Sending HRSG Blowdown to Package Boiler 1

[toothless]

We operate a 1315 psig HRSG 24/7. We'd like to send the continuous blowdown from this HRSG to a standby package boiler (320 psig A-frame)). This blowdown will help keep this standby boiler warm.

I'd appreciate any information in regards to the injection of the blowdown water into the standby boiler drum. For instance, I am assuming I need to inject the blowdown water into the upper drum above the water line. Do I need any kind of dispersion nozzle so that I don't have a jet of water shooting across the drum? Or is there some benefit to injecting this water into the lower drums? If so, how would I avoid hammering (flash steam collapse) if I inject below the boiler water line?

Any advice would be appreciated.

 

[rmw]

Aside from the merit of injecting blowdown into another boiler I laud your trying to recover this heat. If you inject it into the top drum you will just heat the water in the top drum. The colder water in the lower drums will tend to remain there and the hottest water will remain in the steam drum.

If you inject it into the lower drum or drums, you will heat that water which will rise and be replaced by colder water above it creating a natural circulation and more uniformly spreading the heat.

You need a sparger nozzle (do you have a condenser on this unit that you could look at the spargers?) that will spread this flow uniformly along the length of the drum and give area for the flash steam to exit. Underwater spargers are used often in condenser applications for quenching service.

rmw

 

[toothless]

I agree that injecting the hot water into the bottom drums would be better, to spread the heat, but I am concerned about steam implosion. About 20-25% of the hot water will flash to steam at this lower pressure, and if injected beneath the water level the steam will implode. Maybe that is where the sparger device comes in, and that leads back to my original question about needing some sort of "dispersion nozzle". Would the sparger prevent steam hammering due to steam implosion?

We have no condensers in our plant so I am unfamiliar with the technology employed in that service. I will Google steam spargers to see what I come up with. Thx

 

[sailoday28]

I would also check the TDS, etc of the blow down and what effect it will have on the injected boiler

 

[rmw]

Toothless, thinking about this has brought a raft of questions to my mind.

1. To the point that Sailoday has made, where do they take the HRSG continuous blow down from? Often continuous blowdown is a surface blow and its purpose is to remove the 'floaties' that might be on the boiler water's surface. If it is a surface blow, do you want that stuff in your Type A water?

2. How do you deal with the back pressure on your blowdown into the Type A when and if the package boiler has to be started when the HRSG is needing to blow down? I assume that the package boiler is a standby to back up the HRSG, but it is conceivable to me that there might be the time that it is started before the HRSG is brought off line.

3. Have you considered just passing the blowdown through the lower drums in a serpentine "in/out" loop that takes the hot blowdown in and after the heat is transferred out exits the drum without mixing the fluids? A few minutes doing some elementary heat transfer calcs would determine the length of tubing needed and you could enter one lower drum, make several passes, exit that drum and go the second drum and do the same if needed in order to soak out all the heat.

4. How are you going to control the level in the package boiler if you mix the fluids since you will be bringing "X" amount of blowdown into it while it is off line and has no 'mass' exiting in the form of steam.

One concern I have is that if you bring the blowdown in and flash it into the lower drums, since they are by definition the "mud" drums of the type A, the disturbance caused by the violent action of the exit of the flash steam will stir up the contents of the "mud" drum and defeat the purposes of taking your bottom blow down from that point of that boiler.

If you connect it in an 'open' loop where the blowdown water from the HRSG mixes with the package boiler, then you have connected the two boilers (just as they are connected at the steam header) and I think all the code requirements of isolation/non-return valving would apply. Those that lurk here that are more up to snuff on code issues would have to comment on that to be sure.

rmw

 

[toothless]

I appreciate all of the responses. I did forget to mention that this standby package boiler is typically left in run on "pilot" mode. It is equipped with a special pilot burner that is just large enough to hold the boiler at pressure, or even produce 1-2000 #/hr of steam (the boiler is rated for 250,000 #/hr).

The standby boiler will have to be continuously "blowdown" to keep the level in check.

 

[GenB]

you use an HX for the process,
means a holding tank samall capacity then you....
pump/circulate he boiler water through it...
easy xa!
now les us make the system...
genb