(Author’s Note: Much of the information in this series is based on the practical knowledge of real-world lubrication professionals. Once such expert is Mark Kavanaugh, who has over 42 years of experience in large manufacturing operations, and is currently responsible for coordinating the lubrication of thousands of pieces of rotating equipment in a refinery. Mark is certified as a CLS, MTL I and MLA II.)
Let’s start with a recap: Per the discussion in Part IV (A) of this article, “Lubrication of Positive Displacement Compressors," a compressor’s lot in life is to increase the pressure of a gas to the point where it can be used in an industrial facility. Rated by discharge pressure (psi) and capacity in cubic feet/minute (cfm), compressors fall into two major groups: positive displacement and dynamic (Fig. 1). The types of units in these two groups, along with the specific gases they compress (see Table I), call for differ-ent lube strategies. In this article installment, we’ll review the dynamic side of the compressor family tree, specifically the centrifugal and axial branches.
NOTE: Again, as mentioned in Part IV (A), air-compressor lubrication is not part of this discussion. It was, however, the focus of an article in the May/June 2009 issue of LMT (www.mt-online.com/thibault).
Centrifugal compressors, like that illustrated in Fig. 2, deliver gases at higher flow rates than positive displacement compressors, but at lower pressures. They’re used extensively in the refining and petrochemical industry because there is no contact between the gas and the lubricant—meaning that centrifugal compressors produce oil-free gas. Reactive gases can also be compressed without coming in contact with the lubricant.
The typical centrifugal compressor relies on the rotating blades (or vanes) of an impeller to accelerate a gas and, thus, create pressure. The impeller sits in a volute—a widening chamber connected to a gas discharge line. Gas that enters the compressor is swept up by the impeller vanes and moved from the center to the outside by centrifugal force, causing an increase in gas velocity. When the gas leaves the impeller and enters the volute, it slows down as the chamber widens. This slowdown converts velocity or kinetic energy to pressure.
(Not shown in Fig. 3 are stationary diffuser plates that initially slow down the gas from the impeller and direct it into the volute.)
To achieve desired pressures, most centrifugal compressors are designed as multi-stage units. (The cutaway in Fig. 2 reflects a five-stage design.) In a multi-stage unit, the impellers are all mounted on the same shaft along with a volute for each impeller and one suction- and one discharge-line. Gases aren’t cooled between stages, as they would be in a reciprocating compressor. Typical multi-stage operating pressures are in the range of 100-150 psi.
The major lubricated components in a centrifugal com-pressor train include:
Centrifugal compressors can have large lubrication systems consisting of an oil reservoir, pump, filter, cooler and oil lines. The oil is then pumped to the lubricated parts, which are both radial and thrust bearings. Most centrifugal compressors are lubricated with high quality ISO 32 turbine oil. Some compressors may call for an ISO 46. If a step-up gear is present and lubricated with the same oil as the bearings, ISO 68 turbine oil may be recommended. (NOTE: High-speed air compressors, in some cases, are lubricated with synthetics. This was discussed in the previously referenced 2009 LMT article on air compressor oils.)
Some compressors rely on a seal-oil system along with a seal at the end of the shaft to prevent gas from leaking beyond it. This is particularly true when compressing environmentally toxic gases.
A seal-oil system consists of an oil-collection area, oil supply-line with a check valve, oil return-line, gas return-line, head tank and a seal-oil reservoir. During operation, oil is pumped via the oil supply-line through the seal. Because the pressure of the oil is greater than the pressure of the gas, the oil flowing through the seal keeps the gas from leaking. Oil (carrying the absorbed gas) is pumped to a compartment in the reservoir, at which point the gas is separated and returns to the compressor. The gas-free oil then returns to the seal-oil system. As a safety precaution in the event of pump failure, a separate head tank supplies enough oil on a pump shutdown to allow the compressor to be shut down without a gas leak. Most seal-oil systems use the same oil to lubricate the bearings. The oil from the head tank is NOT used to lubricate the bearings.
Axial compressors are used where high volumes of gas at low pressure are needed. Capable of producing up to 1,000,000 ft3/minute, they incorporate a series of rotating and stationary (or fixed) blades (as illustrated in Fig. 4).
In axial units, the rotating blades accelerate gas that comes in contact with the stationary blades, which change the direction of flow—resulting in a lower velocity, but a higher pressure. Each series of rotating and fixed blades is a stage. (Some axial-flow designs have up to 20 stages.)
Axial compressors are used in gas turbines to provide compressed air during the combustion process. They are also used in large air-separation, blast-furnace-air and other applications where a high volume of air is required.
Bearings—both radial and thrust—on axial compressors are lubricated with high-quality ISO 32 turbine oil.
Basic troubleshooting techniques
As discussed in Part IV (A) of this series, successful compressor troubleshooting calls for a strong knowledge of machine component design, operating parameters, lubrication requirements and OEM guidelines. In-depth troubleshooting is usually a one-on-one proposition, with a troubleshooter required to look deeply into each piece of the puzzle.
Keep in mind that certain troubleshooting principles hold true for all com-pressor types. The following recaps basic guidelines, regardless of the group into which a compressor falls—either positive displacement or dynamic.
Temperature. Changes in temperature from an established norm are reliable indicators of changes in machine condition. Daily temperature inspections should, at least, include: suction and discharge of gas, gas interstage coolers, afterstage coolers, lube-oil coolers, cooling water, mechanical seals, crankcase and bearing oils. Periodic checks of bearings, valves and cylinder head temps are advisable.
Levels. Liquid levels in compressor components must be monitored diligently. Correct crankcase, bearing housing, reservoir oil levels, feed rates on cylinder injectors and circulating oil systems must be kept constant. Compressed gas receivers, intercoolers, aftercoolers and process piping must be drained and kept liquid-free. Free water should be drained from oil reservoirs and oil filter housings daily.
Pressures. All compressors are designed to operate in specific pressure ranges; this is one governing factor determining what type of compressor is used in what service. Pressure differentials between suction, interstage and discharge gases must be tracked and variances out of the norm investigated. Bearing, mechanical seal and oil filter pressures should be checked, at least daily. Air compressor inlet filter differential pressure should be checked daily.
Changes in vibration or sound. Knocks, pings, rattles or ticks should be investigated as soon as possible after detection.
Oil analysis should be conducted on no less than a quarterly basis—and on a monthly basis in severe services. Tests should include: viscosity, particle counts, wear metals, water content and FTIR or Ruler for remaining useful oil life. Modifications to this basic test slate will be required, depending on compressor type and service.
Troubleshooting specifics for dynamic designs…
Centrifugal and axial compressors are usually large, very costly, precision machines. Most are computer-controlled, with software that monitors both operating parameters (i.e., pressure, flow, temperatures, etc.), and machine conditions (i.e., vibration, bearing and seal-oil pressures and temperatures, oil-reservoir levels, etc.). Troubleshooting these units is a significant undertaking: It typically involves sifting through mounds of data and conducting in-depth hands-on inspections during outages.
Oil analysis for centrifugal and axial compressors should include annual ferrography, filter-debris and varnish tests. The large circulating-lube and seal-oil systems found in these types of units usually respond well to filter upgrades (higher beta ratios and lower micron ratings). Consult the compressor OEM and lubricant manufacturer before attempting any changes.
As noted in Parts IV (A) and (B), compressors are among the most important (and complex) types of equipment in a plant. Being proactive and reliability-conscious is key to minimizing their failures. This article focused on how various compressor types operate, the correct lubricants to use and valuable troubleshooting tips. Review these points—and keep them handy. They’ll help improve the uptime of some of the hardest working “moneymakers” at your site.
Part V of this series covers best practice for improving run time and minimizing shutdowns of blowers and fans. LMT