Define the section being measured
Mark pressure points on the flow diagram and distinguish the cartridge housing, piping, valves and membrane stage. A differential spanning several components cannot assign the entire loss to membrane elements. Confirm that transmitters use compatible references and have consistent calibration.
Record feed and concentrate flows, temperature and valve positions. Increased flow raises hydraulic resistance even without new deposits. Compare with a baseline at similar conditions or apply the correction method specified for the equipment. Keep the original readings alongside any adjusted figures so the comparison remains auditable.
| Measured differential | Includes | Purpose |
|---|---|---|
| Across the cartridge housing | Safety filter | Track filter loading |
| Across one stage | Membrane feed channel | Locate stage restriction |
| Pump to final concentrate | Several components | System overview with limited localisation |
Locate the change before proposing cleaning
If first-stage pressure drop rises, investigate pretreatment carryover, colloids and biological growth among the possible causes. A final-stage deviation also requires checks of concentration and scaling. Manufacturer manuals describe typical patterns, but mixed deposits and mechanical problems can produce overlapping symptoms.
Review the time trend and feedwater incidents. A sudden increase after a valve movement may indicate an operational restriction or instrument problem. A progressive increase calls for comparison with used cartridges, SDI, chemical analyses and dosing records. One satisfactory water sample cannot represent an entire operating campaign.
Use arithmetic without treating it as a diagnosis
Consider a stage with an inlet pressure of 12.0 bar and concentrate pressure of 11.2 bar: its differential is 0.8 bar. Later readings of 13.0 and 11.8 bar give 1.2 bar, a 50% increase relative to the original 0.8 bar.
This example compares numbers only. It does not establish fouling if flow has also changed, and it sets no acceptable operating limit. Maximum element or vessel pressure drop and intervention criteria must come from the manufacturer and the actual plant design.
Remove the cause and retain a useful new baseline
Higher feed pressure may temporarily sustain permeate output, but it does not remove a restriction in the feed channel. Check mechanical limits before continued operation. Where diagnosis justifies cleaning, the circuit must deliver the required cleaning flow and allow stages to be treated separately when appropriate.
After the intervention, compare pressure drop, normalised production and quality. If resistance returns quickly, investigate the pretreatment or operating condition creating the deposits. Keep the results with the instrument diagram so that the next performance change can be located with less uncertainty.
Common questions
Are pressure drop and TMP the same?
No. Pressure drop compares inlet and outlet on the feed side. Transmembrane pressure includes the permeate-side pressure and describes a different physical relationship.
Does high pressure drop always mean biofouling?
No. Particles, scale, mixed deposits, valves and instrument errors can also contribute. Diagnosis needs location-specific readings and operating context.