double regulating valve
TRANSCRIPT
This guide introduces the Cimberio CIM 737, CIM 3737 Commissioning Set – a combineddouble regulating valve and separate flow measurement device which provides high accuracyflow balancing and measurement across all valve settings.
The commissioning sets are suitable for both heating (LPHW) and cooling applications at workingpressures up to 20 bar. Valve sizes between 15-50mm are available as “CR” brass oblique patternglobe 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 (seesection 7). The results showed an excellent tolerance to such conditions, providingconfidence that the valves will retain a high level of accuracy and repeatability of flowmeasurement under the worst of system conditions.
INTRODUCTION1
2
DZR BRASS
COMPACT VALVE CHAMBER
CLEAR 360º READING
REPLACEABLE HANDLE
VALVE IDENTIFICATION DISK UNDER HANDLE CAP
SMOOTH OPERATION
• Reduced Air & Dirt Accumulation
• Accurate Allen Key Locking
• Non Rising Handle
CIM 727DOUBLE REGULATING VALVE
TESTED FORPERFORMANCE & RELIABILITY
2.1 CIM 727DOUBLE REGULATING VALVE
3
HEAT AND IMPACT RESISTANTNYLON HANDLE
360° RE-SETTABLE INDEX COLLAR
DN
Grms.
A
B
C
D
E
F
CH
1/2
475
137,5
119
68
15
162,5
52
28
3/4
645
157
138,5
77
16,3
190
52
33
1”
860
160
154
91
19,1
201,5
52
40
1 1/4”
1275
171
168,5
108
21,4
220
52
51
1 1/2”
1890
212
211
116
21,4
276
58
56
2”
2800
231
230
143
25,7
301,6
58
71
DN
Grms.
A
C
D1
D2
CH
1/2
161
25
66,5
15
15
28
3/4
207
28
66,5
16,3
16,3
34
1”
252
31
63,5
19,1
19,1
40
1 1/4”
400
36
71
21,4
21,4
51
1 1/2”
460
39
71
21,4
21,4
56
2”
710
45
79,5
25,7
25,7
71
CIM 737DOUBLE REGULATING SET DN 1/2 - DN 2”
2.2 CIM 737DOUBLE REGULATING SET DN 1/2 - DN 2”
320
284
248
212
176
140
104
68
160
140
120
100
80
60
40
206 8 10 12 14 16 18
°C
lbf/in2
bars
°F
116 145 174 203 232 26187 290 31958
4 20 22
PRESSURE/TEMPERATURE RATINGS AT 1/2 TO 2”
20 bar at –10 to 100°C – 290 lbf/in2 at 14 to 212°F16 bar at –10 to 120°C – 287 lbf/in2 at 14 to 248°F
MATERIALS - MAIN FEATURES:Body: cast non dezincifiable brass “CR” CC752S.Bonnet: machined from drawn non dezincifiablebrass “CR” EN 12164 CW602N.Stem and metal components: machined fromdrawn brass bar “CR” EN 12164 CW602N.Packing: O’Ring in HNBR.Shutter: machined from drawn brass bar “CR” EN12164 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.
4
Body CC752SBonnet CW602NStem CW602NGasket EPDMShutter CW602NO-Ring HNBRIndex HostaformSeeger Bronze1/10 turn index HostaformTurn index Hostaform
Memory CW602NO-Ring HNBRPin SteelO-Ring HNBRKnob Nylon 6Entrainer CW602NCap HostaformElastic ring SteelOutdistance Nylon(only for DN 3/4 - 1” - 1 1/4”)
CIM 727 CIM 722
CIM 3737DOUBLE REGULATING SET DN 65 - DN 300
2.3 CIM 3737DOUBLE REGULATING SET DN 65 - DN 300
MATERIALS - MAIN FEATURES:Body valve: GGG40.
Body union: Fe 360B.
Gauged diaphragm: stainless steel AISI 316.
Seat rubber: EPDM.
Disc: stainless steel AISI 316.
Stem: stainless steel AISI 316.
Bushing: polyamid.
Flanged connections: to UNI 2223 - PN 16.
320
284
248
212
176
140
104
68
160
140
120
100
80
60
40
206 8 10 12 14 16 18
°C
lbf/in2
bars
°F
116 145 174 203 232 26187 290 31958
4 20 22
PRESSURE/TEMPERATURE RATINGS AT DN 65 TO DN 300
16 bar at –10 to 100°C – 232 lbf/in2 at 14 to 212°F16 bar at –10 to 120°C – 287 lbf/in2 at 14 to 248°F
Body GGG40Seat rubber EPDMDisc Stainless steel AISI 316Stem Stainless steel AISI 316Bushing PolyamidO-Ring EPDMWasher St 37Bolt M8.8Seeger Stainless steelHandwheel GGG40
Diaphragm Stainless steel AISI 316Body Fe 360B
DN
Grms.
C
C1
Ø D
Ø D1
H1
H2
N
Ø K
Ø Mxn
65
4500
46
50
188
112
74
152
40
145
16x4
80
6200
46
50
204
126
96
159
40
160
16x8
100
7800
52
56,5
234
152
110
177
42
180
16x8
125
10000
56
60,5
258
185
122
190
46
210
16x8
150
12000
56
60,5
290
210
136
203
46
240
20x8
200
18500
60
64,5
343
262
160
241
48
295
20x8
250
27900
68
72,5
412
316
201
273
58
350
20x12
300
44700
78
84,5
486
372
237
311
64
400
20x12
DN
PN
Ø A
B
Ø C
K
N. holes
65
16
185
150
145
90°
4
80
16
200
150
160
90°
8
100
16
220
150
180
90°
8
125
16
250
200
210
90°
8
150
16
285
230
240
45°
8
200
16
340
300
295
30°
12
250
16
405
400
355
30°
12
300
16
460
450
410
30°
12
CIM 3110 DRV CIM 3722
5
ENSURE UNIFORM BUILDING TEMPERATURES:Terminals receiving too little flow may not deliver theirintended amounts of heating or cooling. This will meanthat the areas they serve may fail to reach designtemperatures under peak load conditions. Chilled watersystems where there is a latent cooling function (i.e. de-humidification) are particularly sensitive to variationsfrom design flow rates.
IMPROVE CONTROL VALVE RESPONSE: Modulating control valves may be unable to controlproperly if the circuits they control start off with toomuch or too little flow. In a circuit receiving too muchflow, the first part of the control valve’s travel is wastedjust getting the flow rate back to its design value; in acircuit receiving too little flow, the action of the valve may cause a dramatic drop in heattransfer 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 needto exceed the design value for the building. Furthermore, energy saving controls will operatemore effectively. For example, if the flow balance is poor, different parts of a building willheat up or cool down at different rates. To compensate for this, optimiser controls may haveto bring on heating/cooling systems earlier than necessary to allow for the uneven heatingup/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 overcomplies with the designer’s specified flow rate figures.This will give the client confidence that the system issatisfactory.
FACILITATE TROUBLE-SHOOTING:In the event of poor system performance, the presence ofbalancing valves and flow measurement devices willenable engineers to establish the locations and causes of flow problems.
FACILITATE FUTURE MODIFICATIONS:In the event that the system is modified or changed at afuture date, the presence of high accuracy balancingvalves and flow measurement devices will enable a newbalance of flows to be established.
THE NEED FOR HIGH ACCURACY FLOW BALANCING AND MEASUREMENT3
Balanced system
Unbalanced system
6
The idea to couple a double regulating valve to a fixed orifice device evolved in the UK in the1980s. This combination was designed specifically to overcome the accuracy problemsassociated with flow measurements utilising the pressure drops across variable orifice valves.Variable orifice valves seldomachieve the accuracy andreliability of fixed orifice valves.For a variable orifice valve, thepressure signal across the plug isused for flow measurement. Agraph of the relationshipbetween pressure drop and flowrate is required for each valvesetting.The fundamental weakness ofthis design is that manufacturingtolerances can cause significantflow measurement distortionsbeyond a certain closure point, typically 50% closed. Beyond this point the flow measurementaccuracy can deteriorate dramatically, to ±30% or more! Since most of the valve’s resistance isadded in the last part of its closure, the valve’s balancing range is severely limited. The resultis a valve, which has either limited balancing capability, poor flow measurement accuracy, orboth. The limited operating range of variable orifice valves inevitably makes valve selectionmore difficult, often resulting in valve sizes which are lower than adjoining pipe sizes.Fixed orifice valves have none of these problems. Because the flow measurement function isseparated from the balancing function they can be regulated to nearly closed positions,achieving much higher balancing pressures whilst maintaining flow measurement accuracywithin ±5% at any setting.
Since their introduction, fixed orifice commissioning sets have become by far themost preferred choice for UK design engineers and installation contractors.
ADVANTAGES OF FIXED ORIFICE OVER VARIABLE ORIFICE FLOW MEASUREMENT
CIM 721FIXED ORIFICE
CIM 721 PRESSURESIMULATION
VARIABLE ORIFICEFIXED ORIFICE
4
7
To avoid the requirement for high balancing pressures, a popular approach amongst designengineers has been to design systems such that some degree of self-balancing is achieved dueto the sizing and arrangement of pipe circuits. Typical design solutions might include the useof reverse return circuits, low pressure loss distribution mains or the selection of terminal unitswith equal resistances.
While this approach can help to achieve well balanced system flow rates, care must be takento 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 designing for self-balancing, the designer usually hasto spend more time on the design to ensure that pressure variations are as small aspossible, and that any remaining imbalance can be dealt with by the limitedtrimming ability of a less accurate balancing valve.
DESIGN ADVANTAGES OF FIXED ORIFICE VALVES5
8
FLOW AND PRESSURE SIMULATION GRAPHS6
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
5 turns open 6 turns open 7 turns open
Computational fluid dynamics software has been used to demonstrate the stable pressure and flowpatterns 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, therebyensuring stable performance and resistance to air and dirt related problems.
CIM
727
Flo
w s
imu
lati
on
gra
ph
s C
IM 7
27 P
ress
ure
sim
ula
tio
n g
rap
hs
9
BSRIA is the UK’s leading centre for building services research. BSRIA offer independent andauthoritative 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 theperformance 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.
BSRIA VALVE TESTING REPORT7.1
Results of trapped air on valves at 25% open position.
10
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.
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.”
BSRIA VALVE TESTING REPORT7.2
Results of dirty water on valves at 25% open position.
11
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 beincluded where flow rates need to be checked. The pipework schematic below illustrates typical locations.
The convention in heating systems is to position regulating valves on the return sides of pipecircuits where the water is coolest. In this position, valve pressures are more likely to be abovethe water vapour pressure (and cavitation region), since vapour pressure increases withtemperature. In chilled water circuits the valve location makes little difference, although thesame convention tends to be applied.
Orifice type flow measurement devices usually require a uniform pattern of flowthrough them to ensure measurement accuracy. Therefore, it is recommendedthat at least 5 diameters of straight pipe are allowed upstream of each device.
LOCATING REGULATING VALVES AND FLOW MEASUREMENT DEVICES8
Typical locations for regulating valves and flow measurement devices
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
12
The smallest valve size in the range is 15mm nominal diameter. Because many modern systemsincorporate 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 offlow rates. For this reason, four alternative valve/flow measurement device combinationsare 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.
It can be seen from the Valve Selection Table (Page 15), that the low flow valves and flowmeasurement devices have high � values. This is because, at such low flow rates, a highresistance is required to generate a measureable pressure differential. In practice, becausethe flow rates are so small, the pressure drops across these devices are not excessive(typically 3kPa maximum).
LOW FLOW COMMISSIONING SETS9
CIM 737 LOW FLOW COMMISSIONING SETSONLY FOR 1/2”
727L 727L 727S 727S
721L 721M 721M 721S
+ + + +
= = = =
737L 737ML 737MS 737S
13
Size pipes based on
design flow rates.
From Valve Selection Table
(see opposite page) select
line size valves to suit
design flow rates
Add full open valve
pressure losses to pipe
pressure losses.
Calculate circuit
residual pressures.
Check from Valve SelectionTable that residual pressures
are within the maximumbalancing pressures of
the selected valves
Create a valve schedule
GUIDE TO VALVE SELECTION10.1
Valve Model Size Location Flow kvs Pressureref rate loss signal
(l/s) (kPa)
e.g.
Full open valve pressure losses:
�p=pressure loss(Pa) v=velocity(m/s) �=density(kg/m3) Q=flow rate (l/s)
For residual pressures greater
than the available range try
splitting the pressure loss
between two valves, one on
the flow, one on the return.
e.g.
�p = �� v 2–––2 or �p = 1.296 x 106
Q–kv
2NB� is sometimesreferred to as “k factor”
14
GUIDE TO VALVE SELECTION10.2
VALVE SELECTION TABLE
DRV Double Regulating Valve FMD Flow Measurement Device Q Flow rate (l/s)
* Flow rates required to generate a minimum 1 kPa loss signal across the FMD.
15
Nominal DRV, FMD or Minimum Model Maximum � kv kvs
Diameter DRV+FMD Flow Rate (l/s)* Balancing (k factor)(mm) Pressure
(kPa)
0.015 721L - 414.6 - 0.473FMD 0.028 721M - 92.1 - 0.976
0.055 721S - 21.9 - 1.799
15 DRV- 727L 54200 Q2 65.8 1.278 -- 727S 2366 Q2 7.1 3.905 -
0.015 737L 54200 Q2 480.4 0.473 0.473DRV + FMD 0.028 737ML 54200 Q2 157.9 0.825 0.976
0.028 737MS 2366 Q2 100.4 1.035 0.9760.055 737S 2366 Q2 29.4 1.911 1.799
FMD 0.11 721 - 10.5 - 4.05720 DRV - 727 1250 Q2 6.6 7.281 -
DRV + FMD 0.11 737 1250 Q2 17.8 4.427 4.057FMD 0.21 721 - 8.4 - 7.452
25 DRV - 727 1203 Q2 6.4 11.757 -DRV + FMD 0.21 737 1203 Q2 15.0 7.684 7.452
FMD 0.46 721 - 4.8 - 16.62832 DRV - 727 284 Q2 5.8 21.600 -
DRV + FMD 0.46 737 284 Q2 9.8 16.560 16.628FMD 0.7 721 - 4.5 - 23.000
40 DRV - 727 203 Q2 6.1 28.461 -DRV + FMD 0.7 737 203 Q2 10.7 21.491 23.000
FMD 1.3 721 - 2.2 - 47.35150 DRV - 727 49 Q2 4.9 50.519 -
DRV + FMD 1.3 737 49 Q2 6.6 43.639 47.351FMD 2.7 3721 - 1.5 - 88.7
65 DRV - 3110DRV 225 Q2 7.3 70 -DRV + FMD 2.7 3737 225 Q2 8.5 64.754 88.7
FMD 4.1 3721 - 1.4 - 13680 DRV - 3110DRV 81 Q2 5.6 110 -
DRV + FMD 4.1 3737 81 Q2 6.5 102 136FMD 6.8 3721 - 1.4 - 234
100 DRV - 3110DRV 36 Q2 6.0 180 -DRV + FMD 6.8 3737 36 Q2 7.1 166 234
FMD 10 3721 - 1.4 - 358125 DRV - 3110DRV 13 Q2 4.5 320 -
DRV + FMD 10 3737 13 Q2 5.8 280 358FMD 14 3721 - 1.4 - 512
150 DRV - 3110DRV 5.7 Q2 4.2 470 -DRV + FMD 14 3737 5.7 Q2 5.4 416 512
FMD 25 3721 - 1.5 - 911200 DRV - 3110DRV 2.1 Q2 4.7 790 -
DRV + FMD 25 3737 2.1 Q2 6.0 702 911FMD 38 3721 - 1.6 - 1438
250 DRV - 3110DRV 0.8 Q2 4.7 1250 -DRV + FMD 38 3737 0.8 Q2 6.2 1089 1438
FMD 54 3721 - 1.6 - 2057300 DRV - 3110DRV 0.4 Q2 4.6 1800 -
DRV + FMD 54 3737 0.4 Q2 6.1 1569 2057
GUIDE TO VALVE SELECTION10.3
Flow rate – Q [l/s]
0.0060.004 0.008 0.01 0.02 0.080.06 0.3 0.60.20.1 0.4 0.8 1 2 3 4 6 8 10 200.1
0.2
1
10
100
0.3
0.40.50.6
0.8
2
3
456
8
20
30
405060
80P
ress
ure
Lo
ss –
∆P
[kP
a]
2”
1”1/
41”
1/2
1”3/4”1/
2”
1/2”
LO
W F
LOW
0.04
Recommended operating range of 727 double regulating valves.
CIM 727
Flow rate – Q [l/s]
0.60.4 0.8 1 2 86 30 602010 40 80 100 200 300 400 600 800 1000 20000.1
0.2
1
10
100
0.3
0.40.50.6
0.8
2
3
456
8
20
30
405060
80
Pre
ssu
re L
oss
– ∆
P [
kPa]
4
DN
65
DN
80
DN
100
DN
125
DN
200
DN
250
DN
300
DN
150
Recommended operating range of 3110 DRV double regulating valves.
CIM3110 DRV
16
GUIDE TO VALVE SELECTION10.4
Flow rate – Q [l/s]
0.0060.004 0.008 0.01 0.02 0.080.06 0.3 0.60.20.1 0.4 0.8 1 2 3 4 6 8 10 200.1
0.2
1
10
100
0.3
0.40.50.6
0.8
2
3
456
8
20
30
405060
80P
ress
ure
Lo
ss S
ign
al –
∆P
[kP
a]
721L
–1/2
” K
vs=0
.473
721M
–1/2
” K
vs=0
.976
721S
–1/2
” K
vs=1
.799
721–
3/4”
K
vs=4
.057
721–
1”
Kvs
=7.4
5272
1–1”
1/4
Kvs
=16.
628
721–
1”1/
2 K
vs=2
3.00
072
1–2”
K
vs=4
7.35
1
0.04
Pressure signal graphs for 721 flow measurement devices at DN 1/2 to DN 2”.
CIM 721
Q = kvs� �P36
Flow rate – Q [l/s]
0.60.4 0.8 1 2 86 30 602010 40 80 100 200 300 400 600 800 1000 20000.1
0.2
1
10
100
0.3
0.40.50.6
0.8
2
3
456
8
20
30
405060
80
Pre
ssu
re L
oss
Sig
nal
– ∆
P [
kPa]
3721
–DN
65
Kvs
=88.
70
3721
–DN
80
Kvs
=136
.00
3721
–DN
100
Kvs
=234
.00
3721
–DN
125
Kvs
=358
.00
3721
–DN
150
Kvs
=512
.00
3721
–DN
200
Kvs
=911
.00
3721
–DN
250
Kvs
=143
8.00
3721
–DN
300
Kvs
=205
7.00
4
Pressure signal graphs for 3721 flow measurement devices at DN 65 to DN 300.
CIM 3721
Q = kvs� �P36
17
PROPORTIONAL BALANCING OF FLOW RATES11.1
PROCEDURE
Consider a pipe circuit branch serving several sub-branches. In an unbalanced conditionthe water entering the branch will distribute itself between them, favouring those with thelowest resistances. Therefore, to ensure that each sub-branch receives its correct designflow rate, the flows need to be balanced using the installed regulating valves and flowmeasurement 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 thedesign 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:
�Pmeasured% design flow rate =
�Pdesign
If the signals at any of the installed flow measurement devices are below themeasurement range of the device, further increase the flow rate entering the branch byclosing down adjacent branches.
3. Identify the index sub-branch. This will be the one with the lowest % design flowrate. Usually, but not always, this will be the end sub-branch (furthest from thepump) 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 donewhilst simultaneously measuring the flow at the true index, since its flow rate willchange as the end branch is adjusted. Hence, two operatives each withmanometers and 2-way radios will speed this exercise.
18
PROPORTIONAL BALANCING OF FLOW RATES11.2
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 workingback towards the furthest upstream sub-branch, adjust each sub-branch regulatingvalve 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 sub-branch, since its flow rate will change as upstream valves are adjusted. Hence, twopersons 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 sub-circuit flow rates are now balanced. This balance cannot be disturbed by theadjustment of upstream valves. Hence, upstream branches can be balanced in exactlythe 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 shouldnow 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 regularlyreturned to its design value by adjusting the main branch valve. Systems with fixed orificecommissioning sets can be balanced using the compensated method of balancing, althoughwe believe the compensated method has the following disadvantages:
• A remote indicating manometer is required so that the commissioning specialist atthe 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 therange of a remote indicating manometer) three persons with radios would berequired for balancing.
19
MEASURING EQUIPMENT12
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 relationshipbetween pressure differential and flow rate. Experience has shown that this method ofdetermining flow rate is the most convenient for use in the building services industry.
FLUOROCARBON MANOMETER
The instrument commonly used for measuring pressuredifferential is the manometer. Manometerstraditionally take the form of a U tube arrangementwhereby the pressure differential being measured is usedto displace a fluid of known density, typically mercury ora fluorocarbon. The height of column displaced isdirectly proportional to the pressure differential.Fluorocarbon manometers are typically used to measurepressure differentials in the range 1-4.7 kPa whereasmercury manometers are capable of measuring pressuredifferentials in the range 1-60kPa. In recent years the useof mercury manometers has declined due to the safetyconcerns surrounding the handling of mercury onconstruction sites.
DIGITAL MANOMETER
A digital differential pressure and flow test set is an electronicpressure measuring device which is programmed to enable the directreading of differential pressure and flow. In addition, the regulatingvalve manufacturer’s kv value can be keyed into the instrument sothat the flow rate can be read direct from the manometer therebyavoiding the need to refer to a pressure loss graph. Althoughgenerally reliable, digital manometers do need to be treated with care and regularly calibrated to ensure that accuracy is maintained.
20
DOUBLE REGULATING VALVE
Double regulating valves (DRVs) are so called because they serve the double function of flowregulation and isolation. Once set in their regulated position, they can be locked so that whenclosed 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 andproving a flow balance. Fixed orifices provide a highly accurate means of flowmeasurement 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 flowmeasurement 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 greatestresistance to flow. It can be identified by calculation as the circuit with the highest pressureloss around it when design flow rates are assumed. On site it can be identified by flowmeasurement; it will be the circuit for which the ratio of measured flow rate to design flowrate 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 theindex terminal.
If all terminal branches are of equal resistance, the main system index circuit is likely to befrom 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 mostremote terminal unit they serve. However, if terminal branch resistances vary then the system index and branch indexes will not necessarily coincide with the most remoteterminals. The location of each index will then depend on which circuit has the highestcombination of pipe and terminal branch pressure losses.
As the circuit starting with the highest resistance, there is no need to regulate flow at anindex. At the end of the balancing process, the index circuit should always have a fully openregulating valve.
TERMINOLOGY13.1
21
RESIDUAL PRESSURE
The residual pressure for a particular circuit is the difference between the pressure availableacross that circuit, and the pressure required to achieve the design flow rate. This residual, orexcess pressure, has to be dissipated in some way, and this is usually achieved by addingresistance to the circuit in the form of a regulating valve.
A circuit’s residual pressure is, therefore, critical for sizing regulating valves. A valve’sresistance obviously increases as it is closed, but there is a limit to how much resistance it cangenerate. As a rule of thumb, if a valve is closed beyond its 25% open position it may becomesensitive to air bubbles or prone to blockage from circulating debris.
Valve selection must therefore include a check to ensure that predicted residual pressures arewithin the operating limits of the selected regulating valve. Fortunately, many pipe sizingprograms calculate residual pressures automatically. These values can then be checked againstthe operating range of the selected valve.
TERMINOLOGY13.2
22
KV
The kv value represents the flow rate through a fully open valve at a temperaturebetween 5degC and 40degC, and measured in cubic metres per hour that will induce apressure 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 theresistance 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 theequation:
�p = 1.296 x 106Q–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’sresistance the smaller its kv value. Design engineers are more used to thinking ofresistance in terms of the pressure loss coefficient � (zeta) sometimes also referred to as a "k factor". The pressure loss through any fitting or component can be calculatedfrom the equation:
�p = �� v 2–––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 valveresistances in terms of both kv values and pressure loss coefficients
KVS
This term is usually applied to the pressure loss between the tappings on a flowmeasurement device. The "s" indicates "signal" since it relates to the pressure loss signalmeasured by a commissioning specialist. For a given flow measurement device with aknown kvs value the commissioning specialist can calculate flow rate from the pressureloss signal using the following equation:
Q = kvs� �P where Q = flow rate in l/s, and �P = pressure loss in kPa36
However, the pressure loss between the tappings is not the same as the overallpressure loss across the device. Because there is an increase in static pressuredownstream of the orifice, the overall pressure loss is usually less than the measuredpressure 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).
TERMINOLOGY13.3
23
28017 SAN MAURIZIO D'OPAGLIO (Novara) - ItalyVia Torchio, 57Tel. +39 0322 923001 - Fax +39 0322 967216 / 967755w w w . c i m b e r i o . c o m - e . m a i l : i n f o @ c i m b e r i o . i t