Boiler Bottom Blowdown

[Jason5000]

I have a question on design flow rates for boiler bottom blowdowns. Does anyone know any design criteria, rules-of-thumb, etc. as to how much flowrate the bottom blowdown should be sized for in order to be effective at removing silt from the boiler? The boiler in question has a continuous blowdown for controlling water chemistry. The bottom blowdown is only to be used for quick, high volume blowdowns for the removal of any built up solids.

I have posted a similar question in the Boiler & Pressure Vessel forum, but I got more a a mechanical engineer's answer. I'm looking for a process/chemical engineer answer as to the appropriate process design for a bottom blowdown.

 

[ash9144]

For periodic b/d for solid removal you want high velocity in the boiler, so as you stated high volume. So ideally bigger is better. However too large will upset the process and cause safety issues. Most of the b/d nozzles I've seen are 2" with gate valve for throttling (ball valve would as well for this purpose though)

 

[786392]

2"dia use is commonly witnessed;although smartly throttled to avoid boiler tripping,tubes overheating possibilities.

Somebody will correct if that is incorrect.

Hope this proves useful.

Best Regards
Qalander(Chem)

 

[rmw]

Well, I am a mechanical engineer and I don't think I answered in the Boiler Forum but your water chemistry determines your blowdown rate, whether it is the continuous blow or the bottom blow. You should be sampling suspended solids and dissolved solids and blowing accordingly at the appropriate place. Process considerations like level control and process upset also enter into how long you blow in each case. I think some of the answers you were given over there were good although it has been a while.

rmw

 

[FFV5]

It seems like a material balance problem to me. Perhaps you establish your inlet and outlet solid concentration gains including all your recycle flows and you analyse your accumulation build up. Once that is done you may determine the flow rate of the blowdown you desire, and size accroding to this flow rate