corrosion allowance - piping 31.3, 31.1

[crazypiper]

I'm looking for ways to simplify piping specifications that we are developing.

The traditional corrosion allowance for CS pipe is .0625". This leads to calc wall pipe in sizes 32" to 36" (150# CS). However, we would like to keep everything in this class to a fixed wall. I've noticed that another company has allowed the corrosion allowance to drop to .047" when the calculation is "close" so that they can stick with the thinner wall and no calc wall. This spec is also 150# CS.

Any problems with letting the corrosion allance drop below .0625.? (other than the obvious lesser lifespan of the pipe). Is .0625" CA just a tradition?

 

[SJones]

Look at the name of the parameter: "corrosion allowance", it is not a mandatory element. It is a means of supporting mechanical integrity under conditions where metal loss is expected to occur. If you are expecting metal loss and then reduce the means to support mechanical integrity, the risk of loss of containment will go up. It is up to the end user to ascertain that the increased risk remains acceptable.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[BigInch]

It might be better to establish a criteria for determining what corrosion allowance is necessary and then using it. You must admit it would make little sense to have a corrosion allowance, if the allowance is always going to equal the excess wall thickness after buying some standard schedule pipe. I mean every time you have no excess wall thickness available, then the corrosion allowance is certainly going to be zero, isn't it. If that's the case, why even bother thinking about it.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[BigInch]

BTW, corrosion allowance doesn't mean it isn't required by the codes. If there will be corrosion, the codes require that an appropriate allowance be provided.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[ColonelSanders83]

I routinely see CA of 1/16 for sevices where corrosion is nigh on impossible. (Refrigerant with oil), and then there are instances where no CA is specified for salt water.

The problem with piping specs is that they always try to dumb down engineering to the level where anyone can specify piping, but then no one wants to pay for how expensive things start to get when all is taken into acount (such as CA on large pipes).

The question you should be asking is not "will 0.047 be ok" but rather as BigInch said "what is the right CA for the application". Do you even know where the 1/16 allowance came from? And by the way "thats how we have always done it" is about the most useless answer to that question (or any other). Is that allowance for uniform corrosion over the life of the pipe? Can you review the calcs to see if there is fluff in the number? Is the allowance for local corrosion (pitting)? Is it for errosion due to the medie you are moving? Did it simply come from somones rear 50 years ago (with the stench that implies)and everyone has been using it since?

I would get a firm grasp on why 1/16 is being used and then determine if it can be modified based on calculation in accordance with sound engineering principles.


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