Double Regulating Valve 4g2236

1

INTRODUCTION

This guide introduces the Cimberio CIM 737, CIM 3737 Commissioning Set – a combined double regulating valve and separate flow measurement device which provides high accuracy flow balancing and measurement across all valve settings. The commissioning sets are suitable for both heating (LPHW) and cooling applications at working pressures up to 20 bar. Valve sizes between 15-50mm are available as “CR” brass oblique pattern globe valves; valves from 65mm-1000mm are available as cast / ductile iron butterfly valves.

The main features of the Cimberio commissioning set are as follows: • an orifice type flow measurement device permitting high accuracy flow measurement to within ±5% regardless of valve setting (CIM 721). • a metal to metal thread locking mechanism so that valve settings can be accurately locked enabling the valve to be closed and re-opened to its exact pre-set position. • a flip up cap housing individual Allen keys for the locking of valve positions. • a valve position indicator scale which can be read from any angle. • an EPDM lined valve plug providing tight shut-off for isolation purposes. The valves have been tested by BSRIA in water containing high air and dirt levels (see section 7). The results showed an excellent tolerance to such conditions, providing confidence that the valves will retain a high level of accuracy and repeatability of flow measurement under the worst of system conditions.

2

2.1

CIM 727

DOUBLE REGULATING VALVE

• Reduced Air & Dirt Accumulation • Accurate Allen Key Locking

TESTED FOR PERFORMANCE & RELIABILITY

• Non Rising Handle

VALVE IDENTIFICATION DISK UNDER HANDLE CAP

REPLACEABLE HANDLE

HEAT AND IMPACT RESISTANT NYLON HANDLE

CLEAR 360º READING

SMOOTH OPERATION

360° RE-SETTABLE INDEX COLLAR

COMPACT VALVE CHAMBER

DZR BRASS

3

CIM 737

2.2

DOUBLE REGULATING SET

DN 1/2 - DN 2”

Body Bonnet Stem Gasket Shutter O-Ring Index Seeger 1/10 turn index Turn index

CC752S CW602N CW602N EPDM CW602N HNBR Hostaform Bronze Hostaform Hostaform

Memory O-Ring Pin O-Ring Knob Entrainer Cap Elastic ring Outdistance (only for DN 3/4 - 1” - 1 1/4”)

CIM 727

CIM 722

MATERIALS - MAIN FEATURES:

PRESSURE/TEMPERATURE RATINGS AT 1/2 TO 2”

Body: cast non dezincifiable brass “CR” CC752S. Bonnet: machined from drawn non dezincifiable brass “CR” EN 12164 CW602N. Stem and metal components: machined from drawn brass bar “CR” EN 12164 CW602N. Packing: O’Ring in HNBR. Shutter: machined from drawn brass bar “CR” EN 12164 CW602N. Knob: nylon 6. Disc face: EPDM rubber. Hydrostatic test pressures: shell 24 bar (348 psi); seat 18 bar (261 psi). Threading: parallel threads to ISO 7/1 Rp - BS 21 Rp.

in2

lbf/ 58

87

116

145

174

203

232

261

290

319

160

320

140

284

120

248

100

212

80

176

60

140

40

104

°F

°C

68

20 4

6

8

10

CW602N HNBR Steel HNBR Nylon 6 CW602N Hostaform Steel Nylon

12

14

16

18

20

22

bars

20 bar at –10 to 100°C – 290 lbf/in2 at 14 to 212°F 16 bar at –10 to 120°C – 287 lbf/in2 at 14 to 248°F 1/2

DN

3/4

1”

1 1/4”

1 1/2”

2”

Grms.

475

645

860

1275

1890

2800

A

137,5

157

160

171

212

231

B

119

138,5

154

168,5

211

230

C

68

77

91

108

116

143

D

15

E

162,5

190

201,5

220

276

301,6

F

52

52

52

52

58

58

CH

28

33

40

51

56

71

16,3

19,1

DN

1/2

3/4

1”

1 1/4”

1 1/2”

2”

Grms.

161

207

252

400

460

710

A

25

28

31

36

39

45

C

66,5

66,5

63,5

71

71

79,5

D1

15

16,3

19,1

21,4

21,4

25,7

D2

15

16,3

19,1

21,4

21,4

25,7

CH

28

34

40

51

56

71

4

21,4

21,4

25,7

CIM 3737

2.3

DOUBLE REGULATING SET

Body Seat rubber Disc Stem Bushing O-Ring Washer Bolt Seeger Handwheel

DN 65 - DN 300

GGG40 EPDM Stainless steel AISI 316 Stainless steel AISI 316 Polyamid EPDM St 37 M8.8 Stainless steel GGG40

Diaphragm Body

Stainless steel AISI 316 Fe 360B

CIM 3110 DRV

CIM 3722

PRESSURE/TEMPERATURE RATINGS AT DN 65 TO DN 300 lbf/in2 58

87

116

145

174

203

232

261

290

MATERIALS - MAIN FEATURES:

319

160

320

140

284

120

248

100

212

Body valve: GGG40. Body union: Fe 360B. Gauged diaphragm: stainless steel AISI 316. °F

°C 80

176

Seat rubber: EPDM.

60

140

Disc: stainless steel AISI 316.

40

104

Stem: stainless steel AISI 316.

68

Bushing: polyamid.

20 4

6

8

10

12

16

14

18

20

22

bars

Flanged connections: to UNI 2223 - PN 16.

16 bar at –10 to 100°C – 232 lbf/in2 at 14 to 212°F 16 bar at –10 to 120°C – 287 lbf/in2 at 14 to 248°F DN

65

80

100

125

150

200

250

300

4500

6200

7800

10000

12000

18500

27900

44700

C

46

46

52

56

56

60

68

78

C1

50

50

56,5

60,5

60,5

64,5

72,5

ØD

188

204

234

258

290

343

412

486

Ø D1

Grms.

112

126

152

185

210

262

316

372

H1

74

96

110

122

136

160

201

237

H2

152

159

177

190

203

241

273

311

N

40

40

42

46

46

48

58

64

145

160

180

210

240

295

350

400

16x4

16x8

16x8

16x8

20x8

20x8

20x12

20x12

ØK Ø Mxn DN

65

80

100

125

150

200

250

300

PN

16

16

16

16

16

16

16

16

ØA

185

200

220

250

285

340

405

460

B

150

150

150

200

230

300

400

450

ØC

145

160

180

210

240

295

355

410

K N. holes

84,5

90°

90°

90°

90°

45°

30°

30°

30°

4

8

8

8

8

12

12

12

5

THE NEED FOR HIGH ACCURACY FLOW BALANCING AND MEASUREMENT

3

ENSURE UNIFORM BUILDING TEMPERATURES: Terminals receiving too little flow may not deliver their intended amounts of heating or cooling. This will mean that the areas they serve may fail to reach design temperatures under peak load conditions. Chilled water systems where there is a latent cooling function (i.e. dehumidification) are particularly sensitive to variations from design flow rates.

IMPROVE CONTROL VALVE RESPONSE: Modulating control valves may be unable to control properly if the circuits they control start off with too much or too little flow. In a circuit receiving too much Balanced system flow, the first part of the control valve’s travel is wasted just getting the flow rate back to its design value; in a circuit receiving too little flow, the action of the valve may cause a dramatic drop in heat transfer effectively making the valve behave as an on/off controller.

OPTIMISE ENERGY SAVINGS: By ensuring an accurate balance of flow rates, the total flow rate from the pump will not need to exceed the design value for the building. Furthermore, energy saving controls will operate more effectively. For example, if the flow balance is poor, different parts of a building will heat up or cool down at different rates. To compensate for this, optimiser controls may have to bring on heating/cooling systems earlier than necessary to allow for the uneven heating up/cooling down of the building.

PROVIDE THE CLIENT WITH A RECORD OF FINAL SYSTEM FLOW RATES: Accurate balancing and flow measurement means that the client can be shown, and given clear evidence, that the system to be handed over complies with the designer’s specified flow rate figures. This will give the client confidence that the system is satisfactory.

FACILITATE TROUBLE-SHOOTING: In the event of poor system performance, the presence of balancing valves and flow measurement devices will enable engineers to establish the locations and causes of flow problems.

FACILITATE FUTURE MODIFICATIONS:

Unbalanced system

In the event that the system is modified or changed at a future date, the presence of high accuracy balancing valves and flow measurement devices will enable a new balance of flows to be established.

6

4

ADVANTAGES OF FIXED ORIFICE OVER VARIABLE ORIFICE FLOW MEASUREMENT

The idea to couple a double regulating valve to a fixed orifice device evolved in the UK in the 1980s. This combination was designed specifically to overcome the accuracy problems associated with flow measurements utilising the pressure drops across variable orifice valves. Variable orifice valves seldom achieve the accuracy and reliability of fixed orifice valves. For a variable orifice valve, the pressure signal across the plug is used for flow measurement. A graph of the relationship between pressure drop and flow rate is required for each valve setting. The fundamental weakness of CIM 721 CIM 721 PRESSURE this design is that manufacturing FIXED ORIFICE SIMULATION tolerances can cause significant flow measurement distortions beyond a certain closure point, typically 50% closed. Beyond this point the flow measurement accuracy can deteriorate dramatically, to ±30% or more! Since most of the valve’s resistance is added in the last part of its closure, the valve’s balancing range is severely limited. The result is a valve, which has either limited balancing capability, poor flow measurement accuracy, or both. The limited operating range of variable orifice valves inevitably makes valve selection more difficult, often resulting in valve sizes which are lower than ading pipe sizes. Fixed orifice valves have none of these problems. Because the flow measurement function is separated from the balancing function they can be regulated to nearly closed positions, achieving much higher balancing pressures whilst maintaining flow measurement accuracy within ±5% at any setting.

FIXED ORIFICE

VARIABLE ORIFICE

Since their introduction, fixed orifice commissioning sets have become by far the most preferred choice for UK design engineers and installation contractors.

7

5

DESIGN ADVANTAGES OF FIXED ORIFICE VALVES

To avoid the requirement for high balancing pressures, a popular approach amongst design engineers has been to design systems such that some degree of self-balancing is achieved due to the sizing and arrangement of pipe circuits. Typical design solutions might include the use of reverse return circuits, low pressure loss distribution mains or the selection of terminal units with equal resistances. While this approach can help to achieve well balanced system flow rates, care must be taken to avoid the following disadvantages: •

Reverse return circuits invariably require longer lengths of pipework thereby increasing system costs and increasing pump pressure and energy requirements.

•

Low pressure loss mains are effectively over-sized pipes which are more expensive than necessary and provide low velocity collecting points where air can accumulate and corrosion can take place.

•

Selecting terminal units with equal resistances effectively means that many locations will end up with over-sized terminal units which will cost more and exhibit poor control. In addition to these points, by deg for self-balancing, the designer usually has to spend more time on the design to ensure that pressure variations are as small as possible, and that any remaining imbalance can be dealt with by the limited trimming ability of a less accurate balancing valve.

8

6

FLOW AND PRESSURE SIMULATION GRAPHS

CIM 727 Pressure simulation graphs

CIM 727 Flow simulation graphs

Computational fluid dynamics software has been used to demonstrate the stable pressure and flow patterns across Cimberio balancing valves at different settings. The analysis shows that, due to the compact body of the valve, turbulent zones and eddy currents are minimised, thereby ensuring stable performance and resistance to air and dirt related problems.

2 turn open

3 turns open

4 turns open

5 turns open

6 turns open

7 turns open

2 turn open

3 turns open

4 turns open

6 turns open

7 turns open

5 turns open

9

7.1

BSRIA VALVE TESTING REPORT

BSRIA is the UK’s leading centre for building services research. BSRIA offer independent and authoritative research, information, testing and consultancy and market intelligence. (e-mail: [email protected] web: www.bsria.co.uk) Since trapped air and dirt are the main causes of non-repeatable flow measurements in small sized valves, we commissioned BSRIA to investigate their impact on the performance of Cimberio balancing valves.

OBJECTIVES • Determine the effects of trapped air on the kv values of each of the valves at the 25% open setting • Determine the effects of dirt suspended in the water on the kv values of each of the valves at the 25% open setting

RESULTS The graph below shows the results of the effects of trapped air on each of the three valves when set at their 25% open positions.

Results of trapped air on valves at 25% open position.

10

7.2

BSRIA VALVE TESTING REPORT

The graph below shows the results of the effects of dirty water on each of the three valves when set at their 25% open positions.

Results of dirty water on valves at 25% open position.

BSRIA’S CONCLUSIONS “The results show that the valves tested are not significantly affected by the presence of trapped air or fine dirt material suspended in the fluid stream. For each test, the valve resistance was found to vary by less then 10% which is extremely unlikely to produce a measureable variation in water flow rate. Furthermore, the results show that by closing and re-opening the valve to its locked position the resistance across the valve is repeatable within acceptable limits.”

11

LOCATING REGULATING VALVES AND FLOW MEASUREMENT DEVICES

8

In general, regulating valves should be located on all branches where it is anticipated there will be a significant pressure imbalance. Flow measurement devices should be included where flow rates need to be checked. The pipework schematic below illustrates typical locations.

Isolating valve

Double Regulating Valve

Orifice type flow measurement device

Double regulating valve close coupled to flow measurement device

Motorised three way valve

Non-return valve

Typical locations for regulating valves and flow measurement devices The convention in heating systems is to position regulating valves on the return sides of pipe circuits where the water is coolest. In this position, valve pressures are more likely to be above the water vapour pressure (and cavitation region), since vapour pressure increases with temperature. In chilled water circuits the valve location makes little difference, although the same convention tends to be applied. Orifice type flow measurement devices usually require a uniform pattern of flow through them to ensure measurement accuracy. Therefore, it is recommended that at least 5 diameters of straight pipe are allowed upstream of each device.

12

9

LOW FLOW COMMISSIONING SETS

The smallest valve size in the range is 15mm nominal diameter. Because many modern systems incorporate low duty terminal units with low flow requirements (typically down to 0.02 l/s), the 15mm commissioning sets have to be able to accommodate an unusually large range of flow rates. For this reason, four alternative valve/flow measurement device combinations are available. Flow measurement devices are available as standard, medium or low flow types. Double regulating valves are available as standard and low flow. The combinations in which the Cim 737 15mm can be close coupled are shown below.

CIM 737

LOW FLOW COMMISSIONING SETS ONLY FOR 1/2”

727L

727L

727S

727S

+

+

+

+

721L

721M

721M

721S

=

=

=

=

737L

737ML

737MS

737S

It can be seen from the Valve Selection Table (Page 15), that the low flow valves and flow measurement devices have high values. This is because, at such low flow rates, a high resistance is required to generate a measureable pressure differential. In practice, because the flow rates are so small, the pressure drops across these devices are not excessive (typically 3kPa maximum).

13

10.1

GUIDE TO VALVE SELECTION

Size pipes based on design flow rates.

From Valve Selection Table (see opposite page) select line size valves to suit design flow rates Full open valve pressure losses:

v–––2

p = 2

or p = 1.296 x 10 6

Q – kv

2

Add full open valve

NB is sometimes referred to as “k factor”

p=pressure loss(Pa) v=velocity(m/s) =density(kg/m3) Q=flow rate (l/s)

pressure losses to pipe pressure losses.

Calculate circuit residual pressures.

Check from Valve Selection Table that residual pressures are within the maximum balancing pressures of the selected valves

For residual pressures greater than the available range try

e.g.

splitting the pressure loss between two valves, one on the flow, one on the return.

e.g.

Valve ref

Model

Create a valve schedule

14

Size

Location

Flow rate (l/s)

kvs

Pressure loss signal (kPa)

10.2

GUIDE TO VALVE SELECTION

VALVE SELECTION TABLE Nominal Diameter (mm)

DRV, FMD or DRV+FMD

FMD 15

DRV DRV + FMD

20

25

32

40

50

65

80

100

125

150

200

250

300

FMD DRV DRV + FMD FMD DRV DRV + FMD FMD DRV DRV + FMD FMD DRV DRV + FMD FMD DRV DRV + FMD FMD DRV DRV + FMD FMD DRV DRV + FMD FMD DRV DRV + FMD FMD DRV DRV + FMD FMD DRV DRV + FMD FMD DRV DRV + FMD FMD DRV DRV + FMD FMD DRV DRV + FMD

Minimum Flow Rate (l/s)*

Model

0.015 0.028 0.055 0.015 0.028 0.028 0.055 0.11 0.11 0.21 0.21 0.46 0.46 0.7 0.7 1.3 1.3 2.7 2.7 4.1 4.1 6.8 6.8 10 10 14 14 25 25 38 38 54 54

721L 721M 721S 727L 727S 737L 737ML 737MS 737S 721 727 737 721 727 737 721 727 737 721 727 737 721 727 737 3721 3110DRV 3737 3721 3110DRV 3737 3721 3110DRV 3737 3721 3110DRV 3737 3721 3110DRV 3737 3721 3110DRV 3737 3721 3110DRV 3737 3721 3110DRV 3737

DRV Double Regulating Valve

Maximum Balancing Pressure (kPa)

54200 2366 54200 54200 2366 2366 1250 1250 1203 1203 284 284 203 203 49 49 225 225 81 81 36 36 13 13 5.7 5.7 2.1 2.1 0.8 0.8 0.4 0.4

Q2 Q2 Q2 Q2 Q2 Q2 Q2 Q2 Q2 Q2 Q2 Q2 Q2 Q2 Q2 Q2 Q2 Q2 Q2 Q2 Q2 Q2 Q2 Q2 Q2 Q2 Q2 Q2 Q2 Q2 Q2 Q2

FMD Flow Measurement Device

* Flow rates required to generate a minimum 1 kPa loss signal across the FMD.

15

(k factor)

kv

kvs

414.6 92.1 21.9 65.8 7.1 480.4 157.9 100.4 29.4 10.5 6.6 17.8 8.4 6.4 15.0 4.8 5.8 9.8 4.5 6.1 10.7 2.2 4.9 6.6 1.5 7.3 8.5 1.4 5.6 6.5 1.4 6.0 7.1 1.4 4.5 5.8 1.4 4.2 5.4 1.5 4.7 6.0 1.6 4.7 6.2 1.6 4.6 6.1

1.278 3.905 0.473 0.825 1.035 1.911 7.281 4.427 11.757 7.684 21.600 16.560 28.461 21.491 50.519 43.639 70 64.754 110 102 180 166 320 280 470 416 790 702 1250 1089 1800 1569

0.473 0.976 1.799 0.473 0.976 0.976 1.799 4.057 4.057 7.452 7.452 16.628 16.628 23.000 23.000 47.351 47.351 88.7 88.7 136 136 234 234 358 358 512 512 911 911 1438 1438 2057 2057

Q Flow rate (l/s)

10.3

GUIDE TO VALVE SELECTION

100

CIM 727

80 60 50 40 30

10 8

2”

1”1 /4 1”1 /2

” 1”

”L

3

3/4

OW

1/2

FL

”

OW

6 5 4

1/2

Pressure Loss – ∆P [kPa]

20

2

1 0.8 0.6 0.5 0.4 0.3 0.2

0.1 0.004

0.006 0.008 0.01

0.02

0.04

0.06 0.08 0.1

0.2

0.3

0.4

0.6

0.8

1

2

3

4

6

8

10

20

Flow rate – Q [l/s]

Recommended operating range of 727 double regulating valves. 100

CIM 3110 DRV

80 60 50 40

0 30 0 DN

DN

20

25

0

0 DN

12

15

DN

DN

DN

10

80 DN

65 DN

Pressure Loss – ∆P [kPa]

0

20

5

30

10 8 6 5 4 3 2

1 0.8 0.6 0.5 0.4 0.3 0.2

0.1 0.4

0.6

0.8

1

2

4

6

8

10

20

30

40

60

80 100

200

300

400

600

800 1000

2000

Flow rate – Q [l/s]

Recommended operating range of 3110 DRV double regulating valves. 16

10.4

GUIDE TO VALVE SELECTION

100

CIM 721

80 60 50 40 30

47. 351

16. 628 23. 000

s=

s=

s=

Kv

Kv

Kv

–2”

1/2

1/4

721

–1”

721

–1” 721

721

721

–3/

–1”

4”

Kv

Kv

s=

s=

7.4 52

4.0 57

1.7 99 s= Kv

1/2 ” S–

721

721

2

M– 1/

L–

1/2 ”

3

2”

Kv

Kv

s=

6 5 4

s=

0.4 73

8

0.9 76

10

721

Pressure Loss Signal – ∆P [kPa]

20

1 0.8 0.6 0.5 0.4 0.3

Q=

0.2

kvs P 36

0.1 0.004

0.006 0.008 0.01

0.02

0.04

0.06 0.08 0.1

0.2

0.3

0.4

0.6

0.8

1

2

3

4

6

8

10

20

Flow rate – Q [l/s]

Pressure signal graphs for 721 flow measurement devices at DN 1/2 to DN 2”. 100

CIM 3721

80 60 50 40 30

0 7.0

s= Kv

Kv 50

00 N3

372

1–D

N2

205

0 8.0

.00

143 s=

s=

911

.00 Kv 1–D 372

372

1–D

N2

00

Kv

s=

512

.00 358 50

N1

372

1–D

N1

25

Kv

s=

00 N1

1–D 372

1–D 372

s=

234

.00

Kv

s= Kv

N8 1–D

2

372

1–D

N6

3

0

5

Kv

s=

6 5 4

136

88.

70

8

.00

10

372

Pressure Loss Signal – ∆P [kPa]

20

1 0.8 0.6 0.5 0.4 0.3

Q=

0.2

kvs P 36

0.1 0.4

0.6

0.8

1

2

4

6

8

10

20

30

40

60

80 100

200

300

400

600

800 1000

2000

Flow rate – Q [l/s]

Pressure signal graphs for 3721 flow measurement devices at DN 65 to DN 300. 17

11.1

PROPORTIONAL BALANCING OF FLOW RATES

PROCEDURE Consider a pipe circuit branch serving several sub-branches. In an unbalanced condition the water entering the branch will distribute itself between them, favouring those with the lowest resistances. Therefore, to ensure that each sub-branch receives its correct design flow rate, the flows need to be balanced using the installed regulating valves and flow measurement devices.

Starting at system extremities (typically branches serving terminal units): 1. Ensure that the total flow rate entering the branch is between 110% - 120% of the design flow rate. It may be necessary to close down other branches to achieve this. 2. Measure the flow rates through each sub-branch. For each sub-branch calculate the % design flow rate:

% design flow rate =

Pmeasured Pdesign

If the signals at any of the installed flow measurement devices are below the measurement range of the device, further increase the flow rate entering the branch by closing down adjacent branches. 3. Identify the index sub-branch. This will be the one with the lowest % design flow rate. Usually, but not always, this will be the end sub-branch (furthest from the pump) e.g. Terminal 5 in the above schematic. If the end sub-branch is not the index, then close its regulating valve until its % design flow rate is approximately 10% less than that at the true index (so that the end sub-branch becomes an artificial index). This needs to be done whilst simultaneously measuring the flow at the true index, since its flow rate will change as the end branch is adjusted. Hence, two operatives each with manometers and 2-way radios will speed this exercise.

18

11.2

PROPORTIONAL BALANCING OF FLOW RATES

4. Connect a manometer to the end sub-branch flow measurement device. Starting at the nearest upstream sub-branch (e.g. terminal 4 in the schematic) and working back towards the furthest upstream sub-branch, adjust each sub-branch regulating valve such that its % design flow rate becomes equal to that at the end sub-branch. This needs to be done whilst simultaneously measuring the flow at the end subbranch, since its flow rate will change as upstream valves are adjusted. Hence, two persons each with manometers and 2-way radios will speed this exercise. 5. Having achieved equal % design flow rates for each of the sub-branches, the subcircuit flow rates are now balanced. This balance cannot be disturbed by the adjustment of upstream valves. Hence, upstream branches can be balanced in exactly the same way. 6. Once the entire system has been balanced, adjust the flow from the pump to 110% of the total design flow rate for the system. All branches and sub-branches should now have flow rates close to their 100% design values. For a more detailed description of the balancing procedure complete with a worked example, reference should be made to the Cimberio Commissioning Guide.

An alternative to the proportional balancing procedure described above is the so-called "compensated method" whereby the flow rate at the furthest sub-branch is regularly returned to its design value by adjusting the main branch valve. Systems with fixed orifice commissioning sets can be balanced using the compensated method of balancing, although we believe the compensated method has the following disadvantages:

• A remote indicating manometer is required so that the commissioning specialist at the main branch valve can observe the flow changes at the end sub-branch. • Where the end sub-branch is a long way from the main branch valve, (outside the range of a remote indicating manometer) three persons with radios would be required for balancing.

19

12

MEASURING EQUIPMENT

FLOW MEASUREMENT INSTRUMENTS By measuring the pressure differential across a fixed resistance (such as an orifice plate) the flow rate through a pipe can be determined utilising the square law relationship between pressure differential and flow rate. Experience has shown that this method of determining flow rate is the most convenient for use in the building services industry.

FLUOROCARBON MANOMETER The instrument commonly used for measuring pressure differential is the manometer. Manometers traditionally take the form of a U tube arrangement whereby the pressure differential being measured is used to displace a fluid of known density, typically mercury or a fluorocarbon. The height of column displaced is directly proportional to the pressure differential. Fluorocarbon manometers are typically used to measure pressure differentials in the range 1-4.7 kPa whereas mercury manometers are capable of measuring pressure differentials in the range 1-60kPa. In recent years the use of mercury manometers has declined due to the safety concerns surrounding the handling of mercury on construction sites.

DIGITAL MANOMETER A digital differential pressure and flow test set is an electronic pressure measuring device which is programmed to enable the direct reading of differential pressure and flow. In addition, the regulating valve manufacturer’s kv value can be keyed into the instrument so that the flow rate can be read direct from the manometer thereby avoiding the need to refer to a pressure loss graph. Although generally reliable, digital manometers do need to be treated with care and regularly calibrated to ensure that accuracy is maintained.

20

13.1

TERMINOLOGY

DOUBLE REGULATING VALVE Double regulating valves (DRVs) are so called because they serve the double function of flow regulation and isolation. Once set in their regulated position, they can be locked so that when closed and re-opened, they cannot be opened beyond their set position. FLOW MEASUREMENT DEVICE Flow measurement devices enable flow rate to be measured for the purposes of achieving and proving a flow balance. Fixed orifices provide a highly accurate means of flow measurement in pipe systems. By measuring the pressure differential across an orifice, this can be equated to flow rate using the manufacturer’s published kvs value. CLOSE COUPLED COMMISSIONING SET This term simply refers to the close coupling of regulating valves to orifice type flow measurement devices. The orifice is screwed into the inlet side of the regulating valve. INDEX CIRCUIT This is the circuit which, with the system in an unbalanced state, exhibits the greatest resistance to flow. It can be identified by calculation as the circuit with the highest pressure loss around it when design flow rates are assumed. On site it can be identified by flow measurement; it will be the circuit for which the ratio of measured flow rate to design flow rate is lowest. All systems will have a single overall index circuit against which pump pressure is calculated. Furthermore, for any branch serving sub-branches, there will be an index sub-branch. Similarly, each sub-branch may serve a number of terminal branches, one of which will be the index terminal. If all terminal branches are of equal resistance, the main system index circuit is likely to be from the pump to the most remote terminal, since this circuit has the longest pipe lengths. Similarly, sub-branch index circuits are likely to be from the start of the sub-branch to the most remote terminal unit they serve. However, if terminal branch resistances vary then the system index and branch indexes will not necessarily coincide with the most remote terminals. The location of each index will then depend on which circuit has the highest combination of pipe and terminal branch pressure losses. As the circuit starting with the highest resistance, there is no need to regulate flow at an index. At the end of the balancing process, the index circuit should always have a fully open regulating valve.

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13.2

TERMINOLOGY

RESIDUAL PRESSURE The residual pressure for a particular circuit is the difference between the pressure available across that circuit, and the pressure required to achieve the design flow rate. This residual, or excess pressure, has to be dissipated in some way, and this is usually achieved by adding resistance to the circuit in the form of a regulating valve. A circuit’s residual pressure is, therefore, critical for sizing regulating valves. A valve’s resistance obviously increases as it is closed, but there is a limit to how much resistance it can generate. As a rule of thumb, if a valve is closed beyond its 25% open position it may become sensitive to air bubbles or prone to blockage from circulating debris. Valve selection must therefore include a check to ensure that predicted residual pressures are within the operating limits of the selected regulating valve. Fortunately, many pipe sizing programs calculate residual pressures automatically. These values can then be checked against the operating range of the selected valve.

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13.3

TERMINOLOGY

KV The kv value represents the flow rate through a fully open valve at a temperature between 5degC and 40degC, and measured in cubic metres per hour that will induce a pressure loss of 1bar. Hence the kv value is effectively a measure of the valve’s resistance. Where a valve is close coupled to a flow measurement device, the kv value represents the resistance across the fully open valve and flow measurement device combined. Using SI units, the pressure drop across a fully open valve can be calculated from the equation: p = 1.296 x

10 6

Q – kv

2

where Q = flow rate in l/s, and p = pressure loss in Pa

kv values express resistance as an inverse - in other words the greater the valve’s resistance the smaller its kv value. Design engineers are more used to thinking of resistance in of the pressure loss coefficient (zeta) sometimes also referred to as a "k factor". The pressure loss through any fitting or component can be calculated from the equation: v2 p = ––– 2 where = fluid density in kg/m3, v = velocity in m/s and p = pressure loss in Pa. The higher the loss coefficient, the greater the resistance of the fitting. For convenience, the valve selection charts in this guide express fully open valve resistances in of both kv values and pressure loss coefficients KVS This term is usually applied to the pressure loss between the tappings on a flow measurement device. The "s" indicates "signal" since it relates to the pressure loss signal measured by a commissioning specialist. For a given flow measurement device with a known kvs value the commissioning specialist can calculate flow rate from the pressure loss signal using the following equation:

Q=

kvs P 36

where Q = flow rate in l/s, and P = pressure loss in kPa

However, the pressure loss between the tappings is not the same as the overall pressure loss across the device. Because there is an increase in static pressure downstream of the orifice, the overall pressure loss is usually less than the measured pressure loss across the tappings. To determine the pressure loss across one of our flow measurement devices, use the pressure loss coefficient (zeta) from the Valve Selection Table (Page 13).

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