17 replies to this topic

[sheiko]

Hi,

I was wondering how manufacturers determine the steam specific consumption (expressed in kg/h/kW) displayed in steam turbines data sheets?

I believe it is calculated as follow:

W = 860/(h1-h2)/E, with:
- W = specific consumption (kg/h/kW)
- h1 = steam enthalpy @ specified inlet conditions (kcal/kg)
- h2 = steam enthalpy @ specified outlet conditions (kcal/kg)
- E = efficiency (fraction)

Is it the way they do? My problem is that, in one case, the turbine efficiency is not displayed on the data sheets and I would like to back-calculate it.

PS: 1 KWh = 860 kcal

Thanks

Edited by sheiko, 15 March 2012 - 11:04 PM.

[breizh]

Hi Sheiko ,

Consider this resource , it may help
http://www.katmarsof...com/turbine.htm

Breizh

Edited by breizh, 15 March 2012 - 03:08 AM.

[sheiko]

Thanks Breizh!

Using Katmarsoftware tool, I find design efficiencies around 25% for backpressure turbines and around 30% for condensation turbines.
These results are in line with those found in the litterature.

I would however be happy if Harvey could share with us the theory behind the software. It seems the equations used are different than the formula I have written above (found in a manual from TOTAL), as I find different results.

I would also like to know how to monitor the steam consumption of steam turbines in "real life"?
My understanding is that the specific consumption (expressed in kg/h/kW) is fixed and the steam consumption (kg/h) only vary with power load (kW).
Is it correct?

Edited by sheiko, 15 March 2012 - 11:01 PM.

[S.AHMAD]

1. I worked in an ammonia/urea plant 3 years ago. The plant has several steam turbines and all of them have steam flowmeters installed. Thus, monitoring the actual steam consumption was easy.
2. In your situation, one way I can think of is to calculate from the shaft power.
3. By knowing efficiency, the ASR can be calculated and TSR is fixed by the steam inlet and outlet pressure
4. If the turbine is driving a compressor, for example, you can determine the shaft power from the compressor operating data but need to assume the shaft mechanical efficiency.
5. Steam consumptions can be verified by steam balance.

Edited by S.AHMAD, 15 March 2012 - 09:20 PM.

[sheiko]

Thanks
ASR = Actual Steam Rate
TSR = Theoretical Steam Rate
Right?

Edited by sheiko, 15 March 2012 - 09:32 PM.

[S.AHMAD]

Yes right

[breizh]

In real life , We install a meter and that tell us a lot on the performance of the equipment & process.
This is based on my past experience .

Breizh

Edited by breizh, 15 March 2012 - 09:59 PM.

[sheiko]

Alright.
From your experience, are flowmeters installed uptream or downstream the steam-turbines? Is there a preferred technology?

Edited by sheiko, 15 March 2012 - 11:06 PM.

[breizh]

http://www.rosemount...annubarflow.pdf

It was annubar flowmeter .


Breizh

Edited by breizh, 16 March 2012 - 02:40 AM.

[breizh]

other resources :

http://www.sugartech...calcs/index.php




http://www1.eere.ene...ating_equip.pdf

Edited by breizh, 16 March 2012 - 01:49 AM.

[katmar]

The way a turbine's efficiency is defined is a bit different from most other efficiencies in that the actual steam uasage is compared with the usage in a perfectly isentropic expansion, i.e.

E = (h₁ - h₂) / ( h₁ - h_2S)

The terms are as defined in Sheiko's first post and h_2S is the enthalpy at the outlet pressure after an isentropic expansion

This is covered in many thermo and rotating machinery textbooks, but a useful summary of it all was in an article in Hydrocarbon Processing, November 1988, p105-108 by V Ganapathy. The SugarTech link from Breizh also gives the basics.

The specific steam consumption is not totally fixed, but for small variations you could use that assumption. It would be better to use the estimating method given by SugarTech for the efficiency when the power changes.

The efficiencies you have back calculated seem very low. I would expect them to be >50% and more likely in the 70-80% range, depending on the sizes. You say the 25-30% numbers you found are in line with what you have seen in the literature, but I can only think that those must be based on some other definition of efficiency. If you define E = (h₁ - h₂) / h₁ You would probably get those sort of numbers, but that is not the way I have seen it done - although it does give you a better indication of what fraction of the energy in the steam you have converted to shaft power.

[S.AHMAD]

1. Definition of steam turbine efficiency is well established as mentioned by Katmar and several info rpovided by Breizh.
2. However, in actual practice, the process engineers normally encountered problem in determining the actual enthalpy at the exhaust because of unknown temperature. Pressure gauge is normally installed but not temperature. Without knowing the temperature, enthalpy cannot be determined.
3. For condensing type, the actual steam dryness at the exhaust is unknown.
4. The situation is more problematic for those plants do not have steam flowmeter, like what Sheiko is experiencing.
5. If steam flowrate is available, the efficiency can be estimated by taking the ratio TSR/ASR x 100
6. ASR can be calculated if the shaft power is available, which is normally can be calculated from process conditions of compressor, as an example.
7. TSR can be calculated if the inlet P & T are known and exhaust P is available. If inlet and exhaust conditions unchanged, TSR is also unchanged.

Edited by S.AHMAD, 16 March 2012 - 02:56 AM.

[sheiko]

The way a turbine's efficiency is defined is a bit different from most other efficiencies in that the actual steam uasage is compared with the usage in a perfectly isentropic expansion, i.e.

E = (h₁ - h₂) / ( h₁ - h_2S)

The terms are as defined in Sheiko's first post and h_2S is the enthalpy at the outlet pressure after an isentropic expansion

I understand that E is the isentropic efficiency.
Does this mean that we can always consider the expansion process through the real turbine as an isentropic one?

The efficiencies you have back calculated seem very low. I would expect them to be >50% and more likely in the 70-80% range, depending on the sizes. You say the 25-30% numbers you found are in line with what you have seen in the literature, but I can only think that those must be based on some other definition of efficiency. If you define E = (h₁ - h₂) / h₁ You would probably get those sort of numbers, but that is not the way I have seen it done - although it does give you a better indication of what fraction of the energy in the steam you have converted to shaft power.

Attached is the specification where I have found the formula shown in my first post (by the way, seems that the formula is incorrect right?).

Efficiencies are defined as the turbine efficiency and range from 20% to 70%, depending on the power rating of the turbine. In my case, the steam turbines are rated < 40 kW, so I have considered 20-25% as a sensible range.

For example, one data sheet display:
- inlet pressure = 25 bara
- inlet temperature = 340°C
- outlet pressure = 3.5 bara
- max power = 36.8 kW
- max specific consumption = 37.8 kg/h/kW

Attached Files

•  Steam turbines.pdf   353.36KB   362 downloads

Edited by sheiko, 16 March 2012 - 11:20 AM.

[katmar]

I understand that E is the isentropic efficiency.
Does this mean that we can always consider the expansion process through the real turbine as an isentropic one?

If the efficiency were 100% then the process would be perfectly isentropic. The efficiency is a measure of how far the actual process deviates from being purely isentropic.

Attached is the specification where I have found the formula shown in my first post (by the way, seems that the formula is incorrect right?).

No, the formula looks good to me.

Efficiencies are defined as the turbine efficiency and range from 20% to 70%, depending on the power rating of the turbine. In my case, the steam turbines are rated < 40 kW, so I have considered 20-25% as a sensible range.

I have no experience with these smaller units, so the efficiencies I quoted aren't really relevant. I would be happy to accept that TOTAL know what they are doing when they give those typical efficiencies for the smaller turbines.

[S.AHMAD]

1. Efficiency Terms - Internal efficiency (hi) is the ratio of actual steam enthalpy drop over isentropic enthalpy drop. Turbine exhaust conditions and gross power are determined using internal efficiency.
Overall or external efficiency (ho) is the internal efficiency multiplied by the ratio of net power to gross power. Net power is gross power less mechanical losses which reduces available shaft power without changing the exhaust conditions. Mechanical losses are typically 1% of the total turbine power.
Since the difference between overall and internal efficiency is so small, it may be ignored for process design purposes.
2. From the info given, I have calculated the design efficiency to be around 22%. The actual efficiency is expected to be lower especially at lower turbine load.
3. I attached the Excel file for reference & comments

Attached Files

•  SteamTurbineEfficiency.xlsx   9.31KB   330 downloads

[kkala]

For clarifications on steam turbine efficiency (according to post by Katmar) and an arithmetic example you can look at Gael D. Ulrich and Palligarnai T. Vasudevan, "Chemical Engineering - Process Design and Economics: A Practical Guide", 2nd edition, Process Publishing 2004, Chapter 4, para "Drives and Power recovery machines". Typical efficiencies (per Katmar) of axial steam turbines (and other drives) are given in Figure 4.2. Turbine efficiency in this figure increases with shaft power and ranges from 40% (100kW) to 80% (20000 kW). Of course individual turbines present some deviation from this line of typical efficiencies.
Attached "example.xls" calculates actual shaft power of a turbine, receiving 0.5 kg/s steam (36 bara, 400 oC) and exhausting steam of 5.0 bara (efficiency per above=40%). Ideal isentropic shaft power is 235 kW, actual 94 kW, and actual temperature of exhausting steam 285 oC. The example (based on Ulrich's illustration 4.2, figures modified) may clarify way of estimating actual exit steam temperature and help clarification of other points on the topic.

Attached Files

•  example.xls   36KB   242 downloads

[S.AHMAD]

Dear kkala
1. The calculation method for steam turbine is well understood and your example is excellent.
2. However, to determine the efficiency of existing installation is not as easy as in your example
3. The inlet conditions (P&amp;T) and outlet pressure is normally known but the outlet conditions (T and steam dryness) are normally unknown. Therefore, the actual enthalpy of the outlet stream is not calculable. For back-pressure turbine, knowing the temperature is normally sufficient if the steam at outlet is saturated or superheated. The problem is normally for wet-steam (e.g. in condensing type) we need to know the steam dryness in order to determine the enthalpy. This is the dilemma facing for existing installation.
4. The typical efficiency mentioned in your post is normally for multistage, but typical efficiency for single-valve and single-stage turbine is around 20% depending on power as well as speed. The efficiency normally decreases with decreasing output power.

Edited by S.AHMAD, 24 March 2012 - 08:51 PM.

[kkala]

Dear S.AHMAD, the example aims at clarifying some points developed in the discussion of the topic. For instance, steam entropy increases from 6.83 to 7.41 kJ/kg/oK; steam exhaust temperature is calculated to 285 oC, versus 153 oC in an ideal (isentropic) expansion, etc. It does not represent the performance guarantee test, I agree on this.

In such a test, the interest is on actual steam consumption at nominal conditions; steam consumption (at least for big turbine size) had better be directly measured, as well as shaft power. Actual conditions during test differ somehow to nominal, so "corrections" are made (assuming given performance curve for them). I suppose the test can be optionally performed at vendor's shop (all instruments available). Or vendor can temporarily carry all necessary instrumentation to the field, if steam at its shop is not available.
Note: Concerning two steam boilers ordered here in last decade, fuel efficiency had no financial penalty for vendor, contrary to fuel consumption at nominal (normal) conditions (corresponding to steam consumption for turbines). We judged fuel consumption+corrections simpler and clearer for the performance guarantees, compared to efficiency+corrections. Probably this concept could be applied to steam turbines too.

Said figure 4.2 in Ulrich's book refers to "axial steam turbines", not mentioning stages. Efficiency apparently concerns total of all stages, curve limited between 100 - 15000 kW. See attached "efficiency.xls", probably useful. Another set of typical steam turbine efficiencies (inlet steam 41.4 bar, 399 oC) is presented in Perry (7th ed, Sect 29 - Process Machinery Drives, Steam turbines, Table 29.9). Efficiency of a 373 kW single stage turbine is written as 30%; so it could be 20% for a ~100 kW turbine, if single staged.
Note: Efficiencies quoted in Perry seem to concern total of all stages, not "stage efficiencies" as written.

Attached Files

•  efficiency.xls   23.5KB   202 downloads

Edited by kkala, 25 March 2012 - 10:41 AM.