Motor sizing fails when the calculation stops at flow. Drilling-fluid pumps need power checked against head, specific gravity, pump efficiency, service factor and the way the pump will actually be controlled.
Hydraulic horsepower
Hydraulic horsepower is the useful power transferred to the fluid. It is not the motor horsepower. It is the clean first screen that tells you whether the duty is in the right power range.
Hydraulic HP = Flow (gpm) × Head (ft) × SG / 3960Metric equivalent: Hydraulic kW = Flow (m3/h) × Head (m) × SG / 367.Brake horsepower
The pump is not 100% efficient. Divide hydraulic horsepower by pump efficiency to estimate shaft demand. Then apply service margin according to site practice and duty severity.
Brake HP = Hydraulic HP / pump efficiencyExample: 1,000 gpm, 90 ft, SG 1.25 gives 28.4 hydraulic HP. At 60% efficiency, brake demand is 47.4 HP before service margin.Motor sizing example
| Step | Calculation | Result |
|---|---|---|
| Hydraulic HP | 1000 × 90 × 1.25 / 3960 | 28.4 HP |
| Brake HP | 28.4 / 0.60 | 47.4 HP |
| Service margin | 47.4 × 1.15 | 54.5 HP |
| Practical motor | Next suitable standard size | Often 60 HP or 75 HP depending on site standard and duty |
Why mud weight changes everything
At the same flow and head, heavier mud needs more power. The pump may still develop the head, but the motor current can become the limiting factor.
Power ratio = SG_new / SG_oldIf SG rises from 1.05 to 1.35, hydraulic power rises by about 29% for the same flow and head.VFD checks
A VFD can help match flow, but speed changes affect flow, head and power unequally. Increasing speed is the fastest way to overload a motor if the power curve is not checked.
| Speed change | Flow | Head | Power |
|---|---|---|---|
| 10% speed increase | About +10% | About +21% | About +33% |
| 10% speed decrease | About -10% | About -19% | About -27% |
Selection red flags
Engineering depth: motor approval checklist
Motor approval is not only a horsepower calculation. The duty must be checked against starting method, VFD limits, ambient temperature, enclosure, service factor, cable capacity, overload setting and whether the pump may be operated at higher speed or heavier mud than the design case.
| Check | Why it matters | Field consequence |
|---|---|---|
| Actual SG | Power rises with density | Motor overload in weighted mud |
| Efficiency assumption | Controls brake horsepower | Undersized motor if efficiency is overestimated |
| VFD max speed | Power rises roughly with speed cubed | Fast overload after small speed increase |
| Service margin | Allows field variability | No margin for heavier mud or line changes |
The safest calculation uses the worst credible operating case, not the cleanest expected case. That means maximum expected SG, maximum required head and realistic efficiency.
Related reading
Motor approval should be checked against measured current, actual SG, route head and the maximum speed the crew can select. If those values do not match the calculated envelope, the site should revisit the duty before accepting the motor size.
Motor Power Field Envelope
Motor sizing must account for hydraulic power, pump efficiency, service factor, density, viscosity and the worst credible route condition. A motor that is acceptable on water may overload when mud weight rises or when the discharge route changes.
Brake HP = hydraulic HP / pump efficiencyFinal motor size should include service margin and measured current verification.| Pump Hydraulic Power and Motor HP check | Required field evidence | Stop or redesign trigger |
|---|---|---|
| Duty definition | required flow, actual route, fluid condition, operating margin and downstream process response are recorded with units and operating state. | The duty is described only by equipment name or nameplate capacity. |
| Installed-route confirmation | Line length, elevation, valves, receiving point and material condition are checked. | The route cannot be restarted or isolated safely after a stop. |
| Process acceptance | The downstream process becomes stable after the correction. | One gauge improves while flow, quality or reliability gets worse. |
