Help on calculations on HRSG for CCPP

[FelipeSC]

Hello!

I'm trying to specificate a HRSG and i'm having some troubles, i'm wondering if anyone could help me.

I have these data about the HRSG, wich is one from Aalborg here in Brazil:

Output Power from GT: 8MWe max

Fuel: Natural Gas

Exhaust Gas Temperature from GT: 600 °C max

Mass Flow from GT: 75 kg/s max

Steam Generation: 15 t/h max

Steam Pressure: 25 bar max

Steam Temperature: 50 °C above saturation temperature

Natural Circulation

Pinch Point: 20 °C

Single-Pressure, no additional burning.

That's the only information i've got from the HRSG.

Now, i've selected a Siemens GT SGT-300, wich has:
rpm: 14000
mass flow: 29.8kg/s
exhaust gas temp: 537°C
Output power: 7.9MWe

My doubt is: how do I proceed in order to calculate the amount of steam that will be generated, so I can select a Steam Turbine? Do I just assume a pressure, let's say, 15 bar, and then use the correspondent saturation temperature, 200°C, and assume the steam temperature 250°C, and, using the pinch point, 270°C for the exhaust gas from the hrsg? and the steam generation, how do i calculate? or can i just assume the max the hrsg can produce without considering anything else?

Sorry for my english, and i hope you can help me.

Thank you so much.

Felipe

 

[MrBTU]

Filipe,
Couldn't the HRSG people (Aalborg) do a rating run for you at the conditions you specify and tell you the amount of steam generated given the GT you have selected? There are a lot of variables, and only Aalborg knows how they will all affect the HRSG.

 

[srfish]

I would use the maximum steam pressure of 25 bar. This gives a higher saturation temperature which means a lower and more conservative mean temperature difference.

The outlet exhaust temperature is saturation temperature + pinch point. From this calculate the exchaust gas heat load.
From the heat load back into the amount of steam to be produced.

In calculating the amount of steam to be produced, allow for blowdown.

 

[FelipeSC]

actually, i don't think i can use the max 25 bar pressure.. here's what i think, i don't know if it is the right calculation:

for a 25-bar pressure -> saturation temperature = 225°C (according to the aalborg table,

http://www.aalborg-industries.com.br/downloads/vapor-sat.seco.pdf)

So, it means that the steam that's going to enter a steam turbine after it leaves the hrsg is 225°C+50°C = 275°C (since aalborg specifications say that steam temp is 50°C above saturation temperature).

Now, using the pinch point (wich is temp of exhaust gas that leaves the hrsg minus temp of steam that will enter the ST), we have:
PP = 20°C = Tgex - 275°C -> Tgex = 295°C.

So, i don't think it is possible to enter with 537°C, and have 275°C to the steam and 295°C to exhaust gas of hrsg, since 275+295 = 570°C > 537°C ! That's why i don't think it is possible to work with 25bar with that GT i've chosen.
I've redone the calculations with 15-bar pressure, and I got:
saturation temperature = 200°C
steam temperature = 250°C
gas exhaust temp = 270°C

Wich seems to be appropriate, but probably i'm doing something wrong, maybe some wrong assumptions.

Thank you!

 

[davefitz]

first, there are programs whihc can provide a more accurate estimate, such as GT-PRO. Perhaps there is a uiversity or mfr you know who can run that program and provide a beter estimate.

Second, the Siemens GT has only 29.8 / 75 = 40% of the exhaust mass flow than the max design case, so you will generate not more than 40% of the max HRSG steam , or 0.4* 15 T/hr = 6 T/hr steam.

Third, the combined cycle process is sensitive to ambient air conditions, inlcuidng plant elevation, so you will need to correct for actual plant conditions to get a reliable estimate of steam turbine output.

 

[srfish]

I believe that when the data shows "steam temperature: 50C above saturation temperature", it may refer to the entering steam temperature. The 275C calculation above would mean the steam would be leaving the HRSG 50C above saturation?

 

[boilerone]

FelipeSC,

There are too many variables and considerations there and it is easy to make mistakes. If not being an expert working for a HRSG supplier, I'd simply run GT-PRO mentioned by davefitz to get results. You can first pick your GT model from its build-in database then enter quite a few other required inputs including ambient data, system configurations and some design choices it will then size the HRSG plus almost everything else for you. You might need to tweak and run the program a few times so that the HRSG results are within your supplier's limits/criteria then you can get the output data for the steam turbine that you are looking for.

GT-PRO is by Thermoflow. You can request a free trial CD package from their site so as to get some feelings about it. The software also contains cost estimations.

 

[rmw]

Felipe,

See if this site answers any of your questions.

http://www.hrsgdesign.com/design0.htm

Ate logo,

rmw

 

[FelipeSC]

srfish:
yeah, from what i understood, it means that the steam that leaves the hrsg and activates a steam turbine is 50C above the saturation temperature, but i really don't know if it's correct. The problem is that here in brazil we're on holydays for carnival (you know, music, samba, women!), and Brazil only begins to work after carnival! Every year is the same! ehehehhe


boilerone:

That software sounds amazing. I'm going to request, but i don't know if they deliver it here in Brazil, i guess i'm too far from them =( Isn't there a downloadable free trial version?

------------------------------------------------
I found a website that shows some calculations on the heat balance,

http://www.hrsgdesign.com/design0.htm

It is really simple and useful, and may solve my problem!

The problem is, i don't know how they did the calculations on the first single pressure flow schematic, they don't show. But i guess if I use the values i got from Siemens turbine and aalborg hrsg in those calculations, i might solve the problem!

Does anyone can help?

Again, thanks a lot!

felipe

 

[FelipeSC]

rmw:

what a coincidence! =)

 

[boilerone]

FelipeSC,

I searched their site and found no link for downloading the free trial. It seems you can only request a CD to be sent to you, but I don't think they have problem sending to Brazil.

That hrsgdesign.com is really a good resource, by the way.

Happy Carnival!

 

[FelipeSC]

thank you all people!

Another question just came into my mind... the gas turbine and the steam turbine usually have great rpm's, 7500 rpm, 11000 rpm, sometimes 14000 rpm.... and the turbogenerators are, according to some literature, around 1800 rpm... how do we manage that? a reductor? if so, it would be something like 11000/1800 = 6 times. Isn't it too much for a reductor? (i don't know if 'reductor' is the right term in english!)

felipe