BS 7593-2006 Code of practice for treatment of water in domestic hot water central heating systems《家用热水中央供暖系统水处理实用规程》.pdf

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1、BS 7593:2006Code of practice for treatment of water in domestic hot water central heating systemsICS 13.060.01; 91.140.10NO COPYING WITHOUT BSI PERMISSION EXCEPT AS PERMITTED BY COPYRIGHT LAWBRITISH STANDARDPublishing and copyright informationThe BSI copyright notice displayed in this document indic

2、ates when the document was last issued. BSI 2006ISBN 0 580 48247 2The following BSI references relate to the work on this standard:Committee reference CII/62Draft for comment 05/30133509 DCPublication historyFirst published, December 1992Amendments issued since publicationAmd. no. Date Text affected

3、BS 7593:2006, BSI 2006 iBS 7593:2006ContentsForeword ii1 Scope 12 Objectives of system water treatment 13 Causes of problems in central heating systems 14 Treatment of water 45 Cleansing 56 Special treatments boiler cleansing as a separate process 107 Inhibition 10Bibliography 12List of tablesTable

4、1 Electrochemical series 2Table 2 Classification of waters 3Summary of pagesThis document comprises a front cover, an inside front cover, pages i and ii, pages 1 to 13 and a back cover.BS 7593:2006ii BSI 2006ForewordPublishing informationThis British Standard has been prepared by Technical Committee

5、 CII/62, Treatment of water for boilers, and supersedes BS 7593:1992, which is withdrawn.Information about this documentThis code gives guidance on the preparation of the primary circuit of wet central heating systems prior to initial commissioning or re-commissioning following major remedial work (

6、e.g. boiler replacement) and ongoing water treatment to ensure continued efficiency in operation. Although intended to cover domestic systems with up to 45 kW heat input, it is recognized that the recommendations are also applicable to similar systems of higher heat output.The purpose of this standa

7、rd is to ensure awareness of potential problems, and of the remedies required to maintain efficiency and maximize the life of the system.Use of this documentAs a code of practice, this British Standard takes the form of guidance and recommendations. It should not be quoted as if it were a specificat

8、ion, and particular care should be taken to ensure that claims of compliance are not misleading.Any user claiming compliance with this British Standard is expected to be able to justify any course of action that deviates from its recommendations.Contractual and legal considerationsThis publication d

9、oes not purport to include all the necessary provisions of a contract. Users are responsible for its correct application.Compliance with a British Standard cannot confer immunity from legal obligations.In particular, attention is drawn to the following statutory regulations.The Water Supply (Water F

10、ittings) Regulations 1999 1The Health and Safety at Work, etc. Act 1974 2The Control of Substances Hazardous to Health Regulations 2002 3 The Building Regulations 2000, as amended (particularly Approved Document L1) 4 The Building (Scotland) Regulations 2004 5The Building Regulations (Northern Irela

11、nd) Statutory Rules 2000 6The Trade Effluent (Proscribed Processes and Substances) Regulations 1989 7The Hazardous Waste (England and Wales) Regulations 2005 8The Special Waste (Scotland) Regulations 1996 9The Hazardous Waste (Wales) Regulations 2005 10The Hazardous Waste Regulations (Northern Irela

12、nd) 2005 11 BSI 2006 1BS 7593:20061 ScopeThis British Standard gives recommendations on best practice for the preparation of the primary circuit of wet central heating systems prior to initial commissioning or re-commissioning following major remedial work (e.g. boiler replacement) and the ongoing w

13、ater treatment to ensure continued efficiency in operation.NOTE For specialized systems, such as those containing solar thermal panels and heat pumps, refer to the manufacturers instructions.2 Objectives of system water treatmentThe objectives of water treatment may be summarized as follows:a) to mi

14、nimize 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; ande) to restore energy efficiency of the system where appropriate.3 C

15、auses of problems in central heating systems3.1 CorrosionCorrosion is generally a process of oxidation of metals which, in a central heating system, can result in restriction of circulation and/or failure of components, for example perforation of radiators. Corrosion in a central heating system is p

16、romoted by the following.a) Poor system design and/or installation, e.g. oxygenation of the circulating water by excessive make-up, pumping over or sucking down at the open vent, incorrect sizing of the cold feed pipe, inadequately sized or incorrectly installed feed and expansion cistern.b) Ingress

17、 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 most susceptible to corrosion (see Table

18、1). However, the position of some metals (especially aluminium and stainless steels) in Table 1 can change depending on conditions, e.g. pH.BS 7593:20062 BSI 2006Table 1 Electrochemical seriesd) Deleterious materials, e.g. flux residues, jointing compounds, residual cleanser, and/or foreign matter r

19、emaining after inadequate cleaning or flushing, or oils and greases left over from the manufacturing process of system components.e) Certain characteristics of the system supply water. For example, some naturally soft waters with low alkalinity and low pH are generally more corrosive towards system

20、metals. Some artificially softened waters, particularly those from softeners not conforming to BS EN 14743, might contain high levels of aggressive anions (e.g. chloride) that can promote pitting of ferrous and non-ferrous metals, e.g. stainless steel and aluminium.f) Other sources of chloride, whic

21、h include fluxes, hydrochloric acid and washing-up liquids.g) The presence of anaerobic bacteria resulting in acidity.h) Incorrect, unsuitable or poorly applied or maintained water treatment products.i) The formation of corrosion products deposited or plated out in the system.Corrosion is most aggre

22、ssive when there is a steady source of supporting reactant, e.g. fresh water make-up or re-aeration of the system water.NOTE Further information on corrosion in re-circulating systems is given in BS EN 14868.3.2 Scale and sludge“Hardness” is the term which describes the concentration of calcium and

23、magnesium salts dissolved in water, usually expressed as calcium carbonate (CaCO3) equivalence. Waters may be classified as given in Table 2.Metal ActivityCopper More noble (less active)BrassGunmetalStainless steelLead-free solderTinLeadSteelCast ironGalvanized ironAluminium alloys Less noble (more

24、active) BSI 2006 3BS 7593:2006Table 2 Classification of watersThe hardness due to bicarbonate is termed “temporary” hardness. When water is heated, temporary hardness forms insoluble calcium carbonate. Scaling is the precipitation of hardness salts and/or corrosion debris to form adherent deposits o

25、n surfaces within the system. This reaction is most likely to take place in the hottest part of the system, usually the heat transfer surface in the boiler. Deposited calcium carbonate is usually referred to as lime scale. While it is most likely to form in the boiler heat exchanger, it can also acc

26、umulate elsewhere in the system as 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 the higher temperatures of the heat exchanger surfaces their solubility reduces rapidly and pr

27、ecipitation can occur.The potential for scale formation is greatest in those hard water areas of the United Kingdom where the bicarbonate alkalinity is high. Scale formation will be most pronounced if there is a high rate of water loss.If calcium carbonate is allowed to form in the heat exchanger th

28、is will have a detrimental effect on boiler heat transfer efficiency. Scale and/or sludge can also be responsible for boiler noise.Corrosion products will usually give rise to sludge, e.g. magnetite, in low flow areas downstream of the point of formation, but small particles can be carried round the

29、 system and can deposit, e.g. in the boiler heat exchanger, generating a scale. Where lime scale is formed in the presence of corrosion products a very hard mixed scale might result.System areas most prone to failure due to fouling by scale or sludge include the boiler heat exchanger and circulator

30、pump. Fouling can also restrict flow through components such as thermostatic radiator valves, zone valves, drain valves, etc., and any parts of the system where there is a low water velocity or small pipe diameters (e.g. in radiators or fan convectors and microbore circuits).Sludge accumulation will

31、 result in poor circulation and a decrease in system efficiency. DesignationTotal hardness (as CaCO3)mg/lSoft 0 to 50Moderately soft 50 to 100Slightly hard 100 to 150Moderately hard 150 to 200Hard 200 to 300Very hard Over 300BS 7593:20064 BSI 20063.3 Microbiological contaminationMicrobiological orga

32、nisms ranging from simple bacteria to fungal and yeast spores can cause problems when they enter a central heating system. Common sources of these organisms include feed and expansion cisterns not installed in accordance with BS 5449 (partially superseded by BS EN 12828, BS EN 12831 and BS EN 14336)

33、, the atmosphere, contaminated feed and expansion cisterns, andnon-proprietary cistern lids and insulation.The greatest potential for microbiological proliferation exists in the feed and expansion cistern of open-vented systems. Here the temperature conditions are more favourable for bacterial growt

34、h and there is contact with the air. Aerobic bacteria, fungi and slimes which have developed in the cistern can enter the system with the make-up water and produce debris. Such debris can cause blockages and is liable to foul the boiler heat exchanger. Under-floor heating and other systems which ope

35、rate at lower temperature (below 60 C) can also be prone to microbiological fouling. Even the high temperature in the boiler heat exchanger might not be sufficient to kill all micro-organisms.Anaerobic bacteria can thrive in both open and sealed systems fouled with corrosion and other debris, beneat

36、h deposits where the temperature might be lower and there is an absence of oxygen. This can give rise to microbiological corrosion of ferrous metals. 4Treatment of water4.1 GeneralIn most cases, the quality of the water used in the central heating system is determined by supply to the premises and t

37、his will vary across the United Kingdom. Water treatment should be applied to all primary systems except for single feed indirect hot water cylinders. Consideration should be given as to whether the water is hard or soft, as this might influence the approach to water treatment and the choice of prop

38、rietary product. Refer to the manufacturers specifications.4.2 External Naturally soft waters of low alkalinity or those supplied via abase-exchange resin softener have an increased potential for corrosion and, if they are to be used in any central heating system, a corrosion inhibitor specifically

39、formulated for the purpose1)should be added and properly maintained.1)Check inhibitor product specification. BSI 2006 5BS 7593:20064.3 InternalTo minimize the likelihood of corrosion, scale and sludge formation, the system water in any system should be treated with an inhibitor. Before the inhibitor

40、 is added to the system, the first step should be to render the system in a condition free from foulants. Foulants in new systems can include corrosion debris, flux residues, grease, installation debris, metal swarf, solder pieces, stamping oil and welding rod. Existing systems might also contain bl

41、ack magnetite sludge and scale. If microbiological fouling is found within a system (often detected by the presence of organic slime or a foul odour), the system should be disinfected using a proprietary disinfectant and a biocide added for ongoing protection.Corrosion inhibitors should not be used

42、as a substitute for correct system design, installation and maintenance.5Cleansing5.1 General considerationsBefore cleansing, the system should be examined to determine the system configuration and the age and overall condition of components, in order to ascertain the cleansing regime required. For

43、example, the procedure could remove corrosion debris covering pin-holes in radiators and this could result in leaks.If there is any doubt as to whether a system will withstand any cleansing methodology, then replacement or repair of relevant components will be necessary before continuing. A choice o

44、f cleansing methodologies is available. In most cases involving major work, e.g. commissioning of new systems or boiler change, this will necessitate a chemical cleanse, followed by a fresh water flush, before an inhibitor is applied. For minor work, e.g. change of a circulator pump or individual ra

45、diators, it might be sufficient to isolate and change the component, but always check that the inhibitor protection meets the manufacturers specification whenre-commissioning.NOTE 1 It is essential that pumped circulation is available throughout the system. A fresh water flush alone, whether hot or

46、cold, is not adequate procedure.NOTE 2 Filters which collect magnetite may be beneficial as part of the flushing procedure. These should be used with a chemical cleanser to mobilize the magnetite and aid its transport to the filter. The cleanser manufacturers specification and usage instructions sho

47、uld be studied and product selection made accordingly. Unless the manufacturers instructions state otherwise, products from different manufacturers or different products from the same manufacturer should not be mixed.Adequate time has to be allowed to complete the cleansing procedure.BS 7593:20066 B

48、SI 20065.2 Cleansing and flushing methodologiesThere are three cleansing and flushing options, one of which should be applied. 1) Mechanically-assisted powered cleanse and flush (powerflushing) (see 5.3). 2) Mains pressure cleanse and flush for sealed systems and open-vented systems with the feed an

49、d vent temporarily capped-off (see 5.4).3) Cleanse and flush using gravity, with the assistance of a circulator pump (see 5.5).Powerflushing is most effective as this produces a more thorough clean, but the boiler manufacturers instructions should be checked to establish whether powerflushing is acceptable. Powerflushing is the most effective method of cleansing existing systems, especially those containing a high level of black magnetite sludge. With all methodologies, reversing the flow will help to remove debris