Safe gas!

Ensuring the safety of CNG (compressed natural gas) filling stations with spot checks of moisture

As the popularity of natural gas vehicles (NVGs) grows, so does the need to ensure the quality of compressed natural gas supplied at filling stations. A vital element of CNG quality is ensuring a very low water dew-point at pressure, and the ability for engineers to carry out spot checks of moisture content of compressor-dryers can save companies time and money on maintenance by catching issues early on.

CNG filling stations use network natural gas supplies that are typically at 1 to 1.5 barg and have a water dew point of somewhere between -30 and -20°C. NGVs operate on a full tank of CNG at a pressure of around 200 to 250 barg. Therefore the natural gas must be compressed by a factor of between 133 and 250 times prior to filling. This compression leads to an increase in the pressure dew point that, in most cases, will leave the natural gas saturated with water and often with liquid condensate. This liquid condensate can cause a number of problems including damage to engines.

To avoid this risk, the condensate must be removed by a suitable drain and then, in order to comply with ISO 15403:2000(E), the compressed natural gas must be dried to give a water dew point significantly lower than the lowest ambient temperature that will be experienced either at the filling station or in the vehicle in subsequent use.

The MDM300 I.S. portable dew-point hygrometer from Michell Instruments is ideally suited for spot checks of dryer performance by site or service engineers as it is capable of measurement in pressures up to 250 barg and has full hazardous area certification. The instrument also makes fast measurements consecutively in any direction without a waiting time in between, allowing for many more measurements to be taken each day.