“The product is 5,000 cP. Is that enough to select the pump?” The next questions are where that value came from, at what temperature it was measured, and what the product is like when the pump starts. The number alone leaves those conditions open.
A heated process may run normally all day and restart with colder material left in an unheated line. Another installation may keep the liquid warm through the stop. Describe the actual arrangement; startup should be checked rather than assumed to be a fixed multiple of running viscosity.
1. Put the Measurement Conditions beside the Number
Send the original viscosity report or product data where available. Include units and sample temperature. Dynamic viscosity in cP or mPa·s and kinematic viscosity in cSt or mm²/s are different quantities; converting between them requires density at the corresponding conditions. Do not relabel one as the other.
For a liquid whose apparent viscosity depends on the test conditions, include the instrument, spindle or measuring geometry, speed or shear rate, test duration, and sample preparation where recorded. A value measured after mixing may not describe the same state as an undisturbed sample. Preserve missing fields as unknown rather than filling them with assumptions.
2. Describe What Is in the Pump at Restart
Record the expected liquid temperature after the longest planned stop, including the pump, supply vessel, and unheated sections of pipework. Ambient temperature alone does not establish the liquid temperature. State what is measured, what is estimated, and which heating or insulation arrangements are in use.
Describe settling, gelling, wax formation, or a change after standing if these have been observed. Shear-thinning behavior concerns how apparent viscosity changes with shear rate; changes with time or rest need separate information. Do not infer a particular starting-torque peak from the term “shear-thinning” alone.
If a viscosity-temperature curve or rheology report is available, send it with its applicable range. If the restart state has not been measured, describe it and identify the data gap. Do not invent a cold-start value by multiplying the warm reading or extrapolating outside the available data without a stated basis.
3. Copy These Fields into the RFQ
Give normal operation and restart their own entries. Use the available report values and mark missing information as unavailable. The supplier can then identify what needs clarification before the selection is finalized.
Liquid: [product, available composition, density with temperature, and solids if present]
Normal operation: [liquid temperature, viscosity with units, required flow range, and inlet/outlet pressures]
Restart: [longest stop, expected liquid temperatures at the vessel, pump, and inlet line, and any measured viscosity for that state]
Test details: [report or data-sheet reference, instrument and method, test temperature, speed or shear rate, and sample preparation]
Condition after standing: [observed settling, gelling, or other changes; whether the pump and lines remain filled]
Installation: [pipe inside diameters and lengths, liquid level, valves and restrictions, heating or insulation, and existing drive details]
Unknowns: [values not yet measured or conditions that need confirmation]
For a replacement, add the existing pump model, recorded speed, motor and drive details, trip or alarm logs, and when the problem occurs. Existing records are useful; do not repeat a failed start just to collect a new reading.
4. Have the Inlet, Outlet, and Drive Checked Together
Ask for the inlet assessment and line-loss calculation at the stated flow and liquid conditions. Include the complete pipe route, inside diameters, supply level or vessel pressure, and fitted restrictions. The coldest section of pipework may need attention even where the supply vessel is heated.
The drive review also needs the proposed speed, pressure, pump configuration, and available motor and gearbox torque. Viscosity is an input to that review, not a direct motor-size or reducer-ratio rule. Ask how the proposed starting method meets the equipment limits rather than assuming a larger motor or slower ramp will resolve the issue.
For the inlet layout, see the self-priming lift guide. Its diagram distinguishes vertical lift from total suction-line length.
5. Make the Quotation Assumptions Visible
Ask the supplier to state the normal-running and restart conditions used for selection, the proposed operating range, and any heating, prefill, or other startup provisions assumed. Confirm which conditions still need measurement or testing. An offer based on a warm liquid should not silently become approval for a colder restart.
If the required restart state is outside the supported selection, resolve it before ordering. That may require more liquid data or a revised equipment and process arrangement. Keep the approved operating and startup procedure with the final selection so the site knows what conditions must be met.
