Buffer fluid pressure is needed to operate double mechanical seals. During operation it must always be higher than the pressure to be sealed (normally around 10% or a minimum 2 bar above the highest occurring process pressure). Its function is to prevent any product leakage escaping into the atmosphere. Buffer fluid pressure is generated by buffer fluid units, refill units, nitrogen pressurization in thermosyphon vessels, product pvessure in pressure transmitters or, in rare cases, by the geodetic height of buffer fluid columns.