CIBSE - KS7 - Variable Flow Pipework

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CIBSE Knowledge Series — Variable flow pipework systems: valve solutions

Variable flow pipework systems: valve

solutions

Supplement to CIBSE Knowledge Series KS7

Principal authorChris Parsloe

Editor

Ken Butcher



The rights of publication or translation are reserved.

No part of this publication may be reproduced, sto red in a retr ieval system or t ransmitted

in any form o r by any means w ithout the prior permission of t he Institution.

© August 2009 The C hartered Institution of Building Services Engineers Londo n

Registered charity number 278104

ISBN: 978-1-906846-09-1

This document is based on the best know ledge available at the t ime of publication.

How ever 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

publication. In adopting these reco mmendations for use each ado pter by doing so agrees

to accept full responsibility for any personal injury, death, loss, damage or delay arising out

of or in connection w ith their use by or o n behalf of such adopter irrespective of the cause

or reason therefore and agrees to defend, indemnify and hold harmless the Chartered

Institution of Building Services Engineers, the authors and others involved in their

publication from any and all liability arising out of or in connection with such use as

aforesaid and irrespective of any negligence on t he part of those indemnified.

Typeset by CIBSE Publicat ions

Printed in Great Britain by The C harlesw ort h 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


http://46091_01.pdf/

http://46091_02.pdf/

http://46091_03.pdf/

http://46091_refs.pdf/

http://46091_refs.pdf/

http://46091_03.pdf/

http://46091_02.pdf/

http://46091_01.pdf/




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 thissupplement, which builds on the principles explained in KS7 but offers

additional design approaches both of which have proved successful on actual

projects.

1



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.

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 frequentcleaning. 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,

CIBSE Knowledge Series — Variable flow pipework systems: valve solutions2

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Figure 1:

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basic valve module (see

Figure 3 for definitions

of symbols)




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 themodule 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 toadjust, 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

3



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 15 m 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.

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.


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CIBSE Knowledge Series — Variable flow pipework systems: valve solutions 5

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Figure 3:

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system incorporating

valve modules



— 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 10–15 kPa for 15–32 mm 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 tobe 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 1.2 MPa (12 bar) 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





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.

7



3 Pressure independent control valve solution

Pressure independent control valves (PICVs), sometimes referred to as

‘combination valves’, integrate the functions of flow limitation, modulating

control and differential pressure control within a single valve body. The

layout and appearance of PICVs varies considerably but they all perform thesame basic functions. Figure 4 shows a cross section through a generic valve

type. The valve body contains two main components. In the top half of the

body is the control valve and flow limiting component and the bottom half

contains the DPCV. With regard to the numbered features in Figure 4:

(1) Flow limiting device: some valves have a specially designed flow

regulator to enable the flow through the valve to be set. Once set,

the flow rate through the fully open valve is held constant by the

action of the DPCV (hence it is referred to as a ‘flow limiting device’).

An alternative approach used for some PICVs is to use the travel of

the 2-port valve for flow regulation, as described below.

(2) 2-port control valve: the 2-port control valve is often used for flow

regulation as well as flow control, i.e. the valve is throttled until the

required flow rate is achieved, then the remaining travel on the valve

spindle is used for modulating control of flow rate. (In this case, the

2-port valve is effectively the flow limiting device.) For good

modulating control, the 2-port control valve should be able to achieve

an equal percentage control characteristic (as opposed to an on/off or

linear characteristic). This can be achieved by a combination of the

shape and design of the valve plug and the action of the actuator to

which it is fitted.

(3) Flow setting dial: a flow setting dial at the top of the valve spindle

permits adjustment of the flow limiting device. By turning the dial, the

device can be manually opened or closed until the design flow rate is

achieved. The flow setting dial is usually marked with flow rate values

(or calibrations that can be read from a graph) to enable the flow to

be set without the need for proportional balancing. This is possible

due to the function of the DPCV as explained below.

(4) Differential pressure control: after the 2-port control valve, water

passes through an in-built differential pressure control valve (DPCV).

The DPCV automatically adjusts its position by sensing the differential

pressure across the flow limiting device and/or control valve, i.e.

between points A and B in Figure 4. A small pressure tube transmits

the pressure of the water entering the device to a chamber at the

bottom of the valve which forms one side of the DPCV diaphragm.


3

21

PressuretubeRubber

diaphragm

4

C B A

Figure 4:

Schematic of a typical

pressure independent

control valve (PICV)




Water that has passed through the 2-port valve is in contact with the

other side of the diaphragm. Hence, the diaphragm will move in

response to changes in the pressure differential between these two

points thereby varying the opening through the DPCV. Similarly, if the

overall pressure differential between points A and C in Figure 4

should vary due to other valves closing or the pump varying its speed,

the DPCV will again sense these changes and adjust its position such that the pressure drop between points A and B is unaffected. It can

be seen that by holding the pressure constant between points A and B

with the flow limiting device (or 2-port valve) in its set position, the

result is a fixed pressure differential across a fixed resistance resulting

in a constant flow rate. This explains how it is possible to limit the

flow to a specific value using the flow setting dial, and why this

maximum flow rate will remain set until the 2-port valve begins to

close.

Incorporating PICVs into systems

Figure 5 shows a typical system layout incorporating PICVs and the

accompanying components that are required on connecting branches.

The designer should be aware of the following issues during design:

— Valve selection: due to the function of the integral DPCV, PICVs can

be selected based on terminal unit design flow rates alone. However,

the flow setting range on some PICVs is limited making it difficult to

select valves for some low flow applications.

— Pre-commission cleaning: small bore (i.e. 10 and 15 mm diameter)

PICVs may have a high resistance that could hinder the flushing of

terminal units. For PICVs with a k v value less than 0.4, a flushing drain

should be incorporated in the pipework between the terminal unit

and the PICV.

— Control valve authority : since the pressure differential is held constant

across the 2-port valve section of the PICV, in theory, the control

valve authority achieved will be equal to 1 (i.e. perfect control). For

valves with an equal percentage characteristic, good modulatingcontrol of flow should therefore be possible. It should be noted,

however, that some small diameter PICVs do not necessarily maintain

an equal percentage characteristic under all operating conditions. The

designer may therefore need to clarify the characteristic of each

particular PICV, and judge the level of control achievable, and

whether it is acceptable for the application in mind.

9




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Figure 5:

Schematic of a typical system

incorporating PICVs




— Minimum pressure differential: in order to operate satisfactorily, the

DPCV component of the PICV must have enough pressure across it

to enable the spring to move and control. This minimum is typically in

the range 15–20 kPa for 15–32 mm diameter valves, and must be

added to the pump design pressure value. Specific values are given by

the particular manufacturer. In order to determine whether there is

sufficient pressure, valves are usually provided with pressure testpoints to enable the pressure differentials across the DPCVs to be

measured.

— Flow measurement: since flows can be set without the need to

measure the flow rate in the pipe, there is no need to locate

individual flow measurement devices on every terminal branch. For

checking purposes, flow measurement devices can be located on main

branches and sub-branches upstream of the terminals, as deemed

appropriate by the designer.

— Upstream regulating valves: since the DPCVs inside the PICVs vary

their positions as system pressures vary, there is no need for

upstream regulating valves. In order to maintain a flow balance, the

DPCVs inside the valves located close to the pump will automatically

throttle the flow more than those located further away. 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 inside the PICVs may be

limited with regard to the maximum differential pressure against

which they can operate. This is typically between 200 and 400 kPa

(2 and 4 bar) but should be checked with the particular manufacturer.

The full load pump pressure must not exceed the manufacturer’s

recommended maximum differential pressure value for the PICV.

— 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 sub-branch (i.e. the sub-

branch feeding to the most remote group of terminal units) and to

control pump speed such that the pressure required across thisbranch is always maintained. Each sensor connection should be

provided with test points and a bypass, as shown in Figure 5, 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 PICVs on the

most remote sub-branch were to close, then an additional sensor

might be required on the new index, i.e. the next furthest sub-branch

as shown in Figure 5. The pump would then be controlled to ensure

11



that the pressure required across both sub-branches was always

maintained. For the same reason, zones with different load patterns

fed from the same system should also have their own sensors.

— Maintaining flow at part load: when all of the PICV integral 2-port

control valves are approaching their closed positions, there needs to

be some path open to flow to prevent the pump operating against aclosed system. A simple solution is to incorporate 4-port valves, with

built-in bypasses, on end of branch terminal units, as shown in Figure

5. Across the entire system, 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. Branches incorporating 4-port

valves must be fitted with flow limiting valves (also known as constant

flow regulators). These stand-alone valves will hold flow rate constant

regardless of changes in system pressure caused by the closure of

PICVs on other branches, or variations in pump speed.

References and bibliography

CIBSE (2003) Water distribution systems CIBSE Commissioning Code W (London: Chartered

Institution of Building Services Engineers)

CIBSE (2006) Variable flow pipework systems CIBSE Knowledge Series KS7 (London: Chartered


CIBSE (2007) Commissioning variable flow pipework systems CIBSE Knowledge Series KS9 (London:

Chartered Institution of Building Services Engineers)

Parsloe C J (1999) Variable speed pumping in heating and cooling circuits BSRIA Application Guide

AG14/99 (Bracknell: Building Services Research and Information Association.)Parsloe C J (2002) The commissioning of water systems in buildings BSRIA Application Guide AG2/89.3

(Bracknell: Building Services Research and Information Association)

Parsloe C J (2004) Pre-commission cleaning of pipework systems BSRIA Application Guide AG1/2001.1


Petitjean R (1994) Total hydronic balancing (Ljung, Sweden: Tour and Anderson AB)

Teekaram A and Palmer A (2002) Variable-flow water systems BSRIA Application Guide AG16/2002.





that the pressure required across both sub-branches was always

maintained. For the same reason, zones with different load patterns

fed from the same system should also have their own sensors.

— Maintaining flow at part load: when all of the PICV integral 2-port

control valves are approaching their closed positions, there needs to

be some path open to flow to prevent the pump operating against aclosed system. A simple solution is to incorporate 4-port valves, with

built-in bypasses, on end of branch terminal units, as shown in Figure

5. Across the entire system, 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. Branches incorporating 4-port

valves must be fitted with flow limiting valves (also known as constant

flow regulators). These stand-alone valves will hold flow rate constant

regardless of changes in system pressure caused by the closure of

PICVs on other branches, or variations in pump speed.

References and bibliography

CIBSE (2003) Water distribution systems CIBSE Commissioning Code W (London: Chartered


CIBSE (2006) Variable flow pipework systems CIBSE Knowledge Series KS7 (London: Chartered


CIBSE (2007) Commissioning variable flow pipework systems CIBSE Knowledge Series KS9 (London:

Chartered Institution of Building Services Engineers)

Parsloe C J (1999) Variable speed pumping in heating and cooling circuits BSRIA Application Guide

AG14/99 (Bracknell: Building Services Research and Information Association.)Parsloe C J (2002) The commissioning of water systems in buildings BSRIA Application Guide AG2/89.3


Parsloe C J (2004) Pre-commission cleaning of pipework systems BSRIA Application Guide AG1/2001.1


Petitjean R (1994) Total hydronic balancing (Ljung, Sweden: Tour and Anderson AB)

Teekaram A and Palmer A (2002) Variable-flow water systems BSRIA Application Guide AG16/2002.



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