People of a certain age, and those that enjoy TV reruns, may remember the show called “The Twilight Zone.” In each episode, there was always a surprise plot twist and a subsequent moral or lesson at the end.
In my career, I have witnessed many pump issues that I have assigned to this zone of unexplained reasons for pump failure.
At first I was “mentally filing” them simply as pump mysteries, but later I just started calling it the twilight zone because no one seemed to know what caused the problem. I learned—after some time, persistence and acknowledgment to the twilight of unexplained things—there was always a good lesson at the end.
Before going further, I want to add a disclaimer: I am not blaming anyone for the issues that occurred. There are guilty parties all around and no names will be used. It is the intent of this article that we all learn from the lessons they teach. All of these examples are true; I present the issues first and the answers follow.
What Is this Twilight Zone?
I am talking about that warped period of time between when the pump is shipped from the factory and subsequently started up. I am talking about the time and place between the pump being removed for repair and when it is reinstalled. I am talking about the unexplained changes that occur when pump “A” is pulled from the system and pump “B” is installed in its place.
This is the interim dimension I previously called the magic and mystery place. Lately, this is the dimension I refer to as the twilight zone.
1. The case of the missing impellers. Do not blink twice, or you will miss it.
In a steel mill in the central Midwest, a six-stage boiler feed pump is pulled from service for routine maintenance to reestablish the original operating clearances. Note that the pump is driven by a multistage steam turbine. The pump operated in excellent fashion for more than eight years of continuous service, but it was time for an overhaul.
The pump was rebuilt and returned to the customer by the OEM repair center. Upon initial startup, the pump operated smoothly, but the hydraulic performance was markedly off. The flow and head were both diminished when compared to the manufacturer’s published performance parameters. The upset customer proclaimed that the repair center had left out a few stages (impellers) during the repair. This pump was originally designed as an eight-stage pump, but was built for this customer as a six-stage pump. This is a common practice where blanks are used for impellers when a lower discharge pressure is required. The customer was suggesting the pump was now a four-stage in lieu of a six-stage pump.
The manufacturer’s engineer was called in to troubleshoot the issue. He arrived at the site and went through all the normal performance checks including vibration, valve lineup, rotor settings and the validation of differential pressure and flow. He considered that it was possible for two stages to be left out of the rotor, but not likely, and besides the OEM repair center vehemently insisted the pump had six stages.
Initially, all the indicators appeared to be an issue with low speed. The customer advised several key points during the troubleshooting process, including that they had not worked on the steam turbine during the outage and that the operating speed was the first thing they checked when the issue appeared. The turbine/pump speed had been validated using a calibrated strobe tachometer (stroboscope) at exactly 3,600 revolutions per minute (rpm). Luckily, the young pump engineer had plenty of experience with steam turbines and stroboscopes.
Solution: No missing impellers
In the first case, the pump factory engineer had extensive experience with turbines, steam governors and stroboscopes. He knew how easy it was to get an incorrect reading from the stroboscope if you were not properly trained. The steel mill stroboscope operator had focused on a coupling bolt to “freeze” the rotor at some speed, which he thought was 3,600 rpm because he had no reason to suspect otherwise. The turbine was really operating at 1,800 rpm, and the stroboscope operator was seeing a multiple image (two coupling bolts appearing as one).
For stroboscope work, it is important to always focus on an object that is unique from a shaft circumference perspective, like a key or key seat. If you must use a coupling bolt, then mark one in some way so it differs from other bolts (glow paint works well in these cases).
The customer did not previously mention that the turbine governor valve (W-TG-10) had been rebuilt while the pump was out for overhaul. The factory engineer who discovered the root cause was given the honor of ceremoniously adjusting the steam governor setting to bring the unit up to the correct speed of 3,600 rpm where it performed like new.
2. The case of the hot transformer, and a cool solution.
Scene: an electric power generation plant on the eastern seaboard. At a 20-year-old power plant, a generator step-up transformer (GSU) has severely overheated at full load since the day it was installed. Consequently, the unit was derated and operated at 85 percent of full load. The derated transformer thereafter limited the power output of the entire power generating station.