Pressure relief valves: What makes them different?
Facilities use many different types of valves to keep themselves operating, and each valve within a plant serves a different function. A typical plant may have an installed base of gates, globes, butterfly, plug and other types of valves, each specifically designed to function based on process conditions. Some valves are manually operated, others may be controlled by air, electric, hydraulic, and even weights or springs. Most valves in a plant can be categorized into one of the following three types: control, on/off or pressure relief. One of these three valve types looks and operates differently than most others—the pressure relief valve.
Anywhere a plant has flow and pressure through a pipe or vessel there is a risk of over pressure, and when such an event occurs, the PRV comes into play. Photo Credit: © Dalestagg | Dreamstime.com
Pressure relief valves (PRV) have been referred to as the “silent sentinel,” meaning, they act as a soldier or guard whose job is to stand and keep watch—an accurate statement of what a PRV does in a protected system. Anywhere a plant has flow and pressure through a pipe or vessel there is a risk of over pressure. Depending on the type of process, whether it is compressible or not, there can be a major risk to people, equipment, buildings and the environment. When looking at the pieces of equipment in a protected system, typically you will find a pressure source such as a pump or boiler—and even the sun in thermal expansion scenarios. These systems are normally operated by a control panel, computers, mechanical controls, a human operator or some combination of these.
READY TO SPRING INTO ACTION
In a perfectly designed plant where nothing ever changes, wears, breaks or goes wrong a pressure relief valve would be unnecessary. Yet nothing is perfect or remains perfect indefinitely, which is why the last line of defense in a protected system—the PRV—is needed. A PRV sits idle and if everything else is doing its job as designed it never gets called into action. When other systems fail and pressure builds, the PRV comes into action and relieves the excess pressure safely protecting people and assets.
There are many codes and standards around the design of a typical valve, and the PRV is no different. The purpose of this article is not to drill down how PRVs differ from other valves. Rather, the intent is to focus on how manufacturers or their supporting sales networks bring a PRV to market.
The American Society of Mechanical Engineers (ASME) along with the National Board (NB) write and enforce the rules, regulations and requirements that drive the application, manufacturing and testing of a PRV. There are specifications on how quickly the valve must open when it reaches full lift and how quickly it recloses. Terms like simmer, set pressure, blow down, full lift, rated capacity and others are a result of these ASME requirements. Various sections within ASME are directly related to PRVs. The most common ones are: Section I, which pertains to a fired vessel, boiler or economizer; Section III, for nuclear applications; and Section VIII, covering process applications.
A RIGOROUS PROCESS
An ASME certified manufacturer, assembler or valve repair facility must live by these codes and standards; they must be fully understood and compliance is mandatory. These companies go through a rigorous certification process and must make a major investment in people, equipment and development of work processes to ensure compliance is met, as well as maintained. The barriers to entry to become a PRV assembler or PRV repair facility are much greater than other valves operating in a plant due to reasons stated earlier—when all else fails the PRV needs to work.
Before one can even approach the National Board to apply for certification an investment in a test stand is required. The size of the valve being assembled or repaired will dictate the capacity requirements of the test stand as well as whether the valve is vapor or liquid certified. Another major hurdle is steam: The equipment needed for a steam valve is vastly different than one needed for air or liquid. The most common PRV sizes range from 0.5-inch inlet all the way to 8 inches. Having the ability to set/test the entire range for air, liquid and steam requires an investment well into the six figures; ASME/NB certifications and recertification bring additional recurring expenses as well. This is not the type of business any valve repair facility can just jump into—very specialized equipment, facilities and certifications are required.
KNOWLEDGE IS KEY
Assuming the hurdles of equipment, investment, etc., have been achieved, next comes knowledge. Each facility must have a certified individual with extensive knowledge of the valve being worked on; this person must sign off on the required QC documents to be in compliance with industry codes and standards.
Many types of PRVs exist, but the most common are spring or pilot operated. Other types include weighted, buckling pin and bursting products. For the purposes of this article, we are focusing on spring and pilot operated. Spring-type valves can be conventional or bellows sealed, depending on process fluid or conditions, and pilot valves are either snap acting or modulating. A snap-acting pilot is a quick open/quick close pilot operated valve while a modulating pilot has a more controlled open/close that functions in proportion to the upset condition, relieving only the process fluid greater than standard operating design conditions.
Many options and special materials are available to tailor the valve for the specific application in which it is being used. To know exactly what the customer needs and be able to quote, sell, build, set/test and ship adds another layer of expertise above and beyond the asset investment of a PRV assembly facility. Further, access to data is required; the facility performing PRV assembly needs manufacturer spring charts for new valves and when doing service or repair, critical seat dimensions are a must. To maintain compliance with applicable code requirements, all replacement parts must be OEM components purchased through proper channels.
PRVs do indeed serve a much different purpose in a plant than any other type of valve. Other valves are bought to be opened and closed, some of them constantly. A PRV is bought in the hopes it never needs to open. Manufacturers and their representatives, along with certified valve repair facilities and distributors of valves and parts, are crucial to ensure that when a PRV is called into action it will operate without fail.
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