Compressor Tandem Dry gas Seal : Failure of inboard and outboard seal 2

[ed16]

Hello !

We are just performing a HAZOP for a Compressor Tandem dry gas seal. One question raised was regarding if the inboard (primary) and outboard (secondary) seal fails at the same time. To evaluate the risc i would be interested if this is a theoretical scenario or if someone has had this failure in reality.

Best regards
Ed16

 

[pmover]

i am not aware of any failure occurences of inboard & outboard gas seals simultaneously. however, it does not mean that it won't happen. i would believe that if both failed simultaneously, then a catastrophic condition occurred - could be anything. do be certain that the gas supply for the seals is clean and free of dirt, debris, and other impurities that can cause failure. not only during normal operating modes, but start-up, shutdown, and other plant upsets need to be considered.

good luck!
-pmover

 

[monaco8774]

By amazing coincidence we were recently asked to do the same thing based on a safety alert on our demag compressors, it turns out the alert was raised after a tandem seal failure on a compressor in a factory in Germany. However the seals were different materials (tungsten Carbide) to ours and they had no monitoring on the primary seal vent and they had their vibration monitoring switched off, the result was pretty catastrophic the shaft was wrecked and there was a big gas leak through the seals into the lube oil and out of the lube tank vent. I went in to the meeting thinking this wasnt credible for us but I was starting to change my mind half way through as for us if for some reason the outboard failed first we would not necessarily pick it up on the primary seal vent monitoring and if the vibration monitoring didnt pick it up we could then get the scenario some time later. I thought vibration would pick it up until we checked and found we had had a number of primary seal failures which had not actuallly registered even alarm level on the vibration system

 

[SJones]

http://www.hse.gov.uk/research/otopdf/2000/oto00070.pdf

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[Rotator]

This posting is a little late, but here goes anyway....

We have experienced a failure of both primary and secondary seal on a gas compressor, but I suspect that the outboard seal had already failed, and we had not picked it up due to problems with our detection instrumentation.

monaco8774 we received a similar safety alert from Man Turbo - the important point is to ensure reliable detection of primary or secondary seal failure in various scenarios.

 

[bvsrtt]

Pmover,
Failure of primary and secondary seals simultaneously can happen,due to one of the below or due to all. This can be seen in typical cases where there is no intermediate labyrinth seal between primary and secondary.

1.Defective filters. if the gas filters fail and allow large particles of dirt into the system >3 mirons.
2.Excessive axial thrust or rapid thrust bearing wear out
3.Entry of oil into secondary seal, If no barrier seals are employed and the bearing drains are choked

In the first case, Tandem dry gas seals with intermediate labyrinth seals which ensures clean N2 or air to the secondary seal, will delay the failure of sec seal. Due to failure of primary seal the high primary vent flow alarm and trip can trip the machine. such a seal failure trip will generaly close inlet and discharge valves and vent the compressor loop for safety by opening vent valve.

The seal are wonderful as long as you take all safety precautions.

Hope this will add a bit to what others said,

BVSRTT
bvsrao1949@yahoo.com

 

[bvsrtt]

the reply is meant for ed16 ( mechanical)

 

[CompressorExpert]

Ed16, this issue had been raised also by Siemens as a potential hazard in dry gas seal arrangement. I received a safety notice 3 or 4 months earlier.
Special monitoring on outboard seal should be performed.
Regards;

 

[longeron]

Most of the tandem seals and support systems that I am familiar with monitor the N2 flow or pressure to the outboard or secondary seal. Signs of an outboard or secondary seal failure should be a flow rate increase or pressure decrease.

Seals without an N2 purge would have to rely upon trending of the Primary Vent pressure or flowrate to suggest an outboard seal failure. Another way might be to monitor the N2 flow rate to the bearing isolators or barrier seal. If the pressure of the primary leakage is higher than the N2 purge, the purge could be overwhelmed by the leakage and stop flowing into the barrier seal.

 

[longeron]

Just realized a mistake- the bearing isolator or barrier seal comment won't work- The Primary and Secondary Vents would have to be unable to handle the volume of leakage from the inboard seal before the leakage would attempt to flow into the bearings.