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
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
compressor. The multi-unit systems require
separate (to valve-off) one system from another.
In the combined
lube- and seal-oil systems with turbine drivers,
outer seal-oil drain must be separate from the
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
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
- Access to major hardware and instruments should be
- 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
- 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
Fig. 2. In this accessible
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
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
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 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-
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
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
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
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
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
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
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
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
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
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
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.
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
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
4 DInnocenzio, M., Oil
SystemsDesign for Reliability, First Texas A&M
University (TAMU) Turbomachinery Symposium, College Station,
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,
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.