Pressure and Vacuum Relief Valves

schematic of processing or storage tank venting system
A pressure/vacuum relief valve is part of a complete tank
control system.
Image courtesy Groth Corporation
Storage or processing tanks are a part of almost any industrial liquid processing operation. There are many considerations for the inclusion of a tank in a liquid handling system. One facet, the subject of today's post, is how to accommodate the normal exchange of balance between liquid filled space and non-liquid filled space in a tank as it is filled or drained. Vacuum and pressure relief valves are, in many cases, the best solution for tank venting.

Keeping in mind that some tank involved operations process liquids that must remain isolated or contained from the atmosphere for any of a number of reasons. The tank may be blanketed with nitrogen or another gas. The contained liquid may produce vapor which cannot be freely vented to the atmosphere for regulatory reasons. Whatever the reason, one thing is common among all these applications. The tank must remain a closed system whenever its level is not changing.

When the liquid level in a closed tank changes, the pressure of the air or vapor in the tank will also change. Filling of the tank will increase the pressure, draining will decrease it. It is necessary to offset the change in pressure to avoid tank structural damage or processing difficulties from excessive positive or negative pressure produced by the changing liquid level. Groth Corporation manufactures pressure and vacuum relief valves for processing and storage tanks. Quoting from their installation manual for pressure/vacuum relief valves...
Pressure and/or vacuum relief valves are used on liquid storage tanks and other process vessels or systems to prevent structural damage due to excess internal pressure or vacuum. Storage tanks are pressurized when liquid is pumped in, compressing the existing vapor or when rising temperatures cause increased evaporation or expansion of existing vapor. Conversely, a vacuum condition may be created when pumping out or due to falling temperature. To prevent tank damage, vapor must be allowed into or out of the tank at specified pressure/vacuum conditions. The volume rate of venting depends upon the tank size, volatility of the tank contents, the pumping rates and the temperature.
These valves have specific requirements for installation and maintenance, but their operation is simple. A weighted pallet will move by gravity to a closed position when the liquid level is static. As liquid level changes and the pressure applied to the pallet changes, it will eventually move. It is important to note that each valve is weighted for the customer's specific application, and weight factory applied to the pallet should not be modified or otherwise changed without consulting the valve manufacturer to confirm the new operating parameters.

A pressure/vacuum relief valve is not intended to be a safety valve. It's function is related to the normal operation of the tank. A safety relief valve is an additional component with differing operational requirements. Here is now Groth describes the basic operation of the pressure and vacuum relief valves, with a little editing.
Pressure Relief: As the pressure in the storage tank increases, the vacuum pallet is held shut. When the set pressure is reached, the pressure pallet lifts and relieves tank pressure to the atmosphere (or to a header if it is a pipe away valve). 
Vacuum Relief: As a vacuum is drawn in the storage tank (for example, when fluid is being pumped out), the pressure pallet is held shut. When the vacuum setting is reached, the vacuum pallet lifts and air is drawn into the tank from the atmosphere.
Selecting and specifying a properly sized pressure/vacuum relief valve requires consideration of all aspects of tank operation. Share your project requirements with a product application specialist and leverage your own process knowledge and experience with their product application expertise to develop the best solution.