Sprinkler tanks: inspection, cleaning and when to replace

The tank is the part of a sprinkler system nobody looks at. It has no moving parts to fail loudly, it sits outside or in a corner of a plant room, and it usually looks the same on the day it stops being adequate as it did on the day it was commissioned. That is exactly why it is worth a routine of its own.

A sprinkler system is a water supply with pipework attached, not the other way round. Everything downstream, the valve sets, the heads and the alarms, assumes a stored volume behind it at a known pressure and for a known duration. If that assumption stops being true, nothing on the system tells you.

What the standard actually asks for

Under BS EN 12845 a suction tank sits inside the same routine as the rest of the installation, and it appears at two intervals.

Weekly, the water level. Read it and write it down. The number matters less than the trend: a level that drops steadily is a leak, a level that recovers on its own is a refill working, and a level nobody has recorded for eight months tells you nothing at all. This is the check most often left undone, because it takes a minute and there is rarely anything visibly wrong.

Annually, the tank itself. Drained where the arrangement allows, inspected internally, cleaned out, and the condition of the internal coating recorded. This is the visit that finds the things a gauge cannot show: sediment on the floor, corrosion under a coating that looked sound from outside, a suction screen partly blocked, an anti-vortex plate that has come loose.

Both sit inside the wider regime set out in sprinkler servicing intervals, and neither is optional because the tank happens to be outdoors.

The parts that decide whether stored water is usable

Capacity is the number everyone knows. It is not the only thing that decides whether the water behind your system will do its job.

The automatic refill. BS EN 12845 sets a refill time for the tank, and the float valve or refill arrangement has to be capable of meeting it. Capacity and refill are counted together as the water supply, so a tank that holds the right volume but fills too slowly is not the supply the system was designed around.

Level indication and the low level alarm. A tank that is losing water is a manageable problem. A tank that is losing water and cannot tell anyone is the one that turns into an incident. The alarm is proved, not assumed, and it is proved by dropping the level rather than by looking at the panel.

The suction arrangement. The connection to the pumps, the screen or strainer that keeps debris out of them, and the anti-vortex plate that stops air being drawn in as the level falls. A pump passing air is not passing its rated flow, which is a failure that only shows up under the conditions you least want to discover it in. Whether the pumps themselves can still deliver is a separate question, answered by the annual test described in fire pump testing.

The coating, and what is under it. On a steel sectional tank the coating is the tank’s working life. Once it fails locally, corrosion runs at the seams and around the fixings, and by the time it is visible from outside there is usually more of it inside.

Why a first inspection is worth booking on its own

Plenty of the tanks we are asked to take on have never been opened. That is normal rather than negligent: the tank was commissioned, it filled, and the maintenance that followed concentrated on the parts with gauges on them.

The first internal inspection is the one that produces information rather than a tick. It establishes what condition the steel and the coating are actually in, how much sediment has accumulated, whether the suction arrangement is clear, and whether the capacity still matches what the building now needs. Everything after that is a comparison against a known starting point, which is what makes the record worth keeping.

It is also the point at which a tank stops being a guess in a fire risk assessment. An assessor asking about the water supply wants to know when it was last looked inside, and “we have always assumed it is fine” is a different answer from a dated inspection with photographs.

Losing level: leak, or something else

A dropping level has more than one cause and they are separated by a short test rather than by opinion.

Evaporation is real on an open or vented tank and it is slow. A passing float valve looks like the opposite problem, a level that never quite settles. A genuine leak, at a seam, a fixing or the suction connection, shows as a steady loss that does not recover.

The way to tell them apart is to measure the drop against time with the refill isolated. That is a short piece of work and it decides what you are actually looking for, which saves the far more expensive exercise of hunting a leak that was never there. What it must not become is a reason to leave the refill isolated, because a tank with its automatic supply shut off is a tank on a timer.

When a tank is replaced rather than repaired

Three cases, and only one of them is really about the tank.

The steel has gone rather than the coating. A coating can be repaired and recoated. Once corrosion has taken the plate itself, patching buys a season and the next failure is somewhere adjacent.

A sectional tank is leaking faster than it can be sealed. Bolted sectional tanks are made to be repairable, and they are worth repairing until the point where every visit finds another seam.

The building changed and the tank did not. This is the common one, and it is a design question rather than a maintenance one. Racking that went up, a change of stored goods, or an extension can move the hazard classification, and a higher density over a larger area needs more stored water for the same duration. The tank is not faulty. It is simply no longer the tank the system needs, which is the same problem warehouse systems run into when the storage height moves.

A tank replacement is usually the moment to look at the pumps as well. A tank and its pump set are one water supply rather than two items, and replacing one while leaving the other at the end of its life tends to mean doing the same job twice.

What to have ready before anyone quotes

The survey goes faster, and the quotation is firmer, if you can find these:

• The commissioning documentation and any as-fitted drawings, which give the design capacity and duration
• The hazard classification the system was designed to, if it is recorded anywhere
• The weekly level readings, however patchy
• Any previous internal inspection, with dates
• Whether the tank is divided or single, which decides how the work has to be staged

If none of that exists, that is a starting position rather than an obstacle, and it is a common one. It is also the argument for getting the first inspection documented properly: it is the record every subsequent conversation, with an insurer, an assessor or a contractor, gets measured against. What that evidence needs to look like is covered in what your insurer wants to see.

If you want the tank looked at properly rather than looked at from outside, that is sprinkler tank service and maintenance, and the survey is free.