Use consistent units in the hydraulic equation<\/h2>\n
In SI units, hydraulic power in watts is P = \u03c1gQH. Here \u03c1 is liquid density in kg\/m\u00b3, g is approximately 9.81 m\/s\u00b2, Q is flow in m\u00b3\/s, and H is differential head in metres. Divide by 1,000 for kilowatts. If flow is supplied in m\u00b3\/h, divide it by 3,600 before using that form of the equation.<\/p>\n
For ordinary water near a density of 1,000 kg\/m\u00b3, the same arithmetic can be written as hydraulic kW \u2248 Q in m\u00b3\/h \u00d7 H in m \u00f7 367. The convenient denominator is rounded and incorporates the assumed density and gravitational acceleration. It is not a universal formula for every liquid. Retain the full density-based expression when the liquid or temperature makes that assumption unsuitable.<\/p>\n
Avoid mixing head in feet with flow in cubic metres per hour or inserting litres per second into an equation expecting cubic metres per second. A worksheet should display units beside every input and retain the conversion steps. Plausible-looking numbers are particularly dangerous when an unnoticed unit error introduces a factor of 1,000 or 3,600.<\/p>\n Assume an illustrative water duty of 36 m\u00b3\/h at 25 m differential head, with density taken as 1,000 kg\/m\u00b3. Convert flow to 0.010 m\u00b3\/s. Hydraulic power is then 1,000 \u00d7 9.81 \u00d7 0.010 \u00d7 25 = 2,452.5 W, or approximately 2.45 kW. These values are an arithmetic example and are not specifications for a BORRAPUMP model.<\/p>\n If the pump efficiency at that duty were 70%, shaft power would be 2.45 \u00f7 0.70 \u2248 3.50 kW. If motor efficiency at the relevant load were 90%, electrical input at the motor boundary would be approximately 3.50 \u00f7 0.90 = 3.89 kW. With an illustrative drive efficiency of 97%, supply input upstream of that drive would be about 4.01 kW.<\/p>\n Each efficiency in this example is assumed solely to demonstrate the calculation. An equipment selection needs the actual supplier data at the intended duty, speed, and load. Do not copy the example efficiencies into a procurement worksheet and describe them as verified performance. Rounding should also be appropriate to the uncertainty of the inputs rather than implying laboratory precision.<\/p>\n Use differential head required at the operating duty, not the height of the building alone or a pump’s catalog maximum head. The requirement can include elevation change, friction losses, and pressure differences between the defined system boundaries. The pump duty-point calculation guide<\/a> explains the information to assemble.<\/p>\n For measurements across a pump, gauge locations, elevation differences, liquid density, and velocity-head differences can affect the conversion from pressure readings to differential head. Record the method and references. A discharge gauge reading alone does not generally establish the pump’s differential head when suction pressure is significant.<\/p>\n The IS clean-water centrifugal pump<\/a> is a route for requesting an offered model’s curve and absorbed-power data. A family description or motor size does not identify the efficiency or shaft power of every operating point. Ask for the actual impeller and speed represented by the quotation.<\/p>\n Pump efficiency changes with flow, speed, impeller configuration, and other operating conditions. A maximum efficiency shown in a brochure may occur at a different point from the intended duty. Use the appropriate curve and distinguish a measured value, supplier prediction, and preliminary assumption. The curve-reading guide<\/a> helps locate the required information.<\/p>\n Motor and drive efficiencies also depend on their own operating conditions. A single nameplate or catalog efficiency may not describe every part-load case. When an energy estimate affects a purchasing decision, obtain suitable efficiency information for the expected operating profile and state the limitations. Do not multiply unrelated peak efficiencies and claim a guaranteed wire-to-water result.<\/p>\n The table separates quantities that should not be interchanged. It does not provide a motor-selection margin or certify an efficiency. The supplier’s curve and the applicable equipment requirements remain necessary for selection.<\/p>\n One duty-point calculation can miss the highest shaft-power demand encountered in service. Check the absorbed-power curve across the intended range of flow and speed, including credible control combinations. The shape of the power curve varies with pump design; do not assume that every centrifugal pump has its highest demand at the same relative location.<\/p>\n A system change can move the operating point. Opening a lower-resistance path, changing a valve position, adding a parallel pump, or changing a setpoint may alter both flow and power. The Hydraulic Institute’s discussion of NPSH and operating regions<\/a> also matters because an electrically acceptable power value does not prove that the hydraulic operating point is acceptable.<\/p>\n Motor selection must consider the complete application: permitted operating power, duty, environment, supply, starting method, equipment limits, and applicable requirements. Do not treat an arbitrary percentage added to the example shaft power as a complete selection rule. Ask the supplier and responsible electrical designer to confirm the offered motor against the actual range.<\/p>\n A motor can demand substantially different electrical conditions while accelerating from those seen at steady operation. Running kW alone therefore cannot size a generator, feeder, protective device, or starting system. Obtain the relevant starting information and assess it using the actual supply and control arrangement.<\/p>\n![\"ISG](\"https:\/\/borrapumps.com\/wp-content\/uploads\/2026\/10\/borra-realistic-20261016-img-007.webp\")
Work through a bounded example<\/h2>\n
Obtain head from the system requirement<\/h2>\n
Use efficiency at the operating point<\/h2>\n
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\n \nQuantity<\/th>\n Calculation boundary<\/th>\n Information needed<\/th>\n<\/tr>\n<\/thead>\n \n Hydraulic power<\/td>\n Energy transferred to the liquid<\/td>\n Density, flow and differential head<\/td>\n<\/tr>\n \n Pump shaft power<\/td>\n Mechanical power entering the pump<\/td>\n Hydraulic power and duty-point pump efficiency<\/td>\n<\/tr>\n \n Motor electrical input<\/td>\n Electrical power entering the motor<\/td>\n Shaft power and motor efficiency at the relevant load<\/td>\n<\/tr>\n \n Drive supply input<\/td>\n Electrical power entering the drive<\/td>\n Motor demand and applicable drive losses<\/td>\n<\/tr>\n \n Energy use<\/td>\n Power integrated over operating time<\/td>\n A realistic load and operating-time profile<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n ![\"ISW](\"https:\/\/borrapumps.com\/wp-content\/uploads\/2026\/10\/borra-realistic-20261016-img-012.webp\")
Review power over the permitted operating range<\/h2>\n
Keep starting demand separate from running power<\/h2>\n