A well-maintained steam turbine oil with moderate makeup rates should last 20 to 30 years. When a steam turbine oil fails early through oxidation, it is often due to water contamination. Water reduces oxidation stability and supports rust formation, which among other negative effects, acts as an oxidation catalyst.

Varying amounts of water will constantly be introduced to the steam turbine lubrication systems through gland seal leakage. Because the turbine shaft passes through the turbine casing, low-pressure steam seals are needed to minimize steam leakage or air ingress leakage to the vacuum condenser. Water or condensed steam is generally channeled away from the lubrication system but inevitably, some water will penetrate the casing and enter the lube oil system. Gland seal condition, gland sealing steam pressure and the condition of the gland seal exhauster will impact the amount of water introduced to the lubrication system. Typically, vapor extraction systems and high-velocity downward flowing oil create a vacuum which can draw steam past shaft seals into the bearing and oil system. Water can also be introduced through lube oil cooler failures, improper powerhouse cleaning practices, water contamination of makeup oil and condensed ambient moisture.

In many cases, the impact of poor oil-water separation can be offset with the right combination and quality of additives including antioxidants, rust inhibitors and demulsibility improvers.

Excess water may also be removed on a continuous basis through the use of water traps, centrifuges, coalescers, tank headspace dehydrators and/or vacuum dehydrators. If turbine oil demulsibility has failed, exposure to water-related lube oil oxidation is then tied to the performance of water separation systems.

Heat will also cause reduced turbine oil life through increased oxidation. In utility steam turbine applications, it is common to experience bearing temperatures of 120ºF to 160ºF (49ºC to 71ºC) and lube oil sump temperatures of 120ºF (49ºC). The impact of heat is generally understood to double the oxidation rate for every 18 degrees above 140ºF (10 degrees above 60ºC).

A conventional mineral oil will start to rapidly oxidize at temperatures above 180ºF (82ºC).. High-quality antioxidants can delay thermal oxidation but excess heat and water must be minimized to gain long turbine oil life.