"Ambition means tying your well-being to what other people say or do. Self-indulgence means tying it to the things that happen to you. Sanity means tying it to your own actions." - Marcus Aurelius
NOTE: This post was written a while back. I held off posting it to allow the statute of limitations to expire.
The generating schedule for the power plant where I work is dictated by the electric utility that purchases the power. That generating schedule has changed over the years, and those changes have been noticeable both in the weekly and the seasonal run profile.
The gradual change over the past decade is likely due to increased demand for power - which has risen along with the influx of people into the region. When I first began working here, the plant would frequently be dispatched offline overnight or over a weekend. Some years, in the spring months - when hydro power is plentiful - the plant would be dispatched offline for weeks on end.
From the financial perspective of the utility, this made sense. Hydro power is inexpensive; the dams were built and paid for decades ago, so the electricity they provide is nearly free. Expenses at a hydro facility are equipment maintenance and salaries - there is no huge fuel bill. For the utility, it made financial sense to keep our power plant offline unless needed, and to rely on hydro power as much as possible.
Nowadays though, the facility runs - at varying power levels - non-stop between scheduled maintenance outages. It's apparent that due to increased demand, this plant has morphed into a base-load facility. "Base load" is the minimum power that must always be provided. We are no longer on the edge of being needed during periods of higher demand. The plant is now necessary just to meet routine electrical demand.
Below: A chart showing the power profile demands of an electric utility, and the generation categories for power throughout the day, for both summer and winter. Courtesy of Penn State University.
Because the plant has become part of the Base Load, it has only been dispatched offline a couple of times recently. All of us operators have lost proficiency at starting up and shutting down, and the equipment isn't necessarily used to the cycling either. It's not good to be rusty at important evolutions like starting up and shutting down.
I've always likened starting up a power plant to the pilot taking off a commercial jet. Both jobs involve a heavy mental work-load, complexity, and focus that results in stress. There are a large number of parameters to monitor, any one of which could require an instant decision and response. A pilot has many lives in his hands, while a power plant operator is dealing with energies and chemicals that could destroy a community. With nuclear, there is the added burden of knowing that you could make a region of the earth uninhabitable for several lifetimes.
The difference is that for a pilot, the stress of take-off is over in just a couple of minutes. Once the aircraft has enough altitude and airspeed, the pilot turns on cruise control. At that point a navigation computer flies the aircraft up to the correct altitude and heading, and the pilot can relax and monitor the situation.
On the other hand, a power plant start-up (with hundreds of various alarms) can last for several hours. The pilot in our aircraft may have already landed again before the power plant operator is in a situation where he can relax and monitor the situation. On the other hand, there's no Love Boat uniform to wear in a power plant :)
With that background out of the way, we had a rare dispatch offline for a 24 hour period, and night shift was supposed to have the plant up and running early in the morning. I arrived for day shift, and there was no steam rising up off the cooling tower. There were also *a lot* of cars (maintenance and management) in the parking lot, instead of just the two night shift operators. These are signs that you are about to have a difficult shift.
At shift turnover I learned that night shift had tripped the steam turbine twice during start-up - cause unknown both times - and they had given up after that. It's quite likely that the lack of proficiency played into the steam turbine trips - if you don't happen to be on shift when it's time for the rare start-up, you might go a couple of years without one.
The outside operator had also found the auxiliary boiler engulfed in a cloud of steam after the failed start up - it had sprung a leak. Our maintenance guys had already checked the leak out, and decided that the aux boiler could
not be fixed by staff. The hole was hard to reach, and in their judgement the
plant should remain offline until a Code Welder
could arrive on site to make a repair. Probably the correct call to make. I went out to have a look.
Below: Arrows point to a stream of water that was pissing out, after the boiler had cooled down somewhat.
I thought perhaps the leak could be patched temporarily. It would only have to hold 150 psi (10 bar) for 30 minutes or so. The aux boiler is only used to provide seal steam for the steam turbine during start-up. Once the steam turbine becomes self-sealing, the aux boiler can be shut down.
The leak was difficult to see, let alone to reach and work on, because there is quite a bit of interference from the structural steel that supports the steam drum. I had to crawl underneath some pipes and stand on the recirc pump to reach the leak. Once I had squirmed into a position to reach it, I used a pry bar to shift the sheet metal out of the way. Then I was able to set a small rubber patch in place with a hose clamp, without burning my fingers very much. Normally I'd be the last guy to consider jury-rigging a boiler with a rubber patch to get the power plant running, but here we are.
Below: How an operator makes repairs on a certified pressure vessel. 
The hole quit leaking, which seemed promising. The real question was: Would the patch hold with the boiler up at firing temperature and max pressure? Throughout the start-up I frequently returned to the boiler when possible, and amazingly the jury-rigged patch held. It began to weep a bit toward the end, as the heat had probably softened up the rubber patch.
I wasn't interested in squirming through the clutter of hot pipes to re-tighten the hose clamp - not while the boiler was firing - so I let it weep. The patch more or less held until the boiler could be shut down, which was all I needed from it. The code welder will make a permanent repair before the boiler will be needed again.
EDIT: Yep, it's been fixed. Here's what a legitimate boiler repair looks like, once you have the time for it :) The boiler code welder also cut the 1/2 inch plate steel gussets off the structural steel (see photo above) so he could gain access. Once the water was out of the boiler drum, it was OK to temporarily remove part of the support structure.
There was also *another* troublesome leak, which has been a longer-term problem.
Inside the gas turbine enclosure, we have been dealing with high combustible gas levels for well over a year - the alarm comes in whenever fuel gas is flowing, and clears again once the flame is off. During this recent dispatch offline, our maintenance guys replaced a slew of gaskets in the fuel system, hoping they could stop the leak.
It's been impossible to locate the source of the natural gas leak, as there are hundreds of points on the turbine and on the fuel gas supply lines where it could leak. It's absolutely impossible to hear a gas leak when the turbine is in operation, and there is a lot of airflow inside the enclosure to cool the engine, so the turbulence mixes the gas with air throughout the enclosure. You can smell the gas, but you can't locate the source. Trust me, we all tried - many, many times.
Below: A Turbine enclosure with the roof removed. See all those silvery pipes and flexible hoses? Any one of
those, or the combustion cans that they connect to (removed in this image), could leak natural gas. At the bottom right are a couple of guys, for scale.
Once we had the aux boiler problem temporarily resolved, my shift partner and I started up the plant. He was in the control room this go-around, and I was the outside operator.
After the gas turbine began firing, the combustible gas alarm returned yet again. The maintenance guys were quite discouraged, having spent an entire Saturday at work, replacing gaskets. It's much too hot to crawl around on the gas turbine chasing leaks while it's in operation. We've all looked for the leak over the past year, and had a rough idea where it was, but no one could locate it.
The head maintenance guy was feeling pretty discouraged about the gas leak, after all the gaskets they had replaced. He and I had a short discussion over the noise of the turbine, and he said he was certain he knew the rough location of the leak, but couldn't quite find it. I asked him to show me where he thought it was.
We entered the gas control valve module with a gas detector. He showed me where it went from showing nothing, to reading clear off-scale gas levels. We both recognized that the gas leak had to be *right there* to make such a massive change in the reading.
I ran my bare hand around the area, and felt a jet blowing on it - different from the normal cooling air flow. Two stainless steel instrumentation tubes had rubbed against each other, and vibration had worn a hole in one of them. So there it was - after a year - and help from the maintenance supervisor, I managed to put my hand right on the gas leak.
Below: It's right there. Can you see it?
That's two more days of gasket replacement work that I just eliminated from the next outage. I jokingly told the boss to put the spot bonus in my in-box - like they would ever financially reward a wagie for making a contribution :)
No comments:
Post a Comment