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How Breather Valves operate?
- Aug 27, 2018 -

Most atmospheric tanks require a venting device that will allow large volumes of vapor to escape under relatively low pressures. Usually the allowable set pressure is in inches of water column pressure, both for positive and vacuum conditions. This is because most large storage tanks have a relatively low maximum allowable working pressure.

These tanks are generally large volume welded vessels that are built to API 650 standard. In order to accommodate large volumes at low set pressures, these Valves have ports that are greater in area than the inlet or nozzle connection. The low setting required necessitates weight loading the Valve as opposed to spring loading. Because of the above, a Breather Valve requires approximately 100% over set pressure in order to reach full opening of the Valve.

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However, when deciding on a set pressure, the weight-loaded Valve operation MAWP should be at least twice the required set pressure to obtain optimum flow. If the MAWP is less than 100% above the required set, the Valve could be larger in size than normally required. The possibility of Valve chatter and accelerated seat and diaphragm wear will exist if less than 20% over pressure is allowed. Simply stated, a Pressure/Vacuum Valve is not exactly like a high pressure safety Relief Valve and should not be sized at 10% or 20% over pressure. When sizing a Pressure/Vacuum Valve, consult the manufacturer flow curves and allow sufficient overset pressure.