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While the dry gas seal system has many advantages over the liquid sealing system such as far fewer systems, less maintenance required and greater reliability, there are a few disadvantages which are not insurmountable but must be considered in the design of such a system.
(This article is a best practice presented by Michael Sean and William Forsthoffer at the 2016 Asia Turbomachinery & Pump Symposium).
Sensitivity to dirt – since clearances between seal faces are usually less than 0.0005 inch and seal design is essential to proper operation, the fluid passing between the faces must be clean (5–10 microns maximum particle size). If it is not, the small grooves (indentations) necessary for seal face separation will become plugged thus causing face contact and seal failure.
Sensitivity to saturated gas – saturated fluids increase the probability of groove (indentation) blockage. Lift-off speed – as will be explained below, a minimum speed is required for operation. Care must be taken in variable speed operation to assure that operation is always above this speed. It is recommended that the seal test be conducted for a period at turning gear speed to confirm proper ‘lift off’ followed by seal face inspection.
Positive prevention of toxic gas leaks to atmosphere – since all seals leak, the system must be designed to preclude the possibility of toxic of flammable gas leaks out of the system. This will be discussed in detail below.
Possible oil ingestion from the lube system – a suitable separation seal must be provided to eliminate the possibility of oil ingestion from the bearings. Whenever a gas seal system is utilized, the design of the critical equipment by definition incorporates a separate lube oil and seal system. Consideration must be given during the design or retrofit phases to the separation between the liquid (lube) and gas seal system.
‘O’ ring (secondary seal components) design and maintenance – most seal vendors state that ‘O’ ring life is limited and should be changed every five years for operating seals as well as spare seals. The writer’s experience has shown that dry gas ‘O’ ring seals can exceed this limit. It is recommended that seal vendors be required to provide references for similar applications prior to making a decision to change out the seals after five years. If all of the above considerations are incorporated in the design of a gas seal system, its reliability has the potential to exceed that of a liquid seal system and the operating costs can be reduced.
One must consider that relative reliability between gas and liquid seal systems are a function of proper specification, design, etc. as mentioned previously. A properly designed liquid seal system that is operated and maintained can achieve reliabilities of a gas seal system. Also, when one considers operating costs of the two systems, various factors must be considered.
While the loss of costly seal oil is positively eliminated, with a gas seal system (assuming oil ingestion from the lube system does not occur) the loss of process gas, while minimal, can be expensive. It is argued that the loss of process gas from a liquid seal system through drainer vents and degassing tank vents, is also significant. While this may be true in many cases, a properly specified, designed and operated liquid seal system can minimize process gas leakage such that it is equal or even less than that of a gas seal.
There is no question that gas seal systems contain far fewer components and are easier to maintain than liquid seal systems. These systems will be used extensively in the years ahead. The intention of this discussion is to point out that existing liquid seal systems that cannot be justified for retrofit or cannot be retrofitted easily, can be modified to minimize outward gas leakage and optimize safety and reliability.