Pressure tests can spur successful start-up

March 1, 2007
Push it: if it breaks it probably needed to be replaced, says Dirk Williard, in this month's Field Notes column. Doing so can help improve your processes.

Admiral Hyman Rickover had a simple philosophy: try your damndest to break it! As the father of the nuclear navy, he couldn’t afford to take chances. Rickover was as much interested in why equipment failed as how it operated. This should be the start-up philosophy of all ground-breaking technology. It also is a sound approach for ordinary chemical engineering.

So, use the pressure test mandated by the ASME code as your first chance to verify the working principles of your process. If a gasket leaks, or as I’ve seen many times, a valve stem bubbles air, this could be a sign of problems later on. There are two basic types of pressure tests: component tests and assembly tests.

ANSI standards and API standards, equipment, instruments and piping must be tested at 1.5 times the maximum operating pressure (MOP), according to ASME Pressure Vessel code. This is less than the design pressure. There is often confusion about these terms. Although the MOP frequently is thought to be the maximum pressure for the pipe, e.g., an ANSI flange is rated for 150 psig, abbreviated, 150 lb., this isn’t so. The design pressure is dictated as the highest allowable pressure for the system, i.e., the assembled pipe; as a rule, fittings always have a lower rating than pipe. If you have a bronze valve that is part of this assembly, and it is rated for 125 lb., then that’s your maximum, the design pressure, regardless of flanges rated at 150 lb.

Often the design pressure is set by a plant pipe standard and everything in the assembly must meet that standard. The MOP is a little less confusing. If no standard exists, then the MOP pressure is usually the maximum pressure seen during operation. For example, if a pump operates at 125 ft H2O, then this is often used as convenience as the MOP and the test pressure is 188 ft H2O.

Component testing involves separately checking each spool in the pipeline. In theory, if all of the spools pass pressure testing, then the assembled piping should pass the test. Strictly following ASME Pressure Code, this is dead wrong. For Division 2, lethal service, you always should pressure test the system again once the piping is assembled. This second testing is to assure that spools haven’t been damaged in transit or during assembly and that everything is tight.

The simple truth is that plant testing is often less rigorous. It is quite common to test the spools at code and then test the assembly at the operating pressure. With the exception of lethal service, company standards frequently allow pressure testing of the assembly provided the system is vigorously inspected. This work should involve opening and closing of valves and operating equipment to dynamically test seals. Inspection also permits ample opportunity to evaluate potential process problems like water-hammer and valve performance.

Perhaps the most difficult part of component testing involves spools with delicate instruments and equipment. pH probes, pressure diaphragms, thermometers and pitot tubes comprise a partial list of instruments that are easily damaged during pressure testing. When there is no way to avoid the risk: buy spares and budget accordingly. You should buy equipment that can be easily disassembled so that the pressure seal can be tested without endangering something delicate.

Once a spool has been tested it should be cleaned, or dried to avoid contaminating product or simply for safety. A good shop practice is to cover the spool ends and attach a document tag. Pressure testing is an excellent way to assess the finished product of perhaps years of design work. Take the time to inspect everything carefully. The system may be asked to serve for many years to come.

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