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Anonymous Poster #1

Power Limitation and High Exhaust Temperature of Single stage Impulse (Curtis) S

02/16/2017 1:36 AM

Dear All,

We are operating a large Ammonia Urea Complex in Pakistan. Following are details of Steam Turbine driving Boiler Feed Water pump at our expansion unit.

Type - Single stage impulse with one nozzle ring, one ring of reversing blades and two rows of moving blades

Following are permissible steam inlet and exhaust operating parameters;

Minimum Normal Maximum
Inlet Pressure (bar gauge) 36.3 37.3
Inlet Temperature (

0C)

375 380
Exhaust Pressure (bar gauge) 3.43 3.92

As per OEM

At normal conditions with 37.3 bar gauge inlet pressure, 380 0C inlet temperature and 3.43 bar gauge exhaust pressure turbine will deliver 1742 KW at 23040 Kg/Hr steam consumption with 215 0C exhaust temperature. Refer to attached Annex-I for steam flow diagram at normal conditions.

At most unfavorable conditions with 36.3 bar gauge inlet pressure, 3750C inlet temperature and 3.92 bar gauge exhaust pressure the turbine will deliver 1935 KW at 26510 Kg/hr steam consumption with 220 0C exhaust temperature. Refer to attached Annex-II for steam flow diagram at most unfavorable conditions.

Power requirement of boiler feed water pump to produce desired flow is 1742KW @2985RPM; which is same as nominal power of the turbine; hence turbine is undersized for the application. Refer to attached Annex-III for specification sheet of the turbine.

Turbine is able to meet power requirement of the pump after initial startup following a cooling cycle, however its performance deteriorates with the passage of time. Following is the comparison of operating parameters of the turbine;

Date

Steam inlet Pressure

Kg/cm2

Steam Inlet Temperature

0C

Exhaust Steam Pressure

Kg/cm2

Exhaust Steam Temperature 0C

Turbine Steam Consumption

Ton/Hour

Condition

11-10-16 38.4 386 4.45 244 26.4 Parameters of Start-up after a cooling cycle
13-12-16 38.6 389 4.4 256 27.3 Parameters after 02 months of operation

As per my understanding, increase in exhaust temperature of this type of turbine is a clear indicative of its performance degradation. Interestingly, performance of the turbine is restored after a cooling cycle of 10~12 hrs without doing any maintenance work. This abnormal behavior of the turbine is exhibited every time after a cooling cycle. The turbine has been overhauled several times but no abnormality is ever found in its internals.

It would be a great favor if anyone advise us on following points;

1) What can be the possible reason for this gradual increase in exhaust temperature of the turbine? Why the exhaust temperature and steam consumption restored after a cooling cycle?

2) Is this performance degradation with time related to overloading of the turbine?

Thank you in advance.

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Guru
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#1

Re: Power Limitation and High Exhaust Temperature of Single stage Impulse (Curtis) S

02/16/2017 11:11 AM

Lose the anonymous handle, but maybe the reversing diaphragm is rubbing as the steam turbine warms up. Any rub will reduce output, and result in heat up.

Is your steam source and piping up to this job? Undersized (even slightly) piping could also be a source of problems such as this, as the supply cannot keep up with normal demand at full pressure, but can deliver the required increased mass of steam per hour after piping pressure drop.

Just a wild hunch.

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Guru

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#2

Re: Power Limitation and High Exhaust Temperature of Single stage Impulse (Curtis) S

02/16/2017 12:20 PM

You need a troubleshooter to go over the system and find the problem, if you are sure it's operating out of spec...are you sure it's not within operating parameters?

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#3

Re: Power Limitation and High Exhaust Temperature of Single stage Impulse (Curtis) S

02/16/2017 2:23 PM

KISS !

Sounds like you have an internal expansion issue.

do a time study to see if the start of degradation is consistent.

15psi. variable on inlet and 7 psi variable on exhaust and 5degree in temp

mfg. specs from the book I'm guessing?

if so looks like your over on inlet pressure from the start. and have gotten higher over this 1 month.

your inlet temp is over nominal specs but your exhaust pressure has increased over specs as well.

seems like your overloading the turbine. Gradual increase due to expansion, cool down time restores normal operating parameters.

You may need to look into purchasing a bigger turbine or controlling the inlet temp and pressure to the operating limits of the turbine.

Just a thought. but I think you answered your own question.

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#4

Re: Power Limitation and High Exhaust Temperature of Single stage Impulse (Curtis) S

02/16/2017 10:08 PM

The flow diagrams mentioned as 'Annex' would be helpful. Is there a condenser immediately down stream? ...perhaps some other use of the exhaust steam?

'... increase in exhaust temperature of this type of turbine is a clear indicative of its performance degradation. ..'

.

While it will be indicative of performance degradation, it is not clearly indicative the fault is with the turbine. Higher exhaust steam temperature and pressure for a given input will require more steam through to get the same amount of work done.

So what it comes down to is why is the pressure/temperature higher downstream once warmed up?

Is some equipment downstream causing geater back pressure, using less steam (than just after start up)when steady state long term operating conditions are reached?

Does the steam condense further downstream causing pressure to fall below atmospheric? Could noncondensable gasses be leaking in that exceed ability to remove until a higher pressure is reached later in steady state operation? (Check steam plant chemistry)

Could effective thermal mass downstream be so great that after cooldown, pressures are reduced downstream for a long time until the thermal mass comes up to steady state?

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Guru
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#5
In reply to #4

Re: Power Limitation and High Exhaust Temperature of Single stage Impulse (Curtis) S

02/17/2017 2:45 PM

Sounds like you are inferring loss of heat transport in the condenser to me, but is not a turbine that exhausts into vacuum. 3 bar gauge (above that) is pretty serious pressure. That is service steam in many such plants. Perhaps the service steam pathway is not opening up.

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#6
In reply to #5

Re: Power Limitation and High Exhaust Temperature of Single stage Impulse (Curtis) S

02/17/2017 4:42 PM

That's why I was asking about what's downstream.

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Guru
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#7
In reply to #6

Re: Power Limitation and High Exhaust Temperature of Single stage Impulse (Curtis) S

02/17/2017 4:49 PM

Then we sort of agree on the statement of the first part, combined with the extension of the argument into the second part. LOL.

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#8
In reply to #7

Re: Power Limitation and High Exhaust Temperature of Single stage Impulse (Curtis) S

02/17/2017 7:33 PM

Do the steam conditions seem odd to you? Seems like a lot of superheat both going in and coming out.

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#9
In reply to #8

Re: Power Limitation and High Exhaust Temperature of Single stage Impulse (Curtis) S

03/08/2017 11:32 AM

Actually the temperatures presented goes along fairly well for an older class of double drum HRSG with that operating pressure. We have one on an LM-2500 gas turbine that produces near that steam value although ours is used for co-generation not combined cycle.

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