- Published on Monday, 02 April 2012 09:11
- Written by Sean Croxford and Martin Joyce
While it is an extremely rare event, bellows can and do fail. But bellows failures are often wrongly attributed to the quality of the valve or the bellows while in reality, a more likely scenario is operating conditions or an improperly specified valve that contributed to the failure. Still, whenever a failure occurs, analysis of what happened and why is critical.
THE USE OF BELLOWS
A spring-loaded pressure relief valve (PRV) is a device that reacts based on the amount of static pressure force pushing up on the disc. In normal processing conditions, the valve will remain shut because the upward force on the disc is less than the closing spring force. When the force from the process fluid pushing up and the force of the spring pushing down are at equilibrium, the disc of the valve will begin to lift from the nozzle, and the valve will begin to “simmer.” At this point, a slight increase in process pressure will cause that valve to “pop” open (its set point), thereby relieving the overpressure.
A bellows is typically specified for applications when a spring-loaded PRV will experience backpressure (which can impact the valve’s ability to open at the correct set pressure) or when the internal components of the valve must be isolated from the processing fluid. When selecting the bellows material, consideration of the process material discharging into a common header must be made.
While it is possible for a bellows to fail because of an imperfection in fabrication, failure more commonly can be attributed to the wrong valves being used or operating conditions. Quality control during PRV assembly can prevent a customer from experiencing this type of failure.
Listed below are four scenarios that are common reasons a bellows might fail. Each assumes that a thorough review of the engineering sizing and specifications for a given PRV has been completed since these calculations will aid in diagnosing the problem.
One clue that indicates a valve has been exposed to excessive backpressure is when the bellows has been crushed. There are two types of backpressure in process systems: constant and variable. Variable can be further divided into two subgroups: superimposed and built-up.
Built-up backpressure is defined as the pressure at the outlet of the PRV based on the discharge piping configuration, i.e., pressure that occurs only after the valve has opened. For applications where the flow is compressible, built-up backpressure is based on the piping hydraulics at the accumulation pressure using the maximum actual capacity for the PRV. All too often engineers perform this calculation at the required capacity for the given scenario, not at the device’s actual capacity.
When a bellows failure can be attributed to excessive built-up backpressure, the following options will mitigate the problem:
- Use a bellows with a higher pressure limit.
- Use a pilot valve balanced against backpressure.
- Modify the outlet piping by making it larger or shortening the length of pipe, thereby reducing the effects of built-up backpressure.
While most PRVs are protecting equipment for more than one relief event, the size of the valve is based on the scenario requiring the greatest relieving capacity. An example would be when a PRV is sized for both fire and blocked outlet scenarios. The fire sizing requires significantly greater orifice area than the blocked outlet sizing. However, since the blocked outlet scenario is more common and more likely to occur, then the PRV will be potentially starved for capacity, causing the valve to “chatter” (rapidly opening and closing). Valve chatter, as well as flow instability, could inevitably cause valve damage such as premature fatigue failure of the bellows, as well as galling of guiding surfaces. In our experience, a PRV should not be specified that has an actual orifice area more than 3 to 5 times larger than the required area.
Mitigation strategies for failure in this scenario include:
- Install multiple PRVs and stagger the set pressure for each of the scenarios. Ensure the small valve is properly sized based on the lowest required capacity relief scenario.
- Install a modulating pilot-operated relief valve.