VENTING THE STUFFING BOX IN CENTRIFUGAL PUMPS V016-1
Heat is always a problem with mechanical seals and any heat generated in the stuffing box is never desirable. When we fill the pump with liquid, air frequently becomes trapped in the stuffing box with no logical way for it to vent.
In many slurry applications we connect a recirculation line from the bottom of the stuffing box back to the pump suction as a convenient method of eliminating flushing water with mechanical seals. Unfortunately this arrangement does not help the problem of venting the stuffing box/ The easiest way to vent this stuffing box air is to drill a hole in the back plate at the upper end of the stuffing box, as shown in the following diagram. This hole will vent the stuffing box during the initial filling process.
After the pump is running, the rotating shaft will throw the stuffing box fluid outward, shutting off this vent. This means that trapped air will remain close to the shaft and act as an undesirable heat shield for the seal faces and elastomer (rubber part) because the venting occurs only when the shaft is not turning.
Remember that you cannot vent the volute of a running centrifugal pump. Centrifugal force throws the liquid to the outside leaving the air in the center of the impeller.
Self-priming pumps need a vent or bleed line installed between the pump discharge flange and the discharge check valve, back to the suction source. Without this vent the pump will not be able to deliver enough pressure to open the check valve. There will be some loss through this line when the pump is operating, but the alternative is to have someone open and close the valve every time you start the pump or install some type of an automatic valve that will open and close at the correct time.