If the SG of a liquid is less than water for example and a light oil the SG would be 0.83 meaning the power absorbed would be 0.83 x 7.09kw = 5.88Kw meaning a smaller motor could be installed.Īs a pump operates according to the system it is installed in, care will need to be taken to ensure the absorbed power at the highest point of the curve is larger than the density multiplied by the SG. If the density of a liquid is 1.3 then the power absorbed would be 7.09kw x 1.3 = 9.217Kw at duty, meaning the motor would need to be at least this size to not trip.
If the absorbed power at the duty point is 7.09kw as in the above curve this is based on water. The amount of power absorbed by a motor during operation is multiplied by the SG to calculate the power absorbed.Į.g. Power Absorbed by Pump Calculation The density and pressure directly affect the power absorbed by the motor during operation. As they contain less relative mass (0.83 being < 1) they are lighter, which is why in the attached picture oil floats on top of the water. Water has an SG of 1, with some oils having an SG around 0.83 meaning they are less dense than water. A figure (SG) is given to fluids to identify how dense the liquid is. What is Specific Gravity (SG)? Each liquid has a relative mass dependent on its volume. Although a pump may be fitted with a larger motor than is required for the duty, the power absorbed is usually indicated on the pump curve with an intersecting line of where the pump is expected to perform.
What is absorbed power? The absorbed power is the power consumed by the pump during operation at the required duty point. H = 2.31 * Pressure / SG P= Pressure (PSI) H = Head (ft) SG = Specific Gravity Head unit (H)= 10.197 Pressure (p)= bar SG = Specific GravityĬonvert Pressure in PSI to Head in Feet Formula The formula for converting pressure in Bar to Head in M: The formula is Pressure in PSI (P) x Head (ft) x Specific Gravity (SG) Abbreviated to P x Head(ft) x SG Pressure = 0.434 x Ft (') x SG Meaning if a pressure of 0.952 bar is showing on a pressure gauge the head being produced for HFO is actually 10M Heavy Fuel Oil (HFO) at 10M Head – HFO has a specific gravity of 0.97, therefore 0.0981 x 10 x 0.97 = 0.952 bar The formula is Pressure (P) x Head (M) x Specific Gravity (SG) Abbreviated to P x H x SG Pressure = 0.0981 x M x SGĮxample: Water at 10M Head - Water has a specific gravity of 1, therefore 0.0981 x 10 x 1 = 0.981 Bar To convert Head in Metres to Pressure in Bar a formula is required. Manometers on pumps indicate suction and delivery pressures, and the corresponding differential pressure, which does not take into account the specific gravity of the fluid being pumped meaning any readings provided need to be checked against the fluids specific gravity to calculate the differential pressure being produced. A calculation of the pressure will change according to the fluid’s specific gravity. What is pressure? Pressure is fluid dependent and relative to fluid density. Head is the height of fluid typically expressed in (MLC) Metres of Liquid Column or represented with the unit M. It often causes confusion leading to a miscalculation of the correct pump specification to use. The discharge pressure of a pump is commonly referred to as head or pressure, but head and pressure are not the same across all liquids.
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