Some time ago, we discussed the importance of a strong shaft in extending the life of a mechanical seal in an end suction centrifugal pump. This month well review the importance of the large bore seal chamber for the same purpose.
It is interesting to note that this radical change in pump design came not from one of the many well established pump manufacturers but from a mechanical seal company who decided to explore the business of making pumps. Sadly, that gamble did not last long, but the legacy was the birth of the large bore seal chamber which has spread throughout the industry.
It can be said that the introduction of the larger bore seal chamber has provided two major benefits to mechanical seal operation, both of which contribute to increased reliability.
1. By increasing the radial space around the shaft from 0.375 ins to 0.875 ins. the larger chamber permitted an increased volume of pumpage. This permits the liquid to dissipate the heat generated by the seal faces much more readily than the lesser volume in a stuffing box. Consequently the mechanical seal will run cooler.
2. As the outer wall of the chamber is moved to a greater distance from the seal, seal rub has been eliminated. (Seal rub is a condition where excessive radial shaft movement brings the seal into contact with the bore of the stuffing box, causing premature failure.)
A number of different seal chamber designs are currently in use. The large cylindrical bore chamber is the same design as the stuffing box, except that the bore diameter is larger, but only in the area occupied by the seal. As the traditional close clearance is maintained at the bottom of the chamber, this perpetuates the capability to control the temperature and pressure of the pumpage inside the seal chamber.
The through bore design is also very popular and, while it permits self draining and self venting of the seal chamber, it severely limits the amount of control that can be exerted on the pumpage in that area.
It is important to recognize that, properly selected for the specific application, either one of these designs will effectively increase the reliability of mechanical seals in your pumps.
Author of The Practical Pumping Handbook and a specialist in Pumping Reliability, Ross Mackay can be reached at www.practicalpumping.com or at 1-800-465-6260
