Chemical Processing - December 2007 - (Page 31)

PROCESS PUZZLER < > THIS MONTH’S PUZZLER We have a fixed-roof fiber-glass storage tank. It was originally equipped with a typical weight-type tank pressure relief valve set to open at 2-in. w.c. pressure. After adding in a vapor recovery system to the tank we changed this to an 8-in. w.c. vent. The replacement vent was fabricated by the tank manufacturer. It was installed by cutting out the previous nozzle for the pressure relief valve and making a large riser integral with the tank roof. The replacement vent uses a 15-in.-diameter round riser coming up to a hinged plate. The weight of the plate was set by selecting the right metal thickness to get a weight of 41.6 lb/ft2 or 8 in. w.c. We later ended up changing our vapor recovery system and now the tank could be reset to a 2-in. w.c. relief point again. Unfortunately, we can’t find the original pressure relief valve. When we started a new HAZOP on the tank, our engineering consultants didn’t want anything to do with our current vent valve — they hadn’t seen anything like it before and didn’t know how to evaluate it. They claimed that the documentation on the relief rates and settings was incomplete and, in spite of “not knowing” how to evaluate it, there were obvious errors in the calculations. They only would consider installing a commercially available pressure relief valve for the tank to replace the existing vent. However, that requires taking the tank out of service, cleaning it out and then adding a new nozzle or opening in the tank. We don’t want to do this unless we have to. Do you have experience with a vent like the one now installed or know how to evaluate its opening pressure and flow capacity? The hinge changes the math The hinge holds one edge of the plate down, unless the hinge is designed to shear off. It will take much more pressure to move the plate compared to a plate just resting on the flange. Then as the valve opens the pressure will drop, closing the plate, and reducing the opening, etc. Hydrostatic head must be included. A 10-ft.-tall relief vent full of water would add 5 psi to the pressure inside the tank. Consider a rupture disc. The relief rate depends on many variables. No wonder it is hard to get buy-in from the consultant! Michael Neschleba, senior principal engineer, Polaroid Corp., Waltham, Mass. Learn from mistakes The toughest problem in this calculation is the effect the hinge angle has on the lift (set) pressure. If the hinged plate were horizontal to the tank roof, then the lift pressure would be the www.chemicalprocessing.com http://www.chemicalprocessing.com

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Chemical Processing - December 2007 Contents From the Editor ChemicalProcessing.com Field Notes In Process Energy Saver Compliance Advisor 2007 Vaaler Award - The Quest Ends Take Equipment Diagnostics to a New Level Better Bellows Boosts Blending Process Puzzler Plant InSites Equipment & Services Product Spotlight/Classifieds Ad Index End Point

Chemical Processing - December 2007

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