9 replies to this topic

[parisbab]

Dear all

i am working as a head engineer at a sulphuric acid plant. As it's the usual the acid production is coupled with power generation (steam turbine driven generators). Lately, in one of our steam turbines we are experiencing loss of vacuum, which we have attributed to the gland steam condenser malfunction. More specifically, we believe that the steam trap of this condenser fails open leading to air ingress into the surface condenser (the attached sketch will make my point clear). In this regard i m considering modifying the system by isolating the gland steam condensates from the surface condenser (as shown in the proposed system configuration in the sketch).
I would appreciate having your opinion for the approach in general as well as for the details of the proposed configuration i should consider (e.g. as i' ve read in related threads, attention should be paid to the length of the downward section of the pipe at the exit of the condenser or provision of self-venting).

Thanks in advance
Paris

Attached Files

•  Gland steam condenser.pdf   19.54KB   164 downloads

Edited by parisbab, 02 May 2010 - 02:47 AM.

[Zauberberg]

Paris,

I am not sure if I understood your post in the correct way. How do you know that there is air ingress in question, and what makes you think the air leak would come through the condensate trap?

Are you experiencing higher back-pressure in the surface condenser?

I can only request from you to put some more details into the sketch - design vs. actual pressures, temperatures and flows being the minimum data. For finding the optimum solution, you need to fully convince yourself about what is the true cause of problem.

Meanwhile, have a look at the article on Vacuum Piping System.

Attached Files

•  Proper Piping for Vacuum Systems.pdf   87.04KB   110 downloads

[parisbab]

Paris,

I am not sure if I understood your post in the correct way. How do you know that there is air ingress in question, and what makes you think the air leak would come through the condensate trap?

Are you experiencing higher back-pressure in the surface condenser?

I can only request from you to put some more details into the sketch - design vs. actual pressures, temperatures and flows being the minimum data. For finding the optimum solution, you need to fully convince yourself about what is the true cause of problem.

Meanwhile, have a look at the article on Vacuum Piping System.

Zauberberg,

thank you for your reply.

The air ingress due to the condensate trap's malfunction is considered as the "culprit", since the vacuum in the surface condenser restored when we throttled the block valve upstream the trap (of course at the cost of condensate back up in the gland steam condenser). Anyway for now we have replaced the trap but i believe that the condensate recovery savings doesn't worth the risk of future turbine shutdowns (for the same reason). Moreover checking the design of the second steam turbine operating at our plant, i saw that the gland steam condensates are thrown to the drain (through a seal loop which is not applicable in this turbine due to space limitations).

Unfortunately because the turbine is very old (built and installed in 60s) we don't have detailed material balances and specifications (but i will come up with some calculations tomorrow that i will be back at work).

In any case i think the article you attached (thank you for that) revealed a weakness of the proposed modification (the slope of the downward piping).

Any other comment would be welcome.

Thanks again
Paris

Edited by parisbab, 02 May 2010 - 12:00 PM.

[kkala]

Lately, in one of our steam turbines we are experiencing loss of vacuum, which we have attributed to the gland steam condenser malfunction. More specifically, we believe that the steam trap of this condenser fails open leading to air ingress into the surface condenser (the attached sketch will make my point clear). In this regard i m considering modifying the system by isolating the gland steam condensates from the surface condenser (as shown in the proposed system configuration in the sketch).

Brief understanding is that (contrary to other turbines) steam trap lets chamber of 960 mbar a "communicate" with surface condenser chamber of P=50 mbar a (tempw=33 0C ?), so P increases and turbine efficiency decreases.
Proposed configuration eliminates suspected trap and any "communication" among the two chambers, so problem attributed to it is apparently eliminated.
Line at 40 cm distance from chamber should be inclined (no pockets). The latter needs ability to resist vacuum or pressure vacuum vent (mentioned self venting?), but this must have been provided already (since presently chamber content can be steam).
Distance of HLL of drain tank to the chamber bottom is assuming ~2 m, which means that water flow will go upwards when chamber pressure gets ~ 810 mbar a or lower (normal pressure=960 mbar a). This may occur and has to be assessed for this specific case. It may be good to cover drain tank and install a vent on it to protect condensate purity.
As mentioned in previous post, more details given could result in a "deeper" discussion.

Edited by kkala, 02 May 2010 - 01:12 PM.

[Zauberberg]

Hello Paris,

Thanks for the additional information. Air ingress will have almost the same consequence as condensate backup, and perhaps you could be mixing between the two. Not saying that your diagnostics is incorrect, just make sure that you have done everything to confirm the true culprit of increased condensation pressure. I used to work in refinery Vacuum Distillation Unit and I remember beautiful stories coupled with ejector systems and vacuum condensers. They are almost like live beings - so moody and unpredictable.

A simple "soap test" on the condensate trap or any flange in the loop will likely point at the air ingress (vacuum will suck-in the sprayed foam). Some pinhole leaks due to corrosion will also cause the same kind of problem.

As for the barometric leg modification, you could consider slightly different piping arrangement and make the things work perfectly. One big advantage in your case is that you operate the condenser at a sufficiently high absolute pressure and there is no need for high driving force in terms of hydrostatic head.

[parisbab]

Hi Kkala

your understanding is very correct (actually it's more than the decrease in turbine's efficiency, it's the turbine trip).

The latter needs ability to resist vacuum or pressure vacuum vent (mentioned self venting?), but this must have been provided already (since presently chamber content can be steam).

I really don't get your point here.

Distance of HLL of drain tank to the chamber bottom is assuming ~2 m, which means that water flow will go upwards when chamber pressure gets ~ 810 mbar a or lower (normal pressure=960 mbar a). This may occur and has to be assessed for this specific case.

I don't think that there is any possibility the pressure in the gland steam condenser to fall at such levels (810 mbar). May be it's not clear in the sketch, but the partial vacuum there (enough to suck the steam/air from the turbine's labyrinths)is created by a blower (with limited capacity).

It may be good to cover drain tank and install a vent on it to protect condensate purity.

Condensate purity is not an issue since they will overflow to the sewage system.

Greetings from the North!
Paris

Edited by parisbab, 03 May 2010 - 07:56 AM.

[parisbab]

Hello Paris,

Thanks for the additional information. Air ingress will have almost the same consequence as condensate backup, and perhaps you could be mixing between the two. Not saying that your diagnostics is incorrect, just make sure that you have done everything to confirm the true culprit of increased condensation pressure. I used to work in refinery Vacuum Distillation Unit and I remember beautiful stories coupled with ejector systems and vacuum condensers. They are almost like live beings - so moody and unpredictable.

A simple "soap test" on the condensate trap or any flange in the loop will likely point at the air ingress (vacuum will suck-in the sprayed foam). Some pinhole leaks due to corrosion will also cause the same kind of problem.

As for the barometric leg modification, you could consider slightly different piping arrangement and make the things work perfectly. One big advantage in your case is that you operate the condenser at a sufficiently high absolute pressure and there is no need for high driving force in terms of hydrostatic head.

Hi Zauberberg

probably my description was a bit vague. The explanation we gave for the pressure increase in the surface condenser was that the steam trap failure (stuck open) allowed the gland steam condenser to communicate with the turbine's surface condenser (leading to turbine trip because of vacuum loss, set point: 680 mbar vacuum). The condensate backup i mentioned was referred to the gland steam condenser and as a result of the block valve (before the steam trap) throttling. Actually it was the improvement of the vacuum after that throttling, that led us to our explanation. Anyway as i 've already mentioned we replaced the steam trap and the vacuum restored to the normal levels (950-970 mbar).

I suppose that by "the slightly different piping arrangement" you mean the piping inclination mentioned before. Or may be something else?

Thanks for your interest
Paris

[fallah]

Hello Paris,

Thanks for the additional information. Air ingress will have almost the same consequence as condensate backup, and perhaps you could be mixing between the two. Not saying that your diagnostics is incorrect, just make sure that you have done everything to confirm the true culprit of increased condensation pressure. I used to work in refinery Vacuum Distillation Unit and I remember beautiful stories coupled with ejector systems and vacuum condensers. They are almost like live beings - so moody and unpredictable.

A simple "soap test" on the condensate trap or any flange in the loop will likely point at the air ingress (vacuum will suck-in the sprayed foam). Some pinhole leaks due to corrosion will also cause the same kind of problem.

As for the barometric leg modification, you could consider slightly different piping arrangement and make the things work perfectly. One big advantage in your case is that you operate the condenser at a sufficiently high absolute pressure and there is no need for high driving force in terms of hydrostatic head.

Hi Zauberberg

probably my description was a bit vague. The explanation we gave for the pressure increase in the surface condenser was that the steam trap failure (stuck open) allowed the gland steam condenser to communicate with the turbine's surface condenser (leading to turbine trip because of vacuum loss, set point: 680 mbar vacuum). The condensate backup i mentioned was referred to the gland steam condenser and as a result of the block valve (before the steam trap) throttling. Actually it was the improvement of the vacuum after that throttling, that led us to our explanation. Anyway as i 've already mentioned we replaced the steam trap and the vacuum restored to the normal levels (950-970 mbar).

I suppose that by "the slightly different piping arrangement" you mean the piping inclination mentioned before. Or may be something else?

Thanks for your interest
Paris

Paris:

I think if potential stuck open of steam trap would still be existed,you have to going to perform what you proposed as the solution (isolating gland steam codenser from surface condenser) and you can not just rely on throttling of the isolation valve in upstream of the steam trap.

Regards

[kkala]

The latter needs ability to resist vacuum or pressure vacuum vent (mentioned self venting?), but this must have been provided already (since presently chamber content can be steam).
I really don't get your point here.

As a rule every vessel containing steam should have a vacuum breaker or be vacuum resistant, since steam will create vacuum when it gets cooled / condensed and the vessel is isolated.
The 910 mbar chamber contains steam and air, so I thought it could create vacuum on cooling and there had been provision for facing it. Now I see the induced draft fan at the chamber, able to create -53 mbarg pressure. The chamber is expected to resist this vacuum.
However max steam content in the chamber could create "deeper" vacuum on cooling and this can be estimated. This vacuum could occur only if complete isolation of the chamber is possible, which is not certain (due to air circuit isolation may not be complete).

Edited by kkala, 03 May 2010 - 03:47 PM.

[agorag]

The comments by all are quite insightful.

However, from the practical point of view, since the trap is the thorn, before embarking on changing the scheme, why dont you start off with
1. changing the design of the trap
2. install a simple 2-steam-traps-in-series configuration?
It may end all your woes and save $$!