CE 3372 WATER SYSTEMS DESIGN LECTURE 11 NET
- Slides: 17
CE 3372 WATER SYSTEMS DESIGN LECTURE 11: NET POSITIVE SUCTION HEAD
OUTLINE • Suction Conditions for Effective Pumping • Net Positive Suction Head
POSITIVE DISPLACEMENT PUMPS • Screw Pump • A revolving shaft with blades rotates in a trough at an incline and pushes water up
PUMP CURVES • Provided d.
SUCTION REQUIREMENTS • The most common cause of pumping failure is poor suction conditions • Cavitation occurs when liquid pressure is reduced to the vapor pressure of • the liquid For piping system with a pump, cavitation occurs when Pabs at the inflow falls below the vapor pressure of the water
SUCTION REQUIREMENTS • Liquid must enter the pump eye under pressure; Net Positive Suction Head available (NPSHa). this pressure is called the • A centrifugal pump cannot lift water unless it is primed • the first stage impellers must be located below the static HGL in the suction pit at pump start-up
SUCTION REQUIREMENTS • The manufacturer supplies a value for the minimum pressure the pump needs to operate. • This pressure is the Net Positive Suction Head required (NPSHr). • For proper pump operation (w/o cavitation) NPSHa> NPSHr
SUCTION REQUIREMENTS • Available suction is computed from Frictional head loss in inlet piping Absolute pressure at liquid surface in suction pit Static elevation of the liquid above the pump inlet eye Absolute vapor pressure at liquid pumping temperature
SUCTION REQUIREMENTS • Example
SUCTION REQUIREMENTS • Example
SUCTION REQUIREMENTS • Example
SUCTION REQUIREMENTS • Example
SUCTION REQUIREMENTS • Example
SUCTION REQUIREMENTS • Repeat Example using On-Line tool
SUCTION REQUIREMENTS • Repeat Example using On-Line tool
SUCTION REQUIREMENTS • Repeat Example using On-Line tool • Compare to “by-hand” and obtain same result • Difficulty is actually looking up the vapor pressure and determining the sign of the static head relative to the pump eye
NEXT TIME • Multiple path pipelines (Networks) • Energy equation in a network of pipes
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