Centrifugal Compressors – Lube- And Seal-Oil Systems

The lubrication of centrifugal compressors is generally handled by a pressurized system, which also provides the seal oil and control oil in some cases. One system usually supplies all machines in a given train (such as the compressor, any gears, and the driver).

A basic pressurized lube system consists of a reservoir, pumps, coolers, filters, control valves, relief valves, instrumentation, and other auxiliaries specific to the application.

Seal oil may be provided from a combined lube and seal oil system, or from a separate seal oil system. Generally, combined systems are selected for sweet gas services. Separate seal oil systems are generally selected for compressors in services that contain hydrogen sulfide or other corrosive or toxic gases. In either type of system, the inner (sometimes called ‘sour’) seal oil leakage is normally not returned to the reservoir. The outer (sometimes called ‘sweet’) seal oil leakage is returned to the reservoir. Under certain conditions, it is possible for sour gas to migrate into the outer seal oil stream that is returned to the reservoir. Having a separate system positively avoids contamination of the lubricating oil and subsequent corrosive attack of babbitt-lined bearings and other components served by the lubricating oil system.

API 614, Lubrication, Shaft-Sealing, and Control Oil Systems for Special Purpose Applications, and Specification CMP-MS-4762 cover the design, manufacture, and testing of the overall system, as well as individual components. Used as a reference, they provide guidelines based on user experience which can easily be scaled down or tailored to fit any requirement.

The system may be designed either as a console or baseplate-mounted package, with all components mounted on a single baseplate, or alternately as a multiple-package arrangement, with system components separated into individually packaged units. In this case the individual component packages are piped together in the field.

Oil return lines must slope toward the reservoir(s) to allow gravity draining. This is often overlooked when piping is being laid out. Also, be careful to avoid “head knockers” when laying out pipe.

Off-shore applications may require a system mounted integrally with the compressor/driver baseplate, with off-mounted air coolers.

The console arrangement, because of its compact layout, may limit or restrict access to various components making maintenance difficult. The multiple-package arrangement allows greater flexibility in locating the individual packages for improved maintenance access. A major disadvantage of the multiple-package arrangement is that the complete system is seldom shop tested and therefore performance is not verified prior to arrival on site.

Careful attention at all phases from initial specification through installation and startup will contribute significantly to trouble-free compressor train startup and operation. Historical maintenance data from many compressor installations indicate approximately 20 to 25% of centrifugal compressor unscheduled downtime results from instrument problems (many of these associated with operation and control of the lube and seal system).

When designing or modifying a system, obtain specific input from the field regarding site requirements, preferences, and operating experience. They may have already modified the basic system to correct problems experienced, found a particular type or brand of instrument that functions better under their site conditions, or standardized on components to reduce spare parts inventories, etc.

The following highlights areas requiring special attention:
1. For critical or non-spared equipment, include a main and an identical full-sized auxiliary oil pump (not to be confused with an emergency oil pump which is normally of much smaller capacity, sized only to handle lube and seal requirements during coast-down). A popular drive arrangement for turbine-driven compressors is a steam-turbine driven main oil pump with an electric motor driven auxiliary. This arrangement has the advantage that auto-start control of the electric motor driven unit is relatively simple and reliable with rapid acceleration to full speed and rated pressure output. For installations where steam is not available, several alternate drive combinations are used, including motor, shaft-driven, and in a few cases air or gas expanders. With motor driven main and auxiliary pumps, each should be supplied by an independent power source.
2. Consider adequate oil-flow to bearings and seals during coast-down following a trip of the auxiliary pump. The two approaches used most often involve either an emergency oil pump or overhead rundown tanks. Overhead rundown tanks are typically located to provide an initial pressure (head) equal to the low oil pressure trip pressure. API requires capacity to be sufficient to supply oil for a minimum of three minutes. In the majority of cases this is adequate.
A second method is an emergency oil pump. This pump would probably be DC motor driven, with power supplied by a battery backed UPS system.
3. Manufacturers often insist that the response time of a motor driven auxiliary pump is sufficient to avoid pressure decay tripping the main unit, and therefore accumulators are not required. However, several tests have shown this not to be the case. The option should always be held open so that accumulator requirements are based on the system demonstrating acceptable stability during the prescribed testing.
4. The system rundown tanks, and the accumulators are sometimes confused. The rundown tanks provide lubrication and cooling to bearings and seals during coast-down. The accumulator is designed to maintain system pressure within specified limits during transient conditions or upsets, thus avoiding machinery trips.
5. When oil seals are used, the manufacturer is normally asked to guarantee a maximum value for this inner seal-oil leakage. The guaranteed value is often found to be considerably lower than actual leakage on test or following startup. Since size of the degassing tank is based on this leakage rate, the tank often ends up being undersized.

API specifies that the degassing tank be sized for a minimum of three times the guaranteed inner seal oil leakage. Actual leakage, however, has in some instances exceeded quoted values by more than 10 times. The manufacturer’s sizing criteria should be verified based on review of leakage-rate tests for similar seals.

6. For centrifugal lube-oil pumps, the pump head should be compared to the maximum allowable filter pressure drop (of dirty filters) to ensure that sufficient oil flow is provided to the machinery as the filters become dirty.
7. Shaft-driven main lube-oil pumps are not recommended, since any maintenance or repair of this pump requires the machine be shutdown.

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