(50d) A Novel Method for Testing the NPSHA Level of a Submerged Centrifugal LNG Pump

NPSHR (net positive suction head required) is defined as the NPSH at which the pump total head (first stage head in multi stage pumps) has decreased by three percent (3%) due to low suction head and resultant cavitation within the pump. Cavitation begins as small harmless bubbles before any indication of loss of head or capacity occurs. This is called the point of incipient cavitation. Testing has shown that between two and twenty times the NPSHR is required to fully suppress incipient cavitation, depending on the impeller shape (specific speed number) and operating conditions. To keep a pump from cavitating due to vaporizing or boiling fluid at the low pressure side of the pump, the NPSHA (net positive suction head available) must be equal to or greater than the NPSHR. Current methods used for testing the NPSHR of submerged centrifugal LNG pumps cannot be performed in real time while the pump is running. This is due to the fact that current methods assume ideal methane for fluid properties and assume the vapor pressure is the saturation pressure at saturation temperature. This paper presents a novel method of testing NPSHR levels in real time using pure methane or hydrocarbon mixtures. This is accomplished by using custom data acquisition software that calculates the instantaneous fluid properties of the test liquid, including the vapor pressure, based on the chemical composition delivered by the LNG supplier. With an accurate vapor pressure calculation based on the known working fluid composition, more accurate NPSHR levels can be defined.