Bringing a cooling tower back into service after a shutdown

May is when a lot of towers wake up. Comfort cooling that's been off since October gets switched back on, seasonal process lines start running, and someone in facilities gets asked to "just turn the tower on" before the first warm week arrives.

That last bit is where it can go wrong. A cooling tower that's been sitting idle is at one of the riskiest points in its year, and flicking the fans on without a plan can throw out exactly the aerosol you've spent all year trying to control. Here's why, and how to bring a tower back properly.

Why an idle tower is a risk

When a tower stops, so does everything that keeps it under control. No circulation means no fresh biocide reaching the water, and whatever was in there slowly breaks down or gets used up. Water sits still in the pond, pipework and dead legs. Sludge settles. Biofilm carries on growing quietly on the fill and in the corners.

Then, on restart, the pumps stir all of that up and the fans blow it out as drift. ACOP L8 and HSG274 Part 1 both treat start-up after an idle period as something that needs managing, not something you do on autopilot. Your written scheme should say what happens while the system is idle and what has to be done before it runs again.

Short break or long shutdown?

The right approach depends on how long the tower has been off and how it was left. Broadly there are two situations:

• Short idle periods with treatment maintained. If the system has been kept topped up with treatment going, and circulated regularly so the chemicals reach everywhere, it may only need checks before it goes back into full service: chemical levels, a dip slide, a look at the pond and the dosing kit.
• Longer shutdowns, or idle periods with no treatment. HSG274 Part 1 expects a system that's been out of use for an extended period to be cleaned and disinfected before it goes back into service. If the tower was drained and left dry, it still needs cleaning and disinfecting before restart, because dry doesn't mean clean.

Where exactly the line sits between the two should be set out in your risk assessment and written scheme, not decided on the day. If your scheme is vague on it, that's worth fixing before the next shutdown. Our shutdown and start-up page covers how we help sites plan this.

A sensible start-up sequence

Every site's a bit different, but a typical restart after a longer shutdown looks something like this:

1. Check the records. When did the tower stop? Was it drained, or left wet? Was treatment kept going? When was the last clean and the last Legionella sample? The answers decide how much work is needed.
2. Inspect before anything turns. Look in the pond, check the fill, drift eliminators and distribution, and make sure nothing has moved in over winter. Birds and their droppings are a classic spring find.
3. Check the kit. Make sure dosing pumps are primed, chemical drums are in date and full, the conductivity controller is calibrated and the bleed valve works. A pump that's sat for months may not deliver what the display says.
4. Clean and disinfect where needed. Follow the sequence in HSG274 Part 1: disinfect, drain and clean, refill, disinfect again, then flush and bring the water back to normal treatment levels. Chemical levels and contact times come from your written scheme or the method statement for the work.
5. Re-establish treatment before full load. Get biocide and inhibitor levels where the programme wants them before the tower is running hard.
6. Monitor closely. Dip slides and chemistry checks soon after restart tell you whether the system has settled down. Many schemes also call for a Legionella sample after a restart, timed so the result means something.
7. Write it all down. Dates, what was found, what was done, chemical readings and results, all in the log book.

Better still: plan the shutdown properly

The easiest restart is one that was planned when the tower stopped. Before you switch off for a long period, decide which approach you're taking:

• Keep it wet and treated. Water stays in, treatment stays on and the system is circulated regularly so biocide reaches the whole system. How often should come from your risk assessment.
• Drain it down. Drain the system, clean it, and leave it dry with drains open so water can't collect. Then clean and disinfect again before restart.

What you don't want is the halfway house: full of water, no circulation, no dosing, for months. That's the one that turns into a problem in spring.

Don't forget the standby units

Shutdowns aren't just seasonal. Standby towers, duty-assist cells and towers on sites with N+1 redundancy can sit idle for weeks while the duty unit does the work. A standby tower that's full of water but never runs is effectively a stagnant system. Your scheme should cover rotating duty and standby, or circulating the standby regularly, so it's ready to go and under control when it's called on.

It's also worth checking whether a long shutdown changes anything with the council. A tower that's switched off but still connected is still notifiable. If it's being permanently taken out of use, that's a different matter, and we cover it in our post on notifying the local authority.

The short version

Don't just flick the switch. Check how the tower was left, inspect it, get the dosing working, clean and disinfect if the scheme calls for it, and keep a close eye on monitoring results for the first few weeks. For more on what a proper clean involves, see how often a cooling tower should be cleaned.

If you'd like a hand planning this year's restart, or writing a proper shutdown routine into your scheme, give us a ring. Booking it in a few weeks ahead makes everyone's life easier, including ours.