A pump for tank mixing eductors must be selected from the combined motive-flow requirement and the pressure required at the eductor header, not from tank volume or motor power alone. The pump operating point is where its curve meets the system curve after pipe, valve, fitting, elevation, filter and eductor losses are included.
This guide provides a first-pass workflow. Final selection requires the current pump curve, the selected eductor’s performance data and the actual piping layout.
Define the required motive flow
Obtain motive flow per eductor, Qm, from the selected model’s performance data at the intended inlet pressure. For N equal units, the preliminary pump flow is:
Qpump = N × Qm + other simultaneous process demand
If the recirculation loop also feeds heat exchangers, filters, spray devices or bypass lines, include those branches at their simultaneous operating conditions. Do not use total eductor circulation as pump flow: induced liquid is moved inside the tank and is not all supplied through the pump.
Define pressure at the correct location
The model requires pressure at its inlet or header. Pump discharge pressure measured upstream of long piping or a filter can overstate the available pressure. Add an accessible pressure tap near the header for commissioning.
Build the system head requirement
The pump must overcome the static component where applicable and the dynamic losses at design flow. Include:
- Suction and discharge pipe friction
- Elbows, tees, reducers and branch fittings
- Control, check and isolation valves
- Strainers or filters at their clean and expected dirty conditions
- Heat exchangers or other equipment in the recirculation loop
- Elevation difference and vessel pressure where applicable
- Required residual pressure at the eductor header
Because friction loss rises rapidly as flow increases, adding eductors can move the system to a very different operating point. Recalculate the complete loop after quantity or pipe size changes.
| Frequent shortcut | Why it fails |
|---|---|
| Selecting from connection size | Equal threads can have different motive-flow and pressure requirements. |
| Using pump nameplate flow | Nameplate or best-efficiency data is not the installed operating point. |
| Ignoring a dirty strainer | Pressure available at the header can fall as the pressure drop increases. |
| Throttling without measurement | Valve position alone does not prove branch pressure or flow. |
Check the pump curve against the system curve
Plot or calculate the system requirement over a useful flow range, then locate its intersection with the pump curve. Confirm motor load, allowable operating range and available net positive suction head using the pump manufacturer’s documentation.
Verify branch balance
A single header pressure does not guarantee equal flow through asymmetric branches. Compare branch lengths and fittings, use suitable header sizing and add balancing provisions when required. During commissioning, measure pressure at near and far points.
Check minimum and maximum conditions
Review clean and dirty filters, minimum and maximum liquid levels, viscosity and temperature range, valve states and any simultaneous users. A pump that meets one normal condition may not meet the worst credible case.
Control and protection decisions
- Variable-frequency drive: can adjust pump speed, but eductor performance must be rechecked at every intended operating point.
- Pressure control: useful where stable header pressure is more important than constant speed.
- Low-level interlock: can prevent operation when submergence or suction conditions are unacceptable.
- Differential-pressure indication: helps identify a loaded strainer or filter.
- Flow or pressure alarm: provides evidence of degraded circulation rather than relying on visual surface motion.
Control logic should reflect the process risk and be reviewed by the responsible system engineer.
Hypothetical first-pass example
Values below illustrate the method and are not product specifications.
- Six eductors require 4 m³/h motive flow each at their selected inlet pressure.
- Combined motive flow = 24 m³/h.
- Calculated piping and equipment loss at 24 m³/h = 12 m of liquid head.
- Required residual head at the eductor header = 18 m of liquid head.
- Preliminary pump duty = 24 m³/h at 30 m total head, before project margins and other simultaneous demand.
Next, check the pump curve, suction conditions, motor load, branch distribution and the eductor performance at the resulting pressure. If the curve intersection is different from the assumed duty, iterate rather than forcing the original number.
Information for an engineering review
Send the pump manufacturer and curve, liquid data, suction arrangement, pipe schedule and lengths, fittings, filters, elevation, tank pressure, number of eductors and selected performance point. Pair this page with the sizing calculator and quantity guide.
Review Mixing Eductor options or send the pump curve and piping layout to Jeltecn.
Technical reference
Model-specific circulation, pressure and effective-flow-field examples are available in Spraying Systems Co. 46550 product data. Use the final selected model’s current documentation for design.