B31.1 piping overpressure

[prq123]

A steam desuperheater on our project is designed to B31.1 piping code. B31.1 allows a short term overpressure up to 20%. However, according to the vendor of the desuperheater the B31.3 short term overpressure allowance of up to 33% can be utilized, even though the desuperheater is designed to B31.1. Is it acceptable to use the B31.3 overpressure allowance for an item designed B31.1? The obvious answer is "No" as these are different codes, but wondering if anyone has a different interpetation of the codes.

Thanks.

 

[metengr]

I reviewed the 2004 Edition of B31.1. The specific reference applies to occasional variation from normal operation. In this case you need to review, part 102.2.4. My interpretation of this section is that the abnormal allowable stress does not exceed 15% of the increase in normal operating temperature related to a coincidental allowable stress. You need to back calculate the percent over-pressure event due to either the 15% or 20% increase in allowable stress, depending on your design assumptions for abnormal events.

 

[davefitz]

If the purchase order or contract specification used to buy the desuperheater stated B31.1, then the vendor can only use B31.1 allowable overpressures. B31.3 is irrelevant in that case.

In particular ,for components operating at low temperatures where UTS or YS define the allowable stress, there might not be sufficient safety margin in the B31.1 allowable stress values to permit a 33% sustained overpressure- the allowable stresses were recently increased to be more competitive with european codes, and the 33% overpressure is equivalent to a continuous hydrotest for low temperature components.

Also, the valve design codes only allow a 10% overpressure, and that is only permitted during safety valve lift events. Which birngs up another good point- what exactly are teh safety releif valves doing during this 33% overpressure event?

 

[prq123]

davefitz,
Yes, the purchase order/datasheet for the item states that it is to be designed to B31.1 (there is no mention of B31.3).

The reason for using the short term overpressure is that the water side of the desuperheater has a higher (~30% higher) design pressure than the steam side. There is no safety relief valve protection on the steam side. There is a possibility that the steam side may be blocked in and see the water side design pressure.

 

[stanier]

Do not mix codes. The product is a result of design, fabrication, installation and commissioning to a code. This includes levels of NDE and welder qualifications. You cant cherry pick to suit your problem.

Geoffrey D Stone FIMechE C.Eng;FIEust CP Eng

http://www.waterhammer.bigblog.com.au