June DelGrosso, Sales Director for North America Nuclear and Navy at Emerson, recently published an article in the November 2022 issue of Power. It is titled “In-Service Testing of Pressure Relief Valves” and it discusses equipment that can perform accurate testing of relief valve setpoints without taking them out of operation. The article is summarized below.
Pressure relief valves (PRVs) are a critical line of defense for pressure vessel protection in the power industry, so mechanical engineering regulatory bodies mandate they be tested on a routine basis. Some installations make the option of pulling and testing the valve very difficult, and this is particularly true in the nuclear power industry. Fortunately, there is better approved method of testing relief valves, and that is the subject of this article.
PRV operation and testing
PRVs are relatively simple devices. They consist of an inlet nozzle blocked by a disc held tightly on the nozzle seat by a spring (Figure 1).
When the process reaches set pressure, the upward force of the process offsets the downward force of the spring, and the disc lifts off the seat. The process media vents until pressure falls below setpoint, at which point the spring recloses the valve.
To ensure the PRV will function when necessary, the American Society of Mechanical Engineers (ASME) mandates relief valves be functionally checked on a routine basis. Normally relief valves are pulled during process outages, inspected, and tested to confirm they lift as expected. However, this method is not always possible, as June describes:
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 and other situations, ASME allows in-service test methods, which permit plant personnel to functionally test a relief valve without removing it from the process. This in-situ test method can be quite accurate, but only if it is performed correctly with the right equipment.
In service test methods
In service testing is usually performed at lowered pressures and temperatures by maintenance technicians. Lift assist devices are used in conjunction with these lower system pressures to verify the PRV will operate within tolerance of set point. The lift assist device is attached to the spindle of the valve with adapters, along with other test apparatus to perform set pressure verification testing (Figure 2).
The lift assist device applies a calibrated lifting force to the spindle of the PRV until the disc just lifts off the seat. Since the process pressure and the lift force are known, a computer-driven system can determine the setpoint of the PRV and confirm it falls within tolerance. This specific type of lift assist equipment is known as a set pressure verification device (SPVD).
SPVD design features
The specifics of SPVD construction have a large impact on the accuracy and operability of the device. June explains:
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 detects the earliest sign of valve stem movement, around 0.020”, which allows the SPVD to detect the PRV setpoint without wasteful discharge of the process media or risk of seat damage.
The most important feature of the SPVD is its fully automated test execution system. This system incorporates a laptop computer running automated calibration, test, and diagnostic protocols. The computer can be connected to as many as five relief valves, speeding test execution. This offers significant advantages over competitive units, as June discusses:
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 SPVD can be installed on a variety of PRVs from different manufacturers, and it is lightweight and easily adaptable to fit a wide range of relief valves. It also generates consistent and accurate test results, regardless of personnel experience. The SPVD typically provides ASME-certified test results with a proven test accuracy of less than +/- 1% error, significantly below the ASME accuracy threshold of +/- 3%.
SPVD is often the preferred choice to test very large valves or those that are welded into processing piping (Figure 3). It is also a good option for critical PRVs located within nuclear containment buildings.
In the latter type of application, SPVD lift assist heads and adapters can be permanently installed on the valves, with test cables routed outside the restricted zone. The SPVD does not impede operation of the PRV, but this installation allows the PRV to be tested on an as-needed basis, while avoiding any potential radiation exposure.
Visit the Crosby brand section on Emerson.com for more information on these direct spring-operated pressure relief valves for gas, liquid, steam, and multi-phase applications.