Safe and Efficient Operations Procedures 1

[cc2942]

I am a boiler operator. I currently operate 2 high pressure Cleaver Brooks 100 horsepower boilers. Recently, I had to shutdown one boiler due to a rupture on the feedwater supply line. The boiler I shutdown was the lagging boiler so it already had the main steam stop valve closed. Our normal operating pressure is 116 psi. The Chief Engineer schedhuled it to be repaired the next day so he ordered me to turn the boiler off but not drain the boiler cause it didnt need to be. According to Stationary Engineering books and in the vocational classes I recall that the boiler vent (also referred to as an air cock or test valve) should be opened when the pressure drops to 25 psi in order to prevent a vacuum which could cause internal damages. So i did open the vent . However, is that vacuum that is being referred to, only when draining the boiler or is their actually a vacuum formed as the pressure drops from 116 psi to 0 psi within the boiler? No books or study material seems to make that clear so I am asking an engineer? This is controversial at my job now. Please fill me in.

 

[rmw]

As the pressure drops and the water and steam in the boiler cool off, the steam that is trapped in the boiler above the water level will condense and that will create a vacuum. To a very minor extent the water "shrinks" too as it has a higher specific volume when it is at 116 Psig than it does at 0 psig. All of the above will cause the boiler to go under vacuum, hence the precaution of opening a vent. You did the correct thing. Not to have could have seriously damaged the boiler.

rmw

 

[cc2942]

Thank you Rmw for a detailed explanation. Even though, the minor vacuum may not be strong enough to seriously damage the structural parts in the boiler , some sources say that it will also prevent sensitive critcal components such as the , modulating controller and pressure gage from damage.
Some operators agree with opening the vent regardless if it is to be drained or not and others do not think it is necessary if it wont be drained. I have never heard of any past experiences of damage resulting from not opening the vent after shutdown without draining, however the explanation you provided me lets me be aware that I should not just rely on common sense notions, always consider taking precautions and to operate according to what I have been trained to do regardless of what others say.