Chemical Processing - July 2008 - (Page 42)

plant insites use Water to Check Vessel integrity Hydrostatic testing offers advantages but requires care Pressure testing is a vital step in equipment maintenance and modification. When everything goes well, we have confidence in equipment integrity. However, a failed pressure test creates safety concerns. We must design our equipment, develop testing procedures and implement safety steps that assume failure can occur. After all, if we knew equipment would pass every time — there wasn’t any possibility of failure — we wouldn’t have to pressure test at all. The most important decision in pressure testing is what to select as the test fluid. Most vessels are tested with either air or water. Sometimes other gases (e.g., nitrogen) or liquids are used but these cases are rare. Air and water have overwhelming benefits in being cheap, readily available and safe to handle. The major safety issue in selecting between air and water is what happens if the vessel fails. Pneumatic energy release is much greater than hydrostatic energy release because air expands much more than water when pressure containment fails. Even so, some plants use air for pressure testing. Water contaminates specific processes. If you put water in a sensitive process and can’t get it out again, water may not be a choice. Water testing is more complex than air testing. You must configure the equipment and piping to allow for correctly filling, holding and draining the water. In spite of the added complexity, I prefer the safer approach of using water whenever possible. Here are some of my guidelines and design criteria for using water for pressure testing. Many of these points are especially important in dealing with tall vessels, such as the 130-ft. tall distillation column depicted in Figure 1. There, the water height adds 56 psi of static pressure on the bottom. • Confirm that water delivery pressure is sufficient to get to the top of the vessel or equipment loop. • Have the capability to vent from all enclosed spaces and loops in the process when filling. • Ensure the system is designed for full vacuum or has means of Figure 1. Tall avoiding pulling too low a presdistillation sure when draining the water. column poses five potential • Allow for hydrostatic head trouble spots along the height of the vessel in for hydrostatic testing. setting design maximum allowable working pressure (MAWP) and test pressures. 42 Water testing is more complex than air testing. • Check that the foundations can bear the load when testing. Depending upon location, this may include verification that the water-filled load is acceptable even during an earthquake. You usually don’t need to consider water-filled load at maximum wind speed. You have plenty of warning of coming hurricanes and wouldn’t be pressure testing during them. • Fill and drain the system at a controlled rate. Be tricky tower Valve trays Valve trays 8’ Dead space 13’-6” Dead space 130’ Head July 2008 chemicalprocessing.com http://chemicalprocessing.com

Table of Contents Feed for the Digital Edition of Chemical Processing - July 2008

Chemical Processing - July 2008 Contents From the Editor ChemicalProcessing.com Field Notes In Process Energy Saver Compliance Advisor Who’s a Big Hit? Succeed with Condensate Control Take Some Basic Steps with pH Measurements Steam Projects Provide Fast Payback Process Puzzler Plant InSites Equipment & Services Product Spotlight/Classifieds/Ad-Lits Ad Index End Point

Chemical Processing - July 2008

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