The commonest commissioning fault in dosing
A metering pump meters; it does not regulate. If the path from chemical tank to process line ever offers a pressure gradient that flows by itself — tank above the line, line under vacuum from a pump suction, or simply a full suction line acting as a syphon — chemical will flow whether the pump strokes or not. Syphoning through an unvalved injection point is the classic: the system doses uncontrolled, a day tank empties overnight, and the process gets a slug of chemical no one metered. With hypochlorite or acid, that is a safety event, not just a wasted drum.
The cure is two pieces of standard hardware, each doing its own job. The injection valve — the light spring-loaded check valve supplied with every metering pump — is installed at the injection point, where it keeps the delivery line full and primed, stops process water backing up into it and ends the dribble after each stroke; its 0.3–1 bar spring is not an anti-syphon device, and a day tank a few metres above the line or a sub-atmospheric injection point overcomes it. Syphon protection is the back-pressure/anti-syphon valve: a separate adjustable spring-and-diaphragm valve, normally at the pump discharge, set at least 1 bar above the greatest driving head the tank can ever put on the line. Points that make it work:
- The injection valve goes at the far end, at the quill, so the whole delivery line stays full and under pressure; the back-pressure valve sits near the pump, where it is in series with everything downstream and its setting is easy to adjust and read.
- It is not optional equipment, and it is the first suspect when doses drift: a fouled or scaled injection-valve spring changes the pressure the pump works against.
- Where the tank can ever sit above the injection point, or the injection point can go sub-atmospheric (dosing into a pump suction), the back-pressure/anti-syphon valve is mandatory, not optional. A loop in the delivery line above the tank level only helps if it carries a vacuum breaker at its crown — a full loop over a high point with no air admittance is itself a syphon once the pump has primed it.
Building a proper injection point
- Inject mid-stream, not at the pipe wall. An injection quill carries the chemical to the pipe's centreline, where flow disperses it. Wall-injected chemical dribbles along the pipe surface — concentrated, corrosive, and slow to mix; the corrosion track downstream of a quill-less injection point is a recognisable signature.
- Give it mixing distance. Dose upstream of bends, a static mixer, or a decent straight run before any sensor or take-off. A chlorine probe centimetres after the injection point reads noise and the control loop chases it. Where an acid and hypochlorite are both dosed into one line, give them separate injection points with mixing length between them — the two must never meet undiluted, because that releases chlorine gas.
- A non-return valve in the process line upstream of the injection point stops dosed chemical running back towards the source when the process pump stops (the injection valve is the non-return on the chemical side) — and on RO antiscalant duty, place the injection where the full feed stream passes it, upstream of the cartridge filter, so mixing is complete before the membranes and any quill debris is caught.
- Isolate serviceably. An isolation valve either side of the quill turns an injection-valve clean-out into ten minutes without draining the line. Quills in hard water scale like kettles — hypochlorite injection into hard water grows a calcium stalactite at the quill tip that eventually seals it; make its removal easy because it is a maintenance item.
- On the suction side: flooded suction from a day tank beats suction lift; a foot valve with strainer keeps prime and catches drum debris; and a pulsation dampener near the pump smooths the delivery of diaphragm pumps where instruments or long lines dislike the hammer.
The five-minute commissioning test
Prove the installation, not the pump: with the pump switched off, confirm no chemical moves (syphon test — watch the calibration cylinder level). Then with it on, time the draw-down from a calibration cylinder against the calculated rate. Those two checks catch nearly every fault this article describes, before the process does.
Rule of thumb: injection valve at the quill, back-pressure/anti-syphon valve at the pump wherever the tank can sit above the line, quill to the centre of the pipe, process-line non-return upstream of it, and a switched-off syphon test before handover. A dosing system is proven by what it does when the pump is not running.