Dosing failures: spotting a struggling dosing system early

Dosing systems rarely fail with a bang. There's no alarm bell, no cloud of smoke. A pump loses prime, a drum runs dry over a weekend, or a controller drifts out of calibration, and the tower carries on looking perfectly normal. Weeks later the dip slides start climbing, or a Legionella sample comes back positive, and everyone's wondering what happened.

The good news is that most dosing problems leave clues long before they become a crisis. Here's what tends to go wrong, how to spot it during routine checks and what to do about it.

What the dosing system is actually doing

A typical cooling tower treatment set-up has a few parts working together:

• Biocide dosing to keep bacteria, including Legionella, under control. Often two biocides are used, either alternated or one continuous and one shot-dosed.
• Inhibitor dosing to stop scale and corrosion, usually dosed in proportion to the make-up water or controlled by a sensor.
• Bleed control, normally a conductivity controller and a solenoid valve that dumps concentrated water so fresh water can come in.

If any one of these stops working, the others can't fully make up for it. Our chemical dosing and biocide control page explains the kit in more detail.

The usual culprits

Most dosing problems come down to a fairly short list:

• Empty or nearly empty drums. The simplest one, and still one of the most common. Particularly over holidays and long weekends.
• Loss of prime or air locks. Diaphragm pumps can click away happily while delivering very little, especially after a drum change.
• Blocked injection points or foot valves. Chemical deposits and debris build up over time and restrict flow.
• Wrong drum on the wrong line. Easy to do when drums look alike. It can mean no biocide going in, or two incompatible products ending up in one line.
• Controller faults or drifting probes. A dirty conductivity probe reads wrong, so the bleed stays shut or runs all the time.
• Stuck bleed valves. A solenoid stuck shut lets the water concentrate. One stuck open wastes water and washes treatment out.
• Power or timer issues. A tripped supply, or a timer that resets after a power cut, can stop shot dosing without anyone noticing.
• Old or badly stored chemicals. Some products lose strength over time, or if they're stored too hot or too cold.

Warning signs in your weekly checks

You don't need a lab to catch most of these. Build a few simple checks into your routine:

1. Drum levels. Note the level on the record sheet each week. If a drum isn't going down, the pump probably isn't pumping. If it's going down much faster than usual, something else is wrong.
2. Pump operation. Look at the pump while it should be running. Is it stroking? Is chemical actually moving up the line?
3. Controller readout. Compare the conductivity on the display with previous readings. A sudden jump, or a reading that never moves, is worth questioning.
4. Bleed. Check the bleed valve opens when it should, and that water isn't running to drain constantly.
5. Dip slides and chemistry. Rising TVCs, low biocide or inhibitor residuals and creeping conductivity are often the first measurable sign that dosing has slipped. Our monitoring and sampling page covers what to check and how often.

Write it all down. A single reading doesn't tell you much. A trend over several weeks tells you a lot.

What to do when you spot a problem

• Fix the obvious first. Swap an empty drum, re-prime the pump, reset a tripped supply. Only do this if you're trained and have the right PPE, and follow your COSHH assessment and the product's safety data sheet. Treatment chemicals can be hazardous.
• Tell your treatment contractor. They'll want to know, and they may want to check residuals or add a shot dose to bring levels back up.
• Step up monitoring for a while. An extra dip slide or chemistry check after a dosing fault shows whether the system has recovered.
• Record it. What you found, what you did and when. A dosing fault that's spotted, fixed and written down is good evidence your system works. One that's found later with no record looks very different.

If counts are already high, or a Legionella result has come back above your scheme's action level, follow your written scheme and see our guide to the first 48 hours after a positive result.

Preventing it in the first place

A few habits make a big difference:

• Label every drum and every line clearly, so the right product goes on the right pump.
• Keep a spare drum of each product on site, stored as the safety data sheet says.
• Check drum levels before long weekends and holidays. Christmas is a few weeks away, so now's a good time.
• Have probes cleaned and calibrated, and pumps checked, on every service visit.
• Consider remote monitoring or alarms for low drum level, no flow and high conductivity, especially on critical systems.

As the nights draw in, it's also worth making sure dosing kit and chemical lines are protected from frost. A frozen line is a dosing failure too, and it tends to happen on the coldest night of the year when nobody's around.

Our water treatment service includes routine checks and calibration of all of this. If you've got a dosing set-up that never seems quite right, or the tower's been idle and the kit needs a once-over (see restarting after a shutdown), we're happy to take a look.