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DGS contamination

DGS contamination

(OP)
Dear all forum members
we have a centrifugal compressor. Its suction pressure is 55 bars ( T=56°C) and the discharge pressure is 118 bars (T= 120°C).
The DGS is tandem with no intermediary labyrinth.
The DGS is contaminated with the oil carried over with the process gas; this gas is taken from the discharge of the first stage to be used as the sealing gas in the DGS
For a long period of time we have not opened the purging valves scaling of the filters. Could that lead to this contamination? If so, why does the contamination happens always in the drive through side?

RE: DGS contamination

Check your piping and establish if you have preferential flow to the discharge end. Liquids will take the least resistance you may need to include a heater if you have liquids dropping out and you should drain any potential points to ensure no liquids during start up!

RE: DGS contamination

You first need to determine if the liquids are being carried over in the process gas as free liquid or as vapour, which then condenses out as the pressure/temperature drops across the seal faces.

If it is free liquid then you need a good liquid knock out filter and a coalescing filter in your seal gas system. If it is vapour condensing then you need dewpoint management - typically a heater.

To determine which, you need to examine the removed seals closely. If free liquid is the problem, you will find it on both the process side of the seal and the atmospheric side (usually piped to flare) of the primary seal. If it is a dewpoint problem, you will find liquid only on the atmospheric side of the primary seal and the process side will remain relatively clean.

In either case, even if you install suitable gas conditioning equipment such as liquid knock out, coalescing filter and/or heater, unless you ensure a flow of seal gas through them under all conditions, your seals will become contaminated. The biggest problem here is that dry gas seals leak statically. When the compressor is stopped and under pressure, or during start up, shut down or other transient conditions such as recirc. or line packing, there is often not enough differential over the compressor to drive the seal gas through the system. In consequence, the seals leak raw, untreated process gas directly from the compressor casing. Under these circumstances, no amount of filtering etc. is of any use.

The solution is to install a seal gas booster such as Flowserve's "Ampliflow" which will maintain flow through the conditioning system whenever there is low differential, or to use a clean, dry gas from an external source which must of course be available at all times, at a higher pressure than settle out and compatible with your process.

The above is a greatly simplified explanation for what may be a complex problem. Your DGS supplier should be able to provide you with support to correctly diagnose and cure this problem.

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