Lubrication Systems

Lubrication is a fundamental requirement for all compressors with the exception of those equipped with an alternative form of bearing such as the magnetic bearing. If it is a tiny unit, the lubricant may be sealed into the rolling element bearings by the bearing manufacturer. In process service, lubrication of bearings takes on a more elaborate form. Some of the smaller units will probably use an attached oiler or an oil mist system. Because this affects only the smaller units, this section will deal primari ly with those compressors using force-feed lubrication.

The ring-oiled bearing might be considered the most fundamental and basic of the lube systems (see Figure 8-1). The ring rides on top of the shaft and is dragged at part-shaft speed by friction. The lower portion of the ring resides in a reservoir of oil. In its most primitive state, the reservoir is the lubricant source and heat sink. The rotating ring moves oil from the reservoir to the upper portion of the bearing. Here the ring and shaft interlace causing some of the oil to be removed. The oil enters through grooves cut into the surface of the bearing, where it is carried to the minimum clearance area by the journal pumping action. The next level of sophistication is to add circulation and cooling to the reservoir. An alternative to the separate

Ring Oiled Bearings Journal
Figure 8-1. Ring-oiled sleeve bearing. (Courtesy of Elliott Company

circulation system is to connect the reservoir to a pressure-fed external lube system in use with the balance of the compressor train bearings,

Chapter 3, which discussed various reciprocating compressors, stated that many reciprocators use a pressure-fed lubrication system for the frame bearings. This system is built into the crankcase in many applications. The basics of these systems follow the fundamental criteria which will be discussed with the fully separate system. The larger reciprocating compressors may use a separate frame lubrication system.

For the rotary, centrifugal, and axial compressors, a separate lubrication system is used and in some cases, seal oil and control oil are also supplied from this system. Chapter 4 mentioned that the oil used for flooding is taken from the lubrication system. Because the secondary duties of cooling the process stream and timing the rotors overshadow the primary job of lubing the bearings, the lube oil system may be misnamed for this compressor. As a model to guide the discussion, the basic system as covered by API Standard 614, "Lubrication, Shaft-Sealing, and Control Oil Systems for Special-Purpose Applications," [1] will be used. In the opinion of some vendors, this system is an overkill, but it can easily be tailored to fit any system by scaling down as required. The standard then can be fully or partially invoked or in some smaller system, the standard can be used as an outline and guide.

A basic pressurized lube system consists of a reservoir, pump, cooler, filter, control valves, relief valves, pressure and temperature switches, gauges, arid piping. The oil is pumped from the reservoir, cooled and HI tered, pressure controlled, and directed to the bearings by way of a supply header. A drain header collects the oil exiting from the bearings, and gravity flows it back into the reservoir (see Figure 8-2). If control oil is required for a power positioner on a steam turbine governor valve, addi tional control valves are used to establish the two levels of pressure needed because the control oil is normally at a significantly higher pressure than that needed by the bearings (see Figure 8-3). Note that an accumulator has been added to improve transient response for the turbine governor. Bearings normally operate in a 15 to 18 psig range with some variation from vendor to vendor. Control oil is generally in the 100 to 150 psig range.

If oil film or mechanical contact seals are used, another pressure level must be established. This pressure level is difficult to generalize as the seal pressure is a differential above the process gas pressure. For a mechanical contact seal, it is in the range of 35 to 50 psid to the gas and must follow the gas pressure from startup to shutdown. This generates an

Process Lubrication System DiagramOil Lubrication System Design
Figure 8-3. Lubrication system for a compressor that requires two levels of pressure.

additional design consideration, which will be discussed in more detail later. For the oil film and pumping bushing seals, the pressure is only a few psi above the gas pressure; however, an elevated tank is required. This tank forms the basis for the manometric differential pressure control for the seals as well as a backup supply reservoir to the seals in the case of seal oil supply failure. Figure 8-4 is a block diagram of a lube oil sys-

Compressor Block Diagram
Figure 8-4. Lube oil block diagram used for a compressor lube system with a control system and a seal oil system.

tem similar to the one shown in Figure 8-3 with the seal system added. Figure 8-5 is a schematic drawing of a combined lube and seal system as would be furnished for a compressor with mechanical contact seals.

In the following paragraphs, various available options will be discussed. It is hoped that by using the options best suited for a given application even an inexperienced user might be able to specify a lube system for a compressor.

Reservoir

The reservoir is the lube oil storage tank. In some of the packaged compressors, it is built into the package base, and, in some standardized compressors, it is built into the compressor frame. In a reciprocating

Centrifugal Compressor SchematicsReciprocating Compressor Elliot
Figure 8-6. Lube oil console. (Courtesy of Elliott Company

should be individually returned, including the relief valves, and piped into stilling tubes that discharge below the suction loss oil level. An automatically closing fill opening should be installed in the top of the reservoir and should include a strainer to prevent entrance of foreign material with the oil during fill operations. A breather-filter cap should be used to cover the fill opening. A flanged opening should be placed on the top and blind flanged for an optional, user-furnished vent stack. Some form of level-indicating device should be provided, mounted on the side of the reservoir. A top-mounted dipstick is also required, with the dipstick marked in liquid units of the type in use at the plant location.

The reservoir should be sized for five minutes of normal flow, with a retention time of eight minutes. The retention time should be calculated using normal flow and total volume below the minimum operating level. Provision must be made for the oil rundown from the field located piping. It should be checked on all systems, but particularly on the larger sizes. It is quite embarrassing to take a new compressor through commissioning, have a shutdown and overflow the reservoir on rundown, especially if all the company executives are there to witness the event. Additional features for the reservoirs and the defined operating levels are shown in Figure 8-7.

Heaters should be considered for the reservoir. While they are normally thought of as cold-weather features, they aid in keeping the oil dry if the compressor is shutdown long enough for the oil to cool. The heater

Centrifugal Compressor

In many ways the pump is the heart of the lubrication system as it is the only active element. It must furnish sufficient capacity at a high enough pressure to satisfy the entire train connected to its lube system The pump driver must be sized to be able to start with cold oil, generally 50'F or colder. It must have enough power to operate under all conditions, including the highest seal pressures expected, which, on a refrigeration compressor, is the shutdown stagnation condition. This pressure is equal to the saturation pressure of the refrigerant at ambient temperatures and can be much higher than normal operating pressure. Also the pump should be checked for minimum viscosity operation, particularly at the higher pressures. Pump failures have been traced to violation of the mini mum allowable viscosity limit on higher pressure operation resulting in rotor contact.

The pumps can either be rotary positive displacement or centrifugal. The rotary positive displacement pump of the helical-lobe type is recommended. Centrifugal pumps should only be used for large systems, over 500 gallons per minute, if a suitable rotary screw pump is not available. The centrifugal should have as steep a pressure flow characteristic curve as can be found. The normal low head rise centrifugal pump will lose capacity as the filters get dirty and cause the system to starve for oil and go into an unscheduled shutdown. Oversizing the centrifugal and running it normally in the overload region is not an adequate solution to the problem.

On some of the smaller, standardized compressors, the main lube oil pump may be shaft driven. Others use rotary gear pumps. These have been used for quite some time, and the desire for change or modification usually falls on deaf ears. A good quality standby pump should be connected in parallel to the rotary gear pump in case the first one fails. As a general rule, on even the smaller systems, there should be a main and a full-sized standby pump. On the larger systems, as the equipment is put into unspared service, the two pumps are a must. In some plants a third, smaller pump is used, which is referred to as a coastdown or emergency pump. This pump is used as a contingency in the event of a full power failure when both the main and standby pumps fail due to loss of power or for any other reason. Of course, the main compressor should go into the shutdown mode using the emergency pump to supply oil to the bearings during the coastdown cycle. The whole premise for this course of action is that a source of energy, not affected by a power outage, is available. A better alternative using an overhead tank for compressor coast-down will be discussed later.

The pumps may be driven by any of several drivers. For many years the favorite arrangement has been a steam turbine for the main pump and an electric motor for the standby. It is a nice combination in that if the steam supply is disrupted by power failure, there is some residual pressure as the system decays, in many cases, allowing the pump to deliver some capacity. This may be the only argument for the shaft-driven pump which delivers oil while the compressor shaft is turning. Unfortunately, even though the pump will turn, it doesn't necessarily deliver oil because of long suction lines, air leaks, or the capacity is just not available at the lower speeds. In some cases there is no steam available and some smaller compressors do not have the steam turbine option, primarily because of cost. In these cases, the two pumps should be motor driven. If possible, the motors should receive power feed from two independent power sources. If an emergency or coastdown pump option is selected, it should be run from a totally independent power system. Some large plants use an uninterruptible power supply (UPS) to operate devices during a power outage. Sufficient additional capacity might be added to the batteries to run a small motor. The motor can be direct current and will not tax the inverter on the UPS. If a separate battery system is installed only for the coastdown pump, the odds are it will not be operational after a period of time due to lack of battery maintenance. That is why it should be combined with other plant functions that require the backup system to be operational. Other power sources, such as air motors, have been used but without great success.

Pump casings should be made of steel if possible, which, on the smaller compressor systems, will not be practical. Most petrochemical plants live in the fear of fire, and the use of cast iron casings in a hydrocarbon plant is not a good idea.

The pumps should be piped with a flooded suction to avoid having priming problems. This is difficult to do on a shaft-driven pump. This procedure precludes mounting the pumping equipment on the top of the reservoir. If top-mounted equipment is desirable to keep the system compact, then vertical pumps, operating below the oil level in the reservoir, should be considered. When this arrangement is used, the need for steel in the pump casing is eliminated. The oil reservoir must be strengthened to carry the extra weight without excessive deflection. While this is one way to maintain a compact system, it is only practical on the smaller systems where component maintenance is not as difficult.

A strainer should be used in the pump suction line temporarily for a centrifugal pump and permanently for the rotary positive displacement pump. For the permanent installations, a Y-type strainer with an austenitic stainless strainer basket should be used. The cross-sectional area of any strainer should be at least 150% of the normal flow area. In all strainer installations, a compound pressure gauge should be installed between the strainer and pump suction.

Booster Pumps

In some applications (usually high pressure compressors using oil film seals) alternative pump schemes should be considered. It may be that the desired seal pressure is not achievable by one set of pumps or the quantity required by the seal is small relative to the main pump capacity. There are times when booster pumps are needed; however, if the reason is energy, it would be worth reviewing the economics very carefully, because reliability tends to suffer with the booster. The booster pumps are paired into a main and standby and are configured to take suction from the lower pressure system. Sufficient interlocks have to be supplied to the drivers so that if the main pumps shutdown, the boosters come down. Other problems may arise when the controls are set or trimmed because the system is usually quite sensitive to the high system gain caused b_v the high pressure at the valves.

Pump Sizing

Pumps should be sized for 1.2 times the system's normal flow requirement, with a minimum of 10 gpm above the normal flow. If the booster arrangement is used, an additional capacity is needed equal to the flow of both booster pumps running simultaneously. A centrifugal pump should be within the range of 50 to 110% of the best efficiency point when running at normal capacity. The pump, as was previously mentioned, should have a steep curve. API 614 mandates a minimum of 5% pressure rise to shutoff. This minimum seems to be on the low side. A 15% rise to shutoff would come closer to maintaining minimum oil flow with dirty filters. See Chapter 7 for various driver's sizing guidelines.

Pump Couplings

For pumps above 25 hp, flexible disk, spacer-type couplings should be used. The flexible elements must be selected for compatibility with the plant atmosphere. For the smaller systems, a non-spacer coupling may be adequate, but the coupling should be of good quality. This is not the place to save money. Coupling guards should be furnished as a part of the lube system.

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Renewable Energy 101

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Responses

  • Rahel
    What is the Shutdown pressure for the Lube oil on an Elliot Steam Turbine Model #2BYRIHT?
    5 years ago
  • Bellina Buccho
    How fast should a Elliott lube oil pump tubine go?
    3 years ago
  • gundahar
    Why lube oil circuit used in compressor?
    3 years ago
  • Yrj
    Why the oil temperature not enough in centrifugal compressor?
    3 years ago
  • ADDOLORATA
    Why are compressor seal oil tanks located above the compressor?
    3 years ago
  • wegahta
    What is seal oil system in compressors?
    2 years ago
  • Kim
    What is seal oil used in compressure?
    2 years ago
  • stephanie
    Why centrifugal compressor doesn't require priming?
    2 years ago

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