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By Alton Cox
The first law of physics, conservation of energy, has driven mankind’s pursuit of progress since the dawn of time. Efforts to harness the power of wind, water and fire brought about huge advances in technology. In time, fire and water were combined to bring on a steam-powered industrial revolution. Today, nuclear energy is converted to electricity to power our world.
The second law of physics, increasing disorder, is the reason that the equipment that harnesses, distributes and uses energy requires maintenance to continue operating properly. Machines deteriorate because of friction, vibration, corrosion, etc. Periodic maintenance must be performed to counteract the inevitable deterioration.
“The pressure relief valve (PRV) also is the one piece of equipment we hope never needs to operate.”- Alton Cox
The energy required to operate and maintain manufacturing equipment typically is supplied by boilers or electric motors that power turbines, compressors, pumps and hydraulic systems. Pressurized fluids are capable of performing consistently over an extended period. However, fluids under pressure also pose a threat to property and personnel should an uncontrolled release occur. The catastrophic failure of a pressurized system can be fatal.
That’s why pressurized systems are protected from catastrophe by safety measures including preventive maintenance. The most important piece of equipment in a pressurized system, the pressure relief valve (PRV), is the one piece of hardware that must always be ready to operate properly when needed. However, the PRV also is the one piece of equipment we hope never needs to operate. Because the PRV is the last line of defense against a catastrophic failure of a pressurized system, it must be maintained in “like new” condition if it is to provide the confidence necessary to operate a pressurized system.
The run-to-failure philosophy isn’t an option with a PRV because failure might result in property damage or even loss of life. The minimum acceptable level of PRV maintenance is periodic testing, with internal inspection and repair as dictated by the results of testing.
Troubleshooting PRV problems is the first step in a proper repair. Unless you determine and correct the failure’s root cause, the problem will recur. Repair shops across the country are qualified to perform PRV repair and testing. Working to a nationally recognized standard, PRV service facilities disassemble, clean, inspect, machine or replace parts (as necessary), reassemble, test, seal and apply a repair nameplate for traceability.
Figure 1. Various failure modes. Clockwise from top left: plugged drain, run beyond failure, ruptured bellows, corroded springs and washers, component corrosion, clogged with product.
Troubleshooting begins with the piping configuration, physical orientation and application data such as system pressure, temperature and physical properties of the contained fluid. You’ll need to address installation issues such as stress from vessel and piping expansion and unsupported discharge lines. In addition, improper valve application can result in mechanical damage from elevated temperature, backpressure, material incompatibility or incorrect pressure setting (Figure 1).
Installation problems often result in PRV operational malfunction. Trained and qualified maintenance personnel might recognize and correct such issues. Proper PRV installation is the first step in preventive maintenance. However, an improper installation that causes a PRV failure to shut down the system also causes an unscheduled emergency repair. While installation problems are outside the scope of the typical PRV repair, conscientious PRV repair shops will keep the user informed. Failure to correct installation problems will result in repeated PRV malfunctions, thereby shortening the PRV’s life expectancy and raising the cost of unscheduled downtime, additional repairs, spare parts and possibly fines for fugitive emissions.
After disassembly and cleaning, a trained and qualified PRV technician examines the PRV parts. Such critical inspection is the key to the repair process because the results affect quality as well as economic issues. The inspection includes a comparison of component dimensions to the OEM’s acceptance criteria to determine the disposition of the part — use as-is, remachine in accordance with OEM criteria or replace with OEM parts.
Spring range is compared to OEM charts. Inspection points include the clearance between the adjacent sliding surfaces in the guide area. Valve spindles are checked for concentricity and runout - a bent spindle adversely affects alignment. In some instances, the scope of work the critical inspection suggests makes it uneconomical to repair the valve.
Following the inspection, disc and nozzle seats are reconditioned by machining and lapping (Figure 2) to the proper finish to ensure the seat tightness that reduces fugitive emissions.