Pressure relief valves (PRVs) are a critical line of defense for pressure vessel protection in the power industry. Generating facilities worldwide depend upon these devices to sense and quickly relieve overpressure conditions to avoid catastrophic damage during process upsets. To ensure these valves will perform as expected, mechanical engineering regulatory bodies mandate the valves be tested on a routine basis.
Some installations make the option of pulling the valve for servicing and testing very difficult. This is particularly true for large size valves, and in the nuclear power industry where valves may be located inside containment areas, making valve access particularly problematic. Fortunately, there is another approved method of testing relief valves for this situation, and this alternative solution is the subject of this article.
PRV Operation and Testing
PRVs are relatively, but deceptively, simple devices. They consist of an inlet nozzle attached to the process, which is blocked by a disc held tightly on the nozzle seat (Figure 1). The disc is kept closed by a spring, with adjustments carefully made to dial in the setpoint of the valve.
1. A pressure relief valve (PRV) protects equipment by automatically opening to vent process media when the pressure in the inlet nozzle overcomes the downward force of the spring. Courtesy: Emerson
When the process reaches set pressure, the upward force of the process media offsets the downward force of the spring and the disc lifts off the seat. The process media is relieved through the valve outlet until pressure falls below the setpoint. At this point, the downward force of the spring overcomes the upward force of the process media, and the valve closes.
To ensure the PRV will function when called into action, the American Society of Mechanical Engineers (ASME) mandates relief valves be functionally checked on a routine basis. Typically, a plant will pull smaller valves from their installed position during process outages, and then inspect and test them in a shop environment to confirm they will function as desired and open at the proper pressure setpoint. However, this method of testing is not so easily achieved in certain cases.
Some relief valves are very large and/or located in difficult to reach areas. Others are welded into place and not easily removed from the process. Valves inside nuclear containment areas are particularly troublesome since access to these areas is usually restricted, with strict adherence to extensive protocols required for entry.
To handle these challenging situations, ASME provides alternate means of testing relief valves, as documented in ASME Performance Test Code (PTC) 25 Pressure Relief Devices. These test methods include in-service testing, which allows the plant to functionally test the relief valve without removing it from the process. This in-situ test method can be quite accurate and effective, but only if it is performed correctly with the right equipment.
In-Service Test Methods
Service and testing of PRVs is typically performed during regular maintenance outages as defined by ASME guidelines. Operating pressures and temperatures are brought down to levels conducive for servicing, and the PRVs are tested by maintenance technicians. For this type of in-service, or in-situ testing, lift assist devices are used in conjunction with these lower system pressures to verify the PRV will operate at the setpoint, within allowable tolerances.
ASME-approved, in-service testing allows the use of lift assist devices attached to the spindle of the valve with adapters, along with other test apparatus to perform set pressure verification testing. The equipment shown in Figure 2 allows a carefully monitored lift force to be applied to the spindle of a PRV until the disc lifts off the seat. The process pressure and the lift force are known, enabling this type of a computer-driven system to determine the setpoint of the PRV, and confirm that it falls within tolerance. This specific type of lift assist equipment is known as a set pressure verification device (SPVD).
2. A portable lift assist, or auxiliary lift device, allows a PRV to be functionally tested without removing the valve from the process. Courtesy: Emerson
There are several other types of lift assist devices available to perform in-service set pressure verification testing, with varying degrees of effectiveness. For most applications, an SPVD is the preferred method of in-service testing.
SPVD Design Features
The specifics of SPVD construction have a large impact on the accuracy and operability of the device. High-performing SPVD units employ a variety of design features to ensure consistent test execution and accurate results.
SPVDs utilize highly accurate pressure sensors to measure the process pressure and determine the lift force being applied. These sensors include automatic calibration and diagnostics by the SPVD computer-based controller to confirm the sensors are operating correctly prior to each test.
A linear variable differential transformer (LVDT) is used to detect the earliest sign of valve stem movement, in the range of 0.020 inch, well below the point where the valve will go into full lift. At this time, force and pressure values are obtained, and the test is concluded to avoid wasteful discharge of the process media and minimize seat damage.
Perhaps the most important feature of an SPVD is a fully automated test execution system (Figure 3). This system incorporates an industrially hardened portable laptop computer running automated test protocols, including calibration and diagnostics. The computer can print out certified test results and be connected to up to five relief valves, simplifying and speeding test execution.
3. A technician performs a fully automated set pressure verification device (SPVD) test. Calibration, diagnostics, and test functions are built into the system, allowing plant personnel to execute consistent and accurate PRV tests. Courtesy: Emerson
Some other types of lift devices are more manual and can only be operated by trained personnel, typically provided by the lift device vendor at considerable expense. However, a fully automated SPVD allows most plant technicians to perform PRV set pressure verification tests as needed. The most useful lift assist devices can be installed on a wide variety of PRVs, rather than just on those from specific manufacturers. Ideally, the lift device should be lightweight and easily adaptable to fit a wide range of relief valves.
Cost and scheduling benefits can be realized from self-test execution, and fully automated PRV set pressure testing also helps ensure consistent and accurate test results, regardless of personnel experience. SPVDs typically provide ASME-certified test results with a proven test accuracy of less than +/–1% error, significantly below the typical ASME test accuracy threshold of +/–3%.
SPVD is often the preferred choice to address a number of challenging PRV test issues. Some valves are very large or not easily removed, so an in-service test is clearly the least costly option (Figure 4). This can especially be true for large relief valves that are welded into the process piping.
4. Emerson’s Crosby SPVD is being used to perform an in-service test on this American Society of Mechanical Engineers (ASME) Section III Class 2 safety valve. Installation and testing of the SPVD does not restrict the PRV from operating should process conditions require the valve to open in service. Courtesy: Emerson
Inside nuclear power plants, many large PRVs are located within containment buildings, where access is extremely limited. For critical PRVs in these areas, SPVD lift assist heads and adapters can be permanently installed on the valves, with test cables routed outside the restricted zone and connected to a computer controller.
Since this type of lift device does not impact valve performance during normal operation or overpressure conditions, the PRV can still operate as necessary. Tests can be remotely performed from outside the containment building by simply plugging the cables into a test system and executing the test. Such an installation allows a plant to safely operate under normal conditions and test their critical PRVs on an as-needed basis, while avoiding any potential radiation exposure.
A Sound Option for Difficult Testing Situations
A well-designed lift assist device is a valuable addition to a plant’s PRV maintenance toolset. Every PRV is required to undergo regular in-service testing requirements as defined by the ASME Operation and Maintenance of Nuclear Power Plants code. The code permits use of lift assist devices to perform set pressure verification testing, which is particularly useful in situations where removing the valve from its installed position is not practical. The right lift assist device allows plant personnel to safely execute scheduled maintenance during outage events, or during operation in other cases, ensuring that critical PRVs are functioning per design with correctly adjusted setpoints.
A fully automated SPVD allows plant personnel to perform these tests consistently and accurately, freeing users to schedule and execute PRV tests, without the need for outside vendor involvement. This saves time and cost, and it removes dependence on a single vendor as a service provider. SPVD also provides a means for nuclear power plants to remotely test their critical relief valves, while avoiding exposure in containment areas.
If faced with a PRV testing challenge, plant personnel should consider lift-assist devices, such as an SPVD, as a potential solution. They meet ASME requirements for set pressure verification testing, and the fully automated operation of SPVD guarantees reliable test results, while providing many other benefits noted in this article.
—June DelGrosso is the sales director for North America Nuclear and Navy at Emerson for its flow control products. She has worked for a variety of companies, filling roles such as Valve and Instrument Design Engineer, Product Engineering Manager, and Global Product Technical Leader.