Oil Oxidation Concerns with GE Frame 7EA Gas Turbine Variants

Introduction

In 1994-1995, Unusual oil conditions were observed by plant operators with General Electric Frame 7 gas turbines. The problem manifested itself by clogging lube oil filters. When the filters were examined, a waxy, gel like material was found in the filter cartridges. It had a reddish hue, and was often misidentified as rust particulate. The Northeast GE frame 7EA users group, a group of 14 plants running 41 units total, first helped to identify the problem. The GE frame 7EA is similar to the frame 7 systems, with the A designation having a higher ignition temperature. This variant has problems with oil oxidation when a specific no. of conditions are met. These are:

• Units have between 12000 and 16000 hours of operation clocked (oil is same)
• Units are base loaded
• Plant is grid despatchable( they can be asked to stop supplying power to the grid at off peak times)
• The bearing ambients are usually around 250 F.

Symptoms

What happens is that when the system is base loaded, The No.2 bearing runs much hotter (around 280 F) and the oil is overheated. The oil is cooled with heat exchangers in the system. If the unit is dispatched, the oil has a chance of cooling down by a greater degree. This rapid cooling has a chilling effect on the oil, and a waxy precipitate forms in the oil, appearing as a fine brown gel. This material then clogs the filters up at startup. It occurs with all the refined turbine oils specified for the turbine, and the rapid cooling is "refining" these oils further, by removing the wax. Most of the oil additives are depleted only marginally, however, in the presence of moisture (typically more than 100 ppm), the problem is aggravated. If the customer is monitoring the oxidation package with RBOT (rotating bomb oxidation test), the remaining life numbers may drop at a faster rate. GE was made aware of the problem through the efforts of the users group.

Solutions

In 1997 and 1998, Exxon, Chevron and Mobil worked on introducing a new oil that would combat the problem.

All three companies have introduced oils (listed in the table below), based on a highly refined group 2 basestocks, and then adding in proprietary anti-oxidant additive packages.