A viscosity value without its temperature can mislead a tank dewatering review. Oil may flow easily when warm and become much harder to move after a cold start or long shutdown. Those changes can affect both the available rate and the float response.
Belfield publishes specific viscosity and temperature guidance for defined ranges. The figures are application guidance, not a guarantee of the flow at every site. This article explains how to report the operating envelope and when to request a factory review.
How to assess viscosity and temperature for a tank dewatering valve
Ask what the viscosity number represents
Record viscosity with its unit, test method, sample identity and measurement temperature. A value measured at 20 degrees Celsius cannot be assumed to describe a hot process or a cold start. If the supplier has a viscosity curve, include it with the enquiry.
For a tank duty, note the minimum, normal and maximum temperature, plus any credible shutdown or stagnant condition. If the liquid contains several components or varies by batch, identify that range and the source of the data.
Include both phases when relevant. The heavier liquid drives the drain flow, while the lighter liquid must still move the float and produce the intended interface response. Do not treat the water phase as the only property that matters.
Apply Belfield guidance only within its stated conditions
Belfield performance information says viscosities up to 50 c/s at operating temperature have little or no effect on nominal capacity. For 50 to 100 c/s, it calls for a 25% reduction in nominal capacity. Applications over 100 c/s are referred to the factory with full liquid details and a viscosity curve where available.
The same page gives temperature guidance for atmospheric pressure duties up to 50 feet of water head: normal capacity guidance up to 80 degrees Celsius and a 25% reduction from 80 to 95 degrees Celsius. Higher head conditions depend on temperature and pressure and require individual review.
These published bands do not account for every combination of liquid, pressure, tank, pipework or transient condition. Do not add multiple percentage corrections or extrapolate beyond the stated range unless the current manufacturer documentation provides a method for that exact duty.
Consider the coldest and warmest credible cases
Cold operation can raise viscosity, encourage wax formation or make a product approach its pour or solidification condition. The operating manual specifically asks for viscosity at operating and atmospheric temperatures where a product may become solid at ambient conditions.
Hot operation can affect viscosity and density, while pressure changes may alter the process further. Give the normal production temperature and any startup, cleaning, upset or shutdown temperatures that the equipment could actually encounter.
If the range is broad, present several data points rather than a single average. State how often the extreme occurs, how long it lasts and whether the tank is being drained during that condition.
Keep viscosity separate from head and pipe losses
Available head changes as the tank level falls. The final part of a drain cycle may therefore have a lower driving force than the beginning. Pipe length, bends, reducers, valves and downstream pressure also influence the real flow.
A high viscosity liquid can make restrictions more significant. Do not assume that a bigger outlet or a pump will fix a slow drain; each change alters the system and needs to be checked against the valve inlet, allowable pressure and receiving route.
Provide the minimum, normal and maximum head, desired flow and a sketch of the pipework. If the arrangement is pumped or pressurised, add pump and pressure data and ask for review rather than applying gravity capacity tables.
Include solids and phase behaviour in the same review
A viscosity curve alone does not describe a dirty or multiphase stream. Disclose suspended solids, sludge, wax, sticky deposits, an emulsion or any layer between the liquids. The nature and amount of material can change the practical risk.
The Belfield guidance specifically asks for details of suspended solids when higher viscosity applications are referred. The current selection guide also asks applicants to list solids, wax, emulsions and flashing risks. These details help the manufacturer decide whether further review or testing is appropriate.
Do not omit a solids concern because it is intermittent. Record when it appears, where it comes from and whether it settles, floats or adheres to surfaces. Avoid proposing a screen or flush arrangement without checking the effect on the complete duty.
- Viscosity at the operating temperature and ambient temperature
- A viscosity curve over the full range, if available
- Minimum, normal and maximum process temperature
- Available head as the tank level changes
- Solids, wax, sludge, emulsions or deposits
- Pump, pressure, downstream route and piping diagram
Write a conclusion with a clear boundary
A useful application note states which published guidance was applied, what remains uncertain, and which model or installation details require confirmation. Nominal capacity is a reference, not a promise that a process will meet a production target.
If any condition is outside the published range, ask the Belfield team for a factory review before selecting or changing equipment. Provide the data in its original units and preserve the latest manual and approved drawing with the project file.
Revisit the conclusion when the product, temperature range, tank level, pump or pipe route changes. A documented envelope helps operators recognise when the original basis no longer applies.
If a batch certificate reports one value but the process temperature changes substantially, include both facts in the enquiry. The reviewer can then decide whether a temperature correction, additional data or testing is needed. Do not hide a difficult condition inside a single average.
Frequently asked questions
What viscosity does the Belfield guidance give for nominal capacity?
The published page says up to 50 c/s at operating temperature has little or no effect, 50 to 100 c/s calls for a 25% reduction, and applications above 100 c/s should be referred to the factory.
Is the 25% temperature correction valid for every installation?
No. The page limits that guidance to stated atmospheric pressure and head conditions. Higher head or pressure needs individual review.
Can I use a viscosity value measured at ambient temperature?
Provide it, but also report operating-temperature viscosity and the full temperature range. The manual highlights both operating and atmospheric conditions.
Will increasing the outlet size solve a high-viscosity problem?
Not necessarily. The inlet, head, pipework, pressure, solids and valve configuration all matter. Review the complete system with the manufacturer.
Sources and further reading
The figures above come from Belfield performance guidance and the published operating brochure.
For the broader process information, see the Belfield sizing and selection guide.
Next steps
Send the viscosity curve, temperature range, head, required flow and a piping sketch to the Belfield application team. For more about PMA, visit the PMA website.
