Piping Brittle Fracture Risk During Blowdown 1

121202 · Feb 25, 2005

When plant containing pressurised gases and liquids is blowndown (depressurised), the depressurisation can cause piping and vessels to become chilled. The temperature within the plant drops as the pressure drops and the minimum vessel and piping temperatures coincide with the minimum internal pressure. I am of the opinion that the risk of brittle fracture during blowdown is low, due to the fact that stress levels are low when minimum temperatures are experienced. Despite the fact that it is common practice to set the minimum design temperature of plants based minimum blowdown temperatures I believe that this is overly conservative. I'd be interested in the opinions of experienced engineers on this matter. Is there any provision in piping design codes (B31.3) that allows for this situation?

jcfoley · Feb 25, 2005

You might want to check out B31.3 302.2.4 "Allowances for Pressure and Temperature Variations." This reference is from the 1999 Edition (should be the same reference number in the newer one) but it may help you out.

jsummerfield · Feb 25, 2005

Process simulations predict the outlet temperature for the gas downstream of the blowdown restriction (choke, orifice, etc.). If that temperature is predicted below -20 degree F then a spec change is required. Typically, only isolated applications require the low-temperature treatmement. If few lines are below the -20 F case, then the specification can switch back to the normal process spec for the flare header and branch connections. Thus, only a limited distance downstream of the choke would require cold service specification. A cold flare header is common in applications such as an olefins unit where cryogenic streams require blowdown.

John

4Pipes · Feb 25, 2005

The pd/2t and simple bending stresses could be low. However, unless you have stress relieved you could still have very high and unknown weld stresses. Also temp differential stresses across welded shoes and attachments could be high because of the time lag of temp equalisation.

SJones · Feb 25, 2005

This is a common "clash" between process engineers and materials engineers (and operations engineers who have to deal with cold restart issues). The usual rule applied to the stress level to set the minimum design temperature, as opposed to the minimum metal temperature, is 50 MPa membrane stress level including wind loading etc. This level is related to the Robertson plateau (haven't found exactly what the definition of this phenomemon is yet). I am only aware of its official application in Australian pipeline standards and Shell standards. The principal thing to watch out for is cold restart after blowdown as the plant could be suddenly pressured back up whilst cold.

Steve Jones
Petroleum Development Oman LLC
Muscat
Sultanate Of Oman

bvi · Feb 27, 2005

121202 - Yes, there is a provision in ASME B31.3 that allows for that situation. You can actually take carbon steel down to as low as -155F if the stress is sufficiently low. Note that you must not only consider the hoop stress due to pressure, but also the longitudinal stress do to sustained loads such as weight and pressure and thermal expansion (contraction). The provision is tied into the impact test exemption rules. You can actually follow the depresurization curve, the metal temperature gets lower, but the allowable minimum temperature also gets lower as the stress drops.

Scipio · Feb 27, 2005

Something else to bear in mind, in addition to all the very valid concerns mentioned where residual stress can come from. You might be able to explain carbon steel at low temperatures not having any stress because they've been depressurized, but if you get someone who doesn't know any better start knocking frost off with a wrench (which I've seen happen while starting up natural gas wells downstream of the first choke valve), you could be in for a nasty surprise while repressurizing. You can't cover it with just stress analysis, you also have to make sure operation procedures are in place for safeguards.

bvi · Mar 5, 2005

The requirement for safeguards against loads, such as maintenance, mentioned by Scipio is part of the ASME B31.3 rules when the low-stress exemption from impact testing rules are used.

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Piping Brittle Fracture Risk During Blowdown 1

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