ACC Leak/Performance Issue

[Amir]

Hello Users,

We have a 1+1 configuration with 15 fans and we are having vacuum issues. The pressure does not go below 4.3 in even with the hogger running. The jets (with any configuration) can’t bring the pressure any lower after the hogger comes off.

We performed leak test using Helium and we didn’t find anything noticeable. We hired a leak detection experts and they come by twice and they didn’t find a noticeable leak either. Our pressure rises are so quick that the hogger comes on every 7 minutes at about 6 in and runs about 7 minutes until the pressure get to about 4.3 in.

We leak tested all the piping and systems under vacuum: STG exhaust, steam seals, man ways, rupture discs, condensate removal system (even the isolation valves for back flow), drains, LP blow down tank, by-pass valves, condensate tank,expansion joints, ACC tubes, and …

We have notices that some of our ACC tubes are cold and some hot and they are in a very close proximity. This usually is a sign of excessive leak.

We are open to ideas and suggestion. Please share your experience.

Regards,
-Amir

[Chris Nannemann]

Hi Amir,
Have you performed an inspection of the inlet header? Were you able to spray helium at the top of the tube bundles, right next to the inlet header?
We have experienced leaks at our tube inlets. They are sometimes hard to find with helium leak testing if you cannot spray all the way at the top, or if it is windy, etc.

Also, what chemistry program are you running, and what in what steam pH range do you operate?

I ask this because we found that running too low of steam pH leads to excessive FAC in the STG exhaust header and the ACC inlet header. This was causing a lot of leaks and we had trouble maintaining vacuum for a long time.

Regards,
Chris

[Amir]

We don’t have inlet heaters and I am not familiar with them. What are they for?

We did spray all over the tube bundles with no hits. We used long tubes to reach them.

We keep our PH on our HRH between 9.6-10.4 and the same on HP drum. We are using AVT program and only injecting Ammonia.

Thanks a lot,
-Amir

[Chris Nannemann]

Amir,
Sorry for the confusion. The ACC inlet header, not heater… I mean the point at which the steam enters the ACC tube bundles.

Sounds like your steam pH is kept in a good range. We also operate AVT with ammonia only, and have found keeping the steam pH above 9.8 minimizes the 2 pt FAC in the STG exhaust duct and ACC inlet duct.

We also had the same issue that Chris Karch discussed with the hogger isolation valve.

Good luck sir,
Chris

[chriskarch]

Have you checked your hogger suction isolation valve. Your air leakage may be coming from your hogger exhaust piping leaking through the isolation valve. We had this issue. When we removed the valve for inspection we found that there was a failure in the valve seat. This failure caused damage to the valve disk. Further investigation revealed that the valve was not properly rated for vacuum.

[Amir]

Yes, we did, We sprayed Helium at a drain downstream of the hogger when it was off and we didn’t get any hits. We had the exact same issue as you said in our 2+1 unit few weeks back.

Thanks a lot,
-Amir

[huubhubregtse]

Hi Amir,
helium leak detection is a difficult job.
It took the Rye House power plant 6 weeks to find the leak. (Loose flange of a pressure test line)
Helium leak detection is the best way to find leaks but sometimes it takes time to find it.
we have experienced leaks in bellows as well, especially with multi layer bellows.
Also the turbine exhaust area could give problems, such as near the gland steam or the casing.

Huub

[Rishi Velkar]

Hi Huub,
We are having the same issue where are the bellows located ? are you talking about the bellows located on the exhaust duct inlet headers going into the ACC ?

Thanks
Rishi

[Amir]

Yes, we checked all the areas below the LP section and exhaust.

[huubhubregtse]

Hi Amir,
if you want to eliminate possible leaks of the hogger and or holder you keeep them in operation. during operation no air can flow into the condenser via this route.
If al the ejectors are in operation and the vacuum drop is still unacceptable the leak is not in the extraction section.

Huub

[Amir]

Few weeks ago, when the leak/issue was not as bad, we brought the pressure to 3.2 inch and then isolated both the hogger and jets. Then we let the pressure rise to 3.8 in and we timed it. It took 90 sec. Per OEM this should take 20 minutes.

The other test was I had the jets and the hogger run in parallel and we still could not get the pressure to go down. Then we had the hogger on manual with all the jets isolated manually and the hogger didn’t bring the pressure down to the spec which we used to get to 3.5 in.

[MjP]

Hello Amir,

Do you have cooled water temperature monitoring and non condensable temperature? Look for big difference between those temperature each street since air leakage will decrease non condensible temperature even more than usual. As a rule of thumb, gap should be around 5 C. After locating the street in trouble, shutdown the fan and monitor with an IR camera how the cold area stars increasing, if there are one. It could help you to find the leakage.

Best,
Manuel

[huubhubregtse]

HI

the bellows are installed in the steam duct.
they consist of a SS bellows welded to CS rings.
Some of them are Multi ply where the inner sheet gives the sealing.
the other sheets are mostly having holes to avoid pressure between the sheets.
if the innner ring get a hole due to fatique there is a entrance for air into the ACC.
We have seen this problem before

Huub

[Amir]

We sprayed all over the bellows and didn’t get any indication.

[mcepeda]

We had some issues on ACC performance and we found the next on different events:
– Condensed traps at holding ejectors were not working properly so the condensed were accumulated so quick up to the point where the ejectors were almost flooded and stopped working efficiently. At that time, we purge the condensed traps almost all days until we can perform a comprehensive maintenance.
– Other issue we experienced was on the Steam Turbine Seal system, we found and air leakage through other trap that occasionally empty the water column used to seal the steam extractors from the Steam Turbine seals.
– Have you have identified low temperatures on your ACC just at some cells? Or all your ACC is colder than normal? If there are just some cells colder, you can start looking there…
– Finally, as a comment, we have found air leakage by ultrasound, if you have this as part of your tools, is cheaper and easy to use; it just need be well calibrated in order to identify air leakages.

[Amir]

We did check the ejector condensate traps. I am going to have them flushed again.

We found indication from drain on the bearing housing/seals. We thought it is the steam seal, then we increased the seal steam pressure to 7 psi and nothing changed. we put a bucket on on the bottom of the drain and we filled it with water but the Helium indication is gone but the vacuum problem is still there. On the next outage we will look at our N1 bearing housing (We have A14 unit) to make sure there is no leak there.

The low temp issue is on all cells and they move around. We could not point it to any specific cell. As the hogger comes on the cells get better but the cold temp start developing randomly again.

I have not used ultrasonic for this application. I can look into it.

Thanks a lot,
-Amir

[Nick]

Have you checked upstream of the ACC for air ingress leaks with Helium? Often the ACC is thought to be the source of the leak, yet it is often something upstream of it where the air leak is. If you haven’t done so already, think of helium testing upstream.

[Amir]

We did as much as we could including all the expansion joints and from turbine exhaust all the way to ACC.

Thanks a lot,
-Amir

[Nick]

It sounds like you’ve done a good job of chacking teh ACC, but when I say check upstream I actually mean in the steam bypass lines, any drains leading to the ACC, etc. Basically anything that connects to the ACC and follow it to its source to check a valve isn’t left openm, a flange has become loose, etc.

In my experience, air leaks are as likely to be in the BOP piping and equipment that connect to the ACC rather than the ACC itself.

[udhay61]

Hi
Hope you have taken all possible steps to locate the leaks , nice. One clarification is required whether checked the connecting pipe between the air evacuation header running in the top near the ST Exhaust duct . There will be minimum 2 to 4 connecting pipes for each module. Sometime found cracks in that causing vacuum drop. Also exactly in the ST exhaust penetrating portion sealing portion also gave problem. All syphon loops, vacuum breaker etc maybe checked. Thanks.Regards

[Franco Liza]

Amir, Has the ST a dog bone expansion joint? If it has it, inspect dog bone (exhaust expansion joint of ST), we experienced dog bone leaks, we sealing external temporally, and during an outage we fixed by re-tightening. Other possibility is leak in valve of hogger (usually is a big valve), they could not be properly sealed.

[Amir]

Hello All,

We finally found the leak!!!

We got a He hit on a drain from STG #1 Bearing Packing Head/LP Head drain. (see my comment on May 4, 2017 at 9:29 pm). The #1 bearing housing/head is located in the exhaust tunnel. We disassembled the housing/packing head and found all the joints were not sealed properly and the contractor didn’t torque the bolts to the spec and not enough RTV applied to the surfaces. We found no damages to the steam seals. Note: Our combined cycle was commissioned about 18 month ago.

After we started the combined cycle, our vacuum went to normal. This is a very hard place to find the leak cause there is a great chance there will be no hit on the drain. We believe the air got into the #1 bearing housing (which suppose to be under atmospheric pressure) from the oil lines cause we tested all the other connections. This is something for A14 users to look into in case they have vacuum leak.

I greatly appropriated you all comments and feedback.

[Andy Macqueen]

🙂

[Rishi Velkar]

Hello all,

We are having the same back pressure issue and are not able to find the leak. We have to start the hogger every two hours. We had CONCO (leak detection company) out 4 times so far yet had no success in locating the leak.

The main problem is that even an artificial leak is created by opening a valve and spraying Helium over it is not being detected properly at the air ejector. The test shot takes about 2 mins to be detected a the air ejector further more it is very weak. Has anyone faced this issue before where you were not able to detect even the test leak?

We checked the air ejector nozzles, the valves and the drains , everything is in good working condition. It just seems as though the helium gets diluted in the ACC since the leak is big. The hogger brings the DO from 700ppb to 200ppb for about thirty minutes and gradually goes back up.

We have tested the Steam gland seals, drain tanks, by pass valves, DA/condensate tank and air hogger/ejector skid and haven’t had any luck but doing testing without getting a good hit on the on the test leak is futile.

Anyone have any ideas or suggestions as to why we wouldn’t be able to get a good hit on the test shot? Can the Air leak dilute the Helium ?

• This reply was modified 7 years, 4 months ago by Rishi Velkar.

[vahidjohn]

Hi Rishi,

We have used IR thermography to assist with leak detection. This would require the removal of insulation in some areas. We also test from the ST exhaust all the way to the condensate return tank (removing the insulation along the way). In our experience Helium leak detection is not as effective and can be costly.

Regards,
Vahid John
Lead Operator
Trinidad Generation Unlimited

[Rishi Velkar]

Thanks for the response Vahid.

With IR how do you find the source of the leak , would you see a significant temperature difference on the pipe where the leak is at?

[vahidjohn]

Yes you will see a significant temperature difference on the IR camera

[Marko Lazarevic]

Hello everyone.

I was wondering whether this is the most suitable topic to express our problem, but I ll give a try. Unit (300MW) is running for more than year and a half. ACC consists of 30 cells, 6 streets, we have 3 vacuum pumps (1 is in operation). During last outage we have made several modification related to boiler side, which is now affecting less steam flow coming to HP turbine. Consequently, more steam is coming to IP and LP turbine ending up in ACC. It is possible that slighty increased steam flow is affecting ACC performance which is slightly different (worse) comparing to previous summer. Actually, this summer, for the first time, we were forced to decrease unit load, so we could keep the unit in operation, as backpressure increased up to 400mbar. As per my understanding, ACC operation might be jeopardized by means of heat transfer decreased or in some cases extremely high ambient temperature which cannot be avoided. On a regular basis we are conducting vaccum leakage test by means of stopping vacuum pump operation, keeping the fans running at the same speed along with constant ambient air temperature (as much as possible). Test lasts for 15 minutes and results are always very good (backpressure increase less than 2mbar/min). So, my question is whether it is possible that dirty heat exchange surface area is biggest reason for ACC performance deterioration (comparing to last summer 30-40mbar higher backpressure in same conditions)?

Marko

[Desmond de Haan]

Marko:
Contamination of the heat transfer surface can indeed influence the efficiency of the system, I assume by the fact you are asking you have doubts about the cleanliness of the system.

Also external factors such as ambient air temperature and wind can influence ACC performance. Warmer air will of course cool less, due to lower temperature differential and lower density. Also wind draws extra load on the fans decreasing static pressure output of the fans into the ACC.

You could consider having your bundles cleaned as this is typically good practice, I know many sights prefer the spring for this to be ready for hot summer weather, also it could be a good idea to have your fan peformance evaluated to see if your ACC is receiving sufficient air flow, or if more air flow is required to meet this new cooling capacity requirement.

Please feel free to send me a PM (desmond_DOT_dehaan@Howden_DOT_com) if you need any local references that we may be able to provide assistance for you.

[Maikol Aira]

Hello ACC Users,

I am from Peru. We would like to improve ACC cooling System (water spray or fogging system) to increase Vacuum of Steam Turbine Exhaust and have better performance. We will appreciate if you share with us your experience about it.

Best regards,
-Maikol Aira

[gmirsky@galebreaker.com]

Dear Maikol,

I represent a HP spray cooling system company and Galebreaker wind screens. We have a test facility to prove the benefits of spray cooling for an ACC. We combine wind screens with the spray cooling to best assure the spray goes into the fans it is intended for. I would be pleased to discuss with you further. You may contact me at gmirsky@galebreaker.com

[Desmond de Haan]

Makoi,

Key is to understand how much cooling capacity increase you are looking for.
Are you trying to improve peak performance on hot and windy days only, or do you need to increase your performance in all situations?
Spray systems were recently discussed in a CTI paper (TP18-02 Efficient Water Use to Boost the Air-Cooled Condenser Performance) a very good document to understand the concepts better. Downloads available here:
CTI Thermal Performace papers

You can also increase performance by improving the air flow through the ACC by mitigating wind influences to increase uniformity, cleaning your bundles and of course optimizing your fan system.

Spray systems, especially aftermarket, should be used sparingly as the ACC was probably not designed for high humidity or even wet conditions inside the ACC.
A hybrid cooling system with cooling towers to address peak cooling loads such as at Commanche can have dramatic performance effects, but are of a different investment level than the above options.

[Ockert Augustyn]

Hi Maikol. I agree with Desmond. Are you trying to get back past performance or are you trying to increase performance relative to design?

[rp@ax-system.com]

Hello Maikol,
I am agree with Desmond and Ockert.

If you observe a performance drop, my first advise would be to clean your fin tube with a good cleaning system if it’s not already done.

If during the year the performance is changing due to hot tempreature, fogging can be a good idea only if:
– relative humidity is low during your hot season (so you can vaporise a lot of water and get the adiabatic reaction which decrease air temperature)
– your hot season is short (it will consume a lot of water!).

I can support you about this 2 subjects, at your disposal 🙂 (ax-system.com).

[Author]

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