Corrosion Allowance for Service Pipe Line

You have to calculate it taking into account the corrosivity of the fluid. Once you have defined the corrosion rate you can calculate the corrosion allowance.

You have to define:

-fluid chemistry, is water present?

-operating conditions: pressure and temperature

-flow velocity

What kind of oil do you have? it contains CO2/H2S?

S

corrosion prevention

Corrosion allowances are by my way of thinking a potentially dangerous myth. They lull you into thinking that you've built a safe pressure containment system when in fact you may not have. If a pit forms, it can penetrate far faster than your corrosion allowance accounts for. Instead of using way more of the wrong material than you need, wouldn't it be cheaper in the long run to specify a material that is immune to attack in the given environment and just use enough to satisfy the strength requirements? General corrosion (which is what a corrosion allowance is intended to combat) is the LEAST likely form of corrosion you will ever see. Pitting is far more likely to be the culprit and corrosion allowances do little for that.

I agree with you 100%, pitting, stress cracking you are not protected from by using a corrosion allowance. A proper selection of a material, proper welding procedures and adequate quality control procedures to monitor the process of fabrication and postweld stress relieve are the most important. I have an experience in manufacturing large high pressure heater coils for heaters to heat a high H2S content natutral gas (up to 48%), and the client specified '0' corrosion allowance as it is of no use but the decsribed above.

Corrosion allowance has its place in cases such as where the loss of the material is caused by an erosion over time or where we have an uniform corrosion only.

Frank J. Kana, P.Eng

Others are correct that corrosion allowance should not be used as a crutch for choosing the wrong material. On the other hand not specifying a corrosion allowance says that the first time you measure any point at the minimum thickness you have reached equipment retirement with no lead time for planning or procurement. It means that the equipment should be removed from service immediately with no alternative. A corrosion allowance at least gives some lead time.

• API 570 "Piping System Repair, Alteration, Re-rating & Pressure Testing" is a very good reference for calculating the remaining life for a pipeline in service. Also ASME B31G "Determining the Remaining Strength of Corroded Pipelines" is also a very good reference.

To do that as per API 570, Section 7 "Inspection Data Evaluation, Analysis, and Recording", you have to get a full history of that pipeline regarding the average of corrosion rates that occurred at last 5 years (Short-Term corrosion rate, ST), in addition to the average corrosion rate occurred along the full period from the moment of installation till the instant moment (Long Term corrosion rate, LT).

The corrosion rate used in equation for calculating the remaining life of that pipeline shall be the max. of LT & ST.

Remaining life = (t_actual - t_required) / Corrosion Rate

• Free download an example demonstrating calculation of remaining life of inservice pipeline: Remaining_life_of_inserivce_Pipeline.

• See the following paragraphs are extracted from API 570:

7.1.2 Newly Installed Piping Systems or Changes in Service. For new piping systems and piping systems for which service conditions are being changed, one of the following methods shall be employed to determine the probable rate of corrosion from which the remaining wall thickness at the time of the next inspection can be estimated:

a. A corrosion rate for a piping circuit may be calculated from data collected by the owner/user on piping systems of similar material in comparable service.

b. If data for the same or similar service are not available, a corrosion rate for a piping circuit may be estimated from the owner/user's experience or from published data on piping systems in comparable service.

c. If the probable corrosion rate cannot be determined by either method listed in item a or item b, the initial thickness measurement determinations shall be made after no more than 3 months of service by using nondestructive thickness measurements of the piping system. Corrosion monitoring devices, such as corrosion coupons or corrosion probes, may be useful in establishing the timing of these thickness measurements. Subsequent measurements shall be made after appropriate intervals until the corrosion rate is established.

7.1.3 Existing Piping Systems. Corrosion rates shall be calculated on either a short-term or a long-term basis. If calculations indicate that an inaccurate rate of corrosion has been assumed, the rate to be used for the next period shall be adjusted to agree with the actual rate found.

Thanks for the explanation, that's interesting.

Usually, corrosion rate is calculated based on previous thickness and actual thickness, but it's hard to get both of the thickness on the same exact location. So can we use the nominal thickness? Is that possible, nominal thickness are less thicker than truly thickness of newly installed piping?

"nominal thickness are less thicker than truly thickness of newly installed piping?"

I would not necessarily say that is a valid statement. Steel mills tend to want to make money. So some of them often roll their stuff on the small side of the tolerance band. as a result the starting thickness may actually start out below nominal, but within the standard tolerance. That said, mills that do that also tend to take other shortcuts too, like fudging or outright forging mtr's and other stuff too. So often let the buyer beware is the phrase of the day.

Sorry for the statement, I just past here and read interesting thing. Sorry, if it funny question. So it's likely happens. But my point of previous statement, if the nominal are less thicker, and we used it to calculate the corrosion rate, so we will get slower corrosion rate than the actual corrosion rate. which means it will trick us?

Sorry, I just learn about corrosion.

Thank for the answer.

Dear Abdel Galala

I am looking for an example of Remaining life of inservice pipeline but the link dont work.

Can you help me.

With Best Regards

Luis Carlos

Abdel, this is all well and good when you are discussing erosion or general corrosion, but again, this is completely meaningless when we are discussing other more common forms of corrosion.