Blowdown Flash Steam Heat Recovery

[roasted]

I have suggested to my company that we recover the blowdown flash steam by installing a flash vessel and pipe the recovered steam to the DA. The drum pressure is approx 1350 psig and the DA is approx 21 psig. The LP steam flow to the DA at present is approx 8t/h and the calculated flow of the flash steam will be approx 0.6t/h. Does anyone see a problem with this, does anyone know of a project of similar scale that has already been tried and tested, will the DA performance be affected in any way and what components are essential for safe operation?

 

[toothless]

That is exactly what we do. Our HRSG drum pressure is about 1350 psig. The continuous blowdown goes to a flash tank, and the steam from the flash tank goes to the deaerator which operates at 3-15 psig. The blowdown water that remains in the flash tank is continually drained off.

A makeup steam line is still required on the DA, but using the flash steam provides a savings.

 

[MJCronin]

roasted,

Many operators of smaller boilers utilize a system that not only recovers flash steam, but also preheats incoming makeup water with the condensate generated.

Would this system apply to you ?

http://www.pennseparator.com

Please let us know of your final decision

-MJC

 

[roasted]

Toothless - It is good to hear that a similar sized plant already uses the same system. What is the design flow into the flash vessel compared to the average continuous blowdown flow for your system? Also, did you have any problems with the initial set-up that you could advise on? I would actually like to see how your system is set-up, but I realise that maybe asking too much.

MJC - preheating the makeup water was looked at but many factors ruled this out. Basically, it did not give the required return on investment. The annual savings would cover the cost of the heat exchanger, but the cost of the pipework simply made it uneconomical.

Will keep you updated on the progress of the project.

 

[toothless]

I've never seen the design info on the flash tank, but I can tell you approximately what it handles. Our normal blowdown rate is 2-3%, but I suppose under extreme conditions it might reach nearly 10%. The HRSG steam generation rate varies from 100,000 #/hr up to 240,000 #/hr depending on the GT load and duct burner firing rate.

To estimate the amount of flash steam you will get, the equation below can be used.

% recovery = (h1 - h2) / h3

where h1 is the enthalpy of the blowdown water (at drum pressure), h2 = enthalpy of water at the flash tank pressure, h3 = latent heat of vaporization of water at the flash tank pressure)

I am not aware of any particular problems we had with this system other than a level transmitter issue we had once. The level transmitter read low allowing the water level to rise too high and carryover to our DA.

As MJC stated above, you can, if it is worth it, then use the drain water to cross exhange with your incoming BFW or DA feedwater to preheat it.

Hope this helps.