Auxiliaries are responsible for more downtime events than the main components of a compressor. To improve unit reliability, auxiliaries deserve closer scrutiny; unfortunately, they are often upgraded from the traditional vendors standard configuration. Compliance with an applicable American Petroleum Institute standard (API-614) is helpful. However, engineers should remember that various API standards are intended to explain the minimum requirements. Minimum requirements are not to be confused with best available technology.
As of 2012, only a small percentage of the many thousands of centrifugal compressors operating in modern industry were equipped with magnetically suspended or gas-lubricated bearings. The overwhelming majority of compressors continues to use oil lubrication for bearings that either support the compressor shaft (radial bearings) or limit shaft axial movement (thrust bearings). This article deals with these lubrication systems only. It will emphasize factors that are commonly overlooked.
Seals are used to prevent migration from the pressheaurized compressor interior volume (the compression space) toward the bearings. These seals are available in a variety of configurations, and most seals require oil as a coolant and lubricant. The auxiliary systems that feed oil to the bearings and seals are often combined, in which case, they are aptly called lube- and seal-oil systems. Separate systems are more common and are required if the seal oil is contaminated by entrained sour gases, such as hydrogen sulfide. Fig. 1 shows a simplified schematic of a plain lube-oil system. Several of the most common system instruments are also listed in Fig. 1.
Fig. 1. The simplified, but typical, compressor lube-oil system
includes many auxiliary components in addition to the
compressor. The multi-unit systems require provisions to
separate (to valve-off) one system from another. In the combined
lube- and seal-oil systems with turbine drivers, the compressors
outer seal-oil drain must be separate from the lube-oil drain.
EXAMINING UPGRADE OPTIONS
Oil reservoirs must include valve and space provisions for temporarily or permanently connecting oil purifiers to the low-point drain. In addition to removing water contamination, modern oil purifiers will also remove undesirable gases from the seal oil.
If both drivers are electric motors, different feeder connections are recommended by API-614. It should be of interest that Note 1 (see Fig. 1) also alerts purchasers to locate the suction piping away from reservoir low points where dirt can easily accumulate. A reliability-focused user will take a very active part in the selection and design process for these compressor-support systems. An infinite number of component combinations are possible, and user preferences will be discussed in this article. Guidance can be found in various API specification documents. However, the instrument nomenclature chosen by vendors and manufacturers often differs. Table 1 is one of many hundreds of feasible listings of instruments typically found on lube- and seal-oil systems. The owner-purchasers engineer must understand the purpose and functionality of each of these elements.
All systems must be properly laid out, and supply piping sized for maximum velocities that do not exceed 7 fps (approximately 2 m/s). Stainless steel (SS) is used for all piping, both upstream and downstream of the filters. SS is also needed for vessels, housings, tanks and their respective tops. Only certain valves and a few instruments are (possibly) exempted from this requirement. With high reliability being the first and foremost goal, all supervisory and control instrumentation elements should include stainless steels.
Cost-cutting has made inroads here, although some savings are false economy that will often cost more later. To avoid unavailability, here are some of the key areas to address:
- Access to major hardware and instruments should be easy.
- Filter housings must be vented to a safe location. After replacing a filter, air must be vented to ensure that the standby filter housing is ready for operation. Venting back to the oil reservoir is allowed.
- With the possible exception of valves, all oil-wetted parts of the lube-oil system (but not the pumps) should be made of SS. The top lid of the oil reservoir must be made of SS; moisture condensation can accumulate on this cover.
- The switch-over valve directing oil through either the A or the B filter-cooler set must incorporate provisions to lift its plug off the valve seal before the plug can be rotated in the desired direction.
- If the top lid is made of plain steel, the resulting rust (on the inside) will reduce equipment reliability or require increased preventive maintenance. A nitrogen blanket to fill the space between the liquid oil and top lid will not be a fully effective method to prevent rust on plain steel top lids.
- The top lid is slightly inclined to allow rainwater and spilled oil to drain. Pipe connections and access ports (manways) are flanged with top openings raised at least 1 in. above the reservoir top, and no tapped holes are allowed anywhere on the reservoir.
- All fill openings must be provided with removable strainers.
- Integral internal relief valves are permitted on rotary positive displacement pumps. However, only external relief valves are permitted on pressure vessels.
A small- to mid-sized lube skid is shown in Fig. 2. The photo depicts two horizontally arranged rotary positive displacement lube pumps, which have been sized for oil requirements that include unusual upset conditions. When both pumps are motor-driven, different feeders or a DC supply source are generally specified. The direct-current source must last as long as it takes to secure the main compressor and manipulate all associated valves.
Note how the principal components are readily accessible, as shown in Fig. 2. The suction pipes must be arranged to provide positive suction head for these horizontal pumps, with the line sloped down from the reservoir to the pump. While this recommendation is sometimes contested by pump manufacturers, it will allow gas to be vented back to the reservoir.
Fig. 2. In this accessible skid-mounted lube-
oil system, the filters are in the right
foreground; the coolers are horizontally
arranged on the left.1
To rule out unexpected surprises and the occasional finger-pointing, the compressor manufacturer must be directly responsible for the design, although the manufacturer often asks third parties to fabricate and test the entire skid.
EXAMINE WHAT OFTEN GOES WRONG
Reliability-focused users specify lube- and seal-oil systems that comply with the applicable standards of the American Petroleum Institute (API-614). These standards constitute a detailed and enhanced bill of materials, as well as a description of the redundancies required to ensure years of uninterrupted uptime to such systems. Appropriate instrumentation must be provided. An experienced compressor operator should be involved in selecting these instruments and determining their operator-friendly, optimum mounting locations. Ease of maintenance and accessibility compete with the desire to keep things compact. A measure of judgment must be exercised by both the purchaser and vendor.
With few exceptions, systems that do not comply with API standards will require more frequent maintenance. Regardless of the standards applied, the purchaser should review several pertinent details, as listed here:
Main vs. standby pump
Pumps must be centrifugal or rotary positive displacement. Driving off the main driver or compressor shaft is rarely acceptable, because pump failure would mandate equipment shutdown. If two or three pumps are used, at least one is usually driven by a small steam turbine. Pumps must have carbon-steel casings, and cast-iron casings are allowed only inside the reservoir. Exposed cast-iron pumps would be brittle and more prone to failure when involved, directly or indirectly, in a fire event.
A decision must be made as to which pump is normally on standby (although the turbine-driven pump is usually selected for standby duty). Still, someone must define how quickly the turbine will come up to speed and reestablishes the required oil pressure. The correct electrical classification must be selected for motor drivers. Suitable electronic governors should be chosen even for small steam turbines. If the steam turbine driven pump is in standby mode, it should be kept warm and slow-rolled. But slow-rolling consumes energy. Some bearings will not allow slow rolling at speeds below 100 rpm. Pumps being slow-rolled should have a return line with a restriction orifice back to suction, and dewatering of piping and steam turbine casing must be accomplished by using the right steam trap type and model.
Standby equipment deserves more attention than it usually seems to receive. Pumps and their respective driver shafts must be easy to align.2 Couplings should be designed with a service factor of two or more, and they must be virtually maintenance-free.
The start switch or actuator component for the auxiliary pump must have a manual reset provision. A steam condensate exhaust hood will be needed for steam exhaust lines vented to atmosphere. Without it, operators risk being showered with scalding water whenever the auxiliary steam turbine-driven pump kicks in.
For some steam turbine models, slow-rolling below 150 rpm will not allow establishing an oil film between the journal and bearing bore. Also, consideration must be given to an emergency oil source to be fed to the turbocompressor train during an occasional power failure event. If there is even a remote possibility of neither oil pump being available, an overhead rundown tank should be provided to gravity-feed the turbomachinery bearings.
A pressurized overhead tank is shown in Fig. 3, but non-pressurized (atmospheric pressure) tanks are quite often used as well. An atmospheric breather valve or vent must be used with nonpressurized models, and the user-purchaser must address issues of airborne dirt and birds trying to build nests in or near such vents. A drilled check valve is then used between the lube supply header and atmospheric pressure overhead rundown tank. Regardless of the type of rundown tank selected, elevations should be such that the static head is less than the equipment lube-oil trip pressure. API-614 gives guidance on these and other important matters dealing with lubrication, shaft sealing and control oil systems for special purpose applications.
Fig. 3. Pressurized overhead rundown tank for
centrifugal compressors lists the instrumentation.3
The anticipated time needed for the machine to coast to a stop is 8 minutes, with 15 minutes used as a more conservative limit. This rundown tank should be vented, and the vent oriented and configured to prevent entry of birds and debris. Does the overhead rundown tank need to be heated or insulated for operation in cold weather? Are suitable auto-start facilities provided? Designers should verify that proper dewatering facilities are provided at all points of the steam piping and at the turbine casing.
In installations with two electric motor-driven pumps, the power should come from different feeders or substations. Temporary power dips during the pump switch-over are bridged by using a hydraulic accumulator in the lube supply line. The bladder of the accumulator is usually filled with nitrogen, and the configurations and functionalities of such accumulators are well known. Yet, although widely used, typical bladder-type accumulators (Fig. 4, left side) risk premature failure from the rubbing action of the neoprene or buna-rubber bladder against the accumulator walls. This failure risk is further amplified when dirt particles are carried in the oil.4 Diaphragm-style accumulators (Fig. 4, right side) were used in reliability-focused user companies after 1975 to facilitate condition monitoring and to avoid rubbing-induced failures. Note: The standard diaphragm-type accumulator (Fig. 4, right) is fitted with a vertical indicator rod and a transparent dome at the top.
Fig. 4. Bladder-type accumulator (left) and a rod-
equipped surveillable diaphragm-type
Reliability-focused plants modify the standard diaphragm accumulator, as shown in Fig. 5, by removing the seal ring and screw plug and tightly fitting a tall transparent high-strength plastic dome at the top of the accumulator. A tapped hole is machined into the center of the shut-off button and a long gauge rod is threaded into this tapped hole. The gauge rod extends through the opening created by removing the screw plug. The tip of the gauge rod is seen by the operators making their surveillance rounds. The integrity of the flexible diaphragm and its properly proportioned nitrogen vs. oil fill volumes are visually ascertained, as shown in Fig. 6, which shows a large field installation. The wire-mesh screens are installed to guard against a careless overhead hook or a maintenance tool accidentally hitting the polycarbonate sight-glass dome.
Fig. 5. Cross-view of the diaphragm-type
Fig. 6. Diaphragm accumulators installed at a
If bladder-type accumulators are deemed acceptable, be sure that they have a 10-second or greater capacity and are equipped with fill valves and isolation valves that permit monitoring of bladder condition. Bladderless accumulators will require high-level alarm, low-level alarm and low-level cut-off provisions.
An armored sight glass must be supplied for the reservoir. Because the reservoir should be constructed from stainless steel, its interior should not be coated or painted. Minimum standard practice calls for oil reservoirs to be sized for at least 2.6 minutes of maximum flow. A lube-oil system with pumps supplying 100 gpm would be sized for an operating volume of 260 gallons (1,000 l) or more. A more conservative high-reliability practice defines the system operating range as 2.6 times gpm, to which the greater of 40 gallons or one weeks oil leakage rate is being added. Other rules-of-thumb are noteworthy; one calls for an oil-free surface in the reservoir of at least 0.25 ft2/gpm to promote air disengagement from the oil.
Oil reservoirs are typically rectangular and are provided with a sloped bottom, sometimes called a false bottom. The volume below the sloped false bottom is filled with a heat-transfer fluid for pre-startup heating or for maintaining a controlled temperature. The volume above the false bottom is, of course, the actual working volume of the oil reservoir. Convention calls for a reservoir vent to be one pipe size larger than the sum of the areas of all seal drains.
In installations where steam is available, a thermal fluid with high-temperature capability and low volatility should fill the space below the sloped bottom. If no steam is available, electric heaters sized not to exceed 15 watts/in.2 (the watt density) can be used to heat the thermal fluid. Electric temperature control switches should be provided if electric heat is selected. A high-capacity vent is needed to accommodate thermal expansion of the heat-transfer fluid below the sloped bottom of an oil reservoir. A side-mounted gauge glass or dipstick is required to verify or to monitor the height of thermal fluid under the false reservoir bottom. If a steam coil is used for heating, there should be suitable steam traps.
In some climates, heating will be needed only at startup or in low-temperature ambient conditions. Heaters are generally sized to effect heating from the lowest average ambient to a minimum allowable oil temperature73°F (20°C), for the very typical ISO VG 32 lubricantin four hours. It is possible to pre-heat the lubricant by simply admitting steam into the water upstream of the coolers. However, temperature indicators should be installed, and a responsible operator must be assigned the task of emergency heating operation. For cold-temperature regions or in situations where large ambient temperature swings are common, the reservoir may require external insulation. Such insulation has the associated benefit of reducing condensation of water vapors in the reservoir.
The return oil from the turbocompressor may be at sufficiently elevated temperature to flow freely without further heating. The drain valve at the low point of the working volume serves also as a connection for an onstream lube-oil purifier. Such purifiers are normally sized to handle the entire systems working volume in 24 hours. They must be provided with a piping leg that prevents emptying the reservoir. Some will also require a condensate-removal line.
All reservoirs must be fitted with internal baffles or stilling tubes that allow for contaminants to settle out. Oil returning from the turbocompressor bearings or bypassed from the pressurizing pumps should not fall into the reservoir; this would risk static electricity buildup. The vents from filter housings and other points in the installation should feed back into the reservoir.
Filters and coolers
Suitable instrumentation is also needed on the filters and coolers. The layout should permit the system to operate while maintenance personnel are safely performing routine service on nonoperating redundant elements. Except for the transfer valve (main switching valve) and the structural parts of the mounting skid, stainless steel is the required material of construction. Block valves and check valves are needed, and the user-purchaser must devote time and effort to review the piping and instrumentation diagram for functional completeness.
Kickback valves that route excess oil back to the reservoir must be located upstream of the filters and coolers. They should be sized to pass excess capacity of one pump plus the full capacity of the standby pump. Dual valves may be needed to obtain proper valve coefficients in certain seal systems.
It is usually considered a good move to involve plant operators in the selection process before specifying and purchasing filters and coolers for an existing facility. Blotter paper-style filter cartridges are not acceptable. Allow the operators to ask if they are satisfied with the instrumentation package as shown on the schematics, or on a mockup of the system. Obtaining buy-in from operations staff at this stage will provide great value.
Normally, coolers are bought in compliance with TEMA Class C shell requirements and have removable bundles. Experienced users will not permit tubes with less than 5⁄8-in. OD 18 BWG, but will allow double pipe fin-tube exchangers for small systems. It is usually best to make an experience check and to ask questions. Unlike pumps, coolers are pressure vessels that must be designed and manufactured in accordance with applicable codes. Water should be on the tube side, oil on the shell side. The oil pressure must exceed cooling water pressure to preventor at least reduceleakage of water into the oil system in the event of tube failure. The oil-side design pressure should be equal to, or greater than, the pump relief valve setting with PD pumps and shutoff pressure with centrifugal pumps. Table 2 summarizes the material selection guidelines.
Each cooler must have an oil fill line, a drain and a high-point ventall suitably valved and generously sloped. Cooling water flow enters at the bottom and exits at the top. Drain piping is typically sized for a maximum velocity of 1 fps (approximately 0.3 m/s).
High-pressure gas piping should be seal welded, and all piping should be configured to allow for thermal expansion. Remember: The piping may have to be removed for cleaning prior to compressor commissioning. Flanges and special locations (such as near bearings and seals) are required to insert temporary strainers. Flexible expansion joints are not allowed in the piping due to the danger of fatigue failure. Flexible joints and hoses are also not allowed because they tend to be the first stationary elements to fail during a fire.
To facilitate oil drainage back to reservoirs in gravity systems, each compressor bearing housing typically requires a 1-in. minimum vent. Gear boxes and couplings are generally equipped with 2-in. vents. Coupling guards may require special air exchange provisions to prevent trapped air from overheating the coupling components.
Centrifugal compressor lube/seal reservoir hazards
The static electric charge generation mechanism was investigated by prominent users in the mid-1970s. Static charge buildup in filters was determined to be the root cause; it was confirmed by careful measurements. Several systems that had experienced explosions were equipped with pressure-controlled recycle lines downstream of the seal-oil (or lube/seal-oil) filters. In obvious contrast, systems with recycle lines originating upstream of the filters and with line lengths that allowed relaxation of charges remained trouble-free.
Safe designs allow 30 or more seconds for the oil to travel from the filter outlet to the reservoir inlet. Because undesirable agitation of the oil surface must be avoided, the return line should enter the reservoir below the oil level. Pressurized return lines should not be vented inside the reservoir.
Seal-oil system gas reference lines should be provided with a drilled check valve to prevent disruption of overhead accumulator level control during compressor surge. There is also a need for provisions that allow introduction of a simulated gas signal (sometimes called a false buffer gas) during startup when running a compressor on air, or with a suction pressure below design. These provisions may require control systems that can fully accommodate prevailing running-in conditions.
WHAT WE HAVE LEARNED
Lube- and seal-oil systems must be carefully and conservatively designed. A review of the system design and limitations must begin at the proposal stage. The owner-purchaser must thoroughly understand each design element. Each pipe or control line should be traced back to its origin, the design intent must be well understood, and all of the owner-purchasers questions must be answered by the vendor-manufacturer.
Reliability-focused owner-purchasers go beyond the minimum requirements of API-614 in their efforts to impart the ultimate in maintainability and surveillability to these very important systems.
Special diaphragm-style accumulators are one of many examples where reliability-focused thinking is translated into component selection. They represent best available technology and have been used by best-of-class companies for many decades. HP
This article is based on the book entitled, Compressors: How to Achieve High Reliability and Availability, authored by Heinz Bloch and Fred Geitner. The book was released by McGraw-Hill.
1 IMO-Demag-DeLaval, Trenton, New Jersey, Commercial Literature, 1991.
2 Bloch, H. P., Pump Wisdom: Problem Solving for Operators and Specialists, John Wiley & Sons, Hoboken, New Jersey, 2011.
3 American Petroleum Institute, API Standard 614.
4 DInnocenzio, M., Oil SystemsDesign for Reliability, First Texas A&M University (TAMU) Turbomachinery Symposium, College Station, Texas, 1971.
5 Bloch, H. P., Making machinery surveillablePart 2, Hydrocarbon Processing, July 1993, p. 23.
6 Doddannavar, R. and A. Barnard, Practical Hydraulic Systems, Elsevier Publishing, Burlington, Massachusetts, 2005.
Heinz P. Bloch is a consulting engineer residing in Westminster, Colorado. He has held machinery-oriented staff and line positions with Exxon affiliates in the US, Europe and Japan prior to retirement as Exxon Chemicals regional machinery specialist for the US. Mr. Bloch is the author of 18 comprehensive texts and close to 500 other publications on machinery reliability improvement. He is also the reliability and equipment editor for Hydrocarbon Processing. He is an ASME Life Fellow and maintains registration as a professional engineer in Texas and New Jersey.