When venting hydrogen from the generator casing to the atmosphere, is it better to use the top or bottom vent. It seems if you were to not completely purge the generator, venting from the bottom vent would maintain better purity of the hydrogen but if completely purging the top vent would be better. any thoughts on this would be greatly appreciated.

 

streetglide,

It seems you are using two terms interchangeably: venting, and purging. I guess I could see where you could use the word "venting" for the process of flowing hydrogen out of the generator casing during purging, but you seem to be making a distinction of some kind which isn't clear.

The reason hydrogen is purged using the top pipe/manifold in the generator casing is that it is pushed out by the heavier CO2 during purging. Hydrogen is very light, and CO2 is much heavier, so purging H2 by admitting CO2 from the bottom pipe while allowing the hydrogen to flow out of the top pipe helps to ensure that the hydrogen is more effectively purged and there isn't so much mixing of the two gases during purging. The idea is that the CO2 will "fill up" the generator casing from the bottom because it's heavier, thereby more effectively pushing (purging) the hydrogen, which is much lighter, out of the top pipe

If one is just "venting" hydrogen to atmosphere (why someone would do that is something of a mystery) it really doesn't make any difference what pipe it's "vented" out of. My definition of venting would be relieving hydrogen pressure for some reason to reduce casing pressure.

If you are talking about two separate procedures, then you could use either pipe for "venting"--if we define "venting" in the same way. But, for purging--defined by everyone as displacing the hydrogen in the generator casing either because of a hydrogen purity problem, or a seal oil problem, or a loss of AC problem--hydrogen should be released to atmosphere only through the top pipe. Because heavier CO2 is being used to displace the hydrogen during purging.

Hope this helps!

 

If you are 'venting' hydrogen to elevate purity, it would seem logical to vent from the bottom of the generator, as any impurities (such as oil vapor-introduced by malfunctioning seals) will collect in the bottom of the machine as they are heavier than the Hydrogen. If this is the case, you may need to investigate the generator seal integrity--this would not be a good thing, if the seals are leaking oil. Are you seeing any oil indications in the generator leak detectors?--(if you are, you may be spreading this oil to all parts of the rotor/stator).

Are the generator seals the bolted type or the spring loaded ones?

 

sggoat,

If the unit is running the air and hydrogen will be very well mixed as the axial fans on the generator rotor circulate the hydrogen and air. If the purity is low, the solution is not to "vent" but to increase scavenging until the purity increases. One should not wait until the purity is low before increasing the scavenging rate. It's normal as seal rings wear that it's necessary to increase scavenging rates just to maintain purity. One should also be monitoring seal oil flow rate(s) to relate increased flow-rate(s) to the need to increase scavenging rates to maintain purity. About the only other likely cause of low purity is bad quality hydrogen--presuming no water leaks.

The original poster was not clear about--nor has he clarified--the difference between "venting" (and under what circumstances "venting" would be done, and purging (if there is any difference). Some use words interchangeably when it's inappropriate--especially when speaking/writing about technical issues. It's somewhat understandable in conversation, but not so much when writing. Hopefully we'll get some clarification.

 

Some info that was gleaned from my experience with 7FA+e machines, and GE/Westinghouse Steamers--maybe this will help the poster-maybe spark some things to check--

Our 7FA+es do not have a good history of "scavenging". The purity monitors are of dubious reliability-we relied on portable monitors to confirm purity. We typically 'vented' h2 (from the bottom) and added to the top to get purity back to normal.

I certainly hope there is no 'air' being introduced into the system the poster is referring to. (not possible under normal operating conditions, anyway, as the gen casing is at or above 45 psi H2--air is only ~15 psia) Air can't get in unless it's being forced in.

As for seal oil flow rates-if the regulators are functioning properly, increases in flow rates indicate increased clearances in the seal rings-purity should not suffer at all. Unless to the extreme-where the regulators cannot maintain flow, in which case the H2 will begin leaking out (or getting more and more dissolved in the oil-to be removed by the vacuum in the LO tank). The more likely situation in this case is the oil will begin leaking into the generator--hopefully showing up in the leak detectors (unfortunately, by that time, the gen parts have probably been coated with misting oil).

Water leaks will not get into the generator (from the coolers) if the water system regulators are functioning properly- they are designed (at least most of them are) to maintain water pressure below the hydrogen pressure, so if there is a boundary compromise, hydrogen will leak into the water. The current GE Gas Turbine applications do not have water regulators, so the system pressure is dependent on diligent Operator inspections to maintain and adjust water pressure below hydrogen pressure. These units use bypass orifices to maintain the supply pressure below H2 pressure, but do not take into account fluctuations in system pressures due to other cooled systems that DO have regulators, eg., lube oil.

Maybe some of this helps, maybe none of it does, but there it is.