The presentation at the 7F Users Group’s 2020 Digital Conference through Week 3 (two weeks to go) that has generated the most questions and discussion among attendees concerned a leak in a plate-and-frame lube-oil cooler.
How could a mundane leak generate this much interest at a high-tech meeting? Read on: There are some lessons learned you may benefit from.
The background: One 7FA at a 2 × 1 combined-cycle cogeneration facility was out of service for an outage. Lube oil to the unit was shut off, but cooling water was still running through the plate-and-frame heat exchanger. This had been standard practice for the last 18 years. During that time plant personnel had performed the periodic heat-exchanger cleaning required without incident.
The problem: Water pushed through the exchanger’s gaskets after the lube-oil system was secured. Water then ran through the exchanger discharge and all associated systems, and contaminated the 6400-gal oil reservoir. By the time the leak was found and the water shut off the reservoir level had risen by more than 3 in., causing oil to flow from the explosion doors. A quick calculation revealed that about 400 gal of water had been added to the oil reservoir, creating a milky mixture in the tank.
Staff considered that after its last cleaning the heat exchanger might not have been tightened to the applicable “crush” specifications for that model and the number of plates it has. The exchanger was disassembled and the gaskets inspected. No damage to gaskets or plates was in evidence, so the lube-oil cooler was cleaned and reassembled. Alfa Laval, the manufacturer, was asked to provide a formula to guide reassembly and assure the proper crush. The total inside spread between the end caps of this unit with 106 plates was calculated at 18.56 in.
Given that proper crush is so important to leak prevention, consider verifying the specs for your exchangers. And when using outside labor for cleaning, share this information with that team; it’s not just a matter of “tightening” a few bolts/nuts after cleaning a plate-and-frame exchanger, as some might think.
Another thought was that the leak began when the lube-oil system was shut down because the oil cooled. The logic: When the oil was hot, expansion prevented leakage of water into the oil side of the unit.
In either case, the takeaway is obvious: Avoid leakage by shutting down the water system before taking the lube-oil system out of service. This lesson learned has been incorporated into plant procedures.
However, a couple of attendees listening to the presentation reported having the reverse occur, with lube oil leaking out when water was “isolated in the compartment.” The fix here was gasket replacement and right-torqueing. This exchange among users, and others like it during the 7F event, was proof that a virtual conference done correctly can be as effective as a conventional meeting for sharing experiences—possibly even better.
Another attendee suggested all gaskets be replaced every couple of years or so because they lose their resiliency. Yet another mentioned baking the gaskets to cure them after cleaning. There was no follow-on discussion related to this suggestion, however.
The presenter said a vacuum truck was brought onsite to remove the oil/water solution in the lube-oil sump. The dregs then were mopped up by hand, the lube-oil filters replaced, and the tank refilled. Entire process took three days. The plant didn’t pursue centrifuging/vacuum dehydration to save the oil because the cogen facility was necessary to support process operations.
A similar situation was reported by another attendee who said the issue at his facility was brittle gaskets in the heat exchanger that failed once the oil pressure was off the unit and cooling water was still in service. It was a mess, he said, with oil spilling out the explosion doors as the speaker had reported earlier.
This tank also was drained and mopped clean before new oil was added. Oil could not be salvaged, the user said. Two days were spent trying to save it before deciding on disposal. Next step was to replace gaskets on all of the plant’s plate-and-frame heat exchangers serving the 7Fs and D11 steamer. All those assets were commissioned around 2000.
The takeaway from this session suggests that if you have Alfa Laval lube-oil coolers installed during the bubble years and have not replaced their gaskets it might be time to consider doing so. A user suggested buying a spare set of plates with gaskets (glued on or clipped on) then swapping them out with the plates in service. Job should take about four hours based on his experience.
Someone else added that when you send plates to Alfa Laval for refurbishment a Zyglo inspection also is performed. It detected a pin hole in one of this user’s plates that allowed oil to enter the cooling-water system.