Oct 16 - 22, 2006


a. What is CNG? CNG is compressed natural gas. CNG systems take natural gas from distribution pipelines - at relatively low pressures - and compress it to pressures of 3600 psig or more to fuel vehicles or fill transportation trailers that carry the gas to off-pipeline users. The gas is typically used at lower pressures but compressing it allows it to be transported in much higher volumes, making the overall use more efficient.

b. Why is it a better fuel? CNG as a combustible fuel is much lower in a number of harmful pollutants, particularly sulphur, lead, carbon monoxide, nitrogen oxide, and particulate matter. As well, its overall lifecycle impact (well-to-wheels) is much better for the environment than other refined fuel products such as gasoline and diesel. For these reasons, the implementation of CNG fueling for public transportation is a key component in the clean air strategies of many major cities around the world.

c. Is it safe? Natural gas is neither corrosive nor toxic, its ignition temperature is high, it is lighter than air, and it has a narrow flammability range, making it an inherently safe fuel compared to others. Natural gas cannot contaminate soil or water. It always rises to the atmosphere when outdoors, unlike other fuels, which are heavier than air and can pool, either as a liquid or a vapor, on the ground. Natural gas contains a distinctive odorant (mercaptan), which allows it to be detected at 0.5% concentration in air, well below levels that can cause drowsiness due to inhalation and well below the weakest concentration that can burn. In practice, CNG use worldwide has resulted in only a handful of accidents and incidents which have resulted in damage or injury; most of which were the result of improper equipment use.

d. What does CNG cost? Despite its other compelling advantages, often the greatest impetus for the use of CNG globally is its price relative to gas or diesel. While prices and availability vary from country to country, generally, CNG vehicle fuel - at the "pump" - represents a savings of 30 ~ 50% for the driver or fleet operator. In addition, there are other mechanical advantages for the vehicle engines, which may reduce overall maintenance costs. For the country developing CNG infrastructure, there may be other critical economic advantages such as a reduced reliance on foreign oil and/or a reduction of costly subsidies that may exist on liquid fuels. Worldwide, natural gas deposits are abundant and far better distributed than crude oil deposits, making them a viable resource for many countries.

e. So, why doesn't everybody use it? One of the principal disadvantages of CNG fuel is the relatively high cost of fueling infrastructure development, i.e. CNG fueling stations. Most people take it for granted that gasoline and diesel are widely available; however, because CNG is still at an early stage of market development, its availability cannot be guaranteed in all countries or regions. This can slow down the adoption of CNG vehicles into fleet use, as operators do not want to be confined to limited fueling access. This is the "chicken and egg" conundrum that many potential industry participants struggle with - which comes first, the CNG station or the vehicles? - Yet in many developing countries, proponents have developed a "build it and they shall come" attitude, which has proven to be generally true. In some markets, natural gas is not a domestic resource (i.e. Japan); yet the importation of liquefied natural gas by ocean tanker still provides an overall economic benefit by the time it reaches the CNG vehicle operator.

f. Why does infrastructure cost so much? At the present time, CNG systems are generally custom built to suit the specific site requirements, i.e. electrical supply, gas pressure, overall flow requirements. This factor, coupled with the high-pressure and safety aspects of CNG systems, makes them costly to manufacture. Unlike liquid fuel systems, which require only a storage tank and a pump, CNG systems require multi-stage compression, high-pressure cooling, high-pressure storage, dispensing, and metering of a flammable gaseous medium, which is much more difficult to manage. Despite the high capital cost for infrastructure development, CNG systems have an incredibly positive return on investment in most markets. In many cases, payback (of equipment) can be amortized over periods of less than two years, due to the high latent demand for CNG fuel by vehicle operators. Overall payback periods, of course, depend on the total station development costs (i.e. construction & land) and rate of usage.


In the CNG industry there are mainly two types of compressor technologies:

*Lubricated Compressors - These machines have lubricating oil in the crankcase to provide lubrication to bearings and crank mechanisms. They also provide oil lubrication to packings, cylinders and valves either by injecting oil directly into these components or incorporate a design that purposely allows the oil from the crankcase to migrate into the packings, cylinders and valves from the crankcase.

*Non-lubricated Compressors - These machines have lubricating oil in the crankcase but are engineered to keep the migration of oil into the packings, cylinders and valves to an absolute minimum. These machines do not use any direct injection of oil.


a. Oil Carry-Over Rate - Oil carryover (aerosols and vapors) in CNG has a significant negative influence on natural gas vehicle engines and the maintenance and operation of the CNG station itself. This oil originates largely from the oil lost to the gas stream in lubricated compressors. Non-lubricated equipment minimizes the migration of oil into the cylinders and piping system thereby keeping oil carry-over into the gas to an absolute minimum.

The oil carry-over of lubricated compressors can be as high as 100 ppm (m/m). The oil carry-over of CNG equipment should always be less than 5 ppm.

b. Oil-Carry-Over And System Performance -

The deleterious effects of oil carry-over include the following:

* Oil vapor and small aerosols accumulate in the vehicle fuel tank as liquid oil. Fuel storage capacity is diminished and the vehicle dead weight will increase incurring a fuel penalty paid for lugging around this dead weight.

*Oil collects in ground storage equipment reducing the capacity and performance of the CNG station.

*Oil aerosols accumulate on heat exchanger surfaces causing fouling. As the heat exchanger performance degrades, higher temperatures reduce compressor ring and packing life, reducing the gas storage capacity and reducing energy efficiency.

*Oil carryover increases vehicle emissions in the form of un-burnt hydrocarbons.

*The maintenance and replacement of oil separators and filters will affect the CNG station availability and reliability. It also increases maintenance cost with the increased cost of consumables and additional maintenance labor.

*Inside a CNG vehicle, higher pressure gas, about 20Mpa, is reduced to 0.7Mpa by a regulator and then injected into the 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.

*The most sensitive area to oil contamination is in the engine itself. The oxygen sensors of the engine system are extremely sensitive to any oil contamination and may cause engine shut down.

c. Advantages of Non-Lubricated Compressors -

Non-lubricated compressors offer distinct advantages over lubricated compressors in terms of system reliability, performance and maintenance. Some of the key advantages of the non lubricated feature are as follows:

*Daily or frequent oil replenishment is not required

*Less frequent draining of scrubbers and filters

*Reduction in waste oil disposal cost

*Heat exchanger surfaces remain clean since oil is not contaminating the cooling tubes

*The compressor discharge is fitted with only one coalescing filter and does not require an oil separator

*No oil lubricators required which means reduced equipment maintenance and service

*Reduced oil carry over reducing maintenance requirements on down stream equipment


Emissions of natural gas from compressors can be significant and are usually not considered in performance specifications of CNG equipment. Emissions of natural gas have the following negative effects.

*Result in financial losses

*Negatively impact air quality through smog formation

*Reduce the energy efficiency of CNG stations

*Natural gas losses to the atmosphere are a serious contributor to greenhouse heat entrapment with each molecule of methane trapping 21 times more heat than carbon dioxide

a. Sources of Gas Emissions -

Some of the sources of gas emissions are the following:

*Packing case vent losses

*Leakage from pressurized crankcase seals

*Blow-down vent losses (auto draining scrubbers/filters)

*Maintenance blow-down losses

*stop/start blow down after recovery tank capture

*Gas actuated valves (if used)

*Relief valve venting (intermittent)

*Valve stem, piping and fittings leaks

*Compressor external seal leakage (i.e. gaskets, o-rings, rotating shaft seals, oil filler cap seal)

b. Features to Minimize Gas Emissions -

IMW equipment limits the loss of gas to the environment in the following ways:

*Since the IMW compressor is non-lubricated, the automatic draining of inter-stage scrubbers and oil traps is not required. Some lubricated compressors employ automatic drain systems to drain the traps. Each time the system is actuated a certain amount of natural gas is vented to the environment.

*Recovery tanks are installed on all IMW compressor systems to recover a maximum amount of gas from the compressor during blow-down.

*IMW does not use a pressurized crankcase. Pressurized crankcases are prone to shaft seal failures and may cause continuous gas leakage from compressor shaft seals.

*Packing case vent losses are probably the most important source of natural gas emissions from a CNG stations assuming the station is well maintained. IMW equipment is designed to limit the loss of gas to extremely low levels in the range of 0.05% - 0.15% of total gas throughput.


Pakistan has demonstrated the value and merits of natural gas as a transportation fuel. To achieve maximum benefit from its CNG infrastructure, it is recommended that equipment be built and tested in accordance to best CNG practices and internationally accepted codes and standards.


David S. Gamble - Director, Sales and Marketing
IMW Industries Ltd.
45831 Hocking Avenue
Chilliwack, British Columbia
Fax: 1-604-792-3806


IMW - Canada is the top CNG equipment manufacturer of the world and now being represented under Allied Engg's banner in PAKISTAN. This would be an added advantage to sell CAT G3306TA & CAT G3406NA Industrial Engines with each and every station as well, which will be coupled or driving force of IMW Compressor.