Q&A

Q & A: Sealing Solutions

This article addresses common questions and challenges faced in sealing centrifugal pumps. Readers are encouraged to submit questions for consideration in future print editions of Pump Engineer magazine.
 
By Michael Huebner, Principal Engineer, Flowserve Corporation

Question: I am specifying an Arrangement 2 seal with a containment seal (2CW-CS) for an application. What are the advantages and limitations between using a contacting and noncontacting containment seal?

Answer: A containment seal is a specialized outer seal in an Arrangement 2 seal where the containment seal will
operate on vaporized process fluid or an externally supplied buffer gas. The concept of the containment seal is to provide a redundant seal to capture leakage from the inner seal and direct it to a collection system. If the inner seal fails catastrophically, the containment seal is designed to prevent excessive process fluid from leaking to the environment and to allow for an orderly shutdown of the pump. API 682 Second Edition (2002) formalized the definition of a
containment seal although the concept had been in limited use for a number of years prior to this. The greatest challenge for a containment seal is that it must operate in a dry environment. Most mechanical seals require a liquid
environment where the liquid provides lubrication for the seal faces, as well as cooling the face generated heat.
Containment seals, however, must operate on process vapors or buffer gas. This requires some fundamentally
different seal design details than found in a typical liquid mechanical seal. Seal OEMs have met this challenge with two different approaches:

• a contacting containment seal
• a non-contacting containment seal

  • Both can be considered as technically equivalent but each has characteristics which may make it more suitable for a specific application.

Contacting Containment Seal 
A contacting containment seal operates with the two seal faces in contact while the pump is in operation. This results in a dry sliding interface between the faces. This condition will quickly result in a seal failure in a typical mechanical seal. A contacting containment seal, however, has been designed with very light spring loading, approximately 10 per cent of the loading of a typical seal, to reduce wear and heat generation. In addition, the containment seal chamber is maintained at a very low pressure (less than 10 PSI or 0,7 bar) to further reduce face loading. The seal face materials are also specifically selected for these dry running conditions. Typically, the carbon seal face is formulated with a high graphite content to provide self-lubrication. The seal faces themselves are sometimes provided with face features which help control loading and keep the sliding interface clean and free from wear debris. The advantage of a contacting mechanical seal is that the faces are, by definition, in contact. This means that there is virtually no fluid film between the faces which results in very low leakage. In normal operation, the emissions across the seal are lower than other competing designs. The seal faces can operate successfully over a wide range of speeds, including very low speeds.

To read more of Michael Huebner's article, or to submit a question relating to sealing solutions, please contact the editor Deirdre Morgan

 

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