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Faster refuelling of compressed natural gas

01 September 2008

Compressed natural gas is becoming a popular vehicle fuel, and there is a growing demand for suitably stocked service stations. Essential to the viability of these stations is a faster refuelling dispenser that matches the speed of its petrol equivalent. David Graham describes a new solenoid valve development that is meeting this need

It is the declared aim of the EU to increase the market share of alternative motor fuels. Alongside ethanol, global interest is being focused on the use of natural gas for this purpose, because it has a simple chemical make-up, it burns cleanly, and emissions of carbon monoxide, sulphur dioxide, soot, and particulates are almost completely eliminated when used as a motor fuel.

The benefits of using compressed natural gas (CNG) as a motor fuel include lower costs, increased performance, and reduced emissions. CNG costs 15-40% less than petrol or diesel fuel. In addition to the increased performance experienced by CNG vehicles, drivers also get a longer lasting, more reliable vehicle. Emissions reductions are significant at approximately 90% lower for CO, 35-60% lower for NOx, and 50-75% lower for HC. Of course it should be noted that large global reserves of natural gas exist.

No fewer than 5.7 million natural gas vehicles are currently in use, and the number is expected to rise to 12.7 million by 2011. Fuel for these vehicles is currently supplied by some 10,000 CNG filling stations throughout the world. By 2011, this is predicted to increase to about 20,000.

The principal components of a natural gas filling station are a metered gas supply, a compressor, a storage unit, and a dispenser. Refuelling a vehicle can be accomplished directly by a compressor unit. However, this operation takes several hours, and is usually performed overnight. In order to permit faster refuelling, most filling stations are equipped with single-bank or multi-bank dispensers.

A single-bank dispenser is operated by a single, first-level solenoid valve. Its hydraulic system communicates with a single storage unit that stores the gas at high pressure. A multi bank dispenser (cascade buffer storage system) is operated by two or three solenoid valves (first, second and third level). Its hydraulic system communicates with two or three storage units that store gas at different pressures. A three-level system is generally divided into three ‘banks’ containing low, medium, and high pressure gas.

During refuelling, the storage banks are automatically connected to the vehicle according to the pressure in the tank. When the vehicle tank pressure is low it is connected to the low-pressure bank. As the pressure in the two tanks equalises, the flow rate decreases. Once it reaches a minimum value, a solenoid valve switches the connection to the medium pressure bank, and then to the high-pressure bank. The order is reversed when the signal comes to refill the banks; first the high-pressure bank is filled, then the medium-pressure bank, and finally the low-pressure bank. However, the solenoid valves used in this application need special features to control the flow of natural gas to and from the dispenser.

Asco Numatics has undertaken some development work in this area and recently introduced a compact solenoid valve specifically to cope with the high pressure levels within multi-stage buffer storage systems and the rigours of CNG dispensing applications. The company’s Series 291 CNG solenoid valve is suitable for differential pressures of up to 350 bar and the flow rate is optimised to attain a high filling speed (Kv value of up to 2.3 m3/h). This compact, rugged valve is available in 8mm and 12mm sizes, has an operating temperature range of –40ºC to +70ºC and will install readily in restricted spaces - an important aspect of its design, as pump sizes continue to be reduced.

The valve functions as an internally piloted unit with a pilot orifice in the piston, which is one of the most important components of the valve. In order to withstand the high pressure, the piston is produced from a single piece of special plastic, ensuring reliable operation and keeping maintenance costs low. Another critical component is the housing. The 303 stainless steel body can withstand high pressures, with the maximum bursting pressure four times the maximum allowable pressure drop. The valve can be supplied with numerous explosion proof solenoid operators, rendering it suitable for Zone 1/21 - 2/22 applications, according to ATEX 94/9/EC. Enclosures according to NEMA are also available. Both ISO 228 and SAE threaded versions can be supplied.

Asco Numatic has gained considerable experience in petrol vending applications and has applied this knowledge to the dispensing of ‘clean’ fuels such as liquefied petroleum gas as well as CNG. In both applications the fuels are stored in pressurised tanks and this has led to the development of the innovative three-stage delivery system for LNG applications, and the new high-pressure valve for CNG applications.

Dave Graham is a fluid control product specialist at Asco Numatics

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