Meltdown!

Yesterday was the last day of shift, and the last day for one of my two crew-members. Ed's moving on to a "retirement job" - going to work for the state water district at an aqueduct pumping station. Great retirement benefits and easy work. Sounds nice.

Coincidentally it happened to be an abnormally hot and humid day, with very high electrical demand on the grid. The grid operator had requested full output from our facility, which is never an easy task. The gas turbines (which each have their own generator) exhaust into heat recovery boilers to make steam, which turns a steam turbine and generator.

To make additional power, we have four rows of "duct burners". These are large horizontal natural gas burners that are installed in each boiler. Operating duct burners is not as efficient as burning gas in the gas turbine, but you can get extra steam (and power) without the huge capital cost of installing another gas turbine and boiler. We fire the duct burners fairly frequently - more often when it's hot and the compressor efficiency suffers as the air density drops.

Anyhooo... to reach full load, we have to inject a portion of the steam into the GAS turbine compressor discharge. It rams more mass through the turbine, making it produce more torque. Overall the plant loses efficiency, but the power output goes up - as do the maintenance costs. It's kinda like using nitrous on your car engine. Does wonders for power, but your mileage and longevity suffer. Still, if the economics make sense, we inject steam. The last time we had injected steam was nearly a year ago, last summer

So, we had been dispatched to full power, and an emergency dispatch at that. It took about 45 minutes to properly heat the steam supply lines and get the moisture out. None of the valves wanted to work properly, and some of the instrumentation wasn't functioning, so I ran a couple of things in manual. It was kinda challenging. Emissions also get ugly for a while when you mix steam into the combustion picture. Finally we got everything running and stable, and the plant was churning out every megawatt it was capable of, given the high ambient temperature and humidity.

I left the control room in the hands of my other teammate and went to a meeting to say farewell to Ed. About that time my teammate called me back. We had lost the feedwater pump on Unit 1 boiler! The standby pump had just been sent off to LA for repair... ugh! The High and Intermediate pressure steam drums started losing level, and the gas turbine went into a runback. The runback keeps the gas turbine from putting heat in a dry boiler and melting the tubes down.

We got a re-start on the pump, started to to regain levels in the boiler, and ended the gas turbine runback, when the pump tripped again. We learned that the motor was experiencing high temperatures and the motor protective relay was shutting it down. I requested that one of the maintenance guys go pull the inlet filters off the motor in case they were plugged - they didn't. We got a third start, started filling, and terminated another runback on the gas turbine.

Finally the feedwater pump tripped for the final time, and the protective relay locked us out from starting for an hour. Too many starts will overheat the windings, so you only get three per hour. When it locked out, we were forced to shut down the gas turbine. So right when we were needed most, we lost half the output of the plant. Dammit. Worse, I learned later that the same thing happened here last year (except the standby pump was functional then)

I stayed a little late to help the night shift crew bring the plant back up - after the one hour timer had expired and the filters on the motor had been cleaned :). It wasn't too big of a deal as the boiler was still pretty hot. It was also pretty dry. Took a long time to fill after we finally were able to re-start the feed pump.

After work a few of us had a final beer with Ed at the local brew-pub. I got home afterwards, slumped on the couch, totally drained, and fell asleep right there.