Calculating Intermittent Blowdown Rate for Boiler

[CraigC782]

I've looked around the net, and while there are sources that detail the calculations involved in determining continuous blowdown rate, tips on intermittent blowdown rates amount to little more than "shorter, more frequent blows are better than longer, less frequent blows," and recommend blowdown periods of anywhere between 10 seconds and 2 minutes. 280 psig saturated, 55k lb/hr output, 48k lb/hr condensate return.

HALP!

 

[insult2injury]

Typically, the intermittent blowdown is peformed to lower the water level in the drum a certain distance so the rate would be dependent on the control valve/piping/operating conditions.

As far as frequency is concerned, if your steam quality is poor, then you need to do it more often.

I2I

 

[rmw]

The blowdown amount and interval has to be determined by your water testing. Your blowdown is to try to maintain a certain level of TDS in your boiler water. If you are talking about blowing down from the bottom (mud) drum, the shorter blows are better because in a blow, the non dissolved solids that have accumulated there (hence the name mud drum) will be swept up in the first part of the blow and then only clear water will follow. Shorter blows tend to get more of the solids as they resettle in between blows. The duration as noted above is determined by water tests.

Surface blows are usually continuous and at a very small flow rate. Surface blows are to remove "floaties" as well as TDS control.

rmw

 

[MJCronin]

Craig...

I am assuming that you have a small packaged watertube boiler...

Intermittent blows are typically done once per shift and, yes, they will depend mostly on the policy of the owner and on the desired water quality.

They usually last a few seconds

-MJC

 

[Joesteam]

Lets not confuse things. The surface or continuous blowdown is used to maintain water quality (primarily dissolved solids & hardness in a non-power boiler).

The bottom blowdown is used to remove suspended solids that have settled to the bottom (or 'mud'). A short quick blowdown will suck this muck out. Usually performed once a shift, once a day, or even once a week for cleaner systems.

The rate can be found if you know your bottom blowdown valve, differential pressure, and time you leave the valve open....



Joe Lambert

http://www.control-associates.com/

 

[davefitz]

there are also other uses for the intermittent blowdown. For load cycling variable pressure drum boilers using coordinated phosphate treatment, phosphate hideout can occur, and there may be a need to increase blowdown to minimize hideout effects. Alos, a deliberate blowdown during fast startups can reduce the tendency for the economizer to steam at low loads.