01Define the measurement boundary
Pressure drop is the pressure difference between stated points at stated operating conditions. It is not the vessel design-pressure rating. If the pressure taps include a strainer, valve or long pipe run, the reading includes those losses too.
Read inlet and outlet pressures together where possible. Check gauge accuracy and range, clear pressure taps and their elevations. Significant elevation differences require a static-head correction, and velocity changes can affect the comparison. Subtracting two coarse gauge readings may not resolve a small heater pressure drop reliably.
02Use the operating history
| Symptom | Check first | Evidence to collect |
|---|---|---|
| High from the first start | Cold viscosity, valve positions, actual flow and pipe size | Fluid properties at startup and the applicable pressure-drop data |
| Increasing over time | Blocked strainer, deposits, coking or changed valve position | Trends compared at the same temperature and flow |
| Fluctuation with noise | Entrained gas, vapor formation, pump suction conditions or control instability | Suction pressure, level, temperature and pump data |
| Acceptable hot, excessive cold | Viscosity change and the startup procedure | Viscosity-temperature data and corrected pump performance |
03Check the pump against the full circuit
For a liquid loop, include the heater, piping, valves, strainer and process equipment when checking the system resistance against the pump curve. In a closed recirculating loop, the height of the highest point is not simply added as continuous circulating head. Filling, static pressure and suction conditions still require separate checks.
1 bar = 100 kPa ≈ 14.5 psi. For water at an assumed density of 1,000 kg/m³, a 50 kPa pressure difference is about 5.1 m of liquid head. Use the actual density for another fluid. This conversion does not, by itself, size a pump or establish adequate suction margin.
04Keep the thermal design in the decision
Baffles change flow paths, local velocities and heat transfer. Removing them may reduce resistance while allowing bypass flow or localized overheating. Reducing flow can also reduce cooling of the elements. Any change must preserve the specified minimum flow and temperature limits.
Send the supplier fluid properties, cold and hot flow rates, inlet/outlet pressures and temperatures, tap locations, the pump curve and operating trends. Ask for pressure-drop data at those conditions and a review of the flow passage and startup procedure. Follow site isolation, cooldown and depressurization procedures before inspection.
↗Sources and scope
References support the design principles discussed here. Product-specific limits must come from the selected equipment and fluid documentation. Illustrative values are not purchase specifications.