Learnings & selection guidelines of mechanical seal flush plan for pumps in a typical plant

Abstract

Pump mechanical seal failure incidents out number any other rotating component of a pump in a typical OG & C Plant. This is also well-known fact that up to 70% of pump maintenance expenditure is responsible for seal repair/maintenance or replacements. This arises the need for accurate selection of mechanical seal for given pump & particular service conditions.

As a standard industry practice, Pump Buyer usually follows Pump/Seal Vendor recommendation for final selection of Seal Plan. Although the onus of deciding the seal plan and material selection is fairly & squarely on the Pump/Seal Vendor, however it is recommended that Buyer shall also validate/verify the seal selection during bid evaluation stage himself (based on seal selection guidelines). This will help in avoiding changes in seal plan during detailed engineering (after award) which otherwise led to cost/schedule impact. Before selecting the seal material, type of seal the designer or pump manufacturer either needs to go through exhaustively complete scope of API682 and their field experience (operational experience of using Seal Type X in Pump Type Y with process parameters Z).

This paper will illustrate the selection guidelines based on seal vendor recommendations, feedback from Plant Owner (Plant O&M personals) and in-house lesson learnt. This paper will also discuss the work process diagram for the mechanical seal selection/coordination for OG&C plant along with brief information on various Seal types for Pumps per API 682. 

The recommendations in this paper are based on the “typical cases” of a seal for centrifugal pumps. Other types of machine, installation conditions, modes of operation, designer’s, manufacturer’s andoperator’s specifications, local regulations and so on can result in a different choice of mechanical seal. For complex sealing duties it is always advisable for the user to consult with the specialist engineer/Operator/Consultant/Seal Manufacturer.

A sealing system for hydrocarbons must often make allowance for partial dry running due to their low boiling points. However, with the right design features and face materials, it is possible to guarantee fail safe operation of the mechanical seal.
There is no doubt that the marginally higher cost for the better-quality seals will probably be recovered during the first year of operation of pumps itself. Although this seal selection guideline is applicable for new installations/Procurement of Pumps, however this may be extended for upgrading seals of Pumps in existing OG&C installations where frequent maintenance is encountered.