BS 7074-2-1989 Application selection and installation of expansion vessels and ancillary equipment for sealed water systems - Code of practice for low and medium temperature hot wa.pdf

1、British Standard BS 7074: Part 2: 1989 UDC 621.642.3 : 697.43 : 001.4 : 620.1 : 006.76(083.75) Application, selection and installation of expansion vessels and ancillary equipment for sealed water systems Part 2. Code of practice for low and medium temperature hot water heating systems Utilisation,

2、choix et installation des vases dexpansion et equipements auxiliaires des circuits deau fermes Partie 2. Systemes de production deau chaude a moyenne temperature pour le chauffage Code de bonne pratique Anwendung, Auswahl und Einbau von AusdehnungsgefaBen und Zubehor fur Systeme mit geschlossenem Wa

3、sserkreislauf Teil 2. Leitfaden fur Warmwassererzeugungssysteme fur niedrige und mittlere Temperaturen BS 7074 : Part 2 : 1989 Foreword This British Standard code of practice has been prepared under the direction of the Refrigeration, Heating and Air Conditioning Standards Policy Committee. The code

4、 complements BS 4814 and gives recommendations in its three Parts for the installation of expansion vessels in domestic heating and supply systems (Part 1 ); low and medium temperature hot water heating systems (Part 2); chilled water and condenser systems (Part 3); and boosted hot water supply syst

5、ems*. The code deals with the work involved in the general planning, designing and installation of the various systems when the expansion and contraction of the system water is catered for in a sealed diaphragm type vessel. In all other respects the customary design process should be followed for th

6、e appropriate system. *Planned. , BS 7074 : Part 2 : 1989 Contents Foreword Committees responsible Page Inside front cover Back cover Section one. General 1 Soo 2 2 Definitions 2 3 Symbols, designations and units 3 4 Connection of expansion vessel to the system 3 5 Testing and commissioning 6 6 Main

7、tenance 6 7 Workmanship 6 Section two. Low temperature systems 8 Design considerations 9 Application 10 Ancillary equipment Section three. Medium temperature systems 7 7 9 11 Design considerations 14 12 Application 14 13 Ancillary equipment 15 Appendix A Examples of recommended procedure for the cal

8、culation of the size of expansion vessels 19 Tables 1 Symbols, designations and units 3 2 Expansion percentages (e) for various flow temperatures (t1) from 4 C initial temperature 8 3 Pressure allowance 15 Figures 1 System pressure variations: recommended arrangement 4 2 System pressure variations:

9、neutral point at pump discharge 4 3 System pressure variations: neutral point remote from pump 5 4 Low temperature hot water system 11 5 Multiple boiler installations for low temperature and medium temperature applications 12 6 Typical expansion percentage of water mixed with anti-freezing agent ver

10、sus temperature 13 7 Medium temperature hot water system 18 BS 7074 : Part 2 : 1989 Code of practice. Section one Section one. General 1 Scope This Part of BS 7074 gives recommendations on the application of expansion vessels complying with BS 4814 and having a maximum pressure of 7 bar* for use in

11、hot water heating systems. It includes description, design considerations and application. Sections one and two cover low temperature systems in which the flow temperature does not exceed 100 C. Sections one and three cover medium temperature systems in which the flow temperature does not exceed 120

12、 C. Recommendations are also given on: (a) the application and use of ancillary equipment and (b) testing, commissioning and maintenance. NOTE. The titles of the publications referred to in this standard are listed on the inside back cover. 2 Definitions For the purposes of this Part of BS 7074 the

13、following definitions apply. 2.1 charging pressure. The initial pressure to which the gas/air side of the vessel is charged which is equal to the initial system design pressure. 2.2 closed valve head. The maximum head developed by a pump under a no-flow condition. 2.3 design acceptance factor (a). T

14、he ratio of the volume of water due to expansion, to total vessel volume. 2.4 diaphragm (or membrane). The flexible means by which the chamber of an expansion vessel is partitioned to maintain separation between the expanding hot water and the gas or air which in consequence becomes com pressed. It

15、may be either a literal diaphragm clamped between two parts (or halves) of the vessel, or a bag located by its mouth which is secured to the point of the water connection to the vessel. 2.5 expansion percentage (e). The expansion percentage increase in volume when water is heated to maximum design s

16、ystem temperature. 2.6 expansion volume (Va ). The increase in volume of water due to expansion of water when heated to the maximum design system temperature. 2.7 final system design pressure (Pf). The pressure occurring at the mid-height of the expansion vessel at the maximum design system temperat

17、ure. *1 bar= 10 N/m = 100 kPa. 2 2.8 initial cold water fill temperature (t;). The basic reference temperature is taken to be 4 C. 2.9 initial system design pressure (P;). The pressure occurring at the mid-height of the expansion vessel at cold fill. This is equal to the static height pressure plus

18、the pressure margin. 2.10 lowest working pressure component (LWPC). The component having the lowest working pressure in the system. 2.11 maximum acceptance factor (A). The ratio of maximum acceptance volume of the system to total volume. 2.12 maximum acceptance volume (V). The volume of water which

19、the vessel may be allowed to contain. 2.13 maximum vessel temperature. The maximum water temperature at which the vessel may be allowed to operate continuously. 2.14 maximum vessel working pressure. The maximum pressure that the vessel may be allowed to contain in operation. 2.15 pressure depth (P d

20、). The vertical distance between the component being considered and the mid-height of the expansion vessel situated above it. 2.16 pressure margin (P5). The additional pressure imposed on the circuit to prevent water vaporizing or boiling and/or to exclude air from the system at the highest point. 2

21、.17 safety valve set pressure (Pmaxl- The pressure at which the safety valve is set for operation. 2.18 static height pressure (Ph). The pressure created by the column of water between the uppermost part of the heating circuit and the mid-height of the expansion vessel. 2.19 system flow temperature

22、(tf). The maximum designed temperature of the water circulating in the system. 2.20 total system volume (V5). The total volume of water in the complete system. 2.21 total vessel volume ( Vt). The volume occupied by gas/air when the vessel is empty of water. 2.22 pressure allowance (P a). The pressur

23、e allowance added to the pressure at the highest point in the system to prevent flashing to steam. , , 3 Symbols, designations and units The symbols for physical quantities, their designations and units used in this code of practice are given in table 1. Table 1. Symbols, designations and units Symb

24、ol Designation Unit A Maximum acceptance factor -a Design acceptance factor -e Expansion percentage -pd Pressure depth bar gauge Pt Final system design bar gauge pressure ph Static height pressure bar gauge pj Initial system design bar gauge pressure Pmax Safety valve, set pressure bar gauge Ps Pres

25、sure margin bar gauge tf System flow temperature oc ti Initial cold water fill oc temperature v Maximum acceptance volume L Va Expansion volume L Pa Pressure allowance bar gauge v. Total system volume L vt Total vessel volume L 4 Connection of expansion vessel to the system 4.1 General The neutral p

26、oint of the system is the point of connection of the pipework to the expansion vessel. It is recom mended that the expansion vessel is in the system return pipework close to the heat generator (exchanger). The fill ing position should be into the expansion pipework. The point of connection of the ex

27、pansion vessel (s) into the system having been clearly defined, the physical location of the vessel can be anywhere. 4.2 Location of pump relative to expansion vessel 4.2.1 General. When the pump in the heating system is not operating the only pressure existing varies between Pi and P1 depending upo

28、n the water temperature. When the pump is started the pressure within the system will change from its original condition to a completely new set of pressure conditions. This new pressure condition is defined as the pump head and is indicated by the pressure drop between the suction and discharge of

29、the pump. The pres sure at the pump discharge will be higher than the system pressure at the pump suction by an amount equal to the-3 BS 7074 : Part 2 : 1989 Section one pump head. The pressure drop due to friction within the system will gradually decrease the system pressure from that existing at t

30、he pump discharge to the lower pressure existing at the pump suction. With the pump running, the system pressure will change generally as illustrated in figures 1, 2 and 3. Attention is drawn to the point of no pressure change being relative to the point of connection of the expansion vessel into th

31、e system. The pump only has the ability to create the pressure difference across itself and therefore there is no reason why the full pump head should not appear on its suction side as shown in figure 2. It will be noted that the position of no pressure change (the neutral point) is different from t

32、hat shown in figure 1. By dictating the point of no pres sure change (the neutral point). system pressure changes by pump operation can be controlled. In figure 3, where the neutral point has been changed once again, a new set of pressure characteristics pertain. 4.2.2 Neutral point. The point of no

33、 pressure change, or the neutral point, is that point where the expansion vessel is connected to the system. This is because the air gas cushion in the expansion vessel follows basic gas laws and the change in gas pressure has to be accompanied by a change in gas volume. A change in gas volume in th

34、e vessel has to be accompanied by a change of water volume in the vessel. It follows that a change in water volume in the vessel has to be accompanied by a change of water volume in the system; since water is incompressible, pump operation cannot increase or decrease the system water volume, therefo

35、re pump operation cannot change vessel pressure. Since vessel pressure cannot change due to pump operation, the junction of the expansion vessel with the system has to be a point of no pressure change regardless of whether or not the pump operates. 4.2.3 Neutral point at pump suction. Figure 1 illus

36、trates a system in which the neutral point is located at the pump suction. The pressure changes caused by pump operation are therefore added to the original system pressure. Because all system pressure changes are additive and positive, there is no possibility of pump cavitation or air being drawn i

37、nto the system. 4.2.4 Neutral point at pump discharge. Figure 2 shows a system where the point of no pressure change is located at the pump discharge. All pressure changes caused by pump operation are subtracted from the original pressure and are of a negative type; under certain conditions this can

38、 cause unsatisfactory results. If the pressure decrease below the original pressure is great enough, system pressure could drop to the boiling point, circulation will be unstable and pump cavitation can occur with a resultant pump failure. If the pressure drops below atmospheric pressure, air can be

39、 sucked in at the vent, air pockets are created and circulation blocked. It is strongly recommended therefore that the neutral point should not be located at the pump discharge. BS 7074: Part 2: 1989 Section one Full pump head positive at pump discharge System pressure characteristic pump operating

40、pressure positive r-. !Boiler l I , I “f .-,1-E xpansion vessel Figure 1. System pressure variations: recommended arrangement I I “,-, I I ,_) I L-.-.1 I I I I I I I I I I Original static pressure ( pump not operating ) Expansion-+ vessel Pump head appears as decrease in pump sue tion pressure Figur

41、e 2. System pressure variations: neutral point at pump discharge 4 Original pressure (pump not operating ) System pressure characteristic after starting pump ( negative pressure below system pressure l ,. System pressure :-:=- characteristic pu_P operating pressure part positive part negative Pump h

42、ead BS 7074: Part 2: 1989 Section one Neutral point I I . ,., I I I I L-;:-.J I I I I I I I I I I Original pressure ( pump not operating ) Figure 3. System pressure variations: neutral point remote from pump 4.2.5 Neutral point remote from pump. By locating the expansion vessel at some distance from

43、 the pump, the system pressure change due to pump operation would appear as shown in figure 3. The expansion vessel junction would be the neutral point and all pressure system changes would be referred from that point. Pressure gauge readings at the pump suction and discharge would show partially po

44、sitive and partially negative readings with reference to the original non-operating pressure condition. The pressure changes would be a function of the system pipe friction pressure-drop between the pump and the vessel. When using expansion vessels located at some distance from the pump, perhaps adj

45、acent to the hot water cylinder or in the roof space, it is suggested that the pump should be on the flow from the boiler, providing minimum friction pressure-drop between the vessel and the boiler inlet. It is emphasized that the problems created in systems where the neutral point is such that the

46、pump head is all under suction (figure 2). are more likely to occur in higher head pump systems. Therefore in dealing with micro-bore systems it 5 is important that the relative position of the neutral point and pump be fully understood in order that the most satisfactory system operation can be eff

47、ected. 4.2.6 Recommended arrangement. If the expansion vessel is placed at the suction side of the pump, the suction pressure will not change regardless of whether or not the pump is operated; because the pump suction cannot change, the pump discharge pressure has to change. The pump differential he

48、ad is then manifested as a positive increase in pressure at the pump discharge as shown in figure 1. The pressure increase due to pump head will then decrease around the system, depending upon the various friction losses in the system, until at the pump suction the original pressure is obtained. Bec

49、ause the pump suction pressure is unchanged due to pump operation it is suggested that the boiler be located at the pump suction; this is the recom mended relative arrangement of pump, boiler and expansion vessel within a system. BS 7074 : Part 2 : 1989 Section one 5 Testing and commissioning 5.1 Pressure testing the system Expansion vessels, pressure switches, safety valve(s) and fill unit have to be isolated prior to hydraulic pressure testing. This is to ensure that these components are not damaged by over pressurization. 5.2 Commissioning The recommended