Control Valve Authority and yPressure Independent Systems
TA Hydronic CollegeTA Hydronic College Hailey Mick
Agenda
Why differential controllers?• Controllability in ATC valves• Energy SavingsEnergy Savings• Retrofit-ability
How to use Dp controllers?How to use Dp controllers?• How does it work• Where do they go• Balancing the systemBalancing the system
How TA/Victaulic can help• Hydronic College SeminarsHydronic College Seminars• TA Select 4• Engineering Services Center
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Why Differential Pressure Control?
• Control valves work with improved authority, therefore their performance is improved
• Reducing pump head and keep high controllability in the system
• Control valves are pressure relieved, so low force (= lower cost) actuators can be used
• Noise in control valves is reduced or removed completely
• Circuits are pressure independent modules. Which means:• That the changes in other parts of the system do not affect the circuit• Large plants can be balanced module by module independently• Large plants can be balanced module by module independently• New modules can be added to the system without rebalancing
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Differential Pressure Variations
g se
ason
this
load HeatingHeating 2qP
100%
120%
of h
eatin
gor
bel
ow t
80%
100%Power Dp piping
58%
40%
60%
80%
Thermal plant load [%]%
at
20%
40%
60%
50 % load
0%
20%
0% 50% 100% 150% 200%
easo
ns
load
Thermal plant load [%] 0%
20%
0% 20% 40% 60% 80% 100%
load 4% press.drop
Flow Flow
cool
ing
sebe
low
this 20 %
flowAt constant supply water temperature
4
% o
f at
or
CoolingCooling68% Pressure drops are reduced
to 4% of their design value.
Control Loop
Set value U Disturbances
x =U - x
0 - 10 volts 0-100%k2 k3
0-100% 0-100%k4 k5x
Sensor TerminalController Actuator Valve Room
x
FlowPoweroutputLiftSignal
k1
x = controlled value
Flow in %Heat output in % Heat output in %Power output %Power output % Flow in%10090807060
Flow in %10090807060
10090807060
Heat output in %
=+
Heat output in %pp
5040302010
Lift h in %
5040302010
5040302010
=+
Flow in % Lift h in %
5
00 10 20 30 40 50 60 70 90 10080
00 10 20 30 40 50 60 70 90 10080
00 10 20 30 40 50 60 70 90 10080
Flow in %
Terminal unit characteristic Control valve characteristic
Control Valve Authority
flow design and openfully valve Control
PP
shutfully valve ControlP
Th th it (β) f l t h h
qp
pVH The authority (β) formulates how much the differential pressure builds up on the control orifice of a control valve when it is closing
100% 50% ~35%pV
Bal Valve
H
C
is closing
~15%
Its value indicates how effectively the control valve can reduce the flow while it is closing.
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2-way Control Valve Authority (Variable Flow)
Constant as soon as the valve Cv is chosen (pV).
h tf lllC t l
flow design and openfully valve Control
PP
shutfully valveControlP
V i bl d d fl i th i iVariable, depends on flows in the piping, thus also on the opening of all the other control valves.
In a variable flow distribution, th th it f t l l i i bl
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the authority of a control valve is variable.
Distortion of Valve Characteristic
The lower the authority, the larger the ∆p variations on the control valve, the larger distortion of the valve characteristic
80
90
100
ow % Unstable
control design
40
50
60
70
800.1 0.25 0.5 1Fl
o control
BadFair toGood Excellent
0
10
20
30
0 0.25 0.5 1.0 0 10 20 30 40 50 60 70 80 90 100
Control valve with Lift %
minimumLowest acceptable value
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Equal-percentage characteristic (EQM)
Variable Authority of 2-way Control Valves
Authority in design conditions: ≈ 5/(5+7) = 0.42
H
Authority at half-load: = 5/(5+7+0.96*21) = 0.15 !
Low flow (half load)Low flow (half-load)0.96*21 ft in excessin the valve at half-load
ft
Pump head
5 ft in the valve
7 ft in the circuit
H: 3
3 f
VSP does not allow to compensate for all local
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p 7 ft in the circuit Dp variations in the plant
Control Valve Sizing
Control valves are commercially available with Cv l i i di t th R d ivalues increasing according to the Reynard series:
Cv:....... 2.0 3.0 4.0 5.0 10 20 30 ........
Flow to a FCU of 29 gpm p 5 psi and 2 psi in
Cv: 11Cv: 11 20Cv: 11 20 10
Flow to a FCU of 29 gpm, p 5 psi and 2 psi in connecting pipes. the commercially available control valves create a design pV of:pVH
Cv: 11∆pV [psi] 7
design 0.5
Cv: 11 20∆pV [psi] 7 2.12
design 0.5 0.23
Cv: 11 20 10∆pV [psi] 7 2.12 8.49
NOTHING in betweendesign 0.5 0.23 0.55
STAD
∆H [psi] 14∆H [psi] 14 9.12∆H [psi] 14 9.12 15.49
10Conclusion:Conclusion:Control valves are generally oversized.
Effect of Dp Variations
p variations distort the characteristic of the control valve the nonlinear characteristic of the terminal unit is no longer compensated
90
100Power output in %
90
100Power output in %
90
100Power output in %
H x 10
60
70
80
Design60
70
80 H x 4
Design60
70
80 H x 4
Design
30
40
50
Valve characteristic:30
40
50
Valve characteristic:30
40
50
Valve characteristic:
0
10
20
Control valve lift
Valve characteristic:EQM = 0.33 R = 25Rangeability
area of thecontrol valve 0
10
20
30
Control valve lift
Valve characteristic:EQM = 0.33 R = 25
0
10
20
Control valve lift
Valve characteristic:EQM = 0.33 R = 25
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010 20 30 40 50 60 70 80 90 1000
010 20 30 40 50 60 70 80 90 1000
010 20 30 40 50 60 70 80 90 1000
ON/OFF control
Noise
Sound pressure level [dB]
Cavitatinggvalve
RULE OF THUMBRULE OF THUMB :Static pressure at the inlet of the valve should be at least twice the pressure pdrop in the valve.
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Closing of Control Valves
According to its design, each valve has a required actuation close-off force or torque that depends on:
Tension of the return spring, if any, Friction with o-rings and seals, Differential pressure applied on the plug.
Each control valve/actuator combination has a certainEach control valve/actuator combination has a certain close-off differential pressure
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Agreed?
The differential pressure across control valves must not vary too much.
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Control Valve Authority
To achieve good control it’s recommended to fulfill two rules on authority: 1. Size the control valve with a Cv with design ≥ 0.52 E h 0 2
p circuit2. Ensure that min ≥ 0.25
H
H
p
H pV
pCppipe
pBV
Rule no 1: Rule no 2:p piping
pV ≥ pC + ppipe + pSTADor
pV ≥ 0.5 × H
pV ≥ (ppiping + pC)/3or
pV ≥ 0.25 × H
15design ≥ 0.5 min ≥ 0.25
Improved Control by Correct Control Valve Sizing
IDEAIDEAH
p circuit: 7 ft IDEAIDEAEnsure design authority of at least 0.5 and minimum on 0.25 in all control valves in the worst
Hin all control valves in the worstconditions.
HP
design
flowdesign andopen fully valveControl
p piping: 33 ftH
P flow design and openfully valve Controlmin
Rule no 1: Rule no 2:Rule no 1:For obtaining a design authority of 0.5:
p in control valve must be ≥ 0.5×H
Rule no 2:For obtaining a minimum authority of 0.25:
p in control valve must be ≥ 0.25×H
Since p circuit = 7 ft, p in control valve must be ≥ 7 ft
Final pump head = 40 + 7 = 47 ft
Since p piping + circuit = 33 + 7 = 40 ft, p in control valve must be ≥ 13.3 ft (40/3)
Final pump head = 40 + 13.3 = 53.3 ft
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Final pump head 40 7 47 ftdesign = 7/14 = 0.5 but min = 7/47 = 0.15
Final pump head 40 13.3 53.3 ftdesign = 13.3/20.3 = 0.66 andmin = 13.3/53.3 = 0.25
Improved Control with Reduced Pumping Energy
Hp circuit: 7 ft
H
p riser: 2 ftHead (ft)
5553.3 ft8% savings in
Control valve sizing with Dp control:p piping: 31 ft 50
49 ft
8% savings in Pump Head
For obtaining a design authority of 0.5 and min of 0.25:
p in control valve must be ≥ 0.5×H and ≥ 0.25 of stabilized p
Since p circuit = 7 ft
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Since p circuit = 7 ft, p in control valve must be ≥ 7 ft
Final stabilized p = 7 + 7 + 2 = 16 ft = 7/14 = 0 50 and Fl
0
…
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design = 7/14 = 0.50 and min 7/16 = 0.44
Final pump head = 31 + min p of DpC (2 ft) + 2 + 7 + 7 = 49 ft
Flow
How Does it Work ?
+Measuring valve
Flowmeasurement
+
PHDifferential
- -
DifferentialpressureStabilization
Dp controller
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Dp Controller Position
Depending on project structure, Dp control will be applied:
On risers On branches On control valvesOn risers, On branches, On control valves.
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Your Plant Could Have it All
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Find the best Dp control solution…
First, decompose the plant into modules
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into modules
Dp Control Solution… (1)
Dp control on each control valve
Parameters:On-off or modulating controlor
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Dp in pipes; length of branchesMaterial cost
Dp Control Solution… (2)
Dp control on branches
Parameters:On-off or modulating control
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Dp in pipes; length of branchesMaterial cost
Key 2 Take away …
Using differential pressure controllers is theUsing differential pressure controllers is the most cost effective method to insure a
minimum stabilized control valve authority
Dp Controllers can reduce your operating cost
Maintain control of what gets installed in your design by including in your spec minimum control valve authority ≥ 0 25 to be verifiedcontrol valve authority ≥ 0.25 to be verified
during submittal process
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Thank you for your attention !Thank you for your attention !
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