Cibse Variable Flow Dpcv Cpd
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Transcript of Cibse Variable Flow Dpcv Cpd
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1Andy LucasTechnical Development Manager CRANE BS&U
CRANE BS&U supporting the Building Services
Industry
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Crane Co founded in 1855 by Richard Teller Crane who made the following resolution -
“I am resolved to conduct my business in the strictest honesty and fairness; to avoid all deception and trickery; to deal fairly with both customers and competitors; to be liberal and just towards employees; and to put my whole mind upon the business”
Crane Limited founded in Ipswich in 1919
Crane Building Services & Utilities created 2009
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Building Services
Water UtilitiesGas Utilities
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CRANE BS&U 2010 sales £110m
Water Utilities
Gas Utilities Building Services
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industry organisations
• CIBSE
• SoHPE
• BSRIA
• CSA
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6Andy LucasTechnical Development Manager CRANE BS&U
VARIABLE FLOW SYSTEMSincorporating
DPCVs
CIBSE approved CPD
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VARIABLE FLOW SYSTEMS
To give an overview of Variable FlowSystem design and commissioning using Differential Pressure Control Valves
Title
Objective
this applies to both heating and chilled water systems
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Drivers of Change
• driven by Government legislation
• energy conservation
• subsequent CO emissions 2
The move from constant to variable flow design can give up to 80% pump energy savings; about 6 - 8% total energy saving
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• fixed speed pumps – no energy saving for part load
• constant volume of water is pumped around the system
• 3 or 4 port control valve diverts water through by-pass
• commissioned by proportional balancing or by the use of constant flow regulators – ABV (Automatic Balancing Valves)
Constant volume flow systems
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constant amount of water pumped around a system controlled by 3 or 4 port control valves and would be
Constant flow
• through terminal
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constant amount of water pumped around a system controlled by 3 or 4 port control valves and would be
Constant flow
• through terminal• split between terminal and
by-pass
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constant amount of water pumped around a system controlled by 3 or 4 port control valves and would be
Constant flow
• through terminal• split between terminal and
by-pass• diverted back if not required
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• variable speed pumps – energy saving for part load
• variable volume of water to match demand
• diversity factor
• 2 port control valve
• commissioned by combination of;
proportional balancing
DPCV – Differential Pressure Control Valves
Variable volume flow systems
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Variable flow
open
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Variable flow
open modulating between open and closed
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Variable flow
open modulating between open and closed
closed
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Pump energy saving
Pre
ssur
e
P (K
Pa)
Maximum load operating point
as system demand change, flow rate is changed by varying speed of pump
direct relationship between pump speed and flow rate:
50% pump speed = 50% flow rate
50% flow rate = over 85% energy saving
minimum flow rate approx 20%
Flow Rate Q (kg/s)100%20%
100%
4%
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DPCVs to protect 2 port control valves
DPCV holds pressure constant between points A and B
A
B
to enable modulating 2 port control valves to operate with an acceptable authority, a DPCV is installed to limit the pressure differential against which the 2 port valves have to close. The installation of DPCVs on sub-branches with 2 port control valves is therefore essential to achieve good control, as well as to avoid noise or cavitation.
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Installation of 2 port control valvesdue to fluctuating system pressures created by the 2 port control valves opening & closing, consideration needs to be given to valve authority.
valve authority is;
• is ability of the control valve to control flow
• is calculated by dividing the pressure drop across the 2 port at design flow by the pressure drop at no flow
• should not be below 0.3
• higher authority gives better flow control
the installation of Differential Pressure Control Valves (DPCV) protects the 2 ports from the fluctuating/rising pressures
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Installation of 2 port control valvesexample without DPCV
80kPa
20kPa
at design flow rate
valve authority ß = Δp across 2 portΔp across circuit
ß =20 kPa80 kPa
ß = 0.25
too low - unacceptable
always given as a decimal
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Installation of 2 port control valvesexample with DPCV fitted
80kPa
20kPa
at design flow rate
valve authority ß = Δp across 2 portΔp across circuit
ß =20 kPa40 kPa
ß = 0.5
acceptable
40kPa
controlled by DPCV
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Installation of 2 port control valvesposition of DPCV?
at design flow rate
valve authority ß = Δp across 2 portΔp across circuit
ß =20 kPa25 kPa
ß = 0.8
80kPa
25kPa 20kPa
position can influence authority
on single terminal circuits – as closes as possible to control valves gives higher authority
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Installation of 2 port control valves
Valve characteristic
Per
cent
age
flow
rate Coil
characteristic
Percentage open
for good modulating control the control valve needs to achieve equal percentage characteristic i.e. a characteristic that mirrors the characteristic of the coil.
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% open
Installation of 2 port control valves
% h
eat o
utpu
t
equal percentage control valves will only operate with near to an equal percentage characteristic where authority, β, is greater than 0.3
50% output
20% flow
with β = 1
mirror image of coil characteristic
50% valve opening = 50% heat output
flow rate reduces to 20%
output increases
Coil output characteristic
β = 1
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% open
Installation of 2 port control valves
% h
eat o
utpu
t
equal percentage control valves will only operate with near to an equal percentage characteristic where authority, β, is greater than 0.3
with β = 1
mirror image of coil characteristic
authority is reduced as pressure drop increases
Coil output characteristic
β = 1
β = 0.5
β = 0.3
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% open
Installation of 2 port control valves
% h
eat o
utpu
t
equal percentage control valves will only operate with near to an equal percentage characteristic where authority, β, is greater than 0.3
50% outputwith β = 1
mirror image of coil characteristic
effect of reduction in β to 0.3
output increases
Coil output characteristic
β = 0.3
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2 Port
DPCV – operating principle
Upper chamberLower chamber
rising pressure closes DPCV
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for any flow rate there is only one possible pressure drop between any 2 points
the DPCV identifies the 2 points by the connecting impulse tube
setting the DPCV to control Δp (pressure drop) between A & B at 100% DFR controls flow rate between these 2 points
A
B
DPCV – operating principle
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System layout – position of DPCV
Branches are broken down into sub-circuits, each controlled by a DPCV
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System layout – position of DPCV
Branches are broken down into sub-circuits, each controlled by a DPCV
Note: each circuit must only flow through a single DPCV
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Commissioning features around DPCVs
a ‘Companion Valve’ (FODRV) should be installed so that the DPCV can be adjusted until the required design flow rate is achieved.
if required, pressure test points could be installed so that the pressure controlled by the DPCV can be measured and recorded.
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P
test points to check sensor by-pass with isolating valve
should be included to allow the differential pressure to be checked and zeroed
to control pump speed, differential pressure sensor should be located across the most remote DPCV controlled sub-branch with additional sensors on branches that might become the index circuit under part load conditions.
Differential pressure sensors
sensor
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P
to control pump speed, the differential pressure sensor sends signal to BMS which varies pump speed
Differential pressure sensors
BMS
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at maximum pump turndown, typically 10 - 20%, consideration needs to be given to branches to ensure
pump flow at minimum load
circulation of water treatment
ready supply of heating/chilled water
Minimum flow
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end terminal could have a 3 or 4 port control valve
on larger circuits additional 3 or 4 ports could be added
possible solution
Minimum flow
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a constant flow regulator (ABV) could be used
possible solution
Minimum flow
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a pressure relief valve could be installed
possible solution
Minimum flow
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a RADPCV (Reverse Acting DPCV) could be used
possible solution
Minimum flow
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Commissioning Variable Flow Systems
because each sub-circuit is separated by a DPCV from fluctuating system pressure & therefore holds a constant pressure within the sub-circuit, commissioning sub-circuits can be carried out totally independently
sub-circuits are independent of each other
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Commissioning Variable Flow Systems
commissioning within the sub-circuits is carried out by ‘proportional balancing’ in the conventional manner
reference
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Commissioning Variable Flow Systems
each sub-circuit is balanced by measuring flow thro the ‘Companion Valve’ and adjusting DPCV to regulate flow
commissioning valve normally fully open
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Commissioning Variable Flow Systems
for circuits nearer the pump the regulating function of the commissioning valve may be used to reduce the pressure drop across the DPCV to bring it into a better operating position, ie splits residual pressure
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To summarise • change in system design to variable flow controlled by 2 port
control valve resulting in pump energy saving
• fluctuation in system pressure undermines control valve authority
• DPCV installed into sub-circuits to ‘protect’ control valves from fluctuating pressure to maintain control valve authority
• provision for pump turndown, typically 10 – 20%
• terminal units commissioned by conventional proportional method
• branches commissioned by use of ‘Companion’ Valve & DPCV
• branches commissioned independently of each other
• pressure sensors used to set pump speed
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Written byAndy Lucas
Technical Development Manager CRANE BS&U
VARIABLE FLOW SYSTEMSincorporating
DPCVs