How to safely stop and start circulation

HAZARD WARNING!

There is a risk of the generation and emission of extremely hazardous chemicals when power to the circulation pumps is cut (deliberately or because of power failure). A system should NEVER be restarted following a failure of the circulation system while there are people in the pool. If there is a concern that there may have been inadvertent mixing of chemicals within the pipework of the circulation system, the fire service should be contacted and the circulation and chemical dosing systems left stopped.

Safety Issues When Stopping/Starting Circulation

The video covers loss or prime, chemical injection safety issues and a case study of where it all went horribly wrong.

Here are the slides used in the video above.

In disinfecting systems which incorporate automatic chemical dosing (including those using chlorine, (unless the chlorine injection system is negative pressure and the chlorine gas line has a vacuum-operated regulator), the following precautions should be considered as appropriate:

  • interlocking the dosing system electrically with the water circulating pumps, to prevent the continuation of dosing, should the pumps fail
  • incorporating into the circulation system a fail-safe, 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
  • siting the pool water circulation pumps below the level of the pool water, to minimise the risk of the pumps losing their prime
  • locating an additional sampling point close to the chemical injection point for alarm purposes. (Automatic dosing systems operate by sampling the water and activating or stopping the dosing pumps as required, for example following a change in bather loads.) Disinfectant dosing should cut off when the system fails
  • 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)
  • designing dosing lines so that they are protected from damage, and so that they cannot, inadvertently, be connected the wrong way round
  • displaying notices warning of the risks of mixing calcium/sodium hypochlorite and acids, and the importance of maintaining pool water circulation during dosing
  • ensuring that pressurised chemicals in the line are safely relieved before breaking the delivery line for maintenance work to be carried out
  • 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.

The operator should always check that the chemical dosing equipment has ceased to dose chemicals when the main circulation has stopped, or simply turn it off manually as routine.

Most commercial systems have an interlock. There is usually a flow switch incorporated into the monitoring equipment that stops chemical dosing when main circulation stops. However, it may not be fail-safe. In which case, chemical dosing could continue when the main circulation is stopped.

If power is cut (or the plant fails totally for some other reason), chlorine gas can be released because when the power is returned, the circulation pumps do not restart on their own (they usually need to be restarted manually). However, the chemical dosing pumps may well start pumping again on their own. If both chlorine and acid pumps re-start, acid and chlorine may mix in the pipework system.