Estimation of Leakage Flow Through A Labrynth Gas Seal
Estimation of Leakage Flow Through A Labrynth Gas Seal
(OP)
Can anyone point me to a resource where I can find out how to estimate the gas leakage rate across a labrynth seal. I have the gas pressures on either side of the labrynth, the number of teeth, the inner diameter and the clearance?
Thankyou
Thankyou





RE: Estimation of Leakage Flow Through A Labrynth Gas Seal
Good Luck,
Tim, TurboEngineer
RE: Estimation of Leakage Flow Through A Labrynth Gas Seal
I have a centrifugal compressor (pumping natural gas) fitted with tandem dry gas seals. I know the leakage rates of natural gas from the secondary dry gas seal. To stop contamination of the secondary seal with lube oil, an outboard labrynth tertiary seal is fitted between the dry gas seal and the sleeve bearing - this labrynth is ported, with N2 being pumped through it to stop oil migration across it. The resulting N2 and natural gas mix, in the void between the dry gas seal and the labrynth, is vented to atmosphere.
The need for N2 has been challenged (cost cutting) with a proposal to replace its buffer function with air. Functionally I don't have a problem with this, but from a safety perspective, an air and natural gas mix is not ideal.
Knowing the labrynth parameters, I am looking to estimate air flow across the labrynth to determine the likely mix ratios of air and natural gas to see where we might be relative to explosive limits. Anything that might help put a perspective on this is welcomed
thanks
Terry
RE: Estimation of Leakage Flow Through A Labrynth Gas Seal
Whoever said you should use air instead of GN2 in a purge gas seal with natural gas in it needs to get their head examined! Due to the tight clearances in laby seals, they rub the shaft. This could generate enough heat that, with an oxidizer present, fire is a possibility. Even if there is not enough heat generated to ignite the gas, all substances have an ignition pressure in the presence of an oxidizer. LOX is certainly a better oxidizer then air, but if I can light a piece of wood on fire by throwing a rock at it while it is covered with LOX, then perhaps before your people go to air they should do a little research into ignition pressures. All it takes is one good rub to ruin your day!
I use a similar sealing arrangement in rocket engine turbopump interpropellant seals. My seals are similar to yours except there are two outboard laby galleries on both sides of the purge gas gallery. I have never used anything other then helium or nitrogen gas as a purge gas because it will not support combustion. I have actually worked on the design of centrifugal natural gas compressors at work, but I am not familiar with their sealing arrangement. Take a look at www.barber-nichols.com if you’d like to see what I’m up to here.
Good Luck!
RE: Estimation of Leakage Flow Through A Labrynth Gas Seal
So any methods of calculating labrynth leakage are appreciated
thanks
Terry
RE: Estimation of Leakage Flow Through A Labrynth Gas Seal
The use of air for a buffer gas is quite common for compressors in natural gas (transmission)service. In fact, air is used far more than N2 and I cannot remember seeing N2 used for any recently installed units in gas transmission lines.
The reason is simple. Air is available at most gas transmission stations and N2 bottles are a hassle given that most compressor stations now operate unattended. If you run out of N2 during the night then the unit shuts down. Now the maintenance guy on the call out is perhaps upset and Gas Control is upset because throughput is reduced. And I hope you have spare bottles available.
Furthermore, the amount of seal gas getting through the second seal is nearly zero so I doubt that you would ever be able to get above the LEL.
If there was a problem using air then it would be public knowledge by both the users and the equipment manufacturers.
The vendor should be able to tell you how much buffer air you need if you need to know now. Otherwise, the quantity of buffer air is shown on the P and IDs submitted by the vendor during the drawing review stage.
BTW, the downstream seal in a tandem arrangement is used to provide a safe and orderly shutdown in the event that the first seal fails. It's purpose is not to keep the unit running as a backup.
You should plan on having an air tank in the compressor room in the event of an ESD for the roll down process assuming that the air compressors are not running.
Final thought. You need to use instrument quality air.
RE: Estimation of Leakage Flow Through A Labrynth Gas Seal
RE: Estimation of Leakage Flow Through A Labrynth Gas Seal
It look like israelik has found Egli for you at his recommended website.
“Laby Flex is a Windows program based on A. Egli's equation for calculating
Labyrinth Seal leakage for straight through or for interlocking (stepped) labyrinths”
RE: Estimation of Leakage Flow Through A Labrynth Gas Seal
The reason why air can be used in recent installations of dry gas seals is that they use contacting tertiary seals, with low leakage rates.
The leakage from our dry gas seal cartridges (if fitted with a contacting tertiary seal) is 70sl/min expected and 268sl/min guaranteed (manufacturers figures). We have labrynth tertiaries at the moment and expect a much higher leakage rate (up to 10X in fact) - whatever, the assertion that leakage is nearly zero is wrong and the potential for an explosive mix (air/nat gas) in the secondary vent does exist.
Our N2 buffer gas comes from an N2 skid (running whenever the lube oil pumps are on) where air is pumped through membranes which separate O2 from air: the remainder is used as the buffer gas. The membranes are quite expensive and we would like to go to air but..the use of N2 was HAZOP driven and undoing a HAZOP is not easy.
Going to contacting tertiaries is also expensive and we want to move forward from a position of understanding, where the risks are fully understood.
RE: Estimation of Leakage Flow Through A Labrynth Gas Seal
1) "Steam and gas turbines" by Sodola - volume 1. You can find there a formula of a flow through different seals. This formula is approved by tests for sure...
2) "Gas turbine engineering handbook", M. Boyce.
3) " Jet engine construction", Chronin. - This book is written in russian, however it has the most accurate data. It represents a results of continued work done by Stodola.
RE: Estimation of Leakage Flow Through A Labrynth Gas Seal
WOW- It's interesting to see that someone has the classic stodola.