Vacuum Relief Valve
Table of Contents
Figure 1. Pressure/Vacuum Relief (Breather) Valve from Emerson [1] | |
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What are Vacuum Relief Valves?
Vacuum relief valves are valves designed to open in a case of lower than expected pressures, and allow for the reducing of air pressure in a system. Vacuum relief valves are used to mitigate possible dangers like implosions or damage to a system due to excessively low pressures [2]. Vacuum relief valves are typically used in a water tank/heater system to prevent back siphoning, where low pressures will cause the piping to dry up, and the heater to dry fire [3].
Vacuum relief valves are often combined with a pressure relief valve (Fig. 1), referred to as a breather valve or a pressure/vacuum relief valve. Other designs containing only vacuum relief valves are shown as well (Fig.2 and Fig. 3). All the designs feature the same working principle: when the pressure inside the system drops below a limit, a pallet is pulled open, allowing outside pressure to relieve the internal pressure [1]. The pallet is normally kept down by three different methods: by weight, by springs, or by pressure. Using weights is the most simple and allows for variable pressure settings using weight loading. This design is used in Figures 1 and 3. The design in Figure 2 uses a spring. The method using pressure is referred to as a pilot operated pressure valve, touched on in the "Types of Pressure/Vacuum Relief (Breather) Valves" section of this page.
Figure 2. Vacuum Relief Valve from Watts [4] | Figure 3. Vacuum Relief Valve from Flotech [5] | ||
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Figure 4. Operation of a Pressure/Vacuum Relief Valve [6] |
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Advantages
Protection against pressure induced failures
Protection against vaporization of tank contents
Increased safety of storage
Easy maintenance [6]
Disadvantages
Limited range of pressure, unable to cope with large variations of pressures quickly (due to catastrophic events) [8]
Advantages of Pilot Operated Relief Valves
Easier to control
Small sizing when used in larger pipe applications
Seals more tightly when in normal operating temperatures
Significantly lowers leaks and unwanted discharges [7]
Disadvantages of Pilot Operated Relief Valves
Complexity increases chance of failure and difficulty of repair
Smaller sizes are more expensive
Sensitive to contaminants and crystallizations [7]
Applications
The purpose of a vacuum relief valve is to prevent scenarios where extreme or unexpectedly low pressures cause failures such as tank deformation, pipe blockage, back-flow, implosions, etc.
Applications for Large/Industrial Valves
Large storage tanks, which are usually:
Designed to store large volumes at low pressures
Limited to the amount of pressure variation that can occur before tank deformation or catastrophic failure occurs [6]
Applications for Smaller Valves
Domestic water heaters and supply tanks
Table top heaters
Jacketed steam kettles
Unit heaters
Low pressure steam systems
Steam coil heaters [4]
Types of Pressure/Vacuum Relief (Breather) Valves
Pressure and Vacuum Relief Valves
Combination of a vacuum and pressure relief valve into a single unit [8]
Emergency Vent Valve
An emergency vent valve is intended to mediate sudden rises or drops in pressure that exceed the operating range of a normal pressure/vacuum relief valve. Emergency vent valves are set at a pressure slightly higher or lower than the normal breather valve and are on stand-by unless encountering situations like a fire, unexpected chemical reactions in the tank, etc. [8]
Gauge Hatch
A gauge hatch is used for situations where manual level, temperature measurements, or sampling of tank contents is required [8]
Tank Blanketing Valve
The purpose of a tank blanketing valve is to maintain constant vapor pressure in the tank during unloading of contents or condensation during external temperature fluctuations. The valve allows for inert gasses to enter the tank to maintain vapor pressure and minimize content loss due to vaporization. [8]
Pilot Operated Relief Valve
A pilot operated relief valve replaces traditional springs or weights that keep the piston shut, and instead is controlled by a "control pilot" to modulate the pistons position. This design offers finer control of tank pressure limits and reduces fugitive emissions [7].
Figure 5. Pilot Operated Relief Valve [7] |
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