I will give you some history first, Someone may wish to add or challenge this. For many years LPG pumps were sealed with Single seals and then there was a requirement to contain seal leakage on these applications. Liquid Tandem seals were available at the time and they worked well however there was a cost associated with the ancilary equipment Plan 52 vessels. These vessels were equiped with instrumentation that added to the costs. Contacting Dry running seals were developed to contain any primary seal leakage and these seals were fitted with a Pressure Swithch/Transmitter to alert the operator that the primary seal failed and a controlled shut can be carried out. The issue with these seals was that the operator did not know if the seal was healthy to operate on process fluid if the primary seal failed. There were some sites that introduced a false N2 signal to check the health of the dry running back up seal. These days contacting dry running seals have advanced with new materials and lessons learnt. When selecting a back up seal you need to consider the Piping Plans. Ideally you should have API Plan 11 for the Primary seal (this depends on Pump type) and API Plan 72 which introduces a buffer gas N2and reduces fugative emissions and prevents Icing on cold applications. You will need to include API Plan76 to divert primary seal leakage to the flare or a vapour recovery system. This option has a lower inital cost compared with Plan 52. So the million dollar question is Contacting or non contacting? There are many applications world wide applying both seal types with good experience. I believe that this is a User preference. When using API Plan 76 you can monitor the condition of either seal as Plan 76 applies to both seal types. Some questions I would ask is shaft diameter, speed, operating pressure and temperature before I woud decide Seal type. Trust that this helps
