Start with the supply boundaries
Identify connections to tanks, external pumps, utilities and drains. Distinguish process lines from CIP and backwash lines, and mark flow directions and shared equipment. A connection outside the skid still needs a responsible party, a defined service condition and available capacity even when it is excluded from the supplied frame.
Use unique tags and align the drawing with equipment and signal lists. Establish who measures tank level, who permits startup and where off-specification permeate is diverted. These interfaces can create more uncertainty than the membrane module symbol itself, particularly when the skid is integrated by another contractor.
Measure the variables that explain performance
Define pressure measurements suitable for calculating TMP for the geometry and operating mode. In a crossflow arrangement, mean feed-side pressure relative to permeate may be used with a documented convention. Longitudinal pressure drop and TMP describe different effects and should not be treated as interchangeable readings.
Measure enough flows to close the feed, permeate and concentrate balance. Include temperature and appropriate quality measurements, such as conductivity for RO or turbidity monitoring for UF. Specify range, accuracy, calibration access and historical recording. A reading available only during manual rounds may not explain an overnight event.
Make isolation and maintenance explicit
Show isolation valves, check valves where appropriate, drains, vents and sampling points. Review trapped volumes and the pressure or vacuum protection required with the responsible engineering team. Valve arrangements should permit component removal and condition checks without improvised temporary connections or uncertainty over the isolated volume.
Separate the product-water circuit from chemical circuits. Define compatible materials, backflow prevention and the destination of spent cleaning liquids. Place pressure and sampling connections where they can be used and maintained. A transmitter does not ensure valid measurement if its impulse line becomes blocked by the process fluid.
Document the logic that symbols cannot explain
A cause-and-effect matrix should connect signals, alarms and actions: low level, loss of flow, pressure outside range or failure of a critical measurement. Define the response to loss of electricity or instrument air. Manufacturer limits and risk assessment determine settings and protection, rather than a generic online checklist.
Review production, rinse, cleaning, standby and maintenance states. Check that the selected paths are compatible and cannot unintentionally connect chemicals to useful water. This establishes the design basis; subsequent FAT and SAT activities verify the constructed equipment against the agreed requirements.
Common questions
Is one inlet pressure gauge enough to determine TMP?
Usually not. The feed-side and permeate pressures must reflect the actual configuration. A common header pressure does not automatically describe pressure at every module.
Can an existing skid P&ID simply be copied?
It can inform an initial layout, but geometry, utilities, materials, instruments and operating states require review. An RO drawing does not transfer directly to a UF skid with backwash.