Generator Hydrogen Cooling System

Hello, I am ruiyu chen.

After reading your post, I think you are the one who can help me to get the answer for my question. I want to know that is gasmeter used to detect the hydrogen leaking from the rotor? Do you know where the detectors are placed to monitor the hydrogen leaking from the rotor through the seal oil system?

Thank you very much!

Ruiyu Chen
 
Hello, Ruiyu Chen,

We don't know what kind of turbine-generator you are talking about. My answer presumes a GE-design heavy duty gas turbine generator.

There should be P&IDs provided with any GE-design heavy duty gas turbine generator which would show the presence--not the exact location, but the presence--of any hazardous gas detector installed on the unit (turbine or generator). Sometimes it's shown on the Fire Protection P&ID for the turbine-, accessory or load compartments, and sometimes it's shown on the Generator P&ID.

That's if there are hazardous gas detectors--which check for the presence of combustible gases.

It wasn't until a few years ago that GE began providing hazardous gas detectors to detect hydrogen leaks along the shaft (through the hydrogen seals) of a generator. Sometimes they are located in exhaust ducts of the compartments (Load or Generator Collector) because the air exiting the compartments is "funneled" through the exhaust ducts and that makes it easier to detect the presence of high levels of combustible gases--as long as the hazardous gas sensors are kept clean and oil-free (that includes oil from the hands of well-meaning technicians!). And, that's difficult to do when high volumes of air are continually passing over the sensors.

So, you need to check the hazardous gas sensors for your unit (if there are any), and determine which compartments they are located in, and then go to the compartments and find the sensors to determine exactly where they are located. At least for GE-design heavy duty gas turbines.

Hope this helps! When you write for help, please try to provide as much information as you can about the equipment in question. It will help us to get you as concise a reply as possible in the quickest amount of time. Be aware, though, that while there are some very knowledgeable people here replying to threads on control.com we haven't seen every piece of equipment or configuration, so sometimes if you don't get a response it's because someone who's familiar with the equipment you describe may not be reading your thread today, or this week, or this month. There is a good deal of experience with GE-design heavy duty gas turbines here on control.com, but not so much for other manufacturers' equipment. Many are similar, but some are very different.
 
hi,

Please can someone tell me, what is an alternative to a stableflow (tm) hydrogen controllers manufactured by proton? or alternatively who all are manufacturing a hydrogen controlling system for generator cooling?

thank u
 
May I ask what is the allowable increase in hydrogen consumption if we compare it with what is specified in the operation and maintenance manual?
 
I don't understand the question. If you're saying that hydrogen consumption exceeds the amount "specified" in the manual, then isn't that excessive?

If hydrogen consumption is increasing over time, isn't that an indication of some problem or issue which needs investigation to understand and resolve if necessary?

If hydrogen consumption has been below the level specified in the manual and is now increasing and has exceeded the level specified in the manual, isn't that excessive hydrogen consumption indicative of some problem or issue which needs investigation?

At what point will the cost of hydrogen increase to an unacceptable level?

Or, more correctly, if there is an increase in hydrogen consumption from past levels and it's not known where the hydrogen is leaking/accumulating, isn't that reason enough for investigation and resolution to prevent a fire or explosion from accumulated hydrogen gases?

Every site has to make this determination based on their tolerance for hydrogen leaks and the cost of hydrogen to replace that being lost. There are sites that replace hydrogen bottles/cylinders every two hours and seem to think that's acceptable--and they have no idea where the hydrogen is "going." There are sites that won't accept even the level specified in the manual--and can't calculate the amount flowing through their hydrogen analyzer/scavenging system to see if that's the cause of the consumption (in other words, there are sites that don't think they should be replacing any hydrogen at all, or, if they do, it should be one bottle/cylinder a month or less!--for these sites they just don't want to spend any money replacing hydrogen, it's simply an economic "consideration" for them).

To my way of thinking, losing hydrogen (hydrogen consumption) without knowing where it's going/leaking/accumulating is bad. It's a safety concern. If you've ever seen the results of a hydrogen fire you would understand. When trying to understand this kind of problem, one has to understand a lot about the current condition versus the condition when the consumption was lower--was the generator casing recently opened for maintenance/repair? Has someone changed the scavenging flow rate(s)? Has seal oil flow-rate increased? Has there been a problem with cooling? There are a lot of factors to consider, not to mention the rate of increase of hydrogen consumption. How fast is it increasing?

But, it's not possible to provide a number or percentage at which someone should be concerned--it's always a subjective value, meaning it's subject to the knowledge, experience and concerns of management.
 
Dear CSA sir,

I was searching for some information & I came across this wonderful & very informative thread.

We are operating GE Frame 7FA units with H2 cooler generators, where we are maintaining H2 pressure at 30-31 psi, with purity 98-100%.

Of late 1 of my units has high "Generator common gas temperature" 156 deg F, as compared to other units which have temperatures 115-120 deg F. H2 pressure & purity has been steady for this unit, with last purity improvement being carried out on 14th Jun2015.

I checked the cooling water circuit & found it to be pretty normal with pressure & temperatures normal across all units.

My question is, what can be the possible reason for this issue?

Best Regards
 
Timus,

Are you sure the "Common Gas" temperature is inside the generator casing?

When did this problem start? After a maintenance outage? After a trip from load (high load)?

I don't have any F-class Generator Package Wiring Drawings to refer to, but if I recall correctly there are some RTDs used to monitor the air entering the collector housing and the air leaving the collector housing--and they are given some less-than-descriptive names and locations in the documentation.

If the stator temperatures in the affected generator haven't changed much, if at all, since this high "Common Gas" temperature indication started, and if the gas temperatures entering and leaving the coolers haven't changed much, if at all, then I would suspect the RTD(s) being referred to are not located inside the generator casing, but outside. Dirt and grime can collect on RTDs in the collector housing area very easily and cause problems with readings. Also, if people don't take care when working in the area the RTDs/leads/wiring can be damaged.

You need to find your Generator Package Wiring Drawing to find the locations of all of the RTDs in and on the generator, and then look at the specific RTDs which are indicating this high "Common Gas" temperature, and decide how to proceed.

IF the "Common Gas" temperature RTD(s) are indeed inside the generator casing, then I would suspect there is some issue with one or more of the RTDs used to derive this temperature, OR with the wiring between the generator JB and the Speedtronic. Again, if the stator temperatures and the other internal generator casing temperature measurements haven't changed at the same time as this high "Common Gas" temperature indication started, then it's either a sensor (RTD) problem, or a wiring problem, or a Speedtronic input problem (which would likely be accompanied by a related Diagnostic Alarm).

The only time I've seen this occur before (also on a multi-unit site) was when air in the cooling water system made its way into the hydrogen coolers and got trapped in the top of the coolers, reducing the efficiency of the coolers significantly. BUT, in this case, all of the other internal generator casing RTDs also indicated a loss of cooling (meaning the stator temp's increase, the inlet- and outlet cooler gas temp's also increased).

I have seen pictures of some winding insulation that came loose and migrated to different locations inside the generator casing causing problems with temperature measurement. But, without knowing a lot more about exactly where this Common Gas temperature measurement is being taken, and if other temperatures have also gone higher, there's not much more to recommend.

Far too many hydrogen-cooled generators get a "dose" of lube oil during initial purging/charging when inexperienced commissioning personnel (mechanical and controls, working "together") incorrectly set up the seal oil valving. This can lead to some delayed reactions as oil and sludge, which can't be removed from the generator make their way around the internal passages and collect in unusual places. (These kinds of incidents rarely, if ever, make it into the Start-up Report, so unless someone was around when the units were commissioned this isn't generally known.)

This also occurs during initial purging/charging activities by site personnel as they get accustomed to the operating procedures required for this activity. Unfortunately, when site personnel accidentally "dose" the generator with some oil unless the Gen. Casing Liquid Level alarms go off, usually the oil doesn't get drained out, or doesn't get drained out as well as it should have been....

It would be great if you would write back with the exact location of the RTDs used to measure the "Common Gas" temperature (from the Generator Package Wiring Drawing), and the exact names of the RTDs used in the determination in the Speedtronic. Also, if the Common Gas is indeed inside the generator casing, it would be helpful if you could tell us if the stator temps have also trended higher prior to this Common Gas high temp indication, as well as the cooler inlet- and outlet gas temp's.

But, this should help get you started in your understanding and search. There isn't too much that can go awry in the generator casing.

Look forward to hearing back from you with more information, and maybe that you've found the resolution.
 
D

Dathan Bishop

I don't know if I understand the question completely. I have seen a setup with a Seal Enlargement tank on the TE and CE of the generator. Both of these Seal Enlargemnet tanks drain into a float trap via an internal weir system. When the float trap rises enough it dumps the oil into a larger tank. To prevent vapor lock on the float trap a reference line (vent) is installed back to the CE Seal enlargement tank. The larger tank has an atmospheric tank and also receives the bearing lube oil. In the event there is a high rate of oil through your seals and there isn't enough resonance time in the small Seal Enlargement tanks the larger Bearing/Seal Enlargement has a seal leg design allowing any remaining entrained hydrogen to vent off before returning to the main reservoir.

Hope this helps.
 
A very instructive answer indeed!

My unit is a Siemens 1000 MW one and works slightly different.
I'd like to know, If possible what's the correlation between cold gas temperature and H2 purity, i.e.
How much cold gas temperature would decrease in C for an 1% H2 purity improvement?

Thank you in advance.
 
Gener,

Hydrogen is much better at transferring heat than air--that's why one of the biggest reasons it's used for generator cooling. However, in my personal opinion--because I've never trended cooling and H2 purity--the relationship between them is very small, especially considering the normal range of purity is technically supposed to be very high (above 97%). Perhaps if the purity was much lower--indicating more air or other gas (CO2 not well purged during charging) or air/contaminants from leaking gas-to-water heat exchangers) there would be a larger change in cold gas temperature for a smaller (1%) change in H2 purity( which would mean less air/gas/contaminants which is less effective at transferring heat).

Having said that, a 1000 MW (1 GW) machine is a big machine--with lots of gas volume. It's possible that a 1% change in purity would mean a pretty big change in air/hydrogen concentrations and that could represent a larger change in heat transfer than for a smaller machine--say, 250 MW, or 125 MW, or 80 MW, or 30 MW.

This question was asked recently--though I'm not sure if it was on control.com. And the respondent said basically the same thing: There's no direct correlation between H2 purity at cold gas temperature, and I believe that's mostly true because normal purity is supposed to be maintained in the high 90 percent range. Again, I could imagine if the purity was in the low 90% to high 80% range (which is not recommended) that with less hydrogen and more air/CO2/contaminants a 1% change in H2 purity might make a larger difference in cold gas temperature.

But, it's interesting you're not saying what H2 purity the 1000 MW machine is being operated, and if there are problems with generator temperatures prompting the question about increasing purity and decreasing cold gas temperature and hopefully decreasing generator temperatures. Is there a problem with generator temperatures? More to the point, is there a problem with achieving H2 purity in the high 90% range?

Could there be problems with H2 coolers? Cleanliness?

Explaining the issues--presuming there are issues--would probably yield answers which would be more helpful, again presuming there are issues. But, again, there is typically no direct relationship between a small change in purity and cold gas temperature.

About the best answer for your turbine is to trend purity and cold gas temperature--presuming cooling water temperature/flow-rate is relatively constant as well as load (real and reactive).
 
Thank you very much for your quick answer.

We use to operate our machines in the lowest region of the allowed band,that is 95% H2 purity for us. This summer has been so hot and we have experienced some trouble to keep cold gas temperature within the recommended 46C, so we've had to lower power to meet the Temperature limit. Later on, we decided to increase H2 purity to a 98% and it worked fairly good, so, full power could be recovered. Some still think that such a small detail alone can be responsible for the cold gas temperature decrease we've obtained, and I'd like to know if anybody has experienced a similar situation to confirm or discard this fact.

> About the best answer for your turbine is to trend purity
> and cold gas temperature--presuming cooling water
> temperature/flow-rate is relatively constant as well as load
> (real and reactive).

This is what I've done in my unit and I've found a rough 0.8 C cold gas decrease for every 1% increase in H2 purity. I'd like to know if this figures could be right enough.
 
Gener,

I think we're kind of "uncharted territory" for operations, as, again--it's generally regarded that there isn't a direct correlation between purity and cold gas temperature. But, as was said above, with a large machine and a lot of volume it seems likely there would be some relation between the two--and by trending the signals you have likely developed the relationship for your machine.

I would say it's not possible for anyone other than the machine designers to put a firm number to the relationship between purity and cold gas temperature, but all other variables seemingly remaining relatively constant it's reasonable to presume the decrease in cold gas temperature can be attributed to the increase in purity. That's probably about all that can be said without analyzing the data.
 
buvan,

The cooling gas in the generator should be dry, to protect the windings from moisture being circulated in the cooling gas.

Sources of moisture can include leaky hydrogen coolers and excess moisture entrained in the Seal Oil (along with entrained air--the air can be humid, especially if the turbine driving the generator is a steam turbine and the same oil is used for both bearing lubrication and Seal Oil.

Regardless, there should be a vapour extractor on the oil tank (L.O. or Seal Oil) to help remove any air/gases as well as any humidity.

Again, the idea is not to allow moisture to condense on the generator internals.

Poor purging/charging and contaminated hydrogen can also cause increased moisture in the generator casing. If the hydrogen is produced on site by a local hydrogen-making system they have been known to be problematic. Also, some areas of the world have been known to receive poor quality gas from their suppliers (who tests the gases they receive??? or who even looks at any quality documentation (which usually isn't supplied with many gas cylinders (with the exception of emissions calibration gases--which have also been found to be of poor quality and not within specification).

If a generator is equipped with a hydrogen dew point detector, it probably has a hydrogen drier as well, or at least a way to circulate hydrogen from the generator casing through some kind of drier (absorption beads; etc.). Hydrogen driers are used to remove moisture from the generator casing gas. (Remember to follow all purging and operation procedure before and after running the generator casing gas through any drier vessel and associated piping!)
 
F
Hi Everybody

Can anybody tell me how to compare between the two philosophies for H2 cooled generators:

1- Vent to the atmosphere
2- Return back to generator

Which of these is more useful?
 
faizon aslam,

Every manufacturer has their own philosophy--and sometimes different engineering groups in the same manufacturer have their own philosophies, as well.

Can you be more specific about what you are referring to--is it scavenging/air removal?

Do you have operational experience with either philosophy--or both?
 
j. perez,

Personally, I'm not familiar with a hydrogen seal vacuum pump. What kind of generator are you working on? Can you provide any more details?

Do you have the P&ID for the hydrogen sealing system?
 
P

Paragkumar Bhagat

Dear Gents

we are operating 7FA units in saudi arabia with hydrogen cooled generators. our hydrogen dew point entering to dryer is -30 degc. is this dew point is OK? what should be the ideal dew point temperature of Hydrogen gas in the generator?

regards
paragkumar Bhagat
 
We have intermittent oil leakage excursions that collects in the water drains. sometimes it is one end and other times it tis the other. This is a Westinghouse machine with air and H2 seal oil and air cooled. We have yet to correlate it to any transient parameter such as temperature or pressure. Are there seal failure modes that are transient...such a dirt causing temporary seal "hang up"?

>Failing hydrogen seals; improperly installed hydrogen
>seals; failing or broken hydrogen seal springs; excessive
>Seal Oil pressure are the usual culprits. Dirt/debris has
>also been known to get into the seals and cause problems,
>usually the brass rings get scored and sometimes the shaft,
>also. This is an extreme case, but has happened.
 
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