NG SOLUTION S
Drying natural gas for today’s NGV fueling stations
What you need to know... ISO 15403:2000 – The single most important safety requirement
ISO 15403:2000 (E) standard states (paragraph 5.1): “The single most important safety requirement of compressed natural gas (CNG) fuel is a very low water dew point temperature to preclude the formation of liquid water at any time. Liquid water is the precursor to the formation of corrosive compounds through combination with components in natural gas, namely carbon dioxide and
hydrogen sulphide. The combination of corrosive agents, and the pressure cycling, caused by fuel consumption and subsequent refilling of the fuel storage container, can result in crack growth in metals and ultimately damage and failure. Also, liquid water itself can be detrimental as it may cause blockages, both liquid and solid, in the fuel system.Thus, the water dew point of the fuel gas at the fueling station outlet shall be sufficiently below the lowest ambient temperature in which the fueling station and vehicles will operate.”
If you think your gas is dry, you need to read this
As the effects of moisture in natural gas systems are not always immediately evident, some NGV station builders are reluctant to address the importance of installing desiccant dryer systems in CNG fuelling stations. This is despite the fact that there are industry standards and recommended practices for the required dew point of compressed natural gas used for fuelling stations, such as ISO 15403:2000(E).The most common reason for not installing dryers in NGV stations is the belief that the gas is already dry. However, it should be taken into consideration that although the gas dew point may be as low as -40°C in the pipeline at pipe pressure, the effects of compression will affect the dew point of the gas.Physical laws dictate that the dew point of a gas increases as its pressure increases. Therefore, although the dew point of the gas in a given pipeline may be very low when it reaches the compressor, it will be significantly higher when the gas leaves the compressor. This is why natural gas desiccant dryers are required in the majority of NGV stations in order to conform to the ISO 15403:2000(E) standard, specifically in areas where lower temperatures are encountered during the colder winter months.
Suction side versus high pressure discharge drying technology
Higher concentration of water vapor per volume of gas, caused by high compressor discharge temperatures, affects the adsorption efficiency of desiccant dryers which are installed on the pressure discharge side. Oil vapor found at the outlet of the oil-lubricated compressor can contaminate the desiccant and cause dew point collapse. Lower temperatures and oil vapor free gas on the suction side result in better dryer performance and lower maintenance cost.Allowing “wet” gas to enter the compressor is undesirable, as most reciprocating compressors are extremely sensitive to liquid water entering the compression cylinders, which can have catastrophic results on the entire compressor.Pressure swing dryers use 10 – 15% of the dry gas from their outlet to regenerate the off-line tower. This used gas must then be recovered on the suction side of the compressor, increasing its size and decreasing the output to the storage vessels and dispensers. This is not an economical solution as compared to the suction side dryer which does not utilize any purge gas. 2
