Blowdown line on a Pressure vessel in a cryo plant 1

[rhylsta]

Hi All,

Please i need assistance again on the subject of blowdown system on pressure vessel. I am involved in an LPG plant project where we have a pressure vessel (cold pressure separator) operating at a pressure greater than 300 psig. The detailed designer has omitted blowdown line/valve on this vessel on the premise that this vessel will not carry any significant volume of hydrocarbon at any time. However, one of our third party QA/QC reviewer insists on adding a blowdown since API RP 520 (section 3.19 - Vapour Depressuring) recommends adding a blowdown system to such vessels as long as the operating pressure is in excess of 250 pounds. Please i need your guidance on this in order to resolve this imbroglio. The designer explained that the referenced section of the API is only mandated for refinery application and for fire scenario.

Please help.

Kind regards,

 

[sshep]

Hey rhylsta,

Your reference must surely be API STD 521, section 5.20 and instead of what you have referenced. It has always been my interpretation that this section is to protect vessels in fires where the wall temperature will cause the vessel to fail prematurely at the design pressure, and we at least give the operator a way to do something to do about it. The control valve in any event sometimes proves useful to avoid lifting the relief valve (nonemergency) during a plant trip when the vessel warms due to ambient conditions.

The arguement that this is not a refinery application is exceedingly weak in my opinion. If your QC/QA says you should have it, then the justification not to follow should be based on insulation thickness, vessel material, yeild strength, consequence of rupture, etc. The installation itself is exceedingly minor, typically a control valve (fail open) around the relief valve.

You should probably read the text yourself. This isn't neccessarily cut and dried, but I doubt you can go wrong by including it unless this is a carbon steel vessel and the LPG could lead to brittle fracture if improperly depressured in a non-emergency.

best wishes,
sshep

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FYI, The introductory paragraph (5.1) to Section 5 states:
The basis for determining individual relieving rates that result from various causes of overpressure is presented in
Clause 5 in the form of general considerations and specific guidelines. Good engineering judgment, rather than
blind adherence to these guidelines, should be followed in each case. The results achieved should be
economically, operationally and mechanically feasible, but in no instance should the safety of a plant or its
personnel be compromised.