Unique Failure Mode for API Seal Plan 76

Double mechanical seals with a pressure alarm in the containment chamber are commonly used in LPG and other low boiling hydrocarbon that do not condense at the low pressure in the containment chamber.  This seal plan referred to as Plan 76 in API 682 is considered a robust safeguard in preventing catastrophic releases of toxic or flammable materials. 

As stated in API 682, Plan 76 is intended to work as follows. Leakage from the inner seal is restricted from escape by the containment seal and goes out the containment-seal vent. An orifice in the outlet line of the collector restricts flow such that high leakage of the inner seal will cause a pressure increase and trigger the PSH set at a gauge pressure of 0,07 MPa (0,7 bar) (10 psi).

This paper will discuss a unique failure mode of a cartridge seal conforming to API 682 seal Plan 76 which can lead to a catastrophic release of materials, without actuating the seal failure high pressure alarm in the containment chamber. The failure can occur if the set screws holding the seal on the shaft slip. This can lead to change in seal face settings and cause the shaft to rotate within the drive collar and shaft sleeve. The containment hydrodynamic seal, which is not cooled, fails first followed by the failure of the inner seal. When the inner seal fails the containment chamber high pressure alarm does not actuate, as the containment seal has already failed and, flammable material are released to the atmosphere without any warning.

 A case study of a fire, caused by seal failure on a butane pump equipped with a cartridge type dual mechanical seal will be presented. Factors which led to this type of failure, and the critical steps in the replacement and torquing  of the set screws  will be discussed.