VARIABLE FLOW SYSTEMS
INCORPORATING DIFFERENTIAL
PRESSURE CONTROL VALVES (DPCVs)
Andy Lucas - Crane BS&U
Chris Parsloe - Parsloe Consulting Ltd
Crane Co. was 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 was founded in Ipswich in 1919
Crane Building Services & Utilities was created 2009
including
including
Water, Air and Steam Safety Valves
Automatic Air Eliminators
Differential Pressure Control Valves (DPCVs)
DPCVs hold pressure constant
across a varying resistance
(such as a 2 port valve). An
impulse tube connects upstream
of the variable resistance (A) to
the top side of a flexible
diaphragm. A separate capillary
(or internal tube) connects from
downstream of the variable
resistance (B) to the underside
of the diaphragm. As the overall
pressure available A-C changes,
the DPCV adjusts its position
such that the pressure drop A-B
remains constant. This means
that the 2 port valve only needs
to close against a fixed,
pre-set pressure differential
C
B
A
Two Port Control Valves
This is important because some 2 port control valves
are limited in their ability to close against large
pressure differentials.
TRV
IV
An in-built
temperature sensor
closes the valve
when satisfied
The simplest form of 2 port control valve is the
Thermostatic Radiator Valve (TRV). These valves
sense the temperature in the room and gradually
close when the required temperature is achieved.
They usually rely on the expansion and contraction of
a wax capsule inside the valve.
TRVs are typically only capable of closing against
around 30kPa pressure differential. Hence, left
unprotected in a system with pump pressure greater
30kPa, they may struggle to close and can become
noisy.
2 Port Control Valves
Heating and cooling coil circuits use more
sophisticated 2 port control valves that are
operated by electrically powered actuators.
This means that the valves can shut off
against much larger pressure differentials
(200-500kPa, depending on valve size).
Room
sensor
However, simply because the valve can close
against such a high pressure differential
doesn’t mean it is a good idea to let it do so.
Actuator
Cavitation in 2 Port Control Valves
The closure of valves against excessive
pressures can still cause noise and, if the
static pressure in the system is low enough,
there is a risk of cavitation in the valve.
Cavitation is the localised vaporisation of a
liquid. When the absolute pressure
approaches the vapour pressure of the liquid,
dissolved air is released and small bubbles
of vapour are formed. These bubbles form
and then collapse rapidly releasing
enormous amounts of energy which can
cause damage to metal components such as
valves.
Percentage flow rate
If good modulating control is required, then the control valve needs to
achieve an equal percentage characteristic i.e. a characteristic that
mirrors the heat transfer characteristic of the coil. This will ensure that
closure of the valve will achieve a steady reduction in heating or
cooling output from the coil enabling close control of internal
temperature.
Coil
characteristic
Valve
characteristic
Percentage open
Valve Authority
p1
% flow
An equal percentage control valve
can be specified, but it will only
operate with an equal percentage
characteristic if, when fully open,
the pressure loss across it
represents a significant proportion
of the overall pressure loss in the
circuit it controls. This relationship
p1 / (p1 + p2) in the diagram below
is referred to as “valve authority”.
Ideally the authority should never
be less than 0.3. The graphs
below show the effect of varying
valve authority on the valve’s
characteristic.
% open
DPCVs to Protect Downstream 2 Port Control Valves
In large systems it is usually impossible to select modulating 2 port control valves with an
acceptable authority unless there is some form of differential pressure control that limits the
pressure differential against which the 2 port valves have to close. The positioning of
DPCVs on sub-branches serving downstream 2 port control valves is therefore essential to
achieve good control, as well as to avoid noise or cavitation.
A
B
C
A DPCV holds pressure constant between
points A and B regardless of changes in
pressure between A and C.
System Layout
Branches to each
level feed flow return
circuits which are
themselves broken
down into a series of
sub-circuits. Each
sub-circuit has its own
DPCV.
The secondary
pumps are variable
speed.
Terminal Branches
End terminal units should be given constant flow (3 or 4 port valves) to ensure that:
- there is flow through the pump at minimum load
- water treatment chemicals are circulated to extremities
- when control valves open, there is a ready supply of hot or cold water in the mains.
1 in 5 of the central control valves on each circuit should also be selected as a constant flow (3 or 4
port valves) with the aim of achieving approximately 80% reduction in flow at minimum load.
Locations of DPCVs to Facilitate 2 Port Valve Selection
The constant pressure, controlled by
the DPCV must not exceed 1.5 times
the pressure drop across the end
terminal branch Dptb
Branches should be limited to
no more than 12 terminal
units per DPCV controlled
sub-branch.
Dptb
This will make it possible to select 2 port control valves with acceptable authorities (i.e. >0.3)
2 Port Valve Selection
2 port valves must be sized to achieve an authority of at least 0.3 i.e. p1 divided by p1 plus p2 must be
greater than 0.3 where p1 plus p2 is equal to the total loss through the downstream index branch.
p1 + p2
p1
p1 + p2 = total loss through downstream index branch
Commissioning Features Around DPCVs
A Companion Valve, FODRV, should be
located upstream of the DPCV so that the
DPCV can be adjusted until the required
design flow rate is achieved.
A Capillary tube connects
each side of the controlled
sub-circuit.
Pressure test points should be located
adjacent to each capillary tube
connection so that the pressure
controlled constant by the DPCV can be
measured and recorded.
Pump Speed Control
Pressure Dp (kPa)
Speed control based on
constant pump pressure
Speed control
based on remote
differential
pressure sensors
.
.
.
Flow Rate Q (kg/s) 50%
.
Maximum load
operating point
The pump energy saving achieved is
influenced by how pump speed is controlled.
Integral pump controllers enable the pump to
be controlled to maintain constant pressure or
at an arbitrarily selected pressure proportional
to the reduction in flow rate. The savings
achieved using these methods are not as
good as by using remote differential pressure
sensors.
Speed control based on
proportion of flow rate
Typical energy
saving 30-40%
Typical energy
saving 80-90%
100%
Typical energy
saving 50-60%
Pump Speed Control
Could the index move to an
upstream sub-branch? (Yes if
the entire index branch closes
down) If so an additional
sensor is required here.
Pump speed should be controlled
to maintain the minimum
specified pressure differentials at
all sensors.
P
Differential pressure sensor across the
DPCV controlled index sub-branch.
The pump speed should be controlled
to maintain the minimum specified
pressure differential.
PP
Could the index move to the
floor below? (Yes if the entire
top floor closes down)
If so, an additional
differential pressure sensor
should be installed here.
Features Around Differential Pressure Sensors
Differential pressure sensors should be located across the most remote DPCV controlled sub-branch
with additional sensors on any upstream branches that might become the index under part load
conditions. Pump speed should be controlled such that the minimum specified pressure differential at
each sensor is maintained.
Isolating valves should
be incorporated in pipe
connections so that the
sensor can be isolated
and removed if
necessary.
Tee off connections to the
sensor must be at least
five diameters downstream
of bends or other
restrictions.
Pressure tappings should be
included either side of the
sensor, so that the sensor can
be checked and recalibrated.
P
A by-pass with an isolating
valve should be included to
allow the differential pressure to
be checked and zeroed.
Centralised Valve Modules
Pre-assembled centralised valve modules contain all of the valves required to feed a group of terminal
units. Flexible multilayer pipe connects from the module to the terminals.
Valve Module
Centralised Valve Modules
Commissioning sets on
each return to enable
flow balancing.
A single DPCV
ensures a
constant
pressure across
the flow and
return manifolds.
An integral flushing by-pass in
accordance with BSRIA guidance
Drain cock for back
flushing of terminals
A Ball valve on each
flow to enable isolation.
A large bodied strainer provides
protection to downstream valves.
Centralised Valve Modules
CommPac
Flow
Management
Module