Chemical Processing - October 2007 - (Page 54)

>> PROCESS PUZZLER Keep a float from sinking operation Readers suggest how to achieve reliable level measurement >> THIS MONTH’S PUZZLER A spray condenser tank uses a float as a level instrument. Metal chlorides settle in the tank and are eventually pumped to waste treatment. Unfortunately, solids build up, especially during start-up, causing failure of the float. Last year alone, eight floats were replaced. Two months ago, two floats were replaced during a single outage, causing more than 30 hours of unexpected downtime. The current instrument is looppowered. There are no additional points left in the PLC for self-powered instruments. Can you propose alternatives for measuring level? >> Vent valve Hinge Plate 23 in. long (Free opening) Tank roof 15 in. Avoid precipitation A small amount of condensate flush will help keep solids from precipitating into the still well. The small amount will help to keep the flow outward from the still well (or float column) and the condensate also will dilute the concentration of the liquids that do get into the float column. Bobby J. Hatfield Jr., cat/alky training analyst, ConocoPhillips Co., Sweeny, Texas Make a sound move You could try a sonar transmitter. This could give you a non-contact method of determining level. If you have no room left for a four-wire transmitter, you can use a converter to change the four-wire signal to a loop-powered twowire input and utilize the point that you are replacing! Steven Dow, Yuba City, Calif. Measure pressure I think that I have a very simple answer for your question. You can place a pressure indicator at the lower region of the tank so it senses the liquid level head in the tank. You can calibrate the liquid head (pressure) reading relative to the tank level, when the pressure head varies it is an indication for the liquid level variation. Amr Hatem Rashed, compressor engineer, Abu Qir Fertilizers Co., Alexandria, Egypt Use non-contact technology There isn’t enough information about the installation and available nozzles, but at a guess use radar or sonic. Bob Driskell, PE, project manager, WorleyParsons,Texas City, Texas Figure 1. Unusual design complicates the evaluation of key parameters. >> DECEMBER’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 (Figure 1) 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 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. 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? Send us your comments, suggestions or solutions for this question by November 5. We’ll include as many of them as possible in the December issue and all on CP.com. Send visuals — a sketch is fine. E-mail us at ProcessPuzzler@putman.net or mail to ProcessPuzzler, Chemical Processing, 555 W. Pierce Road, Suite 301, Itasca, IL 60143. Fax: (630) 467-1120. Please include your name, title, location and company affiliation in the response. www.chemicalprocessing.com 54 • October 2007 http://CP.com http://www.chemicalprocessing.com

Table of Contents Feed for the Digital Edition of Chemical Processing - October 2007

Chemical Processing - October 2007 Contents From the Editor ChemicalProcessing.com Field Notes In Process Energy Saver Compliance Advisor Biofeedstocks See Real Growth Become a Cyber-Security Pacesetter Go Beyond Condition Monitoring Disposable Equipment Earns Lasting Role Improve Control Loop Performance Ethanol Plant Boosts Output and Saves Energy Process Puzzler Plant InSites Equipment & Services Ad Lits Product Spotlight/Classifieds Ad Index End Point

Chemical Processing - October 2007

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