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Wednesday, March 12, 2008

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Many years ago, i have seen pressure transmitter with alarm located at the pump suction line. I have spend a lot of time to figure out what is real purpose of this pressure transmitter ?

Why provides Pressure Transmitter (PT) with Low pressure alarm (PAL) at pump suction line ?

i) A drum contain boiling liquid feeding a pump with complicated suction line routing or corroded pipe may leads to NPSH available (NPSHa) very to NPSH (NPSHr) required by the selected pump. NPSHa lower than NPSHr leads to pump cavitation and potential damage the pump. One of the safeguard is to provide a pressure transmitter with low pressure alarm to monitor it pressure level at safe level and alert operator to take necessary actions before the pump is damaged.

ii) Pumping hot fluid (above ambient), the suction line pressure drop is low and NPSHa is maintained above NPSHr of pump. Nevertheless, unavoidable heat dissipation to ambient due to damaged insulation, uncontrolled or fluctuating fluid temperature, etc would results fluid viscosity increase with low temperature. Typical fluid is high viscosity crude. In the vent fluid experience low temperature, pump suction line pressure drop increases due to increases in fluid viscosity and it subsequently lead to cavitation. One way to monitor the potential of cavitation is monitor the fluid pressure at pump suction.

iii) In some low pressure or atmospheric tank and complicated pump arrangement especially there is minimal static head available, proper pressure analysis along pump suction, there may be some location experiencing in-situ pressure is lower than Atmospheric pressure (partial vacuum). Positive displacement pump would have even higher potential of creating vacuum condition at pump suction. This is always unfavored situation because it potentially leads to air ingress into the fluid (via leaked flange) and subsequently results issues like oxidation, poisoning of catalyst, corrosion, accumulation of inert gas, etc. A PT would monitor pump suction pressure and minimizing air ingress potential.

iv) In the event partial or total blocked pump suction line, pump may run on NO flow. Severe internal recirculation (read more about pump internal recirculation in Rule-of-thumb For Minimum Flow Recycle" occur in the dip of pump impeller. It would cause severe damage to pump impeller and severe vibration would potential leads to damage of casing and piping. Again, PT with alarm at pump suction is to alert the operator to take immediate action.

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posted by Webworm, 3:00 PM


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