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Pump Ed 101 Joe Evans, Ph.D http://www.pumped101.com http://www.pump-zone.com Velocity Head
Hint * Centrifugal Pump Impeller Discharge Suction Cutwater Volute What Type of Energy is Added by the Impeller?
* Centrifugal Force It is defined as center fleeing
* Centrifugal Force Instead it actually moves in the same direction it was traveling at the exact instant it is released. When an object is traveling in a circle, it is actually moving in a straight line at any single point in time.
So, How Does It Work? * 1 Rotation of the impeller forces water from its entry point, at the eye, into its vanes. 2 Water moving through the vanes creates a partial vacuum at the eye allowing atmospheric, or some other outside pressure, to force more water into the eye. 3 As water travels through the vanes, it gains rotational velocity (kinetic energy) and reaches its maximum velocity just as it exits the vanes. 4 Upon exiting the vanes, water enters the volute where its kinetic energy of motion is transformed into pressure energy.
Linear versus Rotational Motion * Speed = d / t Rotational Speed (w) = rotations / t Linear Speed (v) radius (r) x w
* Linear Velocity in a Rotational Frame of Reference On the disc to the right there are two points, one at 6 from its center and one at 12. The circle described at 6 has a circumference of 37.7 and the one at 12 a circumference of 75.4. 6 12 At a speed of one rotation per second a point 12 from the center will travel twice the distance of a point that is 6 from the center. Therefore its velocity is twice as great. v = C x w
Water Energy * Water Can Possess Three Forms of Hydraulic Energy Potential Energy Due to Elevation Kinetic Energy Due to Velocity Pressure Energy Due to Weight (force) These Three Forms of Energy Must Live In Harmony Conservation of Energy
* Conservation of Energy Bernoulli s theorem states that, during steady flow, the energy at any point in a conduit is the sum of the velocity head, pressure head, and elevation head. It also states that this sum will remain constant if there are no losses. Daniel Bernoulli 1700-1782 H = v + p + z = Constant 50 PSI 50 PSI 48 PSI 100 GPM
Daniel Bernoulli Hydrodynamica - 1738 * Energy = v + P + z = Constant
Energy = v + P + z = Constant * Piezometer Measurement
Energy = v + P + z = Constant * Piezometer & Pitot Tube Measurement
Energy = v + P + z = Constant *
Total Dynamic Head What is the Total Head produced by a centrifugal pump? Total Head
Total Dynamic Head Total Dynamic Head H = h d - h s Where: h d = discharge head h s = suction head Total Suction Head h s = ± h gs + h vs ± Z s Total Discharge Head h d = h gd + h vd ± Z d Where: h g = gauge head h v = velocity head Z = gauge distance above or below datum Total Head
Velocity Head What is the effect of velocity? KE = 1/2mv 2 Falling Body Equation Velocity Head Equation V 2 = 2gh h = V 2 / 2g At a Velocity of 8 ft/sec h = 1 Total Head
The Performance Curve BEBOP BEP
Pump Testing 247.2 258.3 TDH Error 4.7% TDH Error 0.6% 3X4 End Suction - 650 GPM Velocity 3 = 28.2 ft/sec Velocity 5 = 10.4 ft/sec h v = V 2 / 2g 3 Section h v = 12.4 ft 5 Section h v = 1.7 ft Total Head Actual 3 Pressure = 112.4 PSI (259.6 ft). Actual 5 Pressure = 112.5 PSI (260 ft)
* Pump Testing Lift Station Pump 650 GPM @ 25 (10.8 PSI) 4 Discharge Piping 10 ft Gauge Reading at Valve Box = 6.6 (2.8 PSI) Corrected for Friction = 10.8 (4.7 PSI) Corrected for Elevation = 20.8 (9 PSI) Corrected for Velocity Head (v = 16.4 ft/sec) = 25 (10.8 PSI) TDH Error Ignoring Velocity Head = 19%
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Pump Ed 101 Joe Evans, Ph.D http://www.pumptechnw.com http://www.pumped101.com http://www.pump-zone.com Velocity Head