Application of Dry Gas Seals in the Pumping of Liquid Hydrocarbons

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DÜPERTHAL Sicherheitstechnik GmbH & Co. KG

The Eagle Burgmann product portfolio already includes gas-lubricated seals for pumps. The faces of this type of seal offer non-contacting operation on a stable gas film. The options are for a double seal pressurized with an inert gas or a double seal in which the seal on the product side is pressurized by the medium, while the atmospheric side is operated with an unpressurized buffer gas. Such a gas-lubricated double seal would be perfect for this application, but it does not take account of the imponderables represented by the unpredictable transient conditions and other critical states.

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A few seal manufacturers, including Eagle Burgmann, have been successfully using gas-lubricated pump seal technology for a long time. In critical applications, however, failure is a recurring problem since the seals require clean gas at the faces at all times. If the factors described above are also taken into account, this becomes a high-risk solution.

Another possible approach for Houston was a liquid-lubricated seal with the seal compartment pressurized directly from the pump’s discharge port, as the pressure available at this point is higher. Unfortunately, the seals failed continuously. Particularly on hot summer days when the outdoor temperature rose. What caused this?

As described above, the liquid ethane tends to be a gas. To keep the medium in the liquid state, it had to be held at a specific temperature and pressure (approx. 21…28 bar (300…400 PSI) at 16 °C (60 °F)). When the pump is working optimally, the temperature and pressure are at the ideal point: the liquid ethane flows through the pump and reaches the shaft seal in the liquid state. But when the outdoor temperatures rise in hot weather, the temperature inside the pump rises accordingly. The ethane in the seal compartment vaporizes and so reaches the seal faces as a gas.

Those Capricious Transient Conditions!

Liquid-lubricated mechanical seals are designed for operation in liquid media. The liquid medium penetrates into the sealing gap and lubricates the seal faces. If gas fractions in the medium cause this lubricating film to lift off, the seal faces will run dry and thus be damaged. In practice, however, many liquid-lubricated seals run in ethane applications without problems. What they all need are uniform, stable operating conditions. Under highly fluctuating conditions, these seals cannot function perfectly.

Dry gas seals are able to seal ethane on their own, provided that the ethane reaches the seal as a gas. For ethane to transition to the gaseous state, either the pressure must fall or the specific vapor pressure must clearly exceed the sealing pressure. This can be achieved quickly by heating the liquid. But using an external heating source increases the complexity and maintenance requirement of the plant, which is not what the operator wants at all.

Design modifications such as narrow gaps and a labyrinth on the product side of the seal generate turbulent flows. The resulting fluid friction encourages a build-up of heat — and heat is also generated at the seal faces. The smaller the sealing gap, the less the leakage but the more heat is generated. This may be sufficient for stable sustained operation, but the risks still remain during start-up, in slow-roll or in standby if there is no friction heat.

The team of experts from Eagle Burgmann hit upon a surprising and unconventional idea. See next page ...

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