ADVANTAGES OF IMW NON-LUBRICATED CNG COMPRESSORS
FAISAL MIRZA, Manager Compressor Division
May 19 - 25, 2008
As an alternative fuel, CNG has a number of advantages; less cost, higher efficiency, lower emission, lower engine noise level and longer engine life. The CNG technique was developed and applied for decades and has become a mature and safe technique. As the development of vehicle industry progresses and higher standards of environmental protection are needed, this clean alternative fuel must be extensively used.
As the heart of a CNG fueling station, compressor performance has significant influence on the system operation. There are also some special requirements in CNG compressor design. The natural gas vehicle fuel system and engine and also CNG station itself, requires that CNG has very little oil carryover. Consequently, more and more non-lubricated piston compressors have been developed and applied to CNG stations, especially for low and medium capacity sizes. Experience has shown that non-lubricated compressors have a number of advantages over lubricated compressors in CNG application.
This paper will discuss the advantages on non-lubricated compressor when it is used in CNG service. The features of non-lubricated CNG compressor will be illustrated and explained. The maintenance schedule for a non-lubricated CNG compressor is also presented.
THE INFLUENCES OF OIL CARRYOVER
Oil carryover (aerosols and vapors) in CNG has a significant negative influence on the natural gas vehicle engines and the maintenance and operation of CNG station itself [1~3].
* The oil vapor and small aerosols will accumulate in fuel tanks. Fuel storage capacity will be diminished, the bus dead weight increased and a fuel penalty paid for lugging around this dead weight.
* Oil aerosols affect the heat exchanger surfaces and higher temperatures could result in reducing the storage capacity and more compressor power consumption.
* Oil carryover also increases the vehicle emissions.
* The maintenance and replacement of oil separators and filters will affect the CNG station availability.
* Inside a CNG vehicle, higher pressure gas, about 20Mpa, is reduced to 0.7Mpa by a regulator and then injected into engine. There are many examples of oil soaking the regulator diaphragm, affecting its accuracy, or in some cases forcing a rupture of the regulator internals if oil clogs the regulator.
* However the most sensitive area to oil contamination is in the engine itself. The oxygen sensors of the engine system are extremely sensitive to any contamination.
SELECTION OF THE CNG COMPRESSOR
As the heart of a CNG fueling station; the compressor dynamic characteristic, thermodynamic performance, operation availability and reliability have a significant influence on CNG station operation. The selecting of compressor type is a key step in investigating and building a CNG station.
In order to maximize the vehicle tank capacity, a multiple stage compressor is required to compress low pressure natural gas from pipeline (0.05~2MPa) to high pressure (about 25Mpa). The required compressor flow rate is also high. In addition, because of the low gravity and flammable properties of natural gas, a compressor with a very high sealing efficiency is needed. For such applications, the reciprocating piston type compressor is the best suitable design for a CNG station. In today's market, there are three different types of reciprocating compressors serving the CNG industries.
* Non-lubricated compressor [Oil Free] The lubrication system provides lubrication only to crankshaft bearings, crank pins, cross head pin and cross head guides. The piston rings, wearing bands and packing rings are not lubricated by oil, but use a self-lubricated material. The major compressor construction parameters (speed, stroke, cylinder, piston and piston rod) are specially selected and designed to enhance ring life time and ensure compressor performance. Special distance piece and oil collar designs are used to positively prevent the crankcase lube oil from entering the cylinder.
* Non-lubricated compressor [Mini-lube] The lubrication system again only provides lubrication to crankshaft bearings, crank pins, cross head pin and cross head guides. Piston rings and packing are non-lubricated, but it is possible for a small amount of oil to migrate along the rod from the crankcase. In some cases, the only difference with a lubricated compressor is to remove the cylinder oil lubrication and use self lubricated packing and rings. This is the standard considered acceptable for CNG compression.
* Lubricated compressor The lubrication system not only provides lubrication to crankshaft bearings, crank pins, cross head pins, cross head guides but also the piston rings and packing rings are lubricated by oil injected into the cylinder.
During the earlier beginnings of CNG applications, low flow capacity, small single acting, lubricated compressors were normally more commonly used in the small dispensing station installed at that time. The user recognized the negative influences of oil carryover, but endured this because high pressure, multiple stage non-lubricated compressors were not available.
As the station size increased and carryover of lubricated oil aerosols and vapors became a bigger problem for both the vehicle fuel system and engine [noted above], non-lubricated compressors, where the rings are self lubricated, were developed and applied for the CNG industry. Using a non-lubricated compressor is a better and more reasonable choice to achieve required gas quality. Because the ongoing self-lube material improvement has optimized compressor design, adequate performance and life-time of the self-lube seal rings have been achieved. Non-lubricated CNG compressor operation reliability has been ensured and the oil carryover problem eliminated.
As more and more large fleets and public transportation turn to CNG, the requirement for compressor flow rate has increased. Larger compressor packages are required in many cases. Some users may look only at the equipment capital cost (for a given flow capacity, non-lubricated compressor cost is a little more than a lubricated compressor) and worry about the ring life-time of a non-lubricated compressor. Selection of either a lubricated compressor or non-lubricated for a CNG station is becoming a hard decision for users not conversant with all the facts.
To solve the oil contamination problem in CNG station, a most reasonable way is to eliminate the root of the problem, i.e. avoid using oil to lubricate a compressor cylinder. Actually, all those factors affecting the CNG station -- equipment cost, operation and maintenance cost -- all have to be taken into account when selecting a CNG compressor. Reference  presents a cost comparison between two similar CNG stations. Both stations could have installed two 2400 Nm3/hr, 280KW compressors, but the one using non-lubricated compressors clearly shows the large overall cost savings due to the advantages of non-lubricated compressors.
THE FEATURES OF NON-LUBRICATED CNG COMPRESSOR
A CNG station normally is designed and manufactured as a single skid mounted compressor protected by a weather proof enclosure. There are some application cases requiring a movable mobile station. When designing a non-lubricated CNG compressor the following has to be considered to ensure better compressor thermodynamic and dynamic performance, plus adequate piston ring, packing ring and valve operation life time.
* W or V style Angle mounted cylinders and pistons will be more beneficial to piston ring and packing ring operation life time. These types of compressors have excellent dynamic force balance; the first order reciprocating inertial force, rotation inertial force, and the second order reciprocating inertial force are balanced by counter weights. When it is designed as double acting, the total resistance torque curve is even. The compressor runs very smoothly. Double acting also benefits cylinder cooling. This kind of compressor requires less foundation, and can be designed also as an air-cooled, non-foundation, movable compressor package. It is very suitable for CNG stations, especially in the low to medium capacity, standard stations, daughter stations and mobile stations.
* With cross head Natural gas is a flammable gas and also a greenhouse gas. Gas leaking to atmosphere should be eliminated if at all possible. Crosshead type compressors benefit from the sealing of the gas leaking from cylinder to crankcase by using special seals. Gas leaking past these seal rings will be lower than 0.1% of gas mass flow rate. The compressor crankcase can then be designed as non-pressurized. Special sealing gaskets, mechanical seals and increased oil system pressure are not required.
* Double acting cylinder It can increase cylinder utility and compressor flow rate, reduce compressor dimension and weight. It also benefits piston ring life.
* Lower Piston Speed Piston speed limits (<3.0~3.5 m/s) [600-700 fpm] will improve seal ring and valve life-time. Using the modern self-lube materials, 5000 hours ring life time for non-lubricated compressor pistons has already been achieved. Under ideal conditions life has been seen to already exceed 8000 hours.
* Lower rotational speed (<900RPM) and longer stroke (>100mm) [4.0 in] Flow capacity and reduced rod loading are ensured by smaller diameter cylinders. It also allows optional cylinder cooling, thus reducing power consumption, reducing discharge temperature and increasing piston ring life. With lower rotation speed, the valve open-close frequency is much lower than that of a high speed lubricated compressor. Valve life time is extended.
* Lower pressure ratio, ensures lower discharge temperature The major factor which affects PTFE (TeflonÆ) ring life is high temperature. The discharge temperature of each stage has to be limited. Some new self-lubricated material can handle the higher temperature, e.g. the PEEK ring material can be used up to 160∞C [3200 F] temperature. If necessary, in a non-lubricated compressor, using PEEK rings in some stages with higher temperatures can provide commercial piston ring operation time.
* Longer piston body One or more piston rings can be easily mounted in the longer pistons, and there is room for wearing bands to separate and support the piston from the cylinder. In this way, piston ring sealing efficiency and operation time can be increased.
* Longer piston rod In order to further avoid oil carryover, in addition to using wiper rings, the piston rod and distance piece length for a non-lubricated compressor can be increased by one stroke. This ensures that the same piston rod section entering the crankcase and being contaminated with oil cannot also go into cylinder. This construction would meet the definition of "oil free" noted above.
Compared to lubricated compression, the heat exchanger of non-lubricated compressor has less fouling by the oil film inside the tubing. The heat exchanger will run at a higher efficiency for longer periods. Better storage capacity is also ensured, since the CNG compressor stopping and starting is controlled by the pressure variation signals from the storage. If the heat exchanger cannot cool the compressed high temperature gas effectively, higher temperature gas goes into next stage and eventually into storage. Compressor performance and storage capacity [hotter gas means less weight] is affected. Every 30†C [40 F] temperature approach increase will add 1% to the next stage power consumption. Every 30†C [40 F] temperature approach increase from the aftercooler will also result in a 1% reduction of storage capacity.
As mentioned above, a non-lubricated CNG compressor requires some special design features to ensure better performance, longer ring and valve life-time. Simply taking off the cylinder oil lubricator from a standard oil lubricated design will not offer a good quality CNG compressor.
MAINTENANCE OF NON LUBRICATED COMPRESSORS
In general non lubricated compressors are low maintenance machines. Some of the main advantages considering maintenance of a non lubricated compressor are as follows:
* Daily or frequent Oil filling is not required
* Less frequent draining of scrubbers and filters
* Reduced waste oil disposal cost
* Heat exchanger surfaces remain clean since oil is not contaminating the cooling tubes.
* Compressor discharge fitted with only one coalescing filter. (does not require an oil separator)
* No oil lubricators required which means reduced equipment to service
* Reduced oil carry over reducing maintenance requirements on down stream equipment.