Seal selection for rotary plant

摘要

The choice of what is the best seal for the job often comes down to personal preference as much as objective decisions. For many the choice is between a packed gland or mechanical seal - soft and conformable or hard and rigid. Here Andrew Goulding of James Walker & Co Ltd presents an overview of sealing options for centrifugal pumps. The predominant role of fluid seals in rotary plant such as centrifugal pumps and mixers is to retain pressurised fluid media within a process. Unless a vacuum is involved, the seal rarely needs to prevent the ingress of external contaminants, as the positive pressure differential will achieve that naturally. Two types of rotary sealing device effectively contend for this form of dynamic duty, namely, the simple packed gland (typically containing five rings of pliant material) and the somewhat more complex mechanical seal. Another favoured sealing device for rotary shafts, the elastomeric lip seal, will efficiently protect bearings, but rarely holds a pressure greater than 4bar or works at under-lip temperatures above 200℃.Although compression packing is a traditional sealing device that predates the industrial revolution, constant advances in materials technology, lubricant systems and manufacturing methods over the past 100 years have kept it at the forefront for both OEM and maintenance applications.