Problem on DGS for H2 Recycle Gas Compressor 2

[zozo21]

Hi everybody;

Please help us; we have a new (03 years old) H2 recycle gas compressor for reforming unit equipped with dry gas seals (DGS). For 3 times, when we shut down the plant and we restart again, the DGS fail (high pressure in the primary vent) it mean the primary seal is failed, so, we stop the compressor and we change by a new DGS. As per the vendor of the DGS when he made his repair report, there is some dirty between the two faces of the first seal witch they conduct to deteriorate them. Our question from where this dirty comes and how to avoid this.

Thanks.

 

[rotw]

georgeverghese,

1/ This is steam turbine driven machine; my guess is that when the permissive to start is asserted it is with N2 buffering the primary seals. Then speed controller would ramp up the turbine to the minimum operating speed (controller rated speed) based on predefined start up sequence and logic. When minimum operating speed is reached, the operator would accelerate manually the machine to bring machine to its normal operating condition / speed (common approach). If the operator switches back to normal H2, say when turbine is at minimum operating speed, and there is not enough discharge pressure developed by the machine to overcome system pressure drop in the seal gas supply line (say approx. 2 bar) then flow reversal could occur. Flow reversal can be tolerated by DGS for very short period but it has to be within limits set by the OEM. So things done manned have also their drawbacks as a bottom line.

2/ What I don't get is that during SOP, they are buffering the compressor seals with seal gas taken from compressor discharge. This is NOT advisable. Unless the 15 /20 minutes of settle out condition, before depressurization, have been accepted/mitigated by the OEM, still not a sound practice to my opinion. Even then, if it is a manual depressurization philosophy t applies, there is no guarantee that 15/20 minutes will not be exceeded because of non timely operator intervention. The baseline here is that they would have used the high pressure cylinder to keep positive flow in the DGS during pressurized stand still, but because they don't have let down station from 110 to 15 bar in place, they cannot do it.

 

[georgeverghese]

@rotw,

The 15minute duration for pressurised settleout before blowdown, in my experience, is completely unacceptable. This is probably when the flow reversal occurs and why the shaft seals are getting damaged.

In addition to the FIC regulating seal gas flow, a low flow trip transmitter should also be installed on the seal gas supply line to trip the compressor and blowdown the entire blocked in circuit immediately on detection of low flow. Confirmed seal gas flow should be included as one of the startup permissives for compressor-turbine start sequence.

 

[rotw]

Agree. The duration does not look acceptable to me too. Allthough at the end I still think its the OEM call. Optionally they can include seal gas booster, provide a let down station, thereby satisfying original design, from high pressure cylinder, or buy N2 bottles that would provide enough pressure to buffer during settle out if economically viable. However this last option would require an automatique blow down procedure, otherwise intervention may again cause troubles. regarding adding interlock for low flow trip and blow down, the duration might be restricted by the blowdown system and how it is sized. If it is sized for 15 min, the interlock initiating action wont be of help. It would help avoiding further consequences due to operator delay, if the blowdown is manual and the suggested measure would switch to auto, provided its compatible with process plant philosophy.

 

[gasnaturel]

JJPellin:

I need to undestand the seal sytem, what's the diference between (primary seal gas, secondary seal gas, buffer gas and seperation gas), if you have any document please attach them

Thank you