Variable flow pipework systems : valve solutions - supplement to ks7

16 Pages·2009·0.438 MB·English

Variable flow pipework
systems: valve
solutions
Supplement to CIBSE Knowledge Series KS7
Principal author
Chris Parsloe
Editor
Ken Butcher
CIBSE Knowledge Series — Variable flow pipework systems:
valve solutions
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No part of this publication may be reproduced, stored in a retrieval system or transmitted
in any form or by any means without the prior permission of the Institution.
© August 2009 The Chartered Institution of Building Services Engineers London
Registered charity number 278104
ISBN: 978-1-906846-09-1
This document is based on the best knowledge available at the time of publication.
However no responsibility of any kind for any injury, death, loss, damage or delay
however caused resulting from the use of these recommendations can be accepted by the
Chartered Institution of Building Services Engineers, the authors or others involved in its
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Typeset by CIBSE Publications
Printed in Great Britain by The Charlesworth Group, Wakefield, West Yorkshire, WF2 9LP
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
2 Centralised valve module solution . . . . . . . . . . . . . . . . . . . . . . . . . .2
3 Pressure independent control valve solution . . . . . . . . . . . . . . . .8
References and bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12
CIBSE Knowledge Series — Variable flow pipework systems:
valve solutions
1 Introduction
This publication forms a supplement to CIBSE Knowledge Series KS7:
Variable flow pipework systems (CIBSE, 2006). It explains how to design re-
circulating heating and cooling water systems incorporating variable speed
pumps utilising two alternative valve solutions not covered in KS7, these
being:
— centralised valve modules
— pressure independent control valves (PICVs)
KS7 explains the main principles underlying the design of variable flow
pipework systems, and provides two alternative approaches to system
design. These are:
— self-balancing arrangements in which terminal units are connected
from reverse return branches, and pump speed is controlled to
maintain constant pressure differentials across the terminals
— designs incorporating differential pressure control valves (DPCVs) in
which the system is laid out as a conventional 2-pipe flow and return
configuration but with DPCVs strategically located on sub-branches
in order to minimise the pressure differentials across downstream
2-port control valves.
Knowledge Series KS9: Commissioning variable flow pipework systems (CIBSE,
2007) describes the commissioning procedures for variable flow systems
designed using these two methods.
Both of the above methods give satisfactory results when used on large
re-circulating heating and cooling water pipework systems. Furthermore,
both systems can be designed and specified using a variety of alternative
valve products from different suppliers.
Since the publication of KS7 and KS9, valve technology has advanced to the
point where there are now commonly available products that help to
simplify the design of variable flow systems. Hence, the need for this
supplement, which builds on the principles explained in KS7 but offers
additional design approaches both of which have proved successful on actual
projects.
CIBSE Knowledge Series — Variable flow pipework systems: 1
valve solutions
2 Centralised valve module solution
Valve modules are ‘mini-headers’ that distribute flow from a central valve
manifold arrangement to groups of up to 8 terminal units. The grouping of
terminal units is dictated by their relative locations and flow rates. Modules
are designed, pre-fabricated and pressure tested off-site resulting in reduced
site installation time.
The valve module concept effectively creates exactly the same layout as for
the DPCV-based solution described in KS7, i.e. a DPCV controlling the
pressure differential across a sub-branch serving multiple terminal units.
Hence, the guidance provided in KS7 and KS9 on DPCV systems is equally
applicable to valve modules.
A typical ‘basic’ valve module layout is shown in Figure 1 (although the
layout and valve choice may vary between suppliers). In particular, adjustable
valves (i.e. regulating 2-port control valves or pressure independent control
valves) may be incorporated enabling centralised control functions.
Figure 1:
3
Schematic diagram of
4
basic valve module (see O
P
Figure 3 for definitions DRV TP AV TPIV IV IV IV 2
of symbols)
IV DPCV 5 IVDO IV STR IV
C
6
1
With regard to the numbered features in Figure 1, the common features are
as follows:
(1) Strainer: a strainer is located at the inlet to the module in order to
remove any solid particles from the water before they can become
blocked in downstream control valves or terminal units. This is an
ideal location for the strainer since all of the downstream materials
are non-corrosive, meaning there is less risk of the water becoming
re-contaminated once it has passed through the strainer. The strainer
body should be as large as possible to reduce the need for frequent
cleaning. By incorporating strainers in the module, there is no
necessity for strainers on upstream branches.
(2) Flow manifold with isolation: manifolds provide an effective method for
creating multiple tee connections from a central pipe. Each manifold
port should have some form of isolating valve so that individual
terminal units can be isolated. Where centralised control is required,
2 CIBSE Knowledge Series — Variable flow pipework systems:
valve solutions
an alternative is to install ‘isolatable’ 2-port control valves instead of
isolating valves.
— Flexible multilayer pipe run-outs to terminal units: running rigid pipe
from a central location is often impractical resulting in numerous
elbows and joints, and consequently excessive labour times. A flexible
pipe is preferable, eliminating the need for elbows between the
module and terminal units and requiring only two joints per pipe —
one at the manifold and one at the terminal. Flexible multilayer pipe is
best suited to the pressures and temperatures in large heating and
chilled water systems. The pipe is essentially a butt-welded aluminium
pipe, coated internally and externally with either high density
polyethylene or cross linked polyethylene. It has similar strength and
expansion properties to copper. High strength compression or press-
fit joints are available.
— A flushing bypass arrangement with built-in flushing drain: this is
required in order to achieve compliance with BSRIA Guide
AG1/2001.1: Pre-commission cleaning of pipework systems (Parsloe,
2004). Firstly, by isolating all of the terminal branches and opening
both left and right central isolating valves, dirty water can be flushed
at high velocity out of the upstream pipework system without having
to pass through the terminals. Once the water in the main system is
clean, terminal units can then be forward flushed by opening the left
hand isolating valve and running water out through the central drain.
Terminals can then be back-flushed by opening the right hand valve
and again running water out through the central drain. An air vent
assists final filling of the pipes connecting to the terminal units.
— A return manifold with close coupled commissioning sets: fixed orifice
double regulating valves (commissioning sets) are required to measure
and regulate the flow rates to terminal units. These are orifice plate
type flow measurement devices close-coupled to double regulating
valves. Flow measurement devices must be provided with at least five
diameters of straight, rigid pipe upstream of their inlets to ensure flow
measurement accuracy.
— Differential pressure control valve (DPCV): a DPCV can be used to
adjust, and then hold constant, the pressure differential between flow
and return manifolds. This means that the pressure differential against
which 2-port control valves need to close can be limited. This is
important in order to avoid valve noise, and make it easier to select
valves with good authority. In the case of the valve module, all 2-port
control valves are sized against the pressure differential controlled
constant across the manifolds by the DPCV. Figure 2 illustrates the
CIBSE Knowledge Series — Variable flow pipework systems: 3
valve solutions
principle. Due to the proximity of the DPCV to the control valves,
authorities of 0.3–0.7 are usually achievable provided pipe lengths,
and hence pipe pressure losses, are not excessive. Pipe lengths
greater than 15m between module and terminal might need to be
increased in size in order to reduce their pressure loss and make it
easier to select the 2-port valves.
Figure 2:
Calculation of control
valve authority in valve
2
P
module application V
p
1
(see Figure 3 for symbol
definitions)
O
P
DRV TP AV TPIV IV IV IV
IV DPCV IVDO IV STR IV
C
p+ p
2 1
Incorporating valve modules into systems
As previously stated, valve modules are essentially centralised versions of the
DPCV based solution described in KS7 (CIBSE, 2006), i.e. a DPCV
controlling pressure across a sub-branch serving multiple terminal units. All
of the components required in the branches served by a DPCV controlled
system are incorporated into a valve module. Figure 3 shows a typical
system layout incorporating valve modules and the accompanying
components that are required on connecting branches.
The designer should be aware of the following issues during design:
— Valve selection: terminal branch regulating valves, 2-port control valves
and DPCVs need to be sized and selected to suit each situation.
Because there is some interdependency between the sizes of these
valves, the supplier of the valve module is usually well placed to size
all of them, if provided with details of terminal unit pressure losses
and connecting pipe lengths.
4 CIBSE Knowledge Series — Variable flow pipework systems:
valve solutions
Isolating valve TP Pressure test point
IV
Orifice plate type flow Flexible connection
OP measurement device FC
OP DRV C(cmdlooeomsauesmb-uclreiose usrmiepogelnenuidntla gdtto iens vefgiltco vewa)lve DOC DPuraminp-off cock
STR Y-type strainer DPS Dseinffseorerntial pressure
NRV Non-return valve P Pressure gauge 2PV 2PV 4PV 2PV 2PV 4PV
2PV
2-port control valve VM Valve module
4PV 4-port control valve AV Air vent VM VM
IV IV IV IV TP
IV
TP IV DPS
AV IV
OPDRV
IV
IV OP
2PV 2PV 2PV 2PV 4PV
VM
IV IV TP
IV
TP DPS
IV
2PV 2PV 4PV 2PV 2PV 4PV
VM VM
IV IV IV IV
IV
IV OP
2PV 2PV 2PV 2PV 4PV
VM
IV IV
PTP TP P OPIV Primary
IV NRV FC FC IV TP TP header
IV DOC IV DOC SecondaryIV NRV FPCTP TPFC PIV STP IV
duty/standby
Pump speed
pumps
controller
Figure 3:
Schematic of a typical
system incorporating
valve modules
CIBSE Knowledge Series — Variable flow pipework systems: 5
valve solutions
— Minimum pressure differential: in order to operate satisfactorily, the
DPCV must have enough pressure across it to enable its internal
spring to move and hence control pressure. This minimum is typically
in the range 10–15kPa for 15–32mm diameter valves. Specific values
are given in valve product brochures. In order to determine whether
there is sufficient pressure across each DPCV, modules sometimes
incorporate pressure test points to enable the pressure differential to
be measured.
— Flow measurement: flow rates are measurable to each terminal unit.
For checking purposes, flow measurement devices can be located on
main branches and sub-branches upstream of the valve modules, as
deemed necessary by the designer.
— Upstream regulating valves: since the DPCVs inside the modules will
vary their position depending on system pressures, there is no need
for upstream regulating valves. Due to the action of the DPCVs, the
flow balance will be maintained regardless of subsequent 2-port valve
closures or variations in pump speed.
— Maximum pressure differential: the DPCVs installed within modules
must be able to operate and close against the maximum pressure
generated by the pump. DPCVs with differential pressure ratings of
up to 1.2MPa (12bar) are available which should satisfy the majority
of applications.
— Pump speed control: pump speed must be controlled so as to maintain
a minimum pressure differential at some selected point (or points) in
the system. The most energy efficient approach is to locate a
differential pressure sensor across the index valve module (usually the
one furthest from the pump) and to control pump speed such that
the pressure required across this branch is always maintained. Each
sensor connection should be provided with test points and a bypass,
as shown in Figure 3, to enable the sensor to be calibrated and
zeroed. If there is a risk that the index might move, as would be the
case if all of the 2-port valves fed from the most remote module were
to close, then an additional sensor might be required on the new
index, i.e. the next furthest module as shown in Figure 3. The pump
would then be controlled to ensure that the pressure required across
both branches would always be maintained. For the same reason,
zones with different load patterns fed from the same system should
also have their own sensors.
— Pressure relief at part load: when all of the 2-port control valves are
approaching their closed positions, there needs to be some path open
6 CIBSE Knowledge Series — Variable flow pipework systems:
valve solutions
to flow to prevent the pump operating against a closed system. A
simple solution is to incorporate at least one 4-port valve, with a
built-in bypass, on each group of terminals, as shown in Figure 3. The
selection and positioning of 4-port valves should ensure that the pump
can achieve at least an 80% turndown in flow rate at minimum load
conditions. To give a better match of resistances across control
valves, 4-port valves should be selected as if they are 2-port valves.
CIBSE Knowledge Series — Variable flow pipework systems: 7
valve solutions