Any time you use dual seals (two seals) in an application, you should have a fluid circulating between them.

If the pressure of the fluid between the seals is higher than stuffing box pressure we call it barrier fluid. If the pressure is lower than stuffing box pressure we call it buffer fluid

The liquid can be circulated either by forced circulation, a pumping ring or convection. The method that you will use will be dictated by the pressure in the stufing box, pump speed and shaft size. All seal manufacturers have charts available to give you the correct guidelines.

If you elect to use a forced circulation system be sure to introduce the fluid into the bottom connection and out the top connection of the seal gland. This arrangement will insure that the space between the seals is vented and proper cooling will take place.

Forced circulation is the recommended method with all vertical shaft applications, although it is possible to offset the centering of the seal gland and get a small amount of pumping action as the liquid circulating in the seal changes its velocity at the convection tank connections.

Many of the latest seal designs utilize a built in pumping ring to enhance convection. This pumping arrangement is very necessary whenever oil is used as the barrier fluid because of oil’s low specific heat and poor conductivity.


The illustration shows a typical convection system that can be used with two balanced seals.

Water is one of the best barrier or buffer fluids because of its high specific heat and good conductivity. Petroleum oil is probably one of the worse because of its low specific heat and poor conductivity. Keep this in mind when you select a barrier or buffer fluid for your seals.

The type of seal you select will determine if the fluid between the seals has to be kept higher (Barrier) or lower (Buffer) than the stuffing box pressure. Fluctuating pressures are normal in this business so you should select seals that are hydraulically balanced in both directions to eliminate any problems that might be caused when the barrier fluid or system pressure varies.

Be sure to connect the convection tank or forced lubrication system so that the inlet is at the bottom of the dual seal and the outlet discharges from the top of the seal. This arrangement will allow the seal to vent and insure that the passages are full of liquid.

The low pressure fluid that is circulated between dual mechanical seals is called buffer fluid. If the fluid is at a higher pressure than stuffing box pressure it is called barrier fluid.