Dry gas seal, separation gas loss

[Matchess]

Hi

I have Turbo generator with tandem dry gas seal and carbon ring on gearbox side.
I would like to know, how long the machine can remain running in case of separation gas loss ?
My concern is about DGS potential contamination by oil mist.

In order to avoid oil migration (on DGS) is it recommended to trip the oil pump immediately when the separation gas is loss or is it acceptable to wait for machine complete stop (speed = 0 rpm) before to stop the oil pump.

regards

 

[rotw]

First I dont get generator equipped with DGS. Could you give more details.
You are not explicit on if there intermediate labyrinth or not.

Losing separation gas in the case of compressors does not mean trip but it is recommended controlled shutdown. In case of lost of lube supply from pumps normally the emergency run down tank takes over during ramp down. In no event the supply of oil shall be interrupted.

It is one thing that bearing white surface is damaged, it is another that rotor is damaged.

People with plant experience could tell further.

 

[Matchess]

Thanks for your feedback.

The machine is a exapnder - generator, the expander (on high pressure part) drive the generator (low pressure part)
The gearbox (low presure part) reduce the spped between high and low speed shafts.
The high speed shaft is equiped tandem dry gas seal(with intermediate labyrinth) and carbon ring on gearbox side.

When you say that it is reccommended to do a controlled shutdown rather to a trip it mean that the machine can operate during a some time without separation gas.
Is anyone have an idea of this periode of time (minutes, hours days) ???
And whait is the assosiated risk (working without separation) gas in such configuration ?

This machine is equiped with a run down tank, so in any case the machine will be lubricated when the shaft is rotating.

 

[rotw]

I am not very familiar with turbo-expander, I can only speak regarding centrifugal compressors. You could check if this consideration apply (e.g. your design may be comparable to integrally geared compressor - on compressor side ; lets be careful with comparisons in general)

In general, I would tend to say that if there is intermediate labyrinth then it is likely that when you loose separation gas, you also loose the intermediate buffer because it is most of time the same supply (N2) ; sometimes it is Air but rarely. If that assumption is correct, you may have migration of seal gas into the secondary vent (potential safety issue?) plus migration of seal gas into bearing vent. There is a risk of contamination of DGS (oil). How long can the machine be operated under this condition (machine running without separation gas and possibly intermediate buffer failure), IF at all acceptable, and what are the implications? If this is HAZOP or some sort of feasibility/risk study, I would do NOTHING before contacting the OEM.

 

[georgeverghese]

The risk here is contamination of lube oil with process gas(from the expander) on loss of intermediate separation gas. Agree it is not possible to say how long you would allow this contamination, but note that the longer this happens, the greater would be the risk of corrosion of all lube oil wetted components, plus the risk of a fire at the lube oil tank vent. There is also the risk of flammable process gas leak venting at the local separation gas vent, which this vent may not be designed for. Since sep gas pressure is always kept higher than expander discharge pressure, at the least trip must be triggered when sep gas pressure drops below expander LP end pressure.

 

[Matchess]

In this configuration, the expander proces gas is natural gas @ 30 barg.
The vents are piped to safe area and I'm not really concert about corrosion (tank 100% in stainless steel).
My concern was more about potential DGS contamination by oil mist. Do you think that this is a real risk ?
I can stop the machine with or without the auxiliary oil pump, the stopping time is around 10 mins.
If I stop the machine without the auxiliary pump, I have 10 mins of oil at low pressure (thank to the run down tank) => risk of DGS oil mist contamination limited.
If I stop the machine with the auxiliary pump, I have 10 mins of oil at nominal operating pressure + 10 mins of oil at low pressure =>
risk of DGS oil mist contamination higher but maybe still limited.

 

[rotw]

It is less costly to change a DGS cartridge (15-20k?) then ruin a complete rotor (operation under conditions of oil supply failure) and if you have a trip set on high bearing temperature then the machine is going to stop anyway. What about the process can an immediate trip be accomodated y/n, what is impact of depressurizing the machine and what are the consequences in terms of production ?

How long would it take for operators to move out to the machine/ field in order to "physically" monitor and report the behavior of the machine in such a critical condition before it is tripped (Hours? Day?). Based on this, and not limited to, you should make a decision what is the back up, if any,
(emergency N2 bottle?, N2 generation system 400k excluding installation costs...). This how I would look at it. Is this pre-study? Hazop?

 

[georgeverghese]

Additional risk to consider is fouling of dgs labyrinth seal on process gas side with dirt and viscous corrosion inhibitor and h2s and retrograde condensate liquids from leakage of process gas past labyrinth seal on loss of this clean sep gas. Also lube oil may leak past far side labyrinth seal, and accumulate downstream at the same time.

 

[rQuestionEngineering]

Normally, the control logic should not be configure to stop when separation gas is loss. This will protect the bearing, in case the machine is still spinning.

The logic only prevent the pumps to start without separation gas.