Saturday, August 2, 2014

Stick-Shift Boilers

(Might want to read this first.)

Let's say that you're on a steam ship and the order comes down to increase speed. The throttleman in the engine room acknowledges the bell change and opens the throttle. More steam is admitted to the turbines and they spin faster.

The real fun is in the fire room. When the throttle is opened, the first thing that happens is that steam pressure drops in the main steam line. Because of that drop in pressure, the level of water in the boiler goes up. But that's just a momentary reaction.

In response to the lower steam pressure, the automatic combustion control (ACC) on a modern 1,200lb. steam plant[1] did three things and, in a well-maintained system, did them very well: It would add water to the boiler, increase the firing rate at the burner front and speed up the forced draft blowers. The Burnerman would, as ordered by the Boiler Tech of the Watch (BTOW), cut in more burners.

The Blowerman (or "Lower Levelman") would, if ordered, start an additional forced-draft blower. Each boiler had two blowers, but in normal peacetime steaming, only one blower per boiler would be running.

It took the boiler techs a long time to come to Jesus on ACC systems. There was a special Naval Enlisted Classification code for an ACC technician. One of the things that got Insurv riled up was the failure of surface ships to properly set up, maintain and run ACC systems. When things like that happen, the way that the surface line community[2] handles things is to publicly fire people until everyone gets the message.

The thing was, of course, that the senior boiler technicians had learned their jobs on World War Two-era ships. Those ships had 600lb. steam plants that were manually controlled. The Upper Levelman stood watch by the boiler water-level gauge glasses and he controlled the rate that feedwater was added to the boiler. The Burnerman controlled both the number of burners and the amount of fuel oil that was fed to the burner front. The Blowerman controlled the speed of the blowers.

So now the Throttleman opens up the throttle. The Upper Levelman sees the water level rise in the gauge glass, but he knows that is a temporary effect, so he makes ready to add feedwater. The Burnerman sees the boiler's pressure drop, he increases the firing rate. The Blowerman speeds up his blowers to feed more air to the firebox.

Those three men, naturally, were told what the speed change was and they could react based on experience. But the Blowerman and Burnerman rarely were able to harmonize exactly during big speed changes. Given the choice between too little air and too much air, the Blowerman always opted for too little. Too little air meant that the boiler would emit black smoke out of the stack. Too much air and the boiler would emit white smoke. White smoke was finely atomized fuel and, as you might suspect, a white smoke condition was dangerous: You would get a fuel-air buildup in the upper works of the boiler and the stack and then, if it were not brought under control quickly, very bad things would happen.[3]

Ships with boilers that had tuned ACC systems didn't emit smoke on power changes. If a boiler that was run on an ACC emitted smoke on a power change, that was a sign that the ACC wasn't working properly.[4] So if you see photos or video of a Navy steam-powered warship blasting out smoke as she accelerated, you probably were seeing a ship with a stick-shift plant.
[1]"Modern" being "post Korean War".
[2] Our motto: "We Eat Our Young".
[3] The rule was that if white smoke couldn't be eliminated in a minute or less, the boiler's fires were pulled and the boiler wrapped up.
[4] There was a test called a "boiler flex" in which the Throttleman would rapidly spin the throttle open or shut in order to change the steam demand across 80% of the boiler's operating range, in order to stress-test the ACC system. This was an OPPE fail item.