Before replacing a chlorine analyser, record the disinfectant, sample take-off and flow, panel connections, signal scaling and how the plant responds to lost sample or power. Agree how you will compare readings and hand over the new installation before removing the old instrument.
Record how the existing measurement is used
Start with the residual the site needs to measure and the action operators take when it changes. Record the measurement point, normal and limiting water conditions, reference method and how the result reaches the control room. If the reading feeds a control loop or alarm, document the existing logic and what the site expects to happen when the instrument is unavailable.
Trace the water path on site
Follow the pipework from the sample take-off to the measuring point and on to the drain or return. Mark the isolation valves, pressure changes, flow indication and reference sampling point. Record how conditions change between normal operation and a shutdown. Existing pipework may be worth retaining if it delivers representative water and can be safely isolated and serviced.
Compare the options for the existing panel
A replacement flow cell may suit a panel with a dependable sample supply. Direct in-pipe measurement changes the mounting, access and isolation requirements. Confirm the rating of the complete assembly, including the fittings, against the actual duty. For either option, check whether staff can reach the instrument and take a representative reference sample without interrupting normal operation.
Write the signal and failure schedule
Record power, cable routes, the controller and the inputs available at the PLC or logger. List each measurement, its units, scaling and how the control system marks an unavailable or stale value. Agree who will check the displayed and recorded values before handing over. Verify the fitted interfaces for the exact controller rather than assuming the replacement maps to the old panel.
Set the handover checks before removing the old unit
Agree what the existing and replacement instruments will be compared against, and how the comparison will account for sample delay. Record the reference method, representative conditions and acceptance criteria in the site plan. Include a check of the operator display, alarm indication, reference sampling access and maintenance instructions.
Send PMA a usable survey brief
Include a marked-up sample route, photos of the indoor panel and take-off, expected water conditions, the residual being measured and a list of the signals to retain. Flag any constraints on isolation, outage windows and service access. PMA can review the proposed sensor, mounting and controller against that brief.
