What is the purpose of a desuperheater? 1

[BluSTi]

Hello,

I recently had an opportunity to look at a project at a natural gas-fired generation plant's desuperheater. Neat bit of engineering, but the question I had that nobody seemed to have an answer for is; why? What is the benefit of taking 1800F steam at 2200psi (or so) and injecting cool water? I get that there's a phase change involved, but why not take steam at the desired pressure/temp/quality from the turbine where those conditions exist? Is there some aspect I'm missing?

Thanks in advance!

 

[LittleInch]

To make it more usable at temperatures and pressure which are within the range of more materials.

1800F @ 2200 psi is very hot and high pressure so needs really special materials / turbines etc which cost a lot.

I also don't really know what you do with a fluid that hot.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BluSTi]

I believe the steam to be desuperheated was taken FROM the turbine, at some point in the generating process. The equipment was pretty neat, a 14" dia SCH160 P91 pipe with an Inconel sleeve and the variable orifice injector. I just can't wrap my mind around why one would spend the energy to heat the fluid that much, only to cool it back down.

Maybe a power plant engineer could chime in.

Here's a webpage with the plant I was looking at:

http://www.power-eng.com/articles/p...tments/managing-the-plant/coyote-springs.html

 

[MFJewell]

Desuperheaters and attemperators are used to condition steam. During design of the plant, the combustion turbines would have been selected first, then the HRSG to use the exhaust energy from those particular CTs, then the steam power turbine to use the steam produced. The steam turbine is most likely a tandem compound type with HP/IP/LP sections (so the HRSG has three drums that operate at specific temps/pressures). Each section of the steam turbine has a specific steam requirement. Since they operate as a merchant plant, the steam load to the power turbine was the design conditions for steam. For the cogeneration, they send steam to some other process which is specified by the customer taking the steam. In order to meet that, they had two options, add another section in the HRSG (which nobody would do) or condition the steam to meet the customer spec.

 

[BluSTi]

A HA! I knew there was something I wasn't grasping. The attemperator is conditioning the fluid for some other process (or another user entirely). Thank you, I've been confused about this for a while and couldn't get a decent answer (and the folks at the plant don't offer info if it isn't directly needed for the project).

Have a fantastic day!

 

[stgrme]

I don't believe that the steam temperature in the HRSG (Heat Recovery Steam Generator) will ever reach 1800 deg-F. The exhaust gas from the combustion turbine is about 1100 deg-F. Typical steam temperatures at the inlet of the steam turbine are 1000 deg-F to 1050 deg-F. Maximum pressure at the inlet of the steam turbine is typically 2400 psig plus/minus. Desuperheaters are used to control the temperature leaving the HRSG so as not to exceed the rated inlet temperature at the steam turbine.

If the plant has a turbine bypass system, pressure reducing valves and desuperheaters may be used to limit pressure and temperature at the discharge end of the bypass (typically, the cold reheat system in a reheat application and a condenser in reheat and non-reheat applications). These pressure and temperature reductions are required so design conditions at the discharge are consistent with the design parameters at the destination.

Best of luck!

 

[LittleInch]

If you look at page 30 of this document or in the article you posted it explains why they de-superheat - supply of process steam to other users, which is often a real PITA due to big changes in volume and needing to supply steam even when the GT isn't working...

Nothing I've read says that Coyote Springs is anything other than a decent CCGT, whereby the GT supplies about 60-70% of the electrical power direct from the GT, thus the inlet temps into the power turbine section can be 1800F or higher, but leave at around 1000F before they enter the HRSG. Steam temp used in the steam turbine is lower than that as stgrme states.

https://energy.gov/sites/prod/files/nepapub/nepa_documents/RedDont/EIS-0201-FEIS-v01-1994.pdf

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BluSTi]

strgme - I could have misheard the fluid conditions, it could be 1100F and 1800 psi, a lot in any event.

LittleInch - Thank you for the paper and clarification. I'm trying to get a deeper understanding of their systems so I can be more effective contracting work to them.

 

[Ehzin]

As another note, you're not necessarily JUST heating it to come right back and cool it down. While the temperature is being lowered by the injection of boiler feed water it's also being converted to steam to provide additional steam capacity. The point being it's not a total loss by the injection of BFW. You're decreasing temperature to accommodate temperature/pressure rating issues, controlling steam conditioning/quality to meet your equipment's performance needs, AND generating additional steam at reduced superheat conditions. And I haven't looked at all the details you've mentioned regarding your system but in general I think these comments still apply.

Thanks,
Ehzin

 

[BluSTi]

Thank you for the additional info!

 

[MFJewell]

Desuperheaters are used to control the temperature leaving the HRSG so as not to exceed the rated inlet temperature at the steam turbine.

My experience has been that if the plant is designed correctly, it is not required to have desuperheating for normal operations. For combined cycles, you may want attemperation to help the unit get in emissions compliance sooner. Rather than having to hold lower load on the CTs while the steam turbine warms, you can use attemperators to meet the required warm up rates on while running the gas turbines at full load. Usually this is only considered in the design if it is a unit that has a lot of annual starts.

 

[Krausen]

Don't they also install desuperheaters when pulling superheated steam off a power turbine section to drive other auxiliary equipment in a plant? For example, a turbine-driven boilerfeed pump? The function is exactly as MFJewell has stated - to condition the steam. I'm not sure how common this is in the industry though? This may be more old school.

Tolerance is the virtue of the man without convictions -GK Chesterton

 

[MFJewell]

Don't they also install desuperheaters when pulling superheated steam off a power turbine section to drive other auxiliary equipment in a plant? For example, a turbine-driven boilerfeed pump?

We didn't at the nuke plant I worked at or any of the combined cycles I worked on design (we actually didn't have any steam turbine driven equipment other than the power turbine at the combined cycles) or at the current district energy plant I work at.

Extraction steam from the ST was used for heating processes (feedwater heaters, etc.). The feedpumps at the nuke plant had GE steam turbines as drivers, but they just took LP steam and had an auxiliary condenser below them that fed condensate back to the loop. It all comes down to design stage and matching equipment to available process conditions.

At the district energy plant I am at now, we have steam turbine driven feed pumps, chillers, and chilled water pumps. All of them are designed to take steam at the normal operating temps/pressures of the system. We do have attemperators in the main steam header in case we need to condition steam going out to the adjacent medical center heating process, but I think it is rarely used (I'll check some data tomorrow). It just depends on how our boilers and HRSG are operating.