Proper selection and care of turbine fluids important to plant reliability

The proper selection and care of lubricants and hydraulic fluids for steam turbine/generators are critical to the long-term health and top performance of these machines. So much so that you can expect several presentations and wide-ranging discussion on turbine fluids at the Steam Turbine User Group’s 2017 Conference and Vendor Fair in the Sheraton Grand at Wild Horse Pass, Chandler (Phoenix), Ariz, August 28 – 31. That’s just a few weeks away; register today. 

Highlights of the four presentations on turbine-fluid characteristics and maintenance made at STUG2016 are presented below. They represent the thinking of the user, consultant, alternative-fluid, and filtration communities and attest to the value of this annual meeting. To dig deeper, registered owner/operators of steam turbine/generators can access the PowerPoints on the organization’s website

Varnish control in EHC systems

The hydraulic power unit (HPU) is part of the electrohydraulic control (EHC) system responsible for controlling steam-turbine (ST) operation. It supplies phosphate ester fluid, highly regarded for its anti-wear and fire-resistant properties, to the following:

      • ST valve actuators.

      • Some casing over-pressure trip devices.

      • Over-speed protection circuits.

The speaker told attendees it’s important to maintain EHC fluid chemistry within specified limits to prevent the formation of varnish, which can negatively affect the operation of turbine valves and controls. For example, varnish can restrict flow through pencil filters on actuator servos, causing erratic valve motion.

Next, he discussed acid contamination of phosphate ester, caused by the introduction of water into the fluid. High acidity is associated with foaming, gel formation, and corrosion and etching of system components—including pumps, actuators, solenoids, and servo valves. Initially, Fuller’s earth filters were used to control acid level, but there were side effects—specifically the formation of byproducts that also plugged filters.

Selexsorb filters were developed as a solution, but they too produced byproducts (ultra-fine particulates) that caused valves to stick, and pumps to fail from abrasive wear.

The speaker then introduced the group to an ion-exchange filtration system for acid removal that offers an effective alternative for phosphate ester fluid treatment. It was said to control fluid acidity to levels lower than that that possible using Fuller’s earth and Selexsorb filters.

Next topic was the potential causes of varnish—moisture and acid reaction with the filtration medium, thermal degradation (an issue in D11 steam turbines), and micro-dieseling. These topics have been discussed in the pages of CCJ over the years and background material can be accessed easily by using the search function on the magazine’s website at the top right of the page.

The speaker closed with a discussion of Membrane Patch Colorimetry for measuring the varnish potential of turbine fluids and of RULER for identifying fluid decomposition issues. Information on these also can be accessed at CCJ ONline.

Final recommendations from this user:

      • Use ion-exchange filters for acid control.

      • Install a dry-air blanketing assembly for the EHC reservoir head space. It effectively removes moisture from the fluid while excluding possible contamination.

      • All phosphate ester fluids are not created equal; some are more or less stable than others.

PAG promotes reliability

Varnish issues associated with the use of petroleum lubricants include fouling of servo valves, pencil filters, lube-oil reservoirs, and piping to the point where unit availability is affected. There are several ways to mitigate the effects of varnish, one being a switch to a non-varnishing fluid— such as polyalkylene glycol (PAG).

This synthetic lubricant, a product of Dow Chemical Co, has attracted considerable plant-level support over the last decade for delivering on its promise of preventing varnish formation while providing long-lasting high performance. In June 2013 GE announced that it meets the OEM’s requirements as a gas-turbine lubricant (GEK 32568H).

The two speakers from American Chemical Technologies Inc, the exclusive supplier of PAG, explained how the product works and how it is used in about a hundred generating units today—specifically:

      • As a base-oil modifier, typically in quantities of 10% to 15% of the lubricant volume.

      • As a direct replacement for petroleum lubricants (100%).

      • As a direct replacement for phosphate ester in electrohydraulic control systems.

Case histories were offered for each application. Here are a few thumbnails:

Base-oil modifier

Plant 1 equipped with F-class gas turbines was forced to clean/repair or replace servo valves regularly because of varnish fouling. Installed 10% PAG modifier in November 2014 (four units) and nearly two years later Membrane Patch Colorimetry results were still in the low teens and there had been no varnish-related trips.

Plant 2 equipped with two F-class gas turbines; one placed in service in 2001, the other in 2011. The older unit had operated without issue for nine years when the original charge of commercial turbine oil was replaced with the same product. Three years or so later, oxidation products and varnish were identified; thermal imaging showed fluid temperature had increased by up to 30 deg F. Base-oil modifier was added in fall 2014. Results: Varnish solubilized, surfaces cleaned; flow increased, temperature back to normal. Units were converted to PAG turbine fluid in fall 2015.

Plant 3’s steam turbine started showing signs of a demulsibility issue. Steam leaks were the root cause. Base-oil modifier was injected to prevent an emulsion from forming.

PAG-based turbine fluid

Plant 1, equipped with four F-class gas turbines, struggled to keep its servos operating as intended. Annual spend to keep varnish from tripping the unit was more than $60k in labor and materials. Success! Current annual spend on servo maintenance is $2500.

Plant 2, equipped with four F-class peakers, replaced a petroleum oil with PAG in one unit at the end of 2007. No trips reported for that machine since. Generator and turbine bearing inspections have revealed no issues. Use of the portable varnish filtration skid purchased for all four units was eliminated for this turbine.

Plant 3, equipped with two F-class gas turbines in baseload service, was beset with servo valve failures and frequent trips. Switched to PAG in 2007/2008 and has not experienced a servo failure or trip since. Servo maintenance is done at major inspections. Lube-oil filter never experienced a high-delta-p condition.

PAG-based EHC fluid

Two combined-cycle units at one plant were converted from phosphate ester to PAG (both the steam and gas turbines) with dramatic reductions in total acid number—typically from slightly more than 2 mg KOH/g to less than 1 mg.

Varnish solutions

The Hy-Pro Filtration presentation began with about a dozen slides on how varnish is formed and the problems it causes. Filtration solutions followed—including non-spark-discharge filter elements and their associated positive Membrane Patch Colorimetry results. The benefits of offline kidney loops and acid scavenge loops were included in the discussion.

Next topic was soluble varnish removal, a segue to a skid offering the “complete contamination solution.” The bottom line: Varnish-free turbine oil operation is possible with the proper treatment equipment.

Prevent turbine-oil performance degradation

Lubrication Engineers projects itself as a “one-stop shop for lubrication reliability.” The company provides consulting services, a variety of equipment/components for lube-oil service and storage systems, and boutique formulations of turbine oils. The STUG presentation focused on the following aspects of lost turbine-oil performance: wear protection, oxidation resistance, demulsibility protection, foam prevention, volatility protection, and rust prevention.

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