Best of the Best 2016: PLEASANT VALLEY

Pleasant Valley

Oil change, bypass plate mitigate varnish issues

Historically, most gas-turbine (GT) hydraulic-system varnish issues have been associated with GE units where lube and control oil share a common sump. However, the 501D5A installed at Pleasant Valley Generating Station, Dexter, Minn, owned and operated by Great River Energy, was plagued by sticking servo valves on its liquid-fuel-system actuators. This contributed to some forced outages and the need for expedited servo repairs, both of which can be costly.

By way of background, Pleasant Valley, managed by Tye Stuart, is a nominal 450-MW three-unit peaking facility which primarily operates on gas, but has oil backup. The 501D5A (Unit 13) was installed in 2002, about a year after two V84.3A2 GTs (Units 11 and 12) were commissioned.

Staff had been able to alleviate the sticking problem by cleaning out a 50-micron ruby orifice when it clogged with sludge and varnish—until one day when this “fix” didn’t work. In this case, the failed actuator would not respond after both cleaning the orifice and changing the servo.

Varnish fouling was so bad the inoperable actuator was sent out for cleaning and repair. The shop report stated this was the worst case of varnish contamination it had seen in an actuator. Clearly, remedial action was necessary.

Interestingly, the plant had sent a sample of the offending oil to its lab only two months earlier for varnish-potential analysis. Tests specified included acid number, membrane patch, ultra-centrifuge, Ruler, and Fourier transform infrared spectroscopy (FTIR).

The lab report came back with normal results and indicated a low level of degradation byproducts associated with varnishing, despite the fact that plant personnel noticed considerable darkening of the oil themselves.

Lesson learned: Plant personnel needed a better understanding of how varnish forms and how to conduct a thorough assessment of the hydraulic system. Staff involved included Mike Herman, Preston Walsh, Chuck Condon, Kevin Beske, Craig Birkett, Doug Goodale, and Stuart Crum.

Mind the heat. Because heat is a major factor in varnish formation, station personnel pulled the lube-oil sump heater and found it had failed. There was evidence of “stitching” on the element—a failure mode caused by overheating.

The sump was designed to have the heater in a thermowell, enabling its removal without draining the oil; however, the heater specified was designed for full immersion in the oil, a better heat-transfer medium than the air in a dry pipe. Plus, the heater rating, 22 W/ft², was too high for this application.

After talking to engineers at Chromalox Inc, the heater manufacturer, staff determined that a 10-W/ft² element would be more suitable for a dry-pipe application. It would both prevent overheating of the oil and prolong heater life.

In addition to use of a heater with a lower output, plant personnel wrapped about 90 ft of 8-W/ft² self-regulating heat tracing around the sump. It was equipped with a thermostat set to maintain 115F on the outside surface of the sump. Next, the sump was insulated.

Insulation also was added to the hydraulic power unit (HPU) shack, thereby reducing the amount of heat necessary to maintain the temperature set point and reducing the chances of thermal degradation.

Staff believed it had found the “smoking gun” in the heating system. But after learning more about varnish, they identified additional improvements to mitigate the problem. While heat contributes to varnish formation, cool surfaces in stagnant flow areas are where it “plates out.”

Keep LO moving. With liquid-fuel runs few and far between, there was a lack of circulation in the system, which facilitates varnish deposition. Staff learned that some GE users challenged with similar issues had good success by adding a small bypass plate; they believed it might work on the Siemens unit as well.

A nearby hydraulic repair facility was able to design and manufacture a cross-port relief manifold for Pleasant Valley. The CPRM is sandwiched between the actuator and the servo, providing 0.5 gpm flow through the actuator when it is not in service. This avoids stagnation and helps keep the varnish in solution. The 0.5-gpm flow setting was selected because staff believed it would have minimal impact on system operation.

The shop also refurbished and cleaned the remaining liquid-fuel valve actuators, and also replaced the 50-micron ruby orifice with a 100-micron orifice to minimize the likelihood of clogging. Finally, insulation was added to the access door adjacent to the actuator—to keep that area warmer in cold weather.

Another area explored was the hydraulic fluid itself. Plant management knew it was necessary to change the oil, not a big expense for a 150-gal sump. The lubricant supplier also recommended a flushing oil, which was added and circulated through the system for two days prior to the change-out.

Station personnel wanted to be sure Pleasant Valley had been using the best lubricant for the application before ordering fresh oil. Not! The original oil spec for this system called for an

R&O ISO 100 (R&O for improved anti-rust and –oxidation properties) oil and that’s what the plant had been using since COD.

A lubrication engineer was consulted, and staff polled other 501D5A owner/operators to see what oil they were using. The informal survey revealed most users in climates similar to Pleasant Valley’s rely on a ISO 46 oil.

So the plant switched to Mobil DTE 10 Excel™ 46, believing the lower-viscosity oil would flow better, plus its anti-wear additive would protect the pump and reduce friction. The Mobil formulation also has a more favorable viscosity index and better thermal stability than the plant’s original oil.

Since the menu of varnish-potential analyses specified previously did not detect any varnish precursors, station management decided to forego this comprehensive testing regimen in the future but stick to generic, routine lube-oil tests. However, personnel plan on monitoring for color and changing the oil every two years as a preventive measure.

Best practices for lube storage, handling, use

To eliminate past issues, industry best practices were adopted and several changes made to the methods for storing, handling, and using lubricants and greases. These are highlighted in the bullet points below:

Oils

      • After careful review of existing inventory, and consultation with a tribologist, plant consolidated its inventory of oils from 12 varieties to eight. This lowered inventory costs while reducing confusion and the chance of cross contamination.

      • Transporting full 55-gal drums with a forklift can be risky, especially over uneven terrain. A couple of near misses were an eye opener. For bulk oils, such as turbine and hydraulic, plant purchased 120-gal “forkable” cube-shaped totes, which have a low center of gravity and can be moved to point of use easily and safely.

Desiccant breathers were added to the totes to protect against moisture ingress. Oil is manually pumped into the machinery sump as needed, using a dual-stage coalescing and 3-micron particulate filter cart to ensure a high level of cleanliness.

      • For quantities of oil smaller than those described in the previous item, staff obtained color-coded dispensing jugs for each. Next, plant personnel applied color-coded labels on all lubricated to identify the specific oil for each point of use.

      • A color-coded “oil menu” is posted in the lube room to help identify the oils.

      • Shelves were installed in the lube room to assist with organization and to maximize storage space.

      • Established a more intensive oil analysis program to better indicate the quality of the lubricating fluid. Added tests include, but are not limited to FTIR spectroscopy, foam test, demulsibility, and the ASTM D2272 rotating pressure vessel oxidation test.

Greases

      • Pleasant Valley occasionally experienced problems with “over-greasing” as evidenced by grease migration into motor windings and oil sumps. In addition to reducing the quantity of grease applied, plant eliminated the use of battery-powered grease guns, opting for manual only. Battery-powered guns can pump grease at up to 20,000 psig which can compromise grease seals and cause premature equipment failure.

      • New hand-operated grease guns have a clear barrel for easy identification and inspection of product labeling; plus, color-coded end pieces.

      • New grease guns are located in the lube room on labeled and color-coded caddies.

      • Color-coded grease zerk caps were installed on all greased equipment. Their color corresponds to the applicator color code to reduce cross contamination and misapplication of lubes.

Proper storage protects against degradation of fuel oil

In summer 2014, Pleasant Valley personnel discovered a large amount of demulsified water, biogrowth, and degraded fuel at the bottom of the plant’s 800,000-gal main fuel-oil tank. This “glop” was unusable and had to be disposed of in an environmentally safe manner. Corrosion also was found in several fuel-system components.

Pertinent facts: (1) The tank had been maintained about half-full since 2010; (2) in winter 2014, the centrifugal pumps began losing suction when the oil level fell below about the 300,000-gal mark; and (3) ultra-low-sulfur diesel (ULSD) had been stored in the tank for about the last three years.

The plant’s proactive O&M staff proceeded with research, sludge analysis, and consulted with petroleum engineers to determine root cause and how to prevent this problem in the future. Staff learned that water gets in a diesel tank provides a habitat for hydrocarbon-consuming bacteria, which produce acetic acid as a byproduct of dining. The acid, in turn, degrades the fuel and promotes corrosion.

A lesser-known fact: Today’s ULSD contains surfactants which change the interfacial surface tension of the fuel and makes it difficult to separate water from the oil.

The bottom line: Pleasant Valley had to implement some mods and practices to ensure the backup fuel is kept moisture-free to the degree possibility and maintained at the highest possible quality. Here are some of the steps taken:

      • Eliminate leakage oil/water return to the main storage tank by redirecting it to a holding tank. An operating procedure dating back to COD called for the purging of fuel lines with water when the gas turbines were shut down after burning oil. The resulting water/oil mixture was collected and forwarded to the main oil tank.

      • Insulate the tank breather vent to reduce condensation buildup and the ingress of water into the tank. The tank is insulated so there was the potential for a large amount of warm, humid air to occupy the headspace.

      • Reduce humidity of the headspace of the fuel tank by installing a system to apply a continuous purge of ultra-dry air.

      • Implement a fuel-truck sampling procedure to ensure the oil received does not contain water.

      • Install a 4-micron tank filtration system to remove contaminants and bacteria, thereby prolonging fuel stability.

      • Implement a tank sampling, testing, and bottom-drain procedure to swiftly remove any traces of water before it becomes a problem.

      • Extend the downcomers in the fuel tank from about half height to near the bottom of the vessel to promote more complete mixing and heating.

By eliminating all sources of possible water ingress, staff expects the dramatic improvement in fuel quality will be maintained, thereby assuring reliable operation on fuel oil and eliminating the need to dispose of large amounts of degraded fuel in the future.

Posted in Best Practices |

Comments are closed.