- Published on Sunday, 19 October 2008 19:55
- Written by David Melcher
A memorable story needs an attention-getting beginning, a compelling middle and a strong ending. Shortchange any of the three, and the audience is bound to be disappointed. Similarly, a pressure relief system needs good design, proper installation and a systematic program of ongoing maintenance. Fail to give any of the three its due, and you put products, facilities and people at risk.
Poorly performing pressure relief valves (PRVs) can result in loss of production, loss of revenue, higher operating and maintenance costs and, potentially, catastrophic failure. Our experience shows that between 75% and 85% of PRV performance issues—including leaks, chattering (rapid opening and closing), failure to open at designated set pressure, long blowdown (closing too far below set pressure) and short uptime between repairs—can be attributed to improper system design, incorrect valve installation, poor maintenance practices, or a combination of these factors.
It also is important to remember that 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. And as any valve manufacturer or valve 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 elements of the system.
This article will use real-world case studies to illustrate some of these common problems and why care must be taken at all stages of a pressure relief system’s lifecycle—design, installation and ongoing maintenance.
PHASE ONE: SYSTEM DESIGN
When designing a pressure relief system, engineers must consider all possible service conditions and select the most appropriate PRV to ensure safe operation. An incorrect design can compromise the expected operation and lifespan of all components and increase installation and maintenance costs. In short, if the design is wrong in the beginning, there can be continual performance problems in the future, including turbulence conditions, wandering set pressure issues, overpressure situations and even explosions.
Design Causes the Problem: The Numbers Did Not Add Up
There must be a gap between a PRV’s set pressure and the system’s normal operating pressure to ensure the valve will not leak at the system’s maximum operating pressure. The typical gap requirement for a safety relief valve is 10%.
In the case of a power plant in South America, changes made to the overall plant design were not accounted for in the pressure relief system design. As a result, the boiler engineer’s math no longer “added up.” During pre-startup testing, plant operators discovered the gap between the valves’ set pressure and the system’s normal operating pressure was not sufficient to enable safe operation of the safety relief valves.
The boiler economizer system included a feed pump, the economizer, a control valve and two relief valves. During a review after system installation, engineers discovered that relief valve #2, located at the economizer outlet, would be open at all times during normal operation. Additionally, relief valve #1, located at the inlet of the economizer, could simmer during normal operation, a condition in which the valve disc flutters when it nears set pressure.
The feed pump was designed to have a normal operating pressure of 483 psig. Relief valve #1 had a set pressure of 505 psig, providing less than the 10% operating gap recommended for safety relief valves.
Relief valve #2 had a set pressure of 305 psig. After media flowed through the economizer, the system pressure dropped from 483 psig to 445 psig. Therefore, relief valve #2 was set below the normal operating pressure and would be open at all times.
Further investigation revealed that relief valve #2 had an isolating valve installed on its inlet pipe. This could cause a pressure drop with media flow and cause the valve to chatter, an unsafe condition characterized by the continuous, rapid opening and closing of a valve.
Adjusting relief valve #1 at the pump for appropriate operation was relatively simple. The set pressure was raised to the maximum allowable working pressure (MAWP) for the economizer, 632 psig. (ASME code requires relief valves to be set at or below MAWP.) This increased the operating pressure gap to 24%, meeting the 10% minimum gap standard.
Adjustment of relief valve #2 at the economizer outlet was more difficult. Engineers designed the system to operate at 445 psig, but the set pressure could not be above 459 psig. Changing the relief valve’s set pressure to 459 psig might have been an easy fix, but it would not solve the 10% gap requirement or eliminate chatter caused by the use of the isolating valve.
The solution was replacing the safety relief valve with a pilot-operated safety relief valve (POSRV). A POSRV is designed to operate with gaps as low as 2%. The difference between 459 psig and 445 psig is approximately 3%, so the addition of the POSRV safely achieved the gap requirement. At the same time, the potential for chatter was eliminated by moving the POSRV’s pressure sensing line from the valve’s inlet to a point upstream of the valve inlet pipe—a technique known as remote sensing.
PHASE TWO: INSTALLATION
The best-designed pressure relief systems can be totally undone if not installed correctly. Unfortunately, installation mistakes occur far too frequently.
It also is important to remember that pressure relief systems are not isolated “islands” within a manufacturing facility. They must coexist with the other manufacturing systems surrounding them, while providing protection to a discrete process.
To minimize installation errors, it is essential to maintain up-to-date pressure system plans as well as comprehensive valve data. At the same time, any modifications to the original plans should be noted and maintained as “as built” drawings for future reference.
Installation Causes the Problem: 50 Gallons Don’t Fit in a Five-Gallon Container
Installers at a steam utility/electric generating plant in Europe deviated from the original design and failed to look at the “big picture” of the entire system. As a result, what was intended as a time- and cost-saving measure put the facility and the employees at risk.
A safety valve system was designed to vent steam from the generator through an outlet stack. However, plant operators reported that large amounts of water were being expelled from the top of one safety valve and the outlet drip pan.
Valve technicians determined the leak was not caused by condensation or by rainwater flushing back through the venting system. The volume of water being expelled from the valve indicated it was coming from another source.
As with many PRVs, the valve had a body bowl drain to remove water created by condensation during normal operation. The valve technicians determined that water was entering the valve via the body bowl drain. By tracing the drain piping, they discovered a secondary drain system had been connected to the pressure relief system. The drain for the pressure relief system was too small to accommodate the additional water from the secondary system. As a result, the excess water backed up through the safety valve and spilled out over the top.
System schematics revealed plans for the system as it was designed, not as it was built. The secondary drainage system was not detailed in the plans, masking the probable cause of the leak from the utility company. Had the leaking valve opened during an overpressure situation, high-pressure water droplets could have damaged the valve and injured nearby employees.
The drain system was re-routed and a larger drain was installed. The water no longer backs up through the PRV, enabling it to protect the steam generating system as designed.
PHASE THREE: MAINTENANCE
Depending on a valve’s service conditions, the initial purchase price may represent only about 20% of the valve’s cradle-to-grave cost. (1) If maintained properly, a PRV can stay in service for up to 30 years. These facts make proper maintenance of a network of PRVs, and the equipment around the valves, critical to the safe operation of any plant.
Personnel responsible for maintenance must be properly trained and knowledgeable in pressure relief systems. Many times, a malfunctioning valve is a symptom of a larger problem within a pressure relief system. An untrained employee may focus only on system components, while a more experienced technician can look at the bigger picture, recognize the symptoms and work toward understanding the root cause of the problem.
During a suspected valve malfunction, plant personnel should never feel as if they are facing a crisis alone. In addition to support offered by the valve manufacturer and experienced repair centers, there are tools that can save time and money and help ensure maximum valve performance, safety and compliance while the team diagnoses the problem. These tools include valve maintenance and management software for storing, evaluating and retrieving historical PRV field data, and in-situ PRV testing devices that allow on-line and in-service assessment of valve performance without the need to halt production and pull the valve from service.
Maintenance Practices Cause the Problem: Hand-me-down Training is Not Always Best
It is important that all personnel who maintain pressurized systems receive adequate training. At a power plant in North America, the maintenance team relied on “hand-me-down”-style maintenance training, leaving the plant vulnerable in overpressure situations.
Personnel at the power generating facility reported they had 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 that a valve problem did exist.
Protocols at the facility required that in-house maintenance personnel perform all maintenance activities on the system. For 25 years, maintenance training for the staff had consisted of passing along information and experience from one person to another.
As part of the plant’s established maintenance 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 hook was connected on the end of the lifting levers of the valves. By pulling the handles with the springs attached, the valve began to release. Using a modified yardstick, the length of spring stretch was measured and used as a means to set pressure in the safety valves.
As a result, the valve manufacturer’s technicians found that all of the valves on the boilers were set 30% to 35% 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 to all valves. The pressure relief system for all four boilers now meets the needs of the system.
These case studies clearly show how the improper design, installation and maintenance of safety valves within pressure relief systems can lead to system malfunctions and hazardous conditions that put equipment, facilities and personnel at risk. When combined, mistakes and compromises made at various stages in a PRV’s lifecycle can add up with disastrous results.
By following three simple principles, plant personnel can avoid this situation. First, design work must be done diligently. It is essential to consider not only the pressure relieving devices or safety-related systems, but also the complete pressure relief system so as not to reduce the relieving capacity or adversely affect the proper and expected operation of the safety devices. (2)
Second, valve manufacturers’ guidelines must be followed in valve selection, installation and maintenance.
Finally, maintenance personnel must be given the proper training. Personnel who do not understand a valve’s function cannot be expected to make valves operate consistently at peak performance. Most, if not all, safety valve manufacturers offer comprehensive training courses that provide instruction in basic valve maintenance and are designed to support regulatory standards. Plant, OEM and EPC engineers can also benefit from attending training courses.
Maintenance personnel must have the skills and ability needed to look past a malfunctioning safety valve and consider the entire pressure relief system to make an accurate diagnosis. Identifying the potential hazards during operation must be done from a wide-angle approach; dangerous situations can occur due to many root cause situations.2
A pressurized system is a complex combination of components. Each contributing component must be properly designed, installed and maintained in order to ensure the integrity of the system. When valves fail or do not perform correctly, the entire system must be assessed before making a final diagnosis. A valve that is not operating properly may not be the cause of a problem; it may only be a symptom.
By following these guidelines, engineers and maintenance personnel can help ensure a positive outcome for the process plant, including improved equipment reliability, more consistent product quality, lower operating costs and, most important, the safety of the facility, equipment, products and personnel.
Based on a paper first presented at the Valve World Asia 2007 Conference & Expo, organized by KCI Publishing, Jacob Damsingel 17, NL-7201 AN Zutphen, The Netherlands.
1. Williams M., Dresser Flow Solutions. “Curb Your Valve Costs,” Plant Services, December 2002. Accessed at www.plantservices.com on May 3, 2007.
2. Bours, R. Fike Europe Bvba. “Incorporating safety design considerations in design of pressure relief systems.” Engineer Live, www.engineerlive.com. Accessed May 3, 2007.