Mechanical Seal Selection

Crest offers many types of pumps with a variety of arrangements including magnetic drive, sealless and mechanical seals. Depending upon the operating conditions factors such as liquid, temperature, solids content fluid density (S.G.) as well as the pump installation itself often means that a mechanically sealed pump is the obvious choice. There are several types of mechanical seal and whilst Crest can fit specific seal if required, it is fair to say that 99% of seals fit under three separate categories. We hope you enjoy our mechanical seal selection briefing we have written below.

Single Internal Mechanical Seals (MSH Type)
This is the single most common seal type and consists of a single large 316 stainless steel spring surrounding a rubber bellows and fitted with a silicon carbide stationary seat and rotating face. We fit Silicon carbide as a standard material but other material combinations such as carbon and ceramic can be fitted if required. The rubber bellows is available in either EPDM or Viton though, again, other materials can be used. The seal is mounted internally so the 316 stainless steel spring and other metal components are exposed to the pumped liquid, as this can be a limiting factor it is important to ensure that correct materials are selected. Please refer to Crest if in any doubt.

Double Mechanical Seals Requires Pressurised External Flush (Double MSH/MSH Type)
For liquids that contain solids, especially abrasive solids, or that have a tendency to crystalize a double mechanical seal is needed. This arrangement consists of two MSH seals that are fitted back to back within a seal chamber that will have a separate pressurised flush needing to be at a pressure of one bar above the maximum discharge pressure of the pump. The inboard seal (the seal nearest the pump) will see the pumped liquid but only the rubber bellows, face and seat; the spring and metal components will only be exposed to the flush liquid (usually water). The outboard seal (in the seal chamber) is used to seal the chamber. The pressure is always higher within the seal chamber ensuring that only clean liquid is at the critical area across the face and that solids and / or crystals cannot damage the face or seat. The pressurised flush can be provided by either an external source or by thermosyphon sealing system such as the Crest SS24 sealing system. A specific advantage of using double seals is that the pump can be protected against dry running.

Single External Mechanical Seals (MSC Type)
This is a PTFE mechanical seal with a machined bellows mainly used for handling clean acids and scrubber liquors, the only materials in contact with the pumped medium is PTFE and silicon carbide (rotating face and stationary seat) so this seal is perfect for handling highly corrosive acids etc. Controlled pressure of the face against the seat is effected by a single external spring surrounding the PTFE bellows. MSC seals are simple to set up and maintain.

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