BS 7593:2006 pdf free.Code of practice for treatment of water indomestic hot water central heating systems.
BS 7593 This British Standard gives recommendations on best practice for thepreparation of the primary circuit of wet central heating systems priorto initial commissioning or re-commissioning following major remedialwork (e.g. boiler replacement) and the ongoing water treatment toensure continued efficiency in operation.
NOTE For specialized systems, such as those containing solar thermalpanels and heat pumps,refer to the manufacturer’s instructions.
The objectives of water treatment may be summarized as follows:a) to minimize corrosion of the system metals;
b) to inhibit the formation of scale and sludge;
c) to inhibit the growth of microbiological organisms;
d) to maintain the engineering design specifications and energy efficiency of the system; and
e) to restore energy efficiency of the system where appropriate.
Corrosion is generally a process of oxidation of metals which, in acentral heating system, can result in restriction of circulation and/orfailure of components, for example perforation of radiators.Corrosionin a central heating system is promoted by the following.
a) Poor system design and/or installation, e.g. oxygenation of the circulating water by excessive make-up, pumping over or suckingdown at the open vent, incorrect sizing of the cold feed pipe,inadequately sized or incorrectly installed feed and expansioncistern.
b) Ingress of air, e.g. at mechanical or poorly soldered joints. Some plastics pipe allows the ingress of oxygen into the system water(see BS 5955-8).
c)Electrolytic (galvanic) action between dissimilar metals.Metals/alloys which are less noble (more active) are the mostsusceptible to corrosion (see Table 1).However, the position ofsome metals (especially aluminium and stainless steels) in Table 1can change depending on conditions, e.g. pH.
The hardness due to bicarbonate is termed “temporary” hardness. Whenwater is heated, temporary hardness forms insoluble calcium carbonate.Scaling is the precipitation of hardness salts and/or corrosion debris toform adherent deposits on surfaces within the system.This reaction ismost likely to take place in the hottest part of the system, usually theheat transfer surface in the boiler. Deposited calcium carbonate isusually referred to as lime scale. While it is most likely to form in theboiler heat exchanger, it can also accumulate elsewhere in the systemas sludge, often at places of low flow.
Under normal operating conditions non-bicarbonate or “permanent”hardness salts, e.g. calcium sulfate, will remain in solution, but at thehigher temperatures of the heat exchanger surfaces their solubilityreduces rapidly and precipitation can occur.
The potential for scale formation is greatest in those hard water areasof the United Kingdom where the bicarbonate alkalinity is high. Scaleformation will be most pronounced if there is a high rate of water loss.If calcium carbonate is allowed to form in the heat exchanger this willhave a detrimental effect on boiler heat transfer efficiency. Scale and/orsludge can also be responsible for boiler noise.BS 7593 pdf download.