Dry gas seal

Dears'

we have a screw comp. with a dry gas seal (John Crane) Type Tandom , we used N2 as a buffer gas for this seal.
suddenly we've found a leakege from the seal discharge side for the female rotor.

Q1 : Is changing of the seal the only solution ?
Q2 : What are the possile causes to make the seal leak ?
Original Post
once you are sure your dgs is screwed, then change-out is the only solution. but you need to investigate why your seal failed to prevent recurrence.

you might want to elaborate more on the event. at what speed (operating or during transient)? how is the leak flow trend like, how about your primary seal inlet filter performance trend, how about your suction side dgs leak flow trend, process changes, rotor vib performance..etc
Have you found the leaking fluid? It is nitrogen or system fluid? May be, only outside seal has failed and nitrogen is leaking out. In case of tandem compound seals, only one seal failure will not cause seal leakage. Alos, is it a bellows seal?
Probable causes of seal failure
-Leakge through sleeve. O ring between sleeve and shaft may be damaged.
-Seal faces damaged. Check lightband after seal removal using monochromatic light source.
-O ring between between stationary face and gland damaged.
-O ring between seal ring and sleeve damaged.
- Misalignment.
- High vibration.
- Deposition of material near dynamic elastomer or shaft wear near this elastomer.
-Seal face groove wear.
- Low buffer fluid pressure.
Regards
Irshad
quote:
only outside seal has failed and nitrogen is leaking out.

quote:
In case of tandem compound seals, only one seal failure will not cause seal leakage.


if primary seal fail then you will have high leak flow (primary vent). normally, the final monitoring parameter is the flow going to the primary vent and this parameter is commonly used as trip interlock. if secondary seal fail, then you will have high leak going to secondary vent, not the primary vent. normally secondary vent is not tied to the seal control supply system nor trip interlock. buffer gas supply is subject to primary seal performance. hence, you will always go back to the primary seal peformance if you notice high leak flow at the secondary vent.

big boss, if you hope that issue you are having here is of spurious, then go check the buffer gas PCV for the discharge end DGS, if you have individual buffer gas PCV for each ends. PCV failure (100% open) will cause high buffer gas supply. most of these excessive supply will not pass the secondary seal but it will pass through the intermediate labyrinth and increase the primary leak flow. if you have common PCV for both ends DGS buffer gas supply and you see no increase in primary vent flow of suction side, then your discharge end DGS has failed.

i always prefer to have details on the issue rather than shooting you all the possibilities (in fact you can easily get this by googling) related to DGS failure. by this way, it will not only you, but i will also benefit something from the discussion.
quote:
“we have a screw comp. with a dry gas seal (John Crane) Type Tandom , we used N2 as a buffer gas for this seal. suddenly we've found a leakege from the seal discharge side for the female rotor.”


First of all, being a tandem seal, the primary seal is the one that performs the main sealing function – the secondary seal is only there to maintain containment in the event of a primary seal failure, allowing the machine to be safely shutdown (tripped) without loss of process gas to the local environment.

The buffer gas seal complements the dry gas seal but has a completely separate function. It is usually either a labyrinth or carbon ring and its purpose is to 'protect' the dry gas seal by preventing lube oil from reaching the secondary (or outboard) seal. Buffer gas is injected into the seal to enhance its sealing capability. You can replace ‘buffer’ by ‘separation’ to describe the function of the seal in a different way. In this case, the mention of N2 is most likely irrelevant, but it is important to ensure that the buffer gas supply is turned on before the lube oil pumps/machine is started.

Far and away the most common cause of dry gas seal failure/excessive leakage is contamination of the working faces – the working gap is only about 3 to 5 microns (a human hair is about 125 to 150 microns in diameter) so it doesn’t take much in the way of particulant contamination (think talcum powder) to cause a seal failure. You should check your seal filtration system for damaged/blown filters and have any contaminants analysed (likewise later on if you have the seal removed and overhauled – you will need to identify any contaminants that are found and then figure out their likely source).

With such a small working gap, it is possible that the seal face can be ‘cocked’ and fail to seal – a shock (such as a trip) can often remedy this issue.

I have a couple of compressors with dry gas seals that often leak after startup, but the leakage rates gradually drop off to normal between 6 and 12 hours of operation. I believe that this is due to fine particles being drawn into the seal across the seal between the compressor and the dry gas seal (when the seal gas pressures are in a transient mode during startup and shutdown conditions) and gradually being blown out following a period of operation.

Is this a new machine or has it been operating for a while? Do you get many failures of these seals? Do failures occur more at the suction or discharge end? What is the typical MTBF of the seals? What is the current leak rate compared to alarm and trip settings?

Was the excessive leakage noted immediately after a startup or did it develop over a period of time when the machine was operating normally?

Typically, seal faces are tungsten or silicon based and you should check for compatibility with the sealing gas to ensure that there is no corrosion taking place

If there is high leakage at all times, then you have little choice but to replace the seal. Make sure you get a report describing the reason for failure.
just wanna prevent confusion in my previous respond.

here is my understanding based on few compressors i am dealing with. in tandem DGS, buffer gas is buffer gas, separation gas is separation gas.. buffer gas goes to secondary seal cavity. nitrogen is normal inert gas used as buffer gas.. normally, the same gas i.e the nitrogen from the same source (not through the buffer gas PCV) also goes to outside laby as separation gas. yea we all know separation gas is to prevent oil ingression.. i dont use 'buffer' term as separation gas

the intermediate labyrinth i mentioned in my previous respond is the labyrinth between primary and secondary seal..
Valve posted:
just wanna prevent confusion in my previous respond.

here is my understanding based on few compressors i am dealing with. in tandem DGS, buffer gas is buffer gas, separation gas is separation gas.. buffer gas goes to secondary seal cavity. nitrogen is normal inert gas used as buffer gas.. normally, the same gas i.e the nitrogen from the same source (not through the buffer gas PCV) also goes to outside laby as separation gas. yea we all know separation gas is to prevent oil ingression.. i dont use 'buffer' term as separation gas

the intermediate labyrinth i mentioned in my previous respond is the labyrinth between primary and secondary seal..

Yes Mr Valve you are correct ..Separation is at normally 2 -3 Kg/cm2 by Separate PCV and buffer at 4 Kg/cm2 ..may vary case to case ...

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