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Twite explains 'explosion' at DMU
Nov. 14, 2016


DELANO, MN – Many people heard and some people even felt what seemed to be an explosion in Delano around 1 p.m. Wednesday.

In reality, it was a backfire, or compressor stall, of a generator at Delano Municipal Utilities, and no one was hurt.

DMU General Manager Paul Twite explained the situation and provided some background information to the Delano Herald Journal.

DMU is required by federal law and the Midcontinent System Operator (MISO) to annually test each generator under full-load conditions, a practice referred to as an URGE (Utility Rating of Generating Equipment) Test.

The incident in question occurred while testing Generator 9.

“Generator 9 is a very unique power plant,” Twite said. “It is comprised of three components: a 12-megawatt electricity generator, a power turbine coupling unit, and a liquid fuel combustion turbine. Our combustion turbine is a Rolls Royce Spey unit, which initially saw action in a Vietnam War-era F4 Phantom or Corsair II jet fighter aircraft. This unit is literally a jet engine in a box.”

Twite explained that the utility has recently had some fuel flow and control issues with the generator, leading staff to replace the main fuel control valve on the combustion turbine during the summer.

“Since then, we have run the unit in ‘test’ mode (low RPM) several times to check for leaks and proper functionality of the fuel system,” Twite said Wednesday. “We were notified this week that we needed to run full-load, according to the rules of MISO, so today was the day.

“We successfully achieved 11.5 megawatt of output and we believe there may have been air in the fuel system because the unit ‘back-fired as we began ramping the RPM downward to a cool-down cycle,” Twite continued. “Any interruption in fuel or air delivery causes an aerodynamic stall of the airfoils inside the compressor, which are traveling at nearly supersonic speed.”

That interruption caused the unit to burp out unburned fuel, which ignited in the exhaust port.

“The ignition sounds like a clap of thunder due to the expanding air,” Twite said. “This situation is called a compressor stall.”

Such occurrences are fairly common in jet aircraft if they inject a bird, torrential rain, lose fuel pressure, or have their combustion or airflow otherwise interrupted, according to Twite.

Two employees were operating Generator 9 when the compressor stall occurred. Because they were inside the building at the time of the stall, they were protected from any exposure to flame or combustion and uninjured.

Following the backfire, DMU staff called 911, and the Delano Fire Department responded quickly.

“Again, no injuries to report other than a couple of rapidly accelerating heart rates,” Twite said.

He noted that DMU has experienced a compressor stall in the past, as well as a transformer failure three years ago following a similar situation.

Now that DMU has completed the annual URGE test, the staff will “pickle” the unit by lubricating; sealing; deactivating; and powering down the controls, engine, and accessories for the winter; as well as sealing the exhaust stack to protect the turbine from birds and the elements.

“Before we do that; however, we will be conducting a thorough maintenance and inspection regimen to insure no permanent damage was done to the turbine or generator,” Twite said. “In fact, we ran the turbine for about 30 minutes after the fire department left to make sure everything still worked correctly. Some damage was noted on the exhaust stack itself as a couple of bolts had sheared off from some structural steel. We also broke two light bulbs in the control room.”

DMU staff is now looking to bleed down the fuel lines and improve the fuel system by adding a couple new components to help insure reliability and safety.

Despite the recent malfunction, Twite remains confident in the equipment.

“This unit was installed at its current location 15 years ago and has been extremely reliable and trouble-free, rather remarkable for a 50-plus-year-old piece of equipment,” Twite said.

He noted the value of DMU’s generators.

“It’s important to understand how much energy is located within our substation: enough energy to power a city more than twice the size of Delano,” Twite said.

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