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When blowing down a control at pressure, the blow-down valve should be opened slowly. The piping needs to be warmed up and stagnant water in the drain piping needs to be pushed out. Suddenly opening a blow-down valve causes steam to condense, which creates water hammer.

Water hammer is the steam imploding as it turns from a vapor back into liquid. When steam is made, the water expands 1700 times its original volume to a vapor. Steam hitting cold water reverses this process. The velocity of the water rushing to fill the void created by the collapsing steam creates pressure spikes. The sudden pressure changes associated with these implosions can reach over 1,000 PSI. This severe and sudden pressure can damage low water cut-off floats and bellows.

For these reasons McDonnell & Miller recommends a dual valve blow-down system for each control.

Proper Blow-down Procedure: (Using dual valve system)

1. With water in the boiler at its normal level, open "Positive Shut-off Ball Valve".

2. Open "Throttling Gate Valve" slowly until drain piping heats up and then open

fully.Observe that the water level starts falling in the gauge glass.

3. Close "Throttling Gate Valve" after verifying that the pump contacts have closed and

the burner contacts have opened thus shutting down the boiler.

Note: If this does not happen, Immediately close all valves, turn off burner and correct the problem.

4. Close "Positive Shut-off Ball Valve".

5. Observe that the water level returns to its normal level before leaving site.