Control Valve Authority and yPressure Independent Systems

TA Hydronic CollegeTA Hydronic College Hailey Mick

Hmick@TAHydronicCollege.com

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

45

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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