THE MYSTERY
The customer had issues with two of their Once Through Steam Generators (OTSG). They called ACES to the scene of a large biodiesel production facility. Feedwater and heated air pass through a bank of steel tubes and combine to create high-quality steam. Although these two generators were recently commissioned in the fall of 2014, the tubes in both had failed repeatedly in the past several months.
THE CLUES
The Control Systems Investigator (CSI) performed a thorough inspection of both generators and discovered severe plugging of the instrument ports, tubing, and orifices for water and steam — a scale of iron sulfide had accumulated to a thickness of .125” in some places. Due to neglect, the steam sample system was completely plugged requiring many hours of work to clear plugs on both of the generators. Generator 1 was completed first and restarted, with the addition of a portable data logger.
A week later the tube in Generator 1 failed again. The data showed pressure steady at approximately 1,800 PSI until suddenly it dropped — 30 PSI in 4 minutes — and it continued to plummet. The flow rate from both transmitters mysteriously went to nearly zero.
The pump was still running — and in fact the data showed that it sped up to correct the low flow conditions. When the flow dropped off, the control system calculated a lower steam quality, and automatically increased the throttle burner position to bring quality back to setpoint. The generator continued to run in this no-flow, increased-throttle condition until finally the quality became so high that the alarm timed out and put the generator into shutdown.
In fact, while the CSI was working on the generator, he observed rapid pressure changes at discharge, which caused whistling steam leaks that changed in pitch. The CSI deduced that this was the sequence causing the tube to fail. But why was it happening in the first place? Further investigation uncovered several more clues:
The transmitters showed zero flow, when the CSI knew for a fact that flow never stopped.
The burner flame completely encircled the tubes at exactly the point where both generators were experiencing tube failure.
The CSI questioned the machine operators who revealed that the generators had been performing consistently under a steady oil field steaming load for some time. Then as more field systems were added the load became variable, causing rapid pressure swings.
THE PERP
When the pressure decreased rapidly, this caused a large portion of the water nearest the transmitters to rapidly flash into steam. This led to the loss of feedwater flow measurement, and exposed the flux area of the tubes to sudden, extreme heat, causing them to blow out.
The tube will fail if it is heated to 800-850º F for sufficient time. This temperature could be reached rapidly under these conditions.
THE SOLUTION
Due to the variable load, the CSI installed a pressure control valve on both generators. These valves will hold pressure constant during steam demand changes and prevent the tubes from entering superheated conditions.
The steam oven temperature alarms have been lowered to allow only 4 degrees of superheating to occur for 30 seconds before a trip condition is initiated.
Now, even under variable and inconsistent load conditions, the customer’s generators steadily produce steam of a quality and pressure perfect to the needs of the overall operation. While both tubes remain whole and intact.
If you’re under pressure due to a puzzling equipment failure, let the experts take a look. Give ACES a call for a free estimate.
CASE CLOSED
Have you had issues with your Once Through Steam Generators? Leave us a comment below about The Case Of The Whistling Generator Tubes.