Automatic Dosing Systems
Commercial swimming pools should have a system installed to automatically dose the disinfection, pH control and coagulation chemicals into the pool water circulation system.
Manually dosing chemicals (sometimes referred to as ‘hand-dosing’) is a hazardous activity that can be easily avoided by the installation, use and maintenance of such systems. Automatic dosing systems also provide a much more reliable level of control over the pool chlorine and pH levels.
Chemical dosing should be continuous, 24 hours a day. The automatic dosing system should be backed up by regular monitoring and verification.
Chemical tanks
Day tanks are vessels for holding the chemical solution, from where they are pumped into the circulation system, usually via an injector. They should be constructed from UV-stabilised polyethylene and ideally be fitted with:
- High and low level indicators and alarms
- Overflow pipe
- Water inlet from header tank
- Drain valve
- Agitator
If the plant is to be shut down for longer than 60 hours, valves in filling lines between the day and bulk tanks should not be closed, as decomposition products might otherwise build up. After such a shutdown, the whole of the dosing system should be flushed through gently with low-pressure water.
Chemical pumps
Following correct commissioning, calibration of the chemical pumps should not be necessary. However, it is important that a robust programme of monitoring, both automatic (via the probes integrated into the control panel) and manual (via the DPD1, DPD3 and phenol red reagent tests), is established and maintained.
The pumps pictured are very common in the industry. Click on the icons to learn more about them.
“We have ‘circulation feeders’ not tanks and pumps, how do they work?”
Pools that have the disinfectant pumped in from a chemical tank won’t have these, but circulation feeders are items of equipment that some types of pools use to take dry chemicals and introduce them into the pool. They are mainly used for disinfectants. There are two types:
These are designed so that water flowing through them physically erodes material from a dry tablet; this subsequently dissolves in the water circulation. Calcium hypochlorite (see note 1 below) and trichlorinator (see note 2 below) feeders can be of this type.
These allow water to dissolve material from the tablet directly. Brominators (see note 3 below) are of this type.
Notes:
- Calcium hypochlorite is a type of chlorine-based disinfectant that DOES NOT include cyanuric acid.
- ‘Trichlor’ is a type of chlorine-based disinfectant that DOES include cyanuric acid.
- Bromine is a type of disinfectant that is sometimes used instead of chlorine.
On most types of circulation feeder the water supply to the feeder is taken from the pressure side of the main circulation pumps and returns to the suction side of the pumps. The water passes through the feeder and is returned to the main circulation line. This has the advantage that it fails safe if the water circulation fails. Circulation feeders may be fitted with automatic controls, which will help to prevent overdosing.
Circulation feeder devices should only be used for the purpose, and chemicals, for which they were designed. Calcium hypochlorite, chlorinated isocyanurates and bromochlorodimethylhydantoin (BCDMH) all have specific feeders and it is vitally important that they are only used for the chemical for which they are designed.
WARNING!
Using the wrong chemical in a feeder can result in the formation of dangerous gases, fire or explosion. It is very important that chemicals are not mixed in closed containers/feeders as this may cause explosions.
Any closed vessels used for feeding chemicals need to be safeguarded against pressure accumulation and should be fitted with a pressure relief valve.
Circulation feeder devices should be emptied of chemicals if the pool circulation system is to be closed down for a period of time.
Inspection and maintenance
The entire dosing system should be inspected on a weekly frequency. The inspection should be recorded and any issues dealt with as a priority. Things to look for include:
- Build-up of residue around joints and injection points
- Split chemical feed lines (or areas where a split would likely occur, such as twists and kinks etc.)
- Build-up of sediment in chemical feed lines and chemical storage tanks
- Missing or damaged sheathing of chemical feed lines
It should be noted that the above list is not exhaustive and additional items may need to be added, according the nature and operation of the system.
Preparing chemicals for dosing
- Chemicals should always be added to water and never the other way around when preparing solutions.
- Non-liquid chemicals should be kept dry until dissolved in water.
- Calcium hypochlorite should be kept away from all other chemicals in its preparation for dosing.
- Calcium hypochlorite should be dissolved in water at a ratio of 1:33
- Sodium hypochlorite can be dosed at its delivery concentration (10-15%)
- If hydrochloric acid is not being dosed direct from a container, dilution should be introduced by filling the day tank with a known quantity of water, adding a known quantity of concentrate, and mixing thoroughly.
- Any sludge formed from the incomplete dissolving of chemicals should be cleared periodically.
Chemical Dosing Precautions
A flow measuring device capable of detecting a reduction or cessation of flow and interlocking this with the dosing pumps to prevent the continuation of dosing in the event of flow stoppage.
Most automatic dosing systems will have this as standard but Pool Plant Operators should not assume they are fail-safe. Always check that the chemical dosing system has definitely stopped when cutting power to the circulation pumps, e.g., when backwashing.
Siting the calcium/sodium hypochlorite and acid injection points as far apart as possible (preferably a minimum of 1 m); ideally, the hypochlorite injection point should be located before the filter and the acid dosing point after the filter and heat exchanger (although, this is not possible if using UV or ozone disinfection systems)
Ensuring that pressurised chemicals in the line are safely relieved before breaking the delivery line for maintenance work to be carried out.
Many systems have automatic pressure relief (see graphic). Check your system before attempting and disconnections.
Pipelines and injection points can become blocked by calcium deposits. Removal is usually carried out with acid; therefore the pipes will need to have been flushed out, the acid then added to descale, flushed out again and released for maintenance.
Displaying notices warning of the risks of mixing calcium/sodium hypochlorite and acids, and the importance of maintaining pool water circulation during dosing.
Designing dosing lines so that they are protected from damage, and if possible, so that they cannot, inadvertently, be connected the wrong way round.
