The instance of a CIP pump not turning off during a regular cleaning procedure resulted in the pressure of the airtight stainless steel tank increasing. Meanwhile, the operator was somewhere else. The control system had not yet found the problem. Fortunately, a tiny spring-loaded mechanism was installed in the tank to help avoid an explosion: the pressure relief valve. The valve opened at 80 psi, which is the pressure set for it, released a small amount of hot and caustic liquid into the drain, and then closed again at lower pressure. Thus, the tank was kept intact. The batch was saved and the valve costing $200 saved a tank costing 40 times more in the course of the process. This is how a pressure relief valve works. It is not a control tool. It is not a flow controller. It is an automatic mechanism serving as last strong holding between the system and unregulated discharge.
The Core Function: Automatic Overpressure Protection
PRV, i.e., pressure relief valve, is considered a safety device that gets activated as soon as the level of pressure in a system exceeds previously set limits, allowing discharge of a sufficient volume of fluid for the pressure to return to mandatory levels. In order to operate it does not need any external power, including electrical energy, compressed air, or control signals, its working principle depending solely on the static pressure upstream of the PRV. Thus, the main advantage of PRV is a simple functioning mechanism that enables its operation even in case of the power supply failure, PLC breakdown, and lack of any supervision.
The sanitary pressure relief valve performs the same duties in a sanitary process line, like a dairy pasteurizer, a brewery fermentation tank, and a pharmaceutical bioreactor, but consists of materials and finish that meet all requirements for cleaning in place, draining, and documentation. The valve body is made of 316L grade stainless steel. The seals are composed of ployethylene, Viton, or PTFE. A Tri-Clamp ferrule is one of the means of connection. For a broader introduction to the cleanability requirements that govern every component in a hygienic system, our article on what a sanitary fitting is explains the design principles that apply to PRVs and every other fitting in the line.
How a Pressure Relief Valve Works, Step by Step
The mechanism of the pressure relief valve is neatly simple. The important parts consist of the valve body, the disc or poppet that closes against the seat, a spring which keeps the disc closed, and the adjustable screw that adjusts the spring pressure thus determining the opening pressure. When the pressure is below the set point, the spring force is greater than the pressure force thus the disc stays on the seat. If the pressure is equal to the set point the pressure force becomes greater than the spring force allowing the disc to leave the seat and allowing the fluid to pass through the valve to the outlet. After the fluid volume is enough for the pressure to go down below the closing pressure, which is about several percent less than the set pressure, the spring moves the disc back onto the seat.
In a gas or vapor service application, a pressure relief valve opens quickly with a "pop" and provides the maximum discharge capability as soon as the set point is equaled. In contrast, where liquid service applies, the opening is less abrupt, because liquids cannot be compressed, and the operation needs to be more gradual to avoid water hammer. The safety relief valve has characteristics of both types and is therefore applicable to operations in which liquids and gases (or vapors) coexist. The design and testing requirements for such valve types are specified in ASME Boiler and Pressure Vessel Code and, for sanitary valves, 3-A Sanitary Standards, along with detailed materials from such manufacturers as Swagelok.
Where a Pressure Relief Valve Belongs and Where It Doesn't
A pressure relief valve should be installed at any point in a closed system where the pressure surpasses the maximum allowable working pressure (MAWP) of the vessel, pipe, or connected equipment. The most common places in hygienic process installations where pressure relief valves are typically installed are the top of the steam-jacketed or CIP-heated vessel, the discharge side of a positive-displacement pump that could be deadheaded and any isolated pipe that can be isolated by two closed valves and then heated-up since the thermal expansion of liquid inside a closed pipe produces high pressure without the existence of vapour space to cushion it.
A Relief Valve should not be considered as a replacement for either the controlling valve or a pressure controller. Pressure Controllers facilitate the flow of fluids at normal operating conditions, while the Relief Valve may remain dormant in the automation process for a long time. In addition, the Relief valve is not the same as a backflow valve, since it is designed to relieve pressure in a single direction.
The 3% Rule and Other Sizing Considerations
The correct selection of a pressure relief valve includes matching the set pressure, relieving capacity, and connection size of the valve to the system being protected. The set pressure must be equal to or below the MAWP of the weakest part of the system. The relieving capacity of the valve (the amount of fluid that can be released at the set pressure) must be greater than the maximum possible flow to the protected system from all sources such as pumps, heating, and chemical reactions.
There is a requirement that the pressure drop from vessel to the valve should be below 3% of the set pressure when the valve is fully operational. This is called the 3% rule and is a common rule in process safety engineering which is required in a lot of rules including API 520. If the pressure loss exceeds 3%, the valve will start chattering (operating with rapid opening and closing) which will cause damage to the seat and the spring and reduce its effective relieving capacity. In the case where the valve is installed in the sanitary application (the valve is mounted on a small nozzle), the application of the 3% rule is easily achieved. In the installations with long inlet lines the pressure loss must be calculated and taken into account when sizing the inlet line.
How a Sanitary PRV Differs from an Industrial PRV
A sanitary pressure relief valve resembles its industrial version in appearance and construction. However, the inner features differ greatly to ensure cleanability and better resistance to corrosion than the standard pressure relief valve. The body of a sanitary PRV is made of 316L steel instead of 304 steel to hold up against the acidic, caustic, and chlorinated or other types of chemicals involved in the sterilization cycles.
All surfaces in contact with the product are electropolished to the roughness average of 20–32 micro-inches to remove any possible microscopic unevenness that can harbor bacteria. In addition, the valve utilizes a Tri-Clamp ferrule connection which ensures a smooth and well-sealed connection that can be disassembled in a matter of seconds for inspection. Another important feature of a sanitary valve is that the spring is usually made of a special type of steel suitable for use in high purity applications, which is further isolated from the flows with the help of a diaphragm made of elastomer or PTFE.
What Happens When a Pressure Relief Valve Goes Bad
There are two ways a pressure relief valve can fail and both are dangerous. First, it can fail to open when it reaches its set pressure — this is called "stuck‑closed" failure — meaning that the system has no protection against overpressure. The typical reason for this failure is corrosion or buildup on the seat of the valve, or it could be that the spring’s adjustment setting has been locked. Second, there is a leak.
The PRV may open prematurely or fails to close tightly after a relieving occasion. This can happen due to damage to the seat, disc scoring, or spring fatigue. In the case of a sanitary process, PRVs with a leak can also be considered as contamination pathways, allowing the unsterilized air or liquid to enter the system when the valve is supposed to be closed. Thus, the only means to prevent these failures is to carry out regular inspection and testing of PRVs. PRVs must be checked visually at least once a year and if necessary removed for bench testing or replaced according to the production standards and requirements indicated in the quality system of the plant.
Frequently Asked Questions
What happens when a pressure relief valve goes bad?
If a pressure relief valve fails, it might be in a closed position and unable to release any pressure or cause a leak resulting in loss of product, contamination, and pressure loss, among other issues. It is critical that any valve that has opened in the actual scenario and is obviously corroded or damaged be immediately replaced or repaired.
What is the 3% rule for pressure relief valves?
According to the 3% guideline, the inlet piping non-recoverable pressure drop between the protected vessel and the PRV should remain within 3% of the set pressure of the PRV when the valve is opened fully. If this limit is exceeded, then this could lead to chatter of the valve damaging the seat and reducing its relief capacity.
Should a pressure relief valve be open or closed?
Pressure relief valves should be kept shut under normal circumstances, opening only when the internal pressure of the system has hit its preset figure. If the valve is found to be leaking or seeping under normal circumstances, its state is termed as having ‘failed’, indicating that it must be repaired or replaced.
How long will a pressure relief valve last?
A properly sized PRV, which is correctly installed in a clean and non-corrosive environment, would survive from 10 to 20 years or more, assuming that it is checked on a regular basis and it is not subjected to frequent rupture instances. In a hygienic process characterized by the use of a CIP/SIP tech, a lifespan of 5 to 10 years could be expected, after which the valve should be changed or refurbished for preventive reasons. The seals made of elastomer material have a considerably shorter life quality and should be changed according to the service life schedule influenced by chemical and thermal exposure.
References
- Swagelok — Pressure Relief Valves. Manufacturer of industrial and high‑purity pressure relief valves, with detailed engineering data on set pressure, reseating pressure, and flow capacity.
- Alfa Laval — Sanitary Pressure Relief Valves. Manufacturer of hygienic PRVs for dairy, food, and pharmaceutical applications, with 3‑A and EHEDG certification.
- ASME Boiler and Pressure Vessel Code, Section VIII. The defining standard for pressure relief device sizing, selection, and installation in pressure vessels.
- 3‑A Sanitary Standards — Pressure Relief Devices. The sanitary standard governing the design, materials, and cleanability of PRVs used in dairy and food processing.
A pressure relief valve does one thing, and it does it without power, without a signal, and without anyone watching. It opens when the pressure is too high, and it closes when the pressure is safe. That single, simple function protects vessels from rupture, pipes from bursting, and people from the consequences of a pressurised release. In a hygienic process line, the valve must do all of that while also being completely cleanable, fully drainable, and made of materials that withstand the same aggressive cleaning chemicals that protect the product. Eagle Fittings supplies sanitary pressure relief valves that meet those standards, with the material traceability and the surface finish data that turn a safety device into an auditable, certifiable part of the process.
