engineer firms get paid by the page in their specification. However, there is the thought that they might want to ensure a certain amount of maintenance-free service life.

 

[Giuss]

JimCasey,
i'm a little confused, but i'll agree with you more than with BVValve, i think dynamic sealing deals more with TRIM material than body one, and trim material is usually specified in a more corrosion resistant material.
Anyway i'm not totally sure, probably specifying c.a. is a good precaution of plant designer against valve manufacturer if something goes wrong.

Giuss

 

[Giuss]

Anyway, even if BVValve were totally right,
and i have to specify 6mm of C.A. in valve description,
if i specify a valve according to API 600, in technical bid evaluation, should i verify that the body thickness proposed by manufacturer is 6mm or 3mm or 2,46mm greater than the minimum indicated in the code?
What do you think?

Thank you

Giuss

 

[JimCasey]

Again, if you ask the valve manufacturer to make something totally different from what his expensive tooling is already designed to make, he is either going to decline to bid, or he will charge you for new patterns and engineering time to design and make the patterns.
6mm is a LOT of corrosion. I mentioned seating surfaces before, but gasket surfaces are also prone to corrosion. When a gasket surface corrodes from the inside to the outside it makes people run around excited and use your name in vain. Especially since the stuff that's spraying is so corrosive that it ate its way out of the valve.

Pick a material for 1/2mm corrosion, and it won't be as expensive as paying the valve manufacturer to retool just for you.

 

[terje61]

If you want to protect your valves against corrossion it is much better and easier and cheaper to buy a valve with a more corrossion resistant material than the pipeline. The galvanic principle will make sure that your large amount of pipe with the corrossion allowance will corrode in stead of the high precision controlling mechanism with narrow tolerances that is called a valve.

 

[BillBirch]

Further to Terje61 above, I agree that more corrosion-resistant material should be considered if you feel that 6 mm is needed foe carbon steel. In my experience (upstream), carbon steel piping systems requiring an overall corrosion allowance of 6 mm would usually be complemented with ball valves (carbon steel) with ENP electro-less nickel plating on the body. Typically such high corrosion allowance systems would be drain systems subjected to dead legs of water and the presence of oxygen. Any drain liquid trapped in the valve body cavity would be protected by the ENP (internal) coating.

 

[Giuss]

Terje 61,
i work for a top 5 EPC contractor and i can ensure you that:
''..and cheaper to buy a valve with a more corrossion resistant material...'' is strongly false.
If Client/Licensor or my Process dpt asks for a CS with 6mm of C.A. and i specify SS valves, i'll probably get fired and i'm not totally kidding.
Thank you to everybody, please continue in your advicing.
JimCasey:
''valve shell thickness generally have corrosion allowance built in.'', could you help me to quantify this for most common used STD in refinery envirnoments.
Thank you.

Giuss

 

[JimCasey]

B31.1 governs B16.34, and the specified code thicknesses are based on allowable stresses. The code allowable stresses are much lower than yield. Anything between actual stress in the application and yield stress is essentially sacrificial matrial; or: Corrosion allowance.

Corrosion may happen somewhat uniformly, but as discussed earlier there are places in the valve where a tiny bit of corrosion will cause a problem.
Erosion is also likely if there are any entrained particles (including droplets in a vapor stream) Erosive damage tends to be localized. Then there's the combined effect of corrosion and erosion ( ke-rosion) where the corrosion products flake off easily and are washed away from high velocity areas of the valve.

I still contend that manufacturers will not be willing or inexpensive is you ask them to re-tool to cast a heavier body thickness. If your management is not willing to allow you to consider a more corrosion-resistant material when you obviously have corrosion problems, that scares me. You won't just have valve problems. You will have problems with every component in the system.

 

[Giuss]

JimCasey,
first of all: thank you.
I was exaggerated about willness of saving money of my management, i wrote it just to answer that specifying a more resistant alloy is cheaper;
anyway that's not the only problem:
different rating curves, need to perform bimetallic welds and or a lot of flanged connections beetwen materials with different matrices and therefore strongly different thermal expansions, galvanic corrosion (i'll agree it's just some rust on the boltings but there is), the need to upgrade also trim material (you can't specify austenitic SS valves with 13 Cr trim).
....And try to think if the service contains chloride and you need to pass from a C.S. to a Nickel Alloy........
Honestly when you manage a Lumpsum Turnkey which more than 50000 tons of piping, nobilizing valve material with respect to line one, can't be the solution.

Giuss

 

[bcd]

Hello,

I can confirm that if you ask the valve supplier to provide an additional 6mm for corrosion, they will decline your request to bid or give you a "no thank you" bid. No thank you meaning the price is so high that it will be worth their time to make the valves. Most will not take the time to try and find a reasonable way to comply.

B16.34 tells you how much corrosion allowance is built into the minimum wall thickness tables. However, generally, valve bodies have much thicker walls than the minimum. When cast, the foundries and pattern makers will make the wall thickness as much as it needs to be in order to pour a sound casting. Also holds true for forged and wrought valve bodies. Simply have the valve vendor report a few wall thickness in the thinest areas and confirm for yourself that you will have plenty for corrosion allowance.