How to Select the Right Commercial Pump by Flow Rate and Head Pressure

Commercial pump selection, regardless of brand, reduces to one calculation: matching your system's duty point to a pump's performance curve. The duty point has two coordinates: flow rate (how much water you need moved, typically in liters per minute or gallons per minute) and total head pressure (how much resistance the pump has to overcome, typically in meters or psi). Every manufacturer, including Grundfos and Ebara, publishes performance curves that plot head against flow for each pump model. Find where your duty point lands on that chart and you have a shortlist of models that can do the job. Get either number wrong and the consequences show up immediately. A pump sized for too much flow or head wastes energy and wears out valves and seals faster than it should. A pump sized for too little leaves upper floors or the farthest fixture starved of pressure, a common complaint in Philippine mid-rise buildings where the booster pump was specified from a rough guess instead of a calculation. JOHOB Hardware Trading's pumping and pressure systems range, booster, transfer, and submersible pumps, together with VFD duplex control panels, is specified the same way: flow rate and head pressure first, model and controls second. The brand name on the nameplate matters less than getting these two numbers right before you request a quote.

For a multi-fixture building (hotel, hospital, commercial laundry, residential condominium), the starting point is the Philippine National Plumbing Code (NSPC), which assigns a fixture unit value to every plumbing fixture (lavatory, water closet, shower, kitchen sink) based on its typical draw. Add up the fixture units for the building or the riser you are sizing, then convert that total to a probable peak demand flow rate using the NSPC's fixture-unit-to-flow conversion curve. That peak demand figure, not the sum of every fixture running at once, is your design flow rate, because plumbing codes assume fixtures operate intermittently rather than simultaneously. For an industrial or process application (a boiler feed line, a cooling loop, a wash-down system), skip the fixture-unit method entirely and use the actual flow requirement from the equipment spec sheet or process data, since process equipment has a fixed, known draw rather than a probabilistic one. Whichever method applies, express the result in the same unit the pump curve you are comparing against uses. Grundfos and Ebara catalogs commonly list performance in both LPM and GPM, so confirm the unit before you plot your duty point. A unit mismatch here is one of the most common sizing errors buyers submit in an RFQ.

Total head pressure, often written as total dynamic head (TDH), is the sum of three separate numbers, not one measurement you can read off a gauge. Static head is the vertical lift: the elevation difference between the water source (or suction tank) and the highest point of discharge. Friction loss is the pressure the pipe network itself consumes: it depends on pipe diameter, pipe length, the number and type of fittings, and the flow rate you calculated in the previous step. Longer runs and smaller diameters cost more head. Residual (or discharge) pressure is the pressure you need left over at the farthest or highest fixture for it to function properly. Add static head, friction loss, and residual pressure together and you have your TDH, the second coordinate of your duty point. Buyers frequently calculate flow rate correctly but estimate head pressure by guesswork, which is the more common of the two sizing mistakes because friction loss requires a pipe friction chart or sizing reference rather than a simple sum. If you are unsure of your friction loss figure, JOHOB can review your pipe layout as part of the quote process before confirming a pump model.

A pump performance curve plots head (vertical axis) against flow (horizontal axis) for one pump model, usually with several lines for different impeller sizes or speeds. Your duty point, the flow rate and head pressure you calculated, is a single coordinate on that chart. The pump models worth shortlisting are the ones whose curve passes through or close to that coordinate, not the ones with the highest rated flow or head on the spec sheet. Beyond simply passing through the duty point, check where that intersection falls relative to the pump's best efficiency point (BEP), the zone on the curve where the pump converts the most energy into actual water movement. A pump running far from its BEP, even if it technically meets your flow and head numbers, costs more to run and wears out faster. Grundfos and Ebara, like every other manufacturer in this category, use this same curve format in their catalogs and selection software, so the reading method does not change between brands. What changes is the specific curve library, motor options, and control compatibility each brand offers. This is also where NPSH (net positive suction head) matters: confirm the pump's required NPSH is below your system's available NPSH, or you risk cavitation regardless of how well the duty point matches.

Once you have a duty point and a shortlist of curves that satisfy it, the next decision is pump type, not just model. A booster pump raises pressure on an existing supply line and suits buildings where city or cistern pressure is insufficient at upper floors. A transfer pump moves water between two points, for example from a ground-level cistern to an elevated tank, and is sized primarily on flow rather than fine pressure control. A submersible pump sits inside the water source itself (a deep well or a wet well) and needs a duty point calculation that also accounts for submersion depth as part of static head. JOHOB Hardware Trading supplies all three categories, booster, transfer, and submersible, so the equipment range does not force a type decision before the calculation is done. Motor power should also be checked against the curve's power draw at your specific duty point, not the pump's maximum rated power, since running near maximum continuously shortens motor life. Because duty points vary by project, JOHOB prices pumping systems on a quote basis rather than a published price list, and responds to RFQs within one business day so the sizing conversation can start quickly.

A single fixed-speed pump sized to one duty point works well when demand is steady. Many PH commercial buildings and plants do not have steady demand: a hotel's morning peak, a hospital's 24-hour variable load, or a food facility's wash-down cycles all swing flow and pressure requirements through the day. For variable demand, a VFD (variable frequency drive) duplex panel runs two pumps against a duty point range instead of one pump against a single fixed point, varying motor speed to match actual demand and alternating duty between the two pumps so wear is shared and one unit remains available if the other needs maintenance. Sizing a duplex VFD setup starts from the same two numbers, flow rate and head pressure, but applied across the full demand range rather than a single peak figure. JOHOB supplies VFD duplex panels (built around Inovance or Chutian drives) alongside its booster, transfer, and submersible pump range, and the panels carry CE marking as part of the electrical equipment JOHOB sources. If your flow rate and head pressure calculation shows a wide swing between minimum and peak demand rather than a flat, steady number, a VFD duplex setup is worth pricing alongside a single fixed-speed pump before you finalize the RFQ.