Differential Pressure Control vs Flow Control for Seal Gas Supply to Dry Gas Seal

[Rha257]

Dear All,

This discussion is based on a tandem Dry Gas Seal with an intermediate labyrinth for an API 617 centrifugal compressor.

I have read multiple sources online and in dry gas seal handbooks that differential pressure control of seal gas supplied to the dry gas seal results in higher seal gas consumption when compared to a flow control method. Can someone explain to me how does the flow control method results in lower seal gas consumption when compared to differential pressure control?

Most of the end-users I have worked with prefer to have a PDCV controlling the seal gas supply pressure which ensures a set differential pressure value between the seal gas supply line and the compressor suction pressure or balancing chamber pressure (i.e. 3 bar across the same).

Thank you in advance

 

[georgeverghese]

The dPIC arrangement is more reliable than the FIC loop when there is a risk of condensate or other liquids that may be present in the seal gas supply. In such cases, the alternate FT - FIC may give you wrong readings when the FE is partly flooded with liquid. This is almost always the case with high (hydrocarbon or water) dewpoint seal gas on startup / restart when seal gas supply lines may have cooled down to ambient. Whenever possible, use dewpoint conditioned fuel gas as seal gas.
Dont know if the FIC arrangement result in a lower seal gas consumption that a dPIC.

 

[SB1976]

It all depends upon the flow-pressure curve, specially how flat or skewed the curve is. When the flow requirement is high over a small pressure change, preference is to go for pressure control method while for high-pressure requirement in relatively small flow, we should go for flow control method. This is because in the case of high flow requirement if flow control is employed, a small change in flow might not detect the actual flow occurring. Same thing is true for cases where pressure-flow curve is skewed.