Triple Pressure HRSG's – What are the Advantages?

Written by: johnzactruba • Edited by: Lamar Stonecypher
Updated Jul 14, 2009 • Related Guides: Steam

With the current technology, achieving the optimum performance of an HRSG in a combined Cycle power plant is by the triple pressure mode. What is the advantage of this? How does it work?

In the Heat Recovery Steam Generator (HRSG), as in any boiler, water converts to superheated steam in three different sections. The economizer, the evaporator, and the Superheater.

Heat transfer is dependent on the mass flow of the hot gases, mass flow of the water or steam, the temperature difference, and the surface area. The critical point in the design of an HRSG is to have the gas temperatures leaving the evaporative section as close to the saturated temperature as possible. The difference known as the pinch point is in the order of 5 °C.

The gas temperature entering the HRSG is in the range of 580 °C and gradually reduces to 130 °C or less as it flows over the various heat transfer sections.

For Rankine cycles to be more efficient the steam pressure has to be high. Currently many of the steam turbines in Combined Cycle plants operate at 130 bar - the High Pressure (HP) circuit. At pressures of 130 bar the saturation temperature will be 330 °C. The gas exiting the evaporative section will be in the order of 335 °C. This means absorption of the balance heat of the flue gas has to be in the economizer. This is not possible.

To absorb more heat two additional evaporators are used. One operating at lower pressure 40 bar - Intermediate Pressure (IP) circuit with gas exiting at 255 °C . The second one at 7 bar - Low Pressure (LP) circuit with gas exiting at 170 °C is used. This together with economizers and superheaters in these pressure levels enable absorbing more heat.

This reduces the Flue gas temperature to the range of 130 °C.

Exhaust steam from the HP turbines return to the reheater coils in the HRSG. The IP steam mixes with the reheater an flows to the IP Turbine. The LP steam mixes with IP turbine exhaust steam and flows to the LP turbine.

The IP and LP steam integrates into the steam cycles with the HP, IP and LP steam turbines designed suitably to get the optimum performance. Three different feed water pumps feed the three separate pressure circuits in the HRSG.

By heating the condensate from the condensers in condensate heaters placed in the HRSG, cycle efficiency and heat absorption further increase.

The attached figure shows the flow diagram and the temperature gradient in a Triple pressure HRSG.

Flow and Temperature Gradient

johnzactruba Mar 21, 2011 8:21 PM

reply Zahoor.

"Dear John, Thank you very much for your prompt reply. I had a few more trivial questions on WHRB, which I hope you can address 1. In Oil Refining, I understand WHRB is used to generate process steam in the FCCU unit, SRU and Hydrogen Plant. What is typically the capacity range of the WHRB that is deployed in each of the units - I know this is correlated with capacity and feed (sour vs. sweet), but if you can help me understand this correlation it will be extremely helpful 2. Is WHRB used in petrochemical complexes? If so, where. I understand that naphtha cracking unit is exothermic so I presume WHRB is likely to be used there. 3. WHRB boiler application differs from industry to industry, as the flue gas composition is very different. How drastic is the change in design and configuration of WHRB from industry to industry (Cement, Steel, Petrochemical, Oil Refining, Chemical etc.)? If a WHRB manufacturer is making one type of WHRB, will he have the necessary capabilities to manufacture WHRB for all other industries? Sorry for the basic questions, and I will be really grateful if you can help me understand the same Many thanks for your kind help With my deep gratitude, Zahoor"
Basically it is fluegas. There will be some change in the speciic heat, but the desigan will main;y depend on the mass flow and temperature only. Another main factor to consider will be the prescence of corrossive elements in the gas.

Zahoor Mar 18, 2011 1:55 AM

HRSG

Dear John,

Thank you very much for your prompt reply. I had a few more trivial questions on WHRB, which I hope you can address

1. In Oil Refining, I understand WHRB is used to generate process steam in the FCCU unit, SRU and Hydrogen Plant. What is typically the capacity range of the WHRB that is deployed in each of the units - I know this is correlated with capacity and feed (sour vs. sweet), but if you can help me understand this correlation it will be extremely helpful

2. Is WHRB used in petrochemical complexes? If so, where. I understand that naphtha cracking unit is exothermic so I presume WHRB is likely to be used there.

3. WHRB boiler application differs from industry to industry, as the flue gas composition is very different. How drastic is the change in design and configuration of WHRB from industry to industry (Cement, Steel, Petrochemical, Oil Refining, Chemical etc.)? If a WHRB manufacturer is making one type of WHRB, will he have the necessary capabilities to manufacture WHRB for all other industries?

Sorry for the basic questions, and I will be really grateful if you can help me understand the same

Many thanks for your kind help

With my deep gratitude,
Zahoor

johnzactruba Mar 15, 2011 11:55 AM

reply Zahoor

1. Cement and steel plants
2. CCPP uses triple pressure to increase cycle efficiency. For process applications since the Latent heat of condensation is used in the process the efficiency level are high. so even single pressure cycle will be economical.
3.Supplementery firing increases the cost due to the additional burner system, control system and high temperature materials used in the duct area. Between horizontal and vertical designs the cost factor may not be that much different.
4. Because design is based on the flue gas quantity the fuel effect is not much.
The cost and operating cost depends on very many factors but roughly maybe 600 $ /MW - additional from steam cycle.

Zahoor Mar 15, 2011 7:15 AM

Dear John,

I am doing a study on HRSG and came across your blog. Firstly, I would like to commend you on sharing your knowledge on the subject of HRSG. If only other experts in their respective fields had an iota of selflessness that you display, the world would be a much better place.

I have some basic questions regarding HRSG and I will be truly grateful if you can help me with the same

1. I understand that HRSG are primarily deployed in CCPP. In what industries are you likely to find CCPP aside from Power and Oil Refining & Petrochemical?

2. Is it fair to assume that CCPP with HRSG are likely to be single-pressure (HP) based as the sole function of the HRSG is to generate steam to drive the ST? Whereas, in Oil Refining & Petrochemical, you are most likely expected to find a triple pressure HRSG that is installed as part of a combined-cycle cogeneration plant?

3. What is the average cost of a HRSG (per TPH or MWe - based on rating). How does that vary if the boiler is unfired or it has supplementary firing? How about if it vertical vs. horizontal?

4. Does the design and configuration of HRSG differ based on fuel i.e. natural gas vs. heavy fuel oil vs. naphtha etc. What is relative cost of power generation (per unit)?

I will be truly grateful if you can help answer these very basic questions. My apologies for the trouble.

Kind Regards,
Zahoor

rithik Nov 2, 2010 12:16 PM

Why conventional boilers are single pressure why dont we use multiple pressures.

Dear Sir,

Please tell me y r we going for only single pressures in conventional boilers.

rithik Nov 2, 2010 12:11 PM

How to design conventional Boiler & HRSG

HRSG we are designing based on pinch and approch, then what is the basis of convensional boiler.

johnzactruba Sep 1, 2010 7:08 PM

reply kj george

Unlike the conventional fired boiler the heat transfer surface design in HRSG are very critical. The water entering temperature and the gas tempeerature at the entry point is critical and should be within a small band (aprox 15 deg C) for optimised design. So depending on the configuration of the HRSG, the heat source there may be a requirement for having a high water tempeerature.

kj george Aug 31, 2010 1:44 AM

FEED TEMP IN HRSG

Why feed water temps are high in HRSG boilers?

DMR panda May 27, 2010 4:30 AM

Excellent graphical representation of 3 pr system

excellent and very interesting pictorial representation of heat gradient. It really requires insight and creativity to come with such an excellent idea.

johnzactruba Apr 28, 2010 10:07 AM

reply hawar

Ok.

hawar Apr 28, 2010 8:50 AM

hi Jiri, my thesis is " using steam of HRSG ......:
can we exchange our data about HRSG ?
thanks

johnzactruba Apr 4, 2010 6:13 PM

reply Z.A.ABBASI

Providing a waste heat boiler down stream of the gas engine can recover the heat and produce steam. Pressures and temperatures depend on the capacity and temeperature of the exhaust gas.

Z.A.ABBASI Apr 4, 2010 9:25 AM

HWO TO PRODUCE HIGH PRESSURE STEAM FROM A GAS ENGINE FLUE

PRODUCTION OF HIGH PRESSURE STEAM FROM GAS ENGINE FLUE GASES

johnzactruba Mar 16, 2010 5:30 PM

reply hassan

Vertical HRSG's require more structural steel and bigger foundations, because the weight has to be supported at greater heights.
In horizontal HRSG's proper expansion provisions are to be designed especially the steel supports and in between the sections.
Space required for vertical HRSG's are much less and makes it ideally suitable fro retrofitting in existing plants.

Hassan Mar 15, 2010 4:24 AM

Will you please tell me the advantages and disadvantages for horizontal and vertical HRSG and the difference between its.

thanks

sachin Aug 5, 2009 7:48 AM

hrsg

tel me advantages and disadvantages of triple pressure hrsg in combined cycle power plant

Prakash Jul 26, 2009 11:14 PM

Pressure range for HP, IP & LP steam

Will you please tell me the exact demarcation of pressure in between HP, IP & LP?

I mean to say, How would I differentiate that the steam is HP, IP or LP based on pressure reading ?

Regards,