The term pressure relief valve or PRV is self-explanatory, in a general sense. A PRV is needed in any system, including compressed air, where there is the potential for pressure to increase to an unsafe level in a pressure vessel.
Pressure relief valves are used in a wide array of applications where pressure levels are critically important for smooth operations such as oil and gas, petrochemical, and power generation. They ensure the successful and safe operation of air compressors using steam, air, gas or liquid.
What is a Pressure Relief Valve?
An air compressor pressure relief valve (or sometimes referred to as an air compressor safety relief valve) aims to control or limit the amount of pressure in an air compressor system. It does so to avoid a pressure build-up that could result in failure or breakdown of the system. Therefore, it is vital in the successful operation of an air compressor and ensures less maintenance is needed on the system.
The relief valve is set to detect a certain value of pressure within the system and when it reaches this, it opens to reduce the pressure and ensure the system does not exceed its limit.
Compressed air is explosive when it is released suddenly. To help ensure that this does not happen, the compressed air receiver has a PRV installed that has access to the air pressure inside the tank. The PRV might be part of the check valve, pressure switch, and PRV assembly, where the airline from the air compressor head enters the air tank.
Pressure relief valves can be classified into two main types:
- Pilot operated pressure relief valves – these are operated by hand are opened by the operator turning a wheel, crank, or similar component. This requires the operator to keep an eye on the gauge and open them accordingly when the system reaches a certain pressure and then shut it.. quite a faff right?
- Direct operated pressure relief valves – These valves (most commonly found throughout the industry) eliminate the need for an operator to stand by. They’re programmed to open when the pressure within the system reaches a certain level and to shut automatically when the pressure is relieved. The valves are controlled by a remote, control panel, or computer program.
How Does a Pressure Relief Valve Work?
During the operation of an air compressor, the pressure valve remains closed until the pressure within the system as mentioned reaches the desired set pressure limit. The valve will then crack open, allowing more flow as the overpressure increases. When this system pressure falls a few psi below the set pressure limit, the valve can close again, this cycle repeats over and over to ensure the system stays below limit.
It can be said that the operation of the pressure relief valve has three functional elements; the valve element, sensing element, and reference force element. They will all be discussed in turn.
The valve element of the PRV most commonly uses a spring-loaded poppet valve. During the operation, the spring and system pressure apply opposing forces on the relief valve. It is only when the system pressure exceeds the pressure of the spring that the poppet distances itself from the valve which therefore allows the pressurized air to escape through its outlet port. When the system pressure drops below the set limit then the poppet lowers to the valve and closes it, disallowing any air to escape.
The sensing element of the PRV is often piston-style designs when high relief pressures are required or they do not need to be restricted to fine tolerances. Diaphragm style designs are used when the relief pressures are lower or higher accuracy/abidance to tolerances is required.
The friction from the pistol seal within a piston design tends to make them appear a tad sluggish in comparison to those diaphragm designs. Diaphragms eliminate the friction apparent with piston-style designs, they instead employ a disc-shaped element that senses pressure changes in the air compressor system. For some relief valves (of a certain size) it is certainly possible for the diaphragm design to offer a greater sensing area than a piston style design, which can be very desirable when trying to decide your design.
The Reference Force Element
The reference force element of the PRV is normally a mechanical spring that exerts a force on the sensing element and makes it close the valve. It is possible to change the force exerted by the reference spring and therefore adjust the relief pressure valve set limit.
Many compressor PRV ‘s look a bit like this image below:
There is a ring on the end of the PRV which, when pulled, will activate the PRV and allow air from the receiver to vent to atmosphere.
There are many important parameters that must be considered during the design of a PRV and its elements. These are as follows:
- Relief Pressures – The relief pressure that is expected within the system is a very important factor in determining which design is best suited for the application.
- Flow Requirements – It is also important to consider what the maximum flow rate of the system will be and whether this varies much or at all. This will require special configuration and possible orifices that are sufficient enough.
- Fluid in Use – Before decisions are made on the materials for the desired application, it is important to consider the chemical properties of the fluid in use; is it gas or liquid? toxic? flammable? Every fluid has varying characteristics/properties, so careful consideration must be conducted to select appropriate materials that can handle being in contact with the fluid.
- Size & Weight – In many plants, factories, and similar applications space can be very limited therefore it is important to consider the size of components. Material selection can impact the weight of the components, in particular, the relief valve body. The thread sizes at the port should also be carefully considered along with its adjustment and the options for mounting. These can all influence the size and weight of the component.
- Materials – Common materials like brass, plastic, steel, and aluminum are used for pressure relief valves depending on the required fluid and operating environment. Carbon or stainless steel is often used for the springs inside the valves. Brass tends to be the most common material used and can provide the greatest economical benefits. If weight is an important factor to consider, aluminum can be the better choice and if low costs are desirable, plastic is the better option. When corrosive fluids are used during the operation, stainless steel is the most suitable option. A final important consideration is a material used for the seal and its compatibility. A few options are fluorocarbon, EPDM, silicone, and the most common, Buna-N.
- Temperature – Careful consideration must be taken for the operating temperature of the air compressor as well as the temperature of the fluid. You must ensure that the material will function properly across the entire operating temperature range. The flow capacity can also be affected by the operating temperature, so this must be investigated before coming to a decision.
Pressure Relief Valve vs. Safety Valve
As mentioned, pressure relief valves are sometimes referred to as safety valves due to their perceived operation as safety valves. However, this is not entirely the case as there are some key differences between the two.
We’ll start with their technical definitions:
- Relief valve – a valve within a pressurized system that is used to provide the system with optimal functionality by controlling the pressure within. They work to protect components from over-pressurized conditions and aim to avoid system failures.
- Safety valve – a valve that is the failsafe, the last resort that will release pressure to prevent catastrophic damage from occurring. Comes into action when all other relief valves have failed in controlling the pressure within the air compressor system. They’re designed to protect people, the system, and the environment surrounding the system.
It can be said that both relief and safety valves share a common purpose. They’re both pressure relief valves that are designed to let pressure escape when the air compressor system becomes over-pressurized. The key differences being within their functionality.
As already learned from this article, pressure relief valves are designed to control the pressure within an air compressor system. In comparison, safety valves do not control the pressure. They’re designed to release pressure immediately in an emergency or system failure. Safety valves unlike relief valves, open immediately so that they can completely avoid any potential disasters rather than controlling them.
The Compressor Will Not Stop
Some PRV’s rely on “sticktion”; the friction between its parts, to generate enough internal resistance to keep the PRV from cracking open to vent air. Other PRV’s have an internal spring performing the same task.
In both cases when the pressure inside the tank reaches the PRV set point, the air pressure acting on the piston inside the PRV overcomes the “sticktion” or the spring pressure, forcing the piston out, and opening the flow path from the air tank to the atmosphere, venting overpressure.
Check the PRV regularly. Give the ring a pull to make sure it releases easily and air flows sufficiently. Push the ring and piston back in and pull it out a couple of times, then push back in when the PRV test is complete. I would recommend wearing gloves and air defenders when doing this.
Having the PRV there provides a critical and mandatory additional line of defense against over-pressurization. The PRV is cheap insurance. Having a working PRV in an air circuit backs up the pressure switch and, in extreme cases, will prevent a catastrophic overpressure and failure of an air compressor tank.
Existing Questions/Issues and Answers/Solutions About PRVs
Hello, my tank which max at 175 psi, will run and shut off normally, except PRV at the start of the intercooler
My PRV valve opens before the pressure builds to where it should be. I replaced the PRV and it’s still opening
When the compressor motor is off, usually the unloader valve (often part of the pressure switch) is open to the atmosphere
I recently “horse traded” for an older 20 gallon Speedaire compressor
I have a cast iron, single-piston, oil-filled crank, direct drive, 2 HP Snap On air compressor on a 5 gallon
Every new PRV I have installed on my air compressor leaks air.
In the 2nd stage of my 3 stages, an air compressor is blowing off. what could be the problem here.
I have a central pneumatic 6 gallon 150 psi air compressor.