Cold Startup MDMT issues

[Roland03]

I'm working on a wellsite, and I'm wondering if upon cold startup of the site, there will be an issues with MDMT of the separator piping or vessels.

For example, lets say it is -40oC outside, and so the gas in the piping from the wellhead to the separators is now at -40oC. I have a separator package in a heated building, and the piping and vessel is reg temp (-29C design). On starting things up, the volume of gas in the piping will travel to the separator at -40oC therefore cooling down the metal in the separator package.

Intuitively it would seem that the heat capacity of the gas is very small and so the metal will not cool down significantly. However, at the same time, the metal will have a temperature profile, so although the metal in the piping/vessels may not decrease below -29C, the wetted metal exposed to the gas may.

I'm wondering, when considering MDMT and brittle fracture, does one need to consider localized temperature? Or just the average temperature of the metal?

 

[metengr]

You should base your design on the lowest temperature exposure.

 

[Roland03]

I know that normally that would be the case, but it is alot of extra expense to make all of the materials in the separator packages low temp materials, particularly when they are already in a heated building.

Is it as simple as that? That if the metal is going to be exposed to a fluid at -50 for even a short period of time you need to design to have an design metal temperature of less than -50? I agree that it is a safe approach, but considering the additional cost, I'm looking for a more scientific answer.

Is what you are saying that the localized effects right at the metal cannot be neglected in terms of brittle fracture mechanisms?

 

[metengr]

Roland03;
I understand your concern. The other option is to have a risk evaluation for brittle fracture performed by an engineering organization. You need an engineering evaluation to confirm safe design because what I recommended above is what should have been done.

 

[metengr]

Look at API/ASME FFS-1 (Fitness for Service) regarding risk of brittle fracture.

 

[Roland03]

Thanks. I agree that some engineering calculations need to be done, specifically related to the heat transfer rates between the exposed metal and the gas. They key point here I believe is that the exposure to cold fluid is for a very limited time frame. In that sense, I don't think fitness for service in brittle fracture is relevant because the time frame is very different. The fundamental question I believe is for brittle fracture, is it the localized temperature that matters? or the average material temperature? If it is the local temperature then guaranteed it will be required to use low temp materials, but if it is the average temperature then that is a calculation that I believe we can do.

 

[StoneCold]

I am not saying this is your answer, but we have low temperature equipment that is de-rated for lower temperatures. So you could rate your equipment for a lower pressure at startup until you get warm gas coming in and you can ramp up to full pressure, if the gas is going to be cold and stay cold after starup you need to have the lower MMDT.

Regards
StoneCold

 

[SJones]

...because the time frame is very different

If you are undertaking a risk assessment, what time frame do you think brittle fracture can occur within? That degradation mechanism is not considered to be a time dependent one. By all means, you can get smart with the heat transfer calculations, but it is the flaw size and membrane stresses that will be the drivers. Remember, flaws of critical size may not be present at first commissioning, but several years down the road, they may well have been established by other mechanisms. The cold fluid may then reach further into the wall cross section than your heat transfer calculations show from a pristine surface, so 'average temperature' may not apply.

http://www.eng-tips.com/viewthread.cfm?qid=116637

T.S. Robertson: “Propagation of brittle fracture in steel”, J. Iron Steel Inst., v. 175, (1953), pp.
361-74

Steve Jones
Corrosion Management Consultant

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

All answers are personal opinions only and are in no way connected with any employer.

 

[Roland03]

Thanks for the explanation SJones, this makes sense. Then, what StoneCold mentioned (a proper starup procedure) should be a good approach to mitigating this?

 

[SJones]

It is one way of addressing the situation using the Critical Exposure Temperature approach detailed in API 579-1/ASME FFS-1.

Steve Jones
Corrosion Management Consultant

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

All answers are personal opinions only and are in no way connected with any employer.

 

[StoneCold]

Roland3
Before you sink a lot of time in the de-rating method. Know that to get from -29C to -40C you are going to have to derate at least 30%. (That is based on some tanks we have but the operating presures are low (150 psig at -29C and 109 psig at -40C)

Regards
Stonecold

 

[Roland03]

Thanks Stonecold, appreciate the advice. I think I am going to end up going down this route as we do have ample pressure rating over what we need to have by code. I believe this should be a good solution to the problem.

 

[macmet]

I know nothing about wellsites or the design of systems like the one you're describing, but is it possible to heat up the gas on start-up? In my industry we would look into something like that, but just because it works in one case doesn't mean it will work in all. Thought I'd throw it out there anyway.