Cold Startup MDMT issues
Cold Startup MDMT issues
(OP)
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?
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?





RE: Cold Startup MDMT issues
RE: Cold Startup MDMT issues
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?
RE: Cold Startup MDMT issues
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.
RE: Cold Startup MDMT issues
RE: Cold Startup MDMT issues
RE: Cold Startup MDMT issues
Regards
StoneCold
RE: Cold Startup MDMT issues
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.
RE: Cold Startup MDMT issues
RE: Cold Startup MDMT issues
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.
RE: Cold Startup MDMT issues
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
RE: Cold Startup MDMT issues
RE: Cold Startup MDMT issues