Thermal expansion of the pneumatic valve plug

refrigeration system in the thermal expansion of the pneumatic valve jam fault is often happen, including a 'dirty' and 'ice block'. Dirty wall is the main reason of the impurities exist in the system, such as welding slag, copper scrap, scrap iron, fibre, etc. Is the cause of ice wall system contained too much water ( Moisture) , the ways to produce moisture are:
) When installing system vacuum didn't have enough time, failed to exhaust the moisture inside the pipeline. Pipe joint welding technology is bad, the leak point.
) When filling refrigerant to the system, not to blow out air hose connecting hoses.
) To supply the system with lubricating oil, into the air.