Below are Experts' opinions on this comparison-advantages and disadvantages of oil free and regular compressors.
NGV engines need some oil for gas injectors,
Korea, Thailand and many other NGV countries prefer compressor stations with oil and NGV buses and trucks have no problem at all by being fueled at compressors with lubrication at CNG stations during the last 14 years.
HD NGV buses and trucks may have problems by being fueled at the stations with unknown oil free compressor station equipment. We may end up replacing injectors and other engine components. Too much oil to the NGV engines will be problem but we have many filters at the stations and also on the vehicles and even on the engine side to make sure proper amount of oil goes to gas injectors.
Making right selection of the compressors will be very important for the vehicles.
Oil lube versus oil free is an old argument in the industry, and the following report is presented by industry expert at international ngv association in USA.
Many of the compressors which claim to be oil free (Knox Western, IMW etc.) are really not oil free, some oil from the crankcase does migrate up the piston rod and into the cylinders, so even these "oil free" machines pass oil. The only true oil free machine I have seen in CNG service was the Nuovo Pignone, and it was extremely high priced.
The main question is why do some customers believe he needs oil free? My
guess is that in the past he has had operational issues due to oil in his vehicles; however I would bet a lot of money they were using low cost compressors with poor gas cooling - which accelerates oil passage - and low cost and low quality filtration which does not catch the oil.
The majority of CNG compressors in the US - and around the world - are oil lubricated. Like Korea, the US uses sophisticated engine management systems and injectors, and oil has not recently been an operational issue.
If provided with information of the manufacturer, model and type of compressor both oil lube and oil free - we may be able to give more detailed comparison and information expert opinion on the subject for our Russian
Korean NGV engines use Bosch injectors which need some oil. Oil free compressor stations may damage NGV engines with Bosch injectors on.
This was the reason why Korea supplied regular CNG compressor stations instead of oil free.
In conclusion, Kwangshin offers Oil-Free technology option and Regular Compressors.
___________________________________________________________________________________
CYLINDER LUBRICATION
METHODS
FOR
NGV FUELING
COMPRESSORS
FACTS AND
FRICTION
By
Graham Barker
Director NGV
Fueling Systems, Norwalk Company
Presented at:
13th AGAlNGV Coalition
NGV Conference
October 15-17 1995, Los Angeles, California
______________________________________________________
ABSTRACT
This presentation will examine the various methods
of cylinder lubrication currently being used in NGV compressors, along with the available non lubricated compressor technology and some of the newer developments to components and equipment to meet ever increasing
demands.
It includes definitions and explanations of the terminologies used to describe
each feature, along with their benefits
and drawbacks.
The aim is to provide sufficient information to NGV station operators
and purchasers to help
them evaluate equipment for their particular application.
INTRODUCTION
Oil carry-over from NGV compressors into vehicle fuel systems is a controversial subject,
as it has been cited as a major contributor to engine failures
and poor exhaust emission
results by certain segments
of the NGV industry.
Based on these incidents,
the NGV industry is considering moving
toward "non lubricated" compressor
designs.
Another trend is the compressor
discharge and cascade storage pressures increasing towards
5000 psig,
as many people believe this is the only way to provide a complete
fill in a 3600
psig rated vehicle onboard cylinder.
Individually each of these trends places additional demands upon
a compressor, and together they have already resulted
in some severe operating problems.
Obtaining long reliable life from high pressure components in oil lubricated NGV applications
operating at 3600 psig discharge
pressure has presented
somewhat of a challenge for some
compressors, and this challenge is increased when discharge pressures
reach 5000 psig as
the final stage typically operates
with a differential pressure of around 3000 psig which places
additional strain on the components.
Operating these higher pressure
units in a non lubricated
mode only compounds
the problems, and these aspects will be reviewed.
FUNCTIONS OF A CYLINDER
LUBRICANT
Before discussing the various types of compressor design and lubrication methods, it is worth
reviewing exactly what the lubricant
does in a compressor cylinder.
Lubricant functions
are various and complex,
and the following list
details just a few of them:
a. Reduce
friction loss and power required
by:
i) Separating rubbing parts
ii) Dissipating friction
heat through cooling
and heat transfer iii) Minimizing component wear
b. Flush away entering dirt and wear debris
c. Reduce
gas leakage past rings and packings
Based upon the above, it can be seen that lubrication plays an important
part in the successful operation of a compressor, and serious
consideration must be given to the effects of
removing any of the
above benefits.
OIL CARRYOVER
Oil has indeed been found in some vehicle cylinders
and fuel gas regulators, and several
studies have been made in an attempt to evaluate the effects of this oil on a vehicle's
performance. However the specific reasons high quantities of oil have occurred only in
certain locations have never been fully documented.
Furthermore, there is currently no proven test methodology or readily available
meter to measure oil carryover in the field, therefore
the industry has been unable to establish
a minimum oil content
level. Even
the oil limitation levels contained in the proposed
draft of SAE J1616 (Recommended Practice
for Compressed Natural Gas Vehicle Fuel) were removed in the final published version,
and replaced by statement acknowledging test procedures and acceptable oil levels are required.
Selected compressor manufacturers, conversion suppliers, engine manufacturers and station
operators were polled by phone and fax in an attempt to obtain a consensus on the maximum amount of oil carryover
which can be tolerated in the vehicle fuel system or a station, but at
the time of writing this paper very few responses
had been received.
The only consistent oil limitation amount which has been seen in the industry states a
maximum of one half pound (% Ib) of oil per one (1) million standard
cubic feet (MMSCF) of
gas. This appears
in many specifications, but it is still unclear how a manufacturer is expected to confirm his equipment complies,
or how the purchaser will check in the field.
A
simple calculation is shown
under "Filtration" which has been used as an example to
illustrate the potential quantity of oil
which can be carried over from a given compressor, and though this is not a standardized procedure
it can be used as a basis for filter
.sizinq.
TYPICAL DESIGNS
OF NGV COMPRESSOR
Figure 1 shows three (3) of the more common designs
of compressor currently
in use in NGV fueling stations. Each of these compressors is available in either a "lubricated" or "non
lubricated" configuration.
CYLINDER LUBRICATION
METHODS
The amount of lubricant
required by each cylinder is dependant upon the
operating pressure of the cylinder and its bore size,
therefore care must be taken in selecting
the most efficient lubrication method.
Figure 2 shows a selection of the more common cylinder lubrication types and terminologies used in NGV compressors, and more detailed
descriptions of some of the features are given throughout this paper.
There are several fundamental design differences between lubricated and non lubricated compressors, and there are also differences between lubricated designs
that have a direct
effect on oil carryover. Figure 3 shows cross sections of a trunk type design and a crosshead type design with the major components in each highlighted for comparison.
The trunk type design
has the compression piston directly
connected to the connecting rod, and typically uses SPLASH LUBRICATION.
It may also have another stage of compression (typically higher pressure) mounted
on top of the lower stage in a "stacked piston" layout.
TYPES OF CYLINDER LUBRICATION
Normal Lube The cylinders are oil lubricated
by one of the following methods, and the piston ring materials
are reviewed on a case by case basis and are usually metallic
(cast iron or bronze):
Splash Lube Uncontrolled amounts
of oil from the crankcase are "splashed" onto the cylinder walls as the crankshaft rotates.
Single
Point A single lubricator
meters oil through
a "splitter" which diverts oil to each cylinder.
Point To Point
Individual lubricators meter oil directly
into each cylinder.
Incidental Lube Also known as ACCIDENTAL
LUBE, oil enters the
cylinder in unknown and uncontrolled amounts,
usually from the crankcase
along the piston rod past the wipers. The piston ring and packing materials
are usually non metallic and may include a rider band.
Mini-Lube Approximately 25% of
normal lube rate, sufficient to oil wet rings for gas sealing enhancement. The piston ring and packing materials
are usually non metallic and may include a rider band.
Non Lube No oil enters
the cylinders from ANY source, the piston
ring and packing
materials are non metallic and include
a rider band. This design also requires
the use of an
extra long distance piece of sufficient length to prevent any part of the piston rod from entering
both the frame and
cylinder areas, and a slinger ring to help prevent oil migration along the piston rod.
The trunk design typically starts
out in an underlubricated condition
when the rings are new, particularly on the upper (higher pressure) cylinder
of the stacked design, and can result in an overlubricated condition when the rings are worn. The pistons
are usually equipped
with a special wiper ring in an attempt
to prevent excessive oil carryover.
The crosshead type design
has the piston secured to a piston rod and crosshead before being attached to the connecting
rod, with a distance piece between the compression cylinders and frame. This design can be either LUBRICATED or NON LUBRICATED.
If it is lubricated, it may use either the SINGLE POINT or POINT TO POINT
method.
The SINGLE POINT
design starts out with the appropriate total amount of oil at the pump, but
each cylinder may not receive
the correct amount as the splitter block cannot always be sized to
provide the correct and differing
amounts of oil required
by each cylinder.
The POINT TO POINT design provides the most accurate method of
metering oil,
as the amount of oil going to each cylinder can be precisely adjusted
and monitored.
MINI LUBE, which is often considered a "hybrid" design, can use either of the above lubrication methods. Small amounts of oil can be added to each stage,
or a non lubricated design can be used in the lower
pressure stages with the higher pressure stages receiving full lubrication.
Figure
4 shows two (2) designs of DISTANCE
PIECE used in NGV compressors.
Both designs are often used in "non
lubricated" applications, however the COMBINATION
PACKING design is not a true non lubricated design because a part of the piston rod which
enters the crankcase also passes
through the packing and into the cylinder,
thereby allowing INCIDENTAL lubrication as oil migrates
into the cylinders.
Piston ring manufacturers state that this sporadic method of lubrication is
a major contributor to premature ring wear, and it can also combine with ring dust to create a "sludge" which is known
to cause valves and rings to stick, further contributing to wear.
The extra long DISTANCE
PIECE is normally
used in a true non lubricated
application,
which
prevents the piston rod from entering both the crankcase and the cylinders. A slinger ring
should also be added to horizontal design compressors to further limit carryover possibilities.
A double compartment distance piece is also available, with an additional
packing
in between.
This design is also used on many lubricated compressors to ensure that the only oil entering
the cylinders is 'from the cylinder lubricator,
with none coming from the
frame.
Filtration of the discharge gas from a lubricated compressor should
also be considered as a
method to reduce
oil carryover, as it has provento
be viable
option
in many instances.
Oil is normally carried in the gas in two (2) forms;
an aerosol and a vapor.
Aerosols can be filtered at high pressures using a coalescing
filter.
Vapors
are harder to remove
as they tend
to be absorbed by the gas at normal
compression temperatures, therefore care must be taken to ensure the discharge gas
is cooled to level sufficient
to condense the
oil prior to filtration.
Using synthetic oils in place of mineral
oils can also help to reduce the
vapor phase as they
have less affinity to the natural gas and tolerate higher temperatures.
Most filter manufacturers use
a "DOP"(Dioctyl Phthalate) rating for their filter performance, which is conducted at atmospheric pressure and measures only
dry filter aerosol
removal
efficiency. However, according
to one of the world's
largest fluid clarification equipment manufacturers, these ratings do not depict the true efficiency since DOP conditions
are much
different to actual
operating
conditions
on a compressor.
In particular oil
re-entrainment, saturated
pressure
drop and the effects of fluctuating pressure and flow are not accounted
for.
Therefore, an "In Service" simulation test was developed
which
uses both liquid
(oil) and solid
(particle) contaminants closely matching the particle size and distribution of a typical compressor application. This
method was used to measure the liquid
aerosol
penetration,
saturated differential pressure and dirt holding
capacity
of several commercially available coalescing filter
elements.
Based upon the above test, the performance
of a typical
coalescing
filter
can be quoted
as a certain penetration number in ppm(w) (parts per million
by weight).
The design
of most filters
is usually based upon an inlet challenge of 40-50 ppm(w)
of oil, and a common penetration number for filters used
in this application was
found to be 0.024 ppm(w) or lower. This is much less than one half pound (1/2 Ib) of oil per one (1) million
standard
cubic feet (MMSCF)
Therefore, in order to apply the correct filtration for a particular
compressor,
the oil in the gas
must be calculated in ppm(w)
to determine the level
of oil challenge.
The following table and calculation illustrates the amount of oil used in a hypothetical three (3) stage, 200 scfm compressor using
point to point lubrication
for both full and mini lubrication configurations, and the amount
of oil in ppm(w) which equates to 1/2 Ib/MMSCF of gas limit previously mentioned.
The number of drops of oil which equals one (1) US gallon
varies between 64,000 and
115,200 depending on the source, so the average of
90,000 drops was used.
One (1) US
gallon of ISO 100 oil weighs 7.34 Ibs, therefore 1 Ib of oil = 12,193 drops.
The Specific Gravity of Natural Gas can vary between 0.6 to 0.7, so 0.65. average was used. Using 0.65 SG, one (1) standard cubic foot (SCF) of gas weighs 0.0487 Ib, therefore one (1)
million standard cubic feet (MMSCF)
weighs 48,704 Ibs.
As ppm (w) is a ratio it can be determined for % Ib/MMSCF using the above information:
1/2 Ib/MMSCF = 1/2 Ib/48,704 Ibs
= 10.26 Ibs /1 MM Ibs
Therefore 1/2 Ib/MMSCF = 10.26
ppm (w)
|
Drops/Min
1st
Stage
|
Lube
10
|
Mini Lube
2
|
|
|
2nd Stage
|
6
|
2
|
|
|
3rd Stage
|
4
|
2
|
|
|
Packings
|
6
|
3
|
|
|
Total
|
26
|
9
|
drops/per
200 scfm
|
If 200 scfm carries 26 drops/min
then 1 MMSCF
carries 130,000 drops.
If 90,000
drops = 1 US gallon = 7.341bs then 130,000 drops = 10.6 Ibs.
10.6Ib/MMSCF
= 10.6 Ib/48,704
Ibs = 217 Ibs /1 MM Ibs = 217 ppm (w).
217 ppm(w) exceeds the design limits for a typical coalescing filter,
therefore, two (2) filters in series must be employed, wherein the first filter helps bring the challenge down to around 50 ppm(w) enabling the second filter to function under design conditions and achieve the 0.024
ppm(w) oil carryover.
Even if the efficiency of both filters drops off significantly, it should still be possible to remain below the 10.26 ppm (w)
or 1/2 Ib/MMSCF limitation.
Using the above calculations for the mini lube
configuration,
the oil carryover from the
compressor drops to 3.67 Ibs, which is the equivalent of approximately 75 ppm (w).
It must be pointed out however, that there are several other compressor design features and operating conditions which directly
affect the operation of the filters. These include
correctly sizing the filter bowl, good drainage capabilities to prevent oil re-entrainment, limiting gas discharge temperatures, and the effects of fluctuating pressures and flows common in NGV applications; butthe most
important factor may be relying
upon the station
operator to perform the routine maintenance required.
It must also be remembered that filters are not limited
just to lubricated units. Filtration is also needed on non lubricated compressors.
An inlet coalescing filter is used to remove
oil from the inlet gas, as
the majority of pipeline
transmission compressors are fully lubricated and a discharge
particulate filter is required to contain the dust generated
from wearing piston rings. These filters typically use the same pressure rated housings which are used for the filters on a lubricated unit.
NON LUBRICATED CYLINDERS
Non lubricated cylinder assemblies are usually a more sophisticated design than lubricated units, and Figure 5 shows
the typical differences in the piston and ring layout.
It can be seen that a non lubricated piston is normally longer than a lubricated design. This
is because the non lube unit incorporates a rider band, the piston rings have a larger cross
sectional area, and in some instances an extra ring is also added to help
withstand the operating pressures.
The piston rings have a larger area to provide a better modulus of elasticity and a greater cross sectional width to help prevent extrusion
between the piston and cylinder. The extra ring is usually
added to the higher pressure
stages in an attempt to combat the high
differential pressures.
The rider band is required to support the weight of the piston and provide
a wearing surface to keep the piston away from the bore.
The non lube cylinder
is also longer to accommodate the longer piston, and is usually made from
a harder material
with the bore honed to a much finer finish.
The piston rings and packing components used in non lubricated applications also have a higher initial
cost than the traditional materials used in oil lubricated
units, and have a more
frequent maintenance requirement.
MATERIAL SELECTION
Material selection for the cylinder components, particularly for non lubricated units, is extremely important and depends upon the operating
conditions of the unit.
The following
table shows some of the design limitations imposed by compressor and/or ring manufacturers.
|
FEATURE
|
|
LUBRICATED
|
NON-LUBE
|
|
Piston Speed
|
|
900 FPM
|
750 FPM
|
|
Cylinder Temperature
|
|
350°F
|
300°F
|
|
Compressor Ratio
|
|
6:1
|
4:1
|
|
Cylinder Finish <10"
|
Bore
|
32 microns
|
8 Microns
|
|
10-20 Bore
>20" Bore
|
32 microns
63 microns
|
16 Microns
32 Microns
|
|
Actual field measurements made by piston
ring manufacturers on several compressors have revealed some disturbing facts, with cylinder discharge temperatures in excess of 450°F and piston speeds in excess of 900 FPM being recorded
even on "non lubricated" designs.
This poses problems for selecting the appropriate material.
The duty cycle of the compressor
also has a direct effect on component
life. Compressors tend to be oversized
for the initial application, which results in heavy compressor cycling and accelerated component wear,
even if the correct material selection has been made. Smaller . dual compressors can help to even out the compressor run times, and also ensure that the station can always be on line during maintenance.
Lubricated cylinders normally use cast iron or bronze for the piston rings, although
some manufacturers are now looking
at the harder non metallic compounds
which can
be used
with oil in a mini lubricated
configuration without creating the
previously mentioned "sludge".
A
wide variety of non lubricated material compounds are available, typically
a form of Teflon®
"filled" with other materials such as carbon, graphite,
fiberglass or bronze to enhance certain properties required by the compressor manufacturer and operating conditions.
However, Teflon® has
not performed well at temperatures in excess of 350°F
(which appears
to be common in many of the smaller high speed compressors used in NGV stations), so tests are now under way to determine which of the currently
available ring materials
can be
utilized
for this application.
FUTURE DEVELOPMENTS
Work is currently
under way in the application of new materials
with a better heat tolerance, in conjunction with innovative changes to piston and cylinder
design, specifically for use in high
pressure non lubricated compressors.
One such design is
the CONSUMABLE SLEEVE piston shown in Figure 6, which was initially developed for 5000 psig oil
free air applications for the US Navy. This concept
makes use of the high differential pressure which is such a problem for a conventional ring designs, and directs it to force a specially
designed sleeve against
the bore to maintain an effective seal. This
design has achieved
some significant increases
in ring life, but the cost is significantly higher than that of a conventional non lubricated design piston assembly.
The design of the conventional piston
ring pack is also being examined and reviewed, with some
ring manufacturers starting
to treat it more like a packing case as shown in Figure 7.
This approach uses a selection of different rings on a piston, with each ring designed for a
specific purpose. For example, the illustration shows a final stage piston with a pressure breaker (throttle) ring at the top of the pack to reduce the force on the rest of the rings.
The
next two (2) rings are single acting
"breather rings" which compress on the up stroke but allow
breathing on the down stroke to help dissipate heat. The next two (2) rings are double acting compressing rings,
and the final two rings are positive sealing
rings to prevent bleed back into
the prior stage.
A
rider band completes the pack.
The illustration also shows rings with different
thicknesses, as the manufacturer prefers this method over color coding
to ensure that the pack is
assembled correctly by the technicians.
ECONOMICS OF LUBE VS.
NON LUBE
Compared to lubricated compressors, current non lubricated designs typically have a higher initial cost, a higher maintenance cost and more down time
for maintenance.
The individual component costs (rings, packings etc.) for Teflon® products is approximately 3-
4
times the price of cast iron, and the cost for some of the more exotic materials can be 2-3 times
higher than Teflon®.
If a consumable sleeve design is used,
the complete piston and cylinder assembly
may be 2-3 times the price of the conventional non lube design.
Maintenance requirements for
a non lubricated unit
can be 3-4 times greater
than the equivalent lubricated unit, based upon
a maximum anticipated ring life of 1500-2000
hours for non
lube versus 6000-8000 hours
for lube.
A slight
offset
against
these
higher
costs are items such as the elimination of some oil clean
up equipment and possible
reduced
maintenance
on the vehicle
fuel
systems.
There is no easy solution
to the question of which compressor lubrication type is best suited for NGV applications or what
the discharge pressure
should be, and the debate is sure to
continue long after today's discussion.
However, before wholesale changes
are made to existing NGV compressor designs,
a workable test methodology with a realistic acceptable lube oil content should be established,
along with a minimum storage and
discharge pressure both backed by substantial data.
A
major factor when establishing the oil
limit is the fact that internal combustion engines
already have to contend with burning oil entering
the cylinder from the crankcase
breather and past valve stem oil seals, therefore
the relatively small amount of oil carried over from the
compressor should not be a factor in emission levels.
The severity of oil related problems
on vehicle fueling components
varies by system design
and manufacturer, however
most are similar
to those experienced with "heavies" in propane. Some manufacturers do not consider
these problems major, and simply recommend
routine draining or cleaning,
or mounting the equipment such that any accumulated oil is carried
into the engine and burned;
others recommend the use of filters
in the vehicle fuel system.
With regard to compressor operation, it seems that most of the "non
lubricated" equipment currently being used is suffering from high maintenance problems and prolonged downtime.
It is this author's opinion
that both oil carryover
problems and compressor operating problems can be significantly reduced
from the levels
currently being seen, even on oil lubricated compressors, by ensuring that the package
incorporates some or all of the features mentioned above and by instituting and following good maintenance programs.
Many of the compressors currently being used for NGV fueling
were
not originally designed for this unique
application, and
it also appears that
they have not been maintained in accordance with the manufacturer's recommendations.
If an oil lubricated compressor is equipped with
a properly sized
and maintained filtration system, heat exchangers with a close design approach, and an automatic
filter
drain
system
it will meet the oil carryover requirements.
It will also be a more reliable
and less expensive
compressor
overall
when
compared
to a true
non lubricated design.
Alternate materials are being developed
for use in high pressure
compressor applications, however these
tend to be significantly more expensive
and it is unlikely that
non lubricated
compressors will ever match
the on line record of lubricated
compressors.
It would be a grave error if the NGV industry
arbitrarily
adopts
non lubricated compressors without further
investigating the causes
of the equipment
and vehicle failures more thoroughly, and investiqatinq some of the available options such as filtration
and mini lube in more detail.
There is no reason that a correctly
designed
oil lubricated compressor package cannot deliver
gas of suitable quality to fuel vehicles.
ACKNOWLEDGEMENTS
The author wishes to acknowledge the various manufacturers and station operators
and industry participants who took the time and trouble to complete the questionnaires and supply
information over the telephone. This information was invaluable during the preparation of this presentation.
REFERENCES
The following publications and companies have been
used as references for material contained in this presentation:
American Petroleum
Institute (API) Specification 11P for Packaged Reciprocating
Compressors for Oil and Gas Production
Services (Second Edition).
American Petroleum Institute
(API) Specification 618 for
Reciprocating Compressors for General
Refinery Services (Third Edition)
Compressed Air and Gas Data (Third Edition)
- Ingersoll Rand Compressed Air and Gas Institute (CAGI)
Handbook (Fourth Edition) C. Lee Cook Compressor Products
Finite Filter
- Compressed Natural Gas (CNG) Filtration
for Compressor Stations
France Compressor Products
Institute of Gas Technology
(IGT) -Practical Solutions to Fueling
Station Compressor
Oil Carryover
Institute of Gas Technology (IGT) -The
Effect of Gas Quality on Natural Gas Vehicles
MME Compressor Parts
Norwalk Company
Compressor Design Handbook
Pall Corporation -
Practical In Service Simulation Tests for the Rating of High Efficiency
Aerosol Coalescing Filter Performance
Piston Seals for High Pressure Air Compressors - An ASME Publication.
SAE J1616 - Recommended Practice for Compressed Natural
Gas Vehicle Fuel
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