Turbine Efficiency

[Vraj84]

I am trying to calculate turbine efficiency. I have one high pressure boiler attached with 3 low pressure boiler. High pressure exhaust is going into reheater and then going into low pressure turbine. Can somebody please help me?

 

[georgeverghese]

Process sketch and a more detailed description with some numbers included pls

 

[Vraj84]

I have attached one excel file included sketch and High pressure turbine inlet steam pressure is 4.1345 mpag and
225 C temperature. Exhaust pressure from three low pressure turbines are 3.5 kpaA. Flow rate of steam to high pressure turbine is 1300 kg/s. Output megawatts from generator is 860 Mgh.

 

[MJCronin]

One thing I have learned from over a decade on these fora is that basic English grammar, punctuation and word usage really count.

Based on your complex EXCEL spreadsheet, you seem to be interested in a complete Power plant system efficiency, not just the efficiency of a steam turbine.

This might help .....

https://www.energy.gov/sites/prod/files/2016/09/f33/CHP-Steam Turbine.pdf

Could this be an engineering homework problem ?????

MJCronin
Sr. Process Engineer

 

[georgeverghese]

This diagram looks messed up and poorly arranged. Tell us if the following is correct, and reply with missing info :

a) HP turbine - inlet is 1350kg/sec,4134kpag, 225degC, exit pressure ? and exit temp to reheater ?

b) 3 nos of LP turbines running in parallel feed from reheater, so flow per turbine is 450kg/sec, feed pressure ?, feed temp average of 242degC, exit pressure 3.5kpa abs, temp 148degC

c) Total electrical power generated is (1 x HP + 3 x LP ) = 878 MW

 

[crshears]

OP [ original post ] appears to be a better fit for the Heat Transfer & Thermodynamics forum...but it's here now, so don't bother trying to duplicate it or move it.

Just saying...

One thing I have learned from over a decade on these fora is that basic English grammar, punctuation and word usage really count.

I hear you, MJ, but some posters are doing the best they can; unfortunately it isn't always easy to discern between these and those who can't be bothered...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Vraj84]

georgeverghese,


a) HP turbine - inlet is 1350kg/sec,4134kpag, 225degC, exit pressure ? and exit temp to reheater ?

Exit pressure = 890kPa, saturated , don't have temp gauge at exit.

b) 3 nos of LP turbines running in parallel feed from reheater, so flow per turbine is 450kg/sec, feed pressure ?, feed temp average of 242degC, exit pressure 3.5kpa abs, temp 148degC
Feed pressure - 950kPa at each LP turbine , 237*C superheated.

 

[Vraj84]

George,
I have done some calculations. I have attached sheet. please check and correct me.

 

[georgeverghese]

Before we jump in to the calcs,

a) Plain logic should indicate that HP turbine exit pressure to reheater should be > feed pressure to LP turbine. So if HP turbine exit pressure is 890kpag, then LP turbine pressure would be say 700-750kPag ?? Can you correct the pressure info.
b)Your info in the post says HP turbine inlet temp is 225degC, but your calc says 253degC ??.
c)Also the exit temp for the LP turbine in your calc says 26degC, which cannot be correct ( this sounds more like the downstream surface condensor exit temp, which doesnt figure in LP turbine power); 148degC sounds more realistic.
d)Note that kW is not the same as kWH; 1kW = 3600kWH

All these errors tells me you may be a college / Uni student struggling with tutorial homework. Also read up your lecture notes on how to calculate turbine eff, which may be better termed as isentropic efficiency. That will be as far as we can take you if you are a student - see Eng-Tips policy re student postings.

 

[Vraj84]

It is 950 only. Because exit steam from high pressure gets reheated and becomes superheated steam and inlet pressure to lp turbine is 950 kpa only. I collected all data from control room only.

 

[georgeverghese]

So assume inlet to HP turbine is at 225degC, not 253degC ??

Follow these steps, assuming power factor is 1.0 (in reality it may be 0.98 or so)
a) Work out power extracted from each LP turbine as you have done on the spreadsheet, with the correction that exit temp is 148degC. Say this is A kW
b)Compute kW for all 3 LP turbines = 3xA kW
c)Assume power loss in drive train is say 1-2%, so total net electrical power from 3LP turbines = 3xAx(1-0.02) = B kW
d)So power generated at HP turbine = 878e3-B
e)So shaft power derived at HP turbine = (878e3-B)/(1-0.02)=C, asssuming 2% power loss here also
f)Run trial and error calcs for enthalpy X kJ/kg for exit temp from HP turbine at 950kpag such you finally get 1350x(2799.5-X)= C approx.

g) To find isentropic eff for HP turbine, first get the entropy of the feed gas from steam tables, say this is D kJ/kg/degC
h) Then, follow a constant entropy line at exit press of 950kpag to get enthalpy of steam at P=950kpag,entropy = DkJ/kg/degC. Say this exit enthalpy at constant entropy is E kJ/kg
i)Isentropic eff of HP turbine = (2799.5-X) / (2799.5-E)

Follow a similar procedure for LP turbine by getting the enthalpy of exit steam at 3.5kpa abs following a constant entropy line.

 

[MJCronin]

In my humble opinion, there are big pieces of the steam cycle that are missing.

Condensate is collected, but how is it deaerated ? Where is the deaerator ?

With multiple sources of HP steam, where are the descriptions and efficiencies of multiple boiler feed pumps ? Don't these strongly affect your efficiency calculation ?

Where are the feedwater heaters and the extraction points from the steam cycle ? Do the boilers accept feedwater at ANY temperature ??

Lastly, rather than homemade spreadsheets, there have been software packages available for decades to perform this iterative cycle calculation.

https://www.thermoflow.com/

MJCronin
Sr. Process Engineer

 

[Vraj84]

MJCronin

I have just started my industrial career. I am just a co-op student trying analyze system so I can make mistakes not a big deal.

 

[Vraj84]

I am trying to increase my knowledge and want to do it on spreadsheet rather than purchasing ready made software.

 

[Vraj84]

George,

is it ok if I calculate efficiency of the turbine by below equation,

1)Enthalpy drop in high pressure turbine (kilowatts) =
(specific enthalpy (Kj/KG) at HP turbine inlet – specific enthalpy at HP turbine outlet) x steam flow rate into turbine (kg/s)

2)Enthalpy drop in each low pressure turbine (kilowatts) =
(specific enthalpy (Kj/KG) at LP turbine inlet – specific enthalpy at LP turbine outlet) x steam flow rate into each LP turbine (kg/s)

3) KW produced (J/S) / Enthalpy drop in High and low pressure turbine (J/S)

cause ultimately I want to know how much energy I am putting into the turbine and converting into useful work.

 

[georgeverghese]

Your definition of eff will result in an eff of 1.0 or near to 1.0. This is not the industry definition of eff - see my previous replies. For these large turbines, isentropic eff is usually in the range 70-80% at full load. Pls also see the chapter on Process Machinery Drives in the 6th or 7th edn of Perry Chem Engg Handbook for the discussion on steam turbines; McGraw Hill have done a dis service to working process engineers by stripping this chapter out in the 8th edn.

 

[Vraj84]

Thank You.