Problem on DGS for H2 Recycle Gas Compressor
Problem on DGS for H2 Recycle Gas Compressor
(OP)
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.
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.





RE: Problem on DGS for H2 Recycle Gas Compressor
1. Is the seal a tandem or double configuration?
2. What gas is used for primary seal gas, secondary seal gas, buffer gas and seperation gas?
3. If purified process gas is used for primary seal gas, do you have an alternate gas supply for start-up and shutdown?
4. Do your procedures maintain positive differential pressures during the pressure/depressure process at unit start-up? (O2 free process)
5. If you use a gas conditioning unit, what level of filtration is used?
Johnny Pellin
RE: Problem on DGS for H2 Recycle Gas Compressor
Also, check that your seal gas pressure is adequately dewpointed, ie Tdew < min ambient temp at the required operating pressure of the primary seal.
RE: Problem on DGS for H2 Recycle Gas Compressor
- During shutdown, do you keep the compressor pressurized? Do you have long stand still period or short (<12 ~ 24 hours)
- Regarding process gas, do you have heavy hydrocarbons (e.g. C5 and beyond) mixed with main hydrogen gas and has the DGS vendor or OEM some seal gas condensation risk analysis in the past based on normal conditions and settle out (if applicable) ? has the gas composition changed between design / actual?
In some cases, when the gas is clean (no risk of condensation), negative flow through the primary seal (assuming a tandem configuration, which you need to confirm), can be tolerated - temporarily.
What is the driver type ? steam turbine ? do you have slow roll condition?
Lastly, is it a bidirectional or unidirectional type of seal ? reverse rotation during shutdown, could occur before pressures stabilize and could damage unidirectional seal.
Do you monitor performance of DGS? i.e. flow to primary seal ?
RE: Problem on DGS for H2 Recycle Gas Compressor
We have also seen problems during layup when nitrogen for separation gas was not available. Running the bearing lubrication system without separation gas creates a risk of lube oil getting between the seal faces. This oil would tend to coke off in operation which could be a source of “dirt”. From your original post, it is not clear if the failure is on the inner seal or the outer seal. A schematic of your system would be very helpful.
Johnny Pellin
RE: Problem on DGS for H2 Recycle Gas Compressor
having some hydrogen released in the vent line should not be a big issue (from toxicity / safety view point).
Tandem type DGS is "generally" better referenced than double DGS, so it should be the first selection option unless some strict standard applies on emissions to atmosphere (high H2S content, etc.).
Double DGS should be an option when suction pressure is limited, say below 6 ~ 10 bar, otherwise your plant N2 pressure would not be sufficient to buffer the seal. In addition, you need a back up source of N2 available permanently, either bottles or a small booster, is this your case?
Another question, JJPellin: when separation gas was lost, you also should have lost N2 as main DGS buffer, thereby tripping the machined immediately - am I correct ? otherwise do you apply some particular philosophy for interlock?
Back to Tandem configuration, which is also hypothetical unless OP clarifies it, contamination of the secondary seal by the lubrication oil seems to occur at low speed, e.g. during start-up, and I was told by some OEM that at rated speed, centrifugal forces associated with a specific design of the shaft are normally acting as a temporary obstacle for the oil to flow toward the secondary seal. I also think the OP has issue with primary seal, as secondary seal failure would not trigger high pressure on primary vent, typically.
This is all speculative, unless OP gives more insight.
RE: Problem on DGS for H2 Recycle Gas Compressor
If there is no way of getting the primary seal gas supply pressure to be higher, then you may be forced to auto blowdown the H2 recycle compressor for all shutdown cases to avoid fouling the seal faces with process gas. Auto blowdown of the compression circuit is also the typical course of action for loss of N2 separation gas on the outer seal. We have also used compressed dry air as the separation gas in some compression trains when N2 gas or inert gas was not available.
In all our large GT drive compression trains, an automated purge sequence is used to clear the circuit of any air / O2. A dedicated automated purge gas valve is installed on a parallel line across the main feedgas supply SDV for this purpose.
RE: Problem on DGS for H2 Recycle Gas Compressor
It is most likely that the OP is describing primary seal leakage on a tandem arrangement. Thus, separation gas would not be relevant.
Johnny Pellin
RE: Problem on DGS for H2 Recycle Gas Compressor
I try to give you more details
- Seal arrangement: Tandem with int labyrinth / bidirectional.
- Driver type : steam turbine with slow roll condition
- For primary seal, we use N2 for startup and shutdown, and H2 from 2 nd stage of compressor in normal operation.
- For secondary seal, we use nitrogen (N2) and for separation, we use also N2.
- The supply lines to the gas seal is equipped with coalesce filter elements with grain size < 5µm in addition the use of gas heating (steam tracing).
- Gas composition: H2 + C1+ C2 + C3 + iC4 + nC4 + C5.
- For DGS monitoring : we have PDT for primary seal vent and FT for secondary seal supply.
Finally, I attached two pictures the first DGS failure in 2014 and the second DGS failure in 2016.
RE: Problem on DGS for H2 Recycle Gas Compressor
And still you have, by design, seal gas heating + heat tracing, which means that the gas composition was well known to be prone to condensation, so the seal gas has been overheated in order to keep its actual temperature distant from its dew point, I guess ~20 degree C as minimum margin should be enough. The very fact that C5+ exists in gas composition indicates that this potential for condensation is real. By the way, beware the uncertainty on the components higher than C6+ - trace level can dramatically shift the seal gas dew point.
Fair enough that seal gas heating is included, but we need to be clear on something, that is often an overlooked problem (its only recently that OEM & technical community in general admitted this to be an issue) which is as follows:
You could super heat the seal gas and heat trace it to keep the seal gas distant from the dew point, BUT - as soon as the seal gas enters the seal assembly / compartment, there is no mean to control its temperature. Lets skip the stand still condition and look at normal operation only at low ambient: the seal gas assembly (wall temperature) can happen to be at lower temperature than seal gas dew point whereby condensate can form when the gas contacts the metal. Its quite typical the next planned start-up, is the condition where seal deteriorates mechanically due to the formation of condensate.
I am not saying this is the explanation, but it is one plausible scenario to evaluate.
Question: has the problem same level of severity on both the drive-end and non-drive end DGS's?
RE: Problem on DGS for H2 Recycle Gas Compressor
RE: Problem on DGS for H2 Recycle Gas Compressor
settle out pressure = 12 kg/cm2G
startup N2 pressure = 6,5 kg/cm2G
RE: Problem on DGS for H2 Recycle Gas Compressor
A Haskel booster in this application would use a slipstream of N2 at 6barg to boost the main supply N2 from 6barg to 13-14barg to feed the FCV.
Check that this 12barg is the max possible settleout pressure of the compressor - there may be some failure scenarios that result in higher settleout pressures - ask a process engineer to build a model of this worst case scenario on Pro II or Hysis and run the settleout simulation.
RE: Problem on DGS for H2 Recycle Gas Compressor
I don't think that the original design just overlooked the issue of buffering the compressor for settle out pressure condition. They should have some mitigation in place, whether it is short term or longer pressurized stand still. What I am saying is that its unlikely that such a 'big elephant' in the room would have been missed...
RE: Problem on DGS for H2 Recycle Gas Compressor
RE: Problem on DGS for H2 Recycle Gas Compressor
Following chart, which simply shows, different operating modes of the compressor, in order to ensure availability of external seal gas (Nitrogen in this case) at all transient condition.
Operation Mode Description N2 availability for primary seal
1- Initial Slow Turning at 16 RPM Casing Pressure 0.5 Kg/cm2g (Start-up) Yes
2- Pressurization / Loading 0.5-6.8 Kg/cm2g Yes
3- dynamic operation 6.8 Kg/cm2g Hydrogen from 2nd stage of Compressor
(suction pressure / sealing pressure)
4- Shut-down / Settle Out 9.5 to 12 Kg/cm2 g Hydrogen from 2nd stage
(casing isolated and pressure balanced)
5- Shut-down / Depressurization 6.5 down to 0 Kg/cm2 g Change primary seal gas to Nitrogen
0 RPM, Venting the Casing
6- Cool Down Slow Turning at 16 RPM Yes
Another point change over from 2nd stage discharge to Nitrogen supply is manual.
My first question is with this startup and shutdown procedure there is a risk of damaging the seals?
The second question can you explain me what happens exactly in settle out in this condition (N2 at 6.5kg/cm2G as seal gas for primary seal)?
RE: Problem on DGS for H2 Recycle Gas Compressor
1/ Can you investigate what are those reasons exactly? Is it process-related reasons ?
2/ I am also unclear about start-up from the sequence you described.
You are explicit on use of N2 for shutdown but for start-up, I can only guess as follows
High purity H2 has been used in orginal design to buffer the machine during start up? is this correct ?
But this gas has been replaced with N2 as part of changed philosophy. So during start-up, N2 is used as primary seal, then when compressor ramps up to a threshold speed / pressure level, the seal gas source is switched from N2 to H2 taken from a high pressure source [which could be of different gas composition than compressor process gas, but accounted for by original design]. Question is - is the switch from N2 to H2 automatic or done by an operator?
3/ what is the typical duration of your settle out condition ? Refer to my previous post related to gas entering seal gas assembly...
Do you have risk analysis report performed by OEM/DGS vendor for the gas composition [H2 taken from high pressure source, your so-called '2nd stage' pressure source]. A longer duration for settle out condition, poses even higher risk on dry gas seals integrity if possibility of contamination exists because of the scenario I described.
Is your machine feeding the 2nd stage ? when subject machine is shutdown, is 2nd stage operating or stopped too ? Is not a must to know this but just to complete the understanding as I assume the availability of H2 from that source is warranted anyway.
RE: Problem on DGS for H2 Recycle Gas Compressor
Until now we have not done.
2- The startup procedure, is as your describtion.
3- The switch from N2 to H2 is done by an operator.
4- The duration of settle out condition is about 15 to 20 minutes.
5- The 2nd stage is the second discharge of compressor and in shutdown condition we close the suction and the 2 discharges valves.
RE: Problem on DGS for H2 Recycle Gas Compressor
Reverting to normal supply H2 from the backup source can be manual, since the startup event is manned locally.
RE: Problem on DGS for H2 Recycle Gas Compressor
Because what is the OEM and DGS vendors doing? sitting back and waiting for next failure to happen?
Or maybe owner is happy to buy new DGS cartridges every now and then, so why bother.
Sorry for the direct approach, but it appears it is not machinery process aspects which would be the most worrying.
RE: Problem on DGS for H2 Recycle Gas Compressor
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.
RE: Problem on DGS for H2 Recycle Gas Compressor
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.
RE: Problem on DGS for H2 Recycle Gas Compressor
RE: Problem on DGS for H2 Recycle Gas Compressor
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