best blowdown method?

[Dan67]

I have recently gotten into boilers. I am a trainee. I help maintain two boilers that are used for heating several academic and dorm buildings at a university. The smaller of the two puts out up to 40 tons of steam per hour. During colder periods we start up the larger boiler, which can put out up to 70 tons of steam per hour. The temp. of the steam leaveing the boilers is around 300 F. at 140 psi. There has been a bit of a debate going on, over the best way to bottom-blow the boilers. There is an A valve and a B valve in series. Ideally we would like to have B valve go bad before A valve. If A valve went bad there would be nothing between it an having hot boiler water all over the place. Some say that valve A would hold up better if it is opened (all the way) first, and then valve B is used to meter the blowdown water. Others say valve B should be opened all the way and valve A should be used to actually release the water. Does anyone have an opinion on this? Thanks!

 

[Dan67]

I should note that valve A is a 90 degree bend valve. Valve B is a straight.

 

[GBurns]

The proper procedure is to open fully the valve closest to the boiler and then the down stream valve. When closing them it should be done in the reverse order with the furthest valve being closed first. If the installation is proper the down stream valve will be designed for throttling while the valve closest to the boiler is not. The valve closest to the boiler is designed to be opened or closed and could be damaged if used for throttling.

 

[Dan67]

Thanks! That is what I thought.

 

[JeffHofacre]

I agree with GBurns. This way the valve closest to the boiler is not sacrificed. If you want to know more check out "Steam Plant Operation" by Woodruff and Lammers under blowdown apparatus.

 

[dfurey]

Why is it that bottom/surface blowing a marine boiler, the sequence of opening the valves differ so much from land based. When bottom/surface blowing a marine main propulsion boiler, the sequence for aligning the system is from overboard to source. The valve closest to the boiler header or drum is used to blow. This valve is opened fully and then shut once the water level has dropped the prescribe amount for that header or drum. It seems to me that I would want the outlet open to prevent the high pressure water from slamming into the closed valve you are using to blow the boiler. How does shore base plant differ.

 

[Dan67]

The difference must be either the higher temps. and pressures for driving engines or generating electricity, or the design of the valves themselves?

 

[69691]

Remember a marine propulsion boiler blows down below the waterline of the vessel which means it discharges through the hull on the lower level of the engine room and many of the older boilers do not have low water cutouts. By controlling blow down on a marine boiler with the valve closest the boiler you can watch the water level, whereas if you controlled the blow down with the skin valve on the lower level of the engine room you would have no way to monitor the water level.