Low temperature issue

[engrom]

Friends,

We've a scenario in one of our gas plants: A106B 14" piping sch 80, WT 0.75" (no PWHT) is installed for design pressure 15MPa. Normal operating 3.5MPa, max pressure the system will see is 10MPa. As per curve B, the min temp it can go is -10degC without impact test. In winter months, the pipe can see temp below -15degC becoz of some J-T effect across the throttling valve. Is this permissible to operate below -10deg without impact test. Can we apply temp reduction factor in Fig 323.2.2B?

 

[BigInch]

As I recall you cannot use the pipe without the Charpy test. The temperature reduction is applied, if you do the testing.

From "BigInch's Extremely simple theory of everything."

 

[SJones]

Sorry BI, this is piping to 31.3! The minimum temperature without Charpy testing ajustment can indeed be applied, but you have to be very certain of the stress level.

Steve Jones
Materials & Corrosion Engineer

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

 

[TGS4]

In addition to the hoop stress (from internal pressure) that is coincident with the lower temperature excursion, you also have to calculate the longitudinal stress, and compare that against the allowable, as well. Not a particularly complicated task.

 

[SNORGY]

I think it is also further complicated by the fact that the intent of 322.2.2 appears to be to calculate the forces and moments developed assuming non-corroded pipe but with the corresponding stresses evaluated in the corroded condition.

Regards,

SNORGY.

 

[BigInch]

B31.3 it is. Sorry.

Stress is not evaluated in the corroded state? Forces and moments would be higher, but resulting stresses would be lower. I'd have to take issue with that procedure.

From "BigInch's Extremely simple theory of everything."

 

[SNORGY]

BigInch...
Maybe I was ambiguous, but what I meant to say is that by my interpretation, you assume strong uncorroded pipe to compute the forces and moments, and then take those forces and moments to evaluate the stress levels in corroded pipe. So thick wall pipe force divided by thin wall pipe area makes for higher stress. By my interpretation of 323.2.2 anyway.

Regards,

SNORGY.

 

[BigInch]

That makes more sense. At least it should be the conservative method.

From "BigInch's Extremely simple theory of everything."