CIBSE - KS7 - Variable Flow Pipework

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

The rights of publication or translation are reserved. 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 publication. In adopting these recommendations for use each adopter by doing so agrees to accept full responsibility for any personal injury, death, loss, damage or delay arising out of or in connection with their use by or on 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 the part of those indemnified. 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

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Introduction

This publication forms a supplement to CIBSE Knowledge Series KS7: Variable flow pipework systems (CIBSE, 2006). It explains how to design recirculating 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: valve solutions

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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: Schematic diagram of basic valve module (see Figure 3 for definitions of symbols)

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With regard to the numbered features in Figure 1, the common features are as follows:

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CIBSE Knowledge Series — Variable flow pipework systems: valve solutions

(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,

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 pressfit joints are available.

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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.

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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.

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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: valve solutions

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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.

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Figure 2: Calculation of control valve authority in valve module application (see Figure 3 for symbol definitions)

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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: —

Variable flow pipework systems: 4 CIBSE Knowledge Series — valve solutions

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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Figure 3: Schematic of a typical system incorporating valve modules CIBSE Knowledge Series — Variable flow pipework systems: valve solutions

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Variable flow pipework systems: 6 CIBSE Knowledge Series — 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 10–15 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 to be measured.

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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.

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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.

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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.

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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.

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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.

CIBSE Knowledge Series — Variable flow pipework systems: valve solutions

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3

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 the same 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:

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Pressure independent control valve solution

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(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.

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Figure 4: Schematic of a typical pressure independent control valve (PICV)

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CIBSE Knowledge Series — Variable flow pipework systems: valve solutions

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.

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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 kv value less than 0.4, a flushing drain should be incorporated in the pipework between the terminal unit and the PICV.

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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 modulating control 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.

CIBSE Knowledge Series — Variable flow pipework systems: valve solutions

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Figure 5: Schematic of a typical system incorporating PICVs

Variable flow pipework systems: 10 CIBSE Knowledge Series — valve solutions

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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 test points to enable the pressure differentials across the DPCVs to be measured.

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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.

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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.

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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.

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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 subbranch feeding to the most remote group of terminal units) 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 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 CIBSE Knowledge Series — Variable flow pipework systems: valve solutions

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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 a closed 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 Institution of Building Services Engineers) 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 (Bracknell: Building Services Research and Information Association) 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. (Bracknell: Building Services Research and Information Association)

Variable flow pipework systems: 12 CIBSE Knowledge Series — valve solutions

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 a closed 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 Institution of Building Services Engineers) 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 (Bracknell: Building Services Research and Information Association) 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. (Bracknell: Building Services Research and Information Association)

Variable flow pipework systems: 12 CIBSE Knowledge Series — valve solutions