By David Melcher, Dresser Consolidated
Pressure relief valves (PRVs) help ensure the reliability of equipment, help plants maximize output and help control operating costs. They also are crucial safety devices, typically serving as the last line of defense in an overpressure situation, protecting equipment, facilities, products and people.
This article will use real-life case studies to illustrate the key components of an effective maintenance program and some of the tools that can help maintenance teams save time and money.
Proper Training of Plant Personnel
Personnel who do not understand a valve’s function cannot be expected to make it operate consistently at peak performance. Those responsible for plant maintenance and operation must be properly trained and knowledgeable in pressure relief systems and how operational decisions affect a valve’s maintenance cycle.
Personnel at a North American power plant reported a boiler pressure excursion far above the level of the lowest-set safety valve in the line. None of the valves lifted during the overpressure situation. The valve manufacturer’s technicians confirmed the level of the pressure excursion and checked the nameplate sets of the safety valves. They determined a valve problem did exist.
Facility protocols required that in-house maintenance personnel perform all system maintenance activities. As part of the procedures, all valve calibration had been executed using a homemade device consisting of a spring with a hook on one end and a handle on the other end. The valve manufacturer’s technicians found all of the boiler valves set 30 percent to 35 percent higher than factory-set pressure limits. To correct the problem, all safety valves were properly full-pressure actuated with appropriate set pressures. Blowdown adjustments also were made.
Look at the Big Picture
A PRV is a single component of a larger system. Valves, tubing and connections—and the operating system itself—all must be working properly for the PRV to function to spec. As any valve manufacturer or technician will attest, a problem with valve performance may not be caused by the valve: leaks, chatter, galling and valve body degradation can also be caused by problems with other parts of the system.
At another power plant, a safety valve used in a steam application continually chattered. Replacing the valve did not correct the problem; the newly installed valve was damaged and chattered during overpressure situations.
Technicians found the connection pipe between the drum and the safety valve measured approximately 1 meter (3 feet) and was too long to effectively support safety valve operation. The connection pipe inner diameter also was found to measure less than the inner diameter of the valve inlet.
This combination caused the pressure to continually drop and the valve to chatter. The pipes were installed such that any safety valve integrated into the system would have the same problem. The system was reconfigured with correctly-sized inlet piping.
Follow Manufacturer’s Guidelines
Plant maintenance personnel should consult maintenance manuals—and an experienced valve technician—in developing and executing an ongoing maintenance program. This was not done in the case of a power generating facility in North America. Personnel reported a boiler pressure excursion far above the level of the lowest-set safety valve in the line. None of the valves lifted during the overpressure situation.
A valve manufacturer technician disassembled the valves and found the lower adjusting rings tightly screwed down to the bottom of each valve base. This prevented the valves from creating a proper reaction chamber to lift the valves at set point. The adjusting rings, which the plant staff mistakenly believed to be lock nuts, were the primary cause of the problem. A secondary cause was that valve calibration was not done after valve maintenance.
The manufacturer’s technician recalibrated the valves using an on-line testing device and provided additional documentation of factory valve specifications. Safety valves now open when required.
Helpful Tools
In addition to support offered by the valve manufacturer and experienced repair centers, multiple tools can save time and money and help ensure maximum performance, safety and compliance. They include:
- Facility-wide PRV audits, during which all valves are inventoried and their condition assessed
- Valve maintenance and management software for storing, evaluating and retrieving historical PRV field data.
- Software that estimates appropriate maintenance intervals. Among the factors taken into account are boiler type, valve type, operating gap, valve location, the part of the boiler the valve is protecting, whether the boiler has gone through a start-up cycle since the valve was last maintained, stack material, the number of known valve actuations and sonic flow vibration.
- In-situ PRV testing devices that allow assessment of valve performance without the need to halt production and pull the valve from service.
Pressure relief valves are an investment in reliability, efficiency and safety. It only makes sense to protect these assets by making the relatively small investment needed to plan and execute an ongoing maintenance program.
Author: David Melcher is Senior Product Manager, Aftermarket for Dresser Consolidated pressure relief valves and Dresser Masoneilan control valves. He has a Master’s of Business Administration degree from Grand Canyon University and a Bachelor’s of Science degree from the University of Kentucky.