The How and Why of Lubricant Selection and Consolidation for Rotating Machinery

Frank J. Hayes, Petro-Canada
Tags: industrial lubricants, bearing lubrication, gear lubrication, electric motor lubrication

Lubricants are formulated for specific applications and often used for general applications. Within one processing facility, there may be dozens of equipment types and dozens of equipment manufacturers. The operation of each equipment type may require specific lubricants.

Each manufacturer of the same equipment type may require a different lubricant. Operators and mechanics are responsible for different phases of machinery lubrication, but their primary responsibility is to keep processing units on stream. Understanding lubricants is typically a secondary priority for operators and mechanics. Therefore, avoiding confusion through lubricant consolidation is key to avoiding costly misapplications.

Common Rotating Equipment

Common rotating equipment in petrochemical processing facilities include steam turbines, centrifugal gas compressors, positive-displacement charge pumps, centrifugal pumps, reciprocating gas compressors, rotary screw air compressors, electric motors, tank mixers, etc. Assessed individually, all these different pieces of equipment may each require a different lubricant. With careful consideration, common acceptable lubricants can be identified for multiple types of equipment.

Lubricated Components

If you look inside rotating equipment, you find that you are lubricating and/or cooling rolling-element bearings, sleeve bearings, gears, slides, cylinders, mechanical seals, transformers and heat-transfer systems. Each lubricated component individually may be optimally protected with a different type of oil or grease.

For ease of use and to reduce lubricant cross-contamination and misapplication, the number of oils and greases should be minimized without sacrificing protection. Ideally, you want to consolidate to one oil for each type of component, such as parallel-shaft gearboxes, right-angle gearboxes and steam cylinders, rolling-element bearings, sleeve bearings, mechanical seals, transformers, heat-transfer systems, hydraulic systems and slides.

Bearing Lubrication

The primary and sometimes only lubricated component in steam turbines, centrifugal gas compressors, axial gas compressors and centrifugal pumps are either rolling-element bearings or sleeve bearings. These bearings are shaft-mounted and may expose the lubricant to high temperatures up to 350 degrees F for a short time frame.

Rolling-element and sleeve bearings are lubricated based on hydrodynamic and elastohydrodynamic concepts. In both cases, the lubricant is relatively low in viscosity and formulated with rust and oxidation (R&O) inhibitors, anti-foam additives and demulsifiers. Anti-wear additives are not required. This type of fluid is called a turbine oil, R&O oil, circulating oil or bearing oil.

Depending on the variety of shaft speeds, it is conceivable that one turbine/R&O oil can be used to lubricate these bearings. Even though the oil required is one of the most basic lubricants offered, it is important to choose the right one. In the petrochemical manufacturing industry, the environment often has high temperatures with possible water contamination. Therefore, a high-quality turbine/R&O oil must be used so it can resist oxidation breakdown and sludge formation.

In the case of steam turbines, condensate may pass through the seals and into the lubricant at the bearings. In some systems, the bearing oil is also used to operate control systems. Thus, the primary performance properties of the chosen turbine/R&O oil should include component rust resistance, oxidation resistance and water demulsibility.

Pump Bearing/Mist Oil Lubrication

Centrifugal pump shaft bearings typically are lubricated with ISO viscosity grade (VG) 32, 46 or 68 R&O oils. Oil-mist lubricating systems are frequently used to distribute lubricant to centrifugal pump bearings. Due to the small orifices and liquid state phase changes (mist to drop), a diester-based fluid is most ideal for use in an oil-mist lubrication system. This type of fluid can be consolidated to one viscosity grade of ISO VG 46 or 68.

Gearbox Lubrication

There are two general gearbox configurations: parallel shaft and right angle. Parallel-shaft gearboxes can be configured with spur, bevel, spiral bevel, helical, double helical and/or herringbone gears. They generally can be lubricated with just one type of fluid, which is ISO VG 220 rated. Industrial right-angle gearboxes can be configured with worm and/or hypoid gears and usually can be lubricated with an IS0 VG 460 worm gear or synthetic lubricant.

Since right-angle gearboxes may contain brass and/or steel gears, it is a good idea to default to a synthetic gear oil, because most mineral-based extreme-pressure (EP) gear fluids are aggressive to soft metals. Most gearboxes on tank mixers require lubrication with ISO 150 to 220 non-EP gear fluids. Either a conventional mineral-based R&O fluid or a non-EP synthetic polyalphaolefin (PAO) can be used.

Reciprocating Compressors

Reciprocating gas compressors are manufactured with two separate lubrication systems: one for crankcase lubrication and the other for cylinder lubrication. An ISO VG 150 or 220 R&O fluid is normally required for the crankcase. The lubrication regime is primarily hydrodynamic in a reciprocating gas compressor crankcase.

The cylinder lubricant in a reciprocating gas compressor typically relies on heavy viscosity fluids from ISO 150 to ISO 320 to provide sliding wear protection between the piston rings, slides and cylinder wall. The additive level in the cylinder lubricant must be at a minimal level for two reasons.

Ash buildup in the plate valves must be maintained at a minimum to enable proper function (seal and movement) of the valve plates. The other reason for low additive levels is to protect catalysts in downstream processing units from fouling with additives from the compressor cylinder lubricant. The viscosity requirements for the cylinder lubricant are relatively high because the lubrication regime in the cylinder is a mix of boundary and hydrodynamic.

Mechanical Seal Barrier Fluid

The ideal barrier fluid for mechanical seals that contain process fluids flowing at temperatures higher than 140 degrees F is a very low viscosity PAO-based fluid with highly stable additives at minimal concentrations. When a barrier fluid is in continued service in the heated dead leg of a mechanical seal, it can thermally degrade and leave deposits that lodge in the seal face, thus causing seal wear and leaks.

Barrier fluid consolidation considerations are usually between viscosity grades of ISO VG 5, 15, 22 and 32. This type of fluid is unique within the petrochemical industry. Typically, ISO VG 5 and 15 as well as ISO 22 and 32 barrier fluids can be consolidated.

Grease Applications

Petrochemical processing facilities are mostly exposed to environmental conditions. The greased components are usually in the housing, but they are still subject to extreme temperature swings and fluid or water contamination. The actual service of greased bearings at petrochemical facilities is not among the most severe applications for grease.

A complexed soap, such as lithium complex, aluminum complex or calcium sulfonate complex, should be used in the wet and hot conditions of outdoor service. The most popular grease base oil viscosity and type for bearing lubrication is an ISO VG 220 mineral oil. The most common grease stiffness (worked penetration) is NLGI #2.

Some facilities have consolidated to a polyurea grease designed for electric-motor bearing lubrication as the lone grease for all applications throughout the facility because of the low load demands.

Hydraulic Systems

Hydraulic power systems are not as common in the petrochemical industry as they are in the durable goods or food manufacturing industries. Typical hydraulic fluids are referred to as anti-wear (AW) fluids and are formulated to protect hydraulic pumps, valves and cylinders under high load conditions.

Control systems in petrochemical facilities may include hydraulic direction-control components. These should not be placed under load-carrying service. Under these conditions, an R&O circulating oil can be used. For hydraulic lifting and/or control, either an ISO VG 32 or 46 fluid is required. Therefore, consolidating to an AW hydraulic fluid may not be necessary, and a turbine/R&O oil could be used instead.

Electric-Motor Bearing Lubrication

Rolling-element bearings are employed in electric motors to support balanced shaft, rotor and fan rotation. Many large electric-motor bearings are oil lubricated. The fluid requirement for these rolling-element bearings is an ISO VG 32 or 46 ashless (non-detergent and non-zinc) R&O oil.

It generally is safer to consolidate to the higher viscosity. Most electric-motor bearings are grease lubricated. In the petrochemical processing industry, only one electric-motor bearing grease is necessary. The best value and stable type of grease for electric-motor bearing lubrication is a mineral-based diurea type of polyurea-thickened grease.

Consolidated Lubricant Table

Use the following table as a starting point for making lubricant consolidation decisions. Careful consideration of typical and specialized equipment lubrication specifications and all operating environments must be taken before making consolidation decisions.

This article was previously published in the 2018 Machinery Lubrication Conference Proceedings.