Variable-flow water systems Design, installation and commissioning guidance

Transcription

1 APPLICATION GUIDE AG 16/2002 Variable-flow water systems Design, installation and commissioning guidance By Arnold Teekaram and Anu Palmer Supported by

2 PREFACE There are significant benefits to variable speed pumping in terms of both energy cost and capital cost. The main energy savings derive from the pump law relationship which indicates that (for a system with fixed resistance) halving the flow rate would reduce the pump power to roughly one eighth its previous value. This theoretical saving is however unlikely to be achieved on an actual variable flow installation because of the following: Changes in the system resistance as valves close Reduction in the pump efficiency as the system characteristics change Variable speed drive losses. Capital cost saving opportunities are likely to arise from the potential diversity of load during pipe and pump sizing. Constant flow heating and chilled water systems are generally well understood by designers and commissioning specialists. The commissioning procedure involves setting up systems to achieve fixed flow rate parameters. Most of the valves used for commissioning are manually adjusted during commissioning, and three-port valves are used to modulate flow rate and hence heat transfer across coils. There are significant differences in systems where pump speed is allowed to vary. Some of the valves used may be self-acting types responding to changes in pressure. Control of heat transfer is more likely to be achieved through the use of two-port valves which need to be able to shut off against the system pressures. Inevitably, the designer and commissioning specialist must take into account part load conditions as part of commissioning and system proving. For these reasons, many of the commissioning procedures laid down in the BSRIA Application Guide AG 2/89.3: Commissioning Water Systems - Application Principles 1 are not applicable to systems with variable speed pumps. This Guide is therefore aimed at these specific types of system. The format of the Guide is the same as the aforementioned Commissioning Water System AG 2/89.3 in that guidance is split into three parts: The Design of Commissionable Systems, The Installation of Commissionable Systems and Commissioning Procedures. Cross reference is made to other BSRIA commissioning guides as appropriate, although every effort has been made to make this guide a standalone document. It is hoped that the introduction of this guide, and other supporting guidance on variable speed applications, will encourage their implementation on a wider scale. This will hopefully result in a realisation of the significant energy saving potential that such systems have to offer. The recommendations given in this document are for guidance only. VARIABLE-FLOW WATER SYSTEMS

3 CONTENTS DEFINITIONS LIST OF SYMBOLS PART A: THE DESIGN OF COMMISSIONABLE SYSTEMS 1 1 INTRODUCTION General Commissioning information 1 2 PIPEWORK SYSTEM DESIGN Self-balancing arrangements Flow velocities Pump selection Inverter drives System cleanliness Venting Access 9 3 FLOW MEASUREMENT AND REGULATING DEVICES Regulating valves Flow measurement devices Flow measurement/regulating/isolating devices 12 4 GUIDELINES FOR COMPONENT SELECTION AND APPLICATION Flow measurement/regulation options Selection criteria 22 5 SPECIFYING FLOW RATES AND TOLERANCES Accuracy of flow measurements 24 6 DESIGN CONSIDERATIONS Return temperature and demand in variable volume systems Pressure distribution Benefits and design considerations Design considerations 30 7 EXAMPLE OF VARIABLE FLOW SYSTEM DESIGNS The Bodle-Orchard circuit Boiler house circuit Sub-circuits and branches 40 PART B: THE INSTALLATION OF COMMISSIONABLE SYSTEMS 45 1 MANAGEMENT General Organisation and planning 45 2 PIPEWORK INSTALLATION PROCEDURES Good housekeeping Good workmanship Good pipework arrangements Adequate accessibility 53 vi viii VARIABLE-FLOW WATER SYSTEMS

4 CONTENTS 3 INSTALLATION OF VARIABLE SPEED DRIVES 54 4 INSTALLATION INSPECTIONS 56 PART C: COMMISSIONING OF VARIABLE FLOW HYDRONIC SYSTEMS 57 1 INTRODUCTION 57 2 CONSTANT VOLUME COMMISSIONING PROCEDURES Least favoured and most favoured circuits Proportional balancing Balancing of branches and risers 61 3 SYSTEM COMPONENTS COMMISSIONING PROCEDURES Inverter-driven pumps DPCV in sub-circuits controlling single loads DPCV in sub-circuits controlling serveral loads Primary constant volume circuit serving a secondary circuit Primary variable serving constant-volume secondary flow Variable volume served by variable primary flow Variable primary serving a variable secondary at different temperatures Use of differential pressure control valve across a single control valve Combination valves in sub-circuits Two-port valves Thermostatic radiator valves Variable speed drives 71 4 VARIABLE FLOW COMMISSIONING PROCEDURES Case study Commissioning procedure Information to be recorded in final commissioning report 78 5 HYDRONIC VARIABLE FLOW COMMISSIONING PROBLEMS 80 REFERENCES 82 TABLES Table 1: Recommended range of water velocities (CIBSE Guide, Volume C, 2000). 4 Table 2: Valves and flow measurement devices. 11 Table 3: Typical flow rate accuracies for flow measurement devices 24 VARIABLE-FLOW WATER SYSTEMS

5 FIGURES Figure 1: Fixed-speed pump performance characteristics 6 Figure 2: Variable-speed pump performance characteristics 7 Figure 3: An example of a double regulating valve 11 Figure 4: Examples of fixed-orifice fittings 12 Figure 5: Examples of variable-orifice double regulating valves 13 Figure 6: Typical fixed-orifice double regulating valve 13 Figure 7: Cartridge type constant flow regulator 14 Figure 8: An adjustable flow controller 15 Figure 9: Typical differential pressure control valve 16 Figure 10: Combination valve 17 Figure 11: The arrangement of a combination valve 17 Figure 12: Relationship between flow rate and pressure drop for a combination valve 18 Figure 13: Practical construction of a combination valve 19 Figure 14: Pipework schematic showing valve locations 21 Figure 15: Return temperature variation in heating systems 25 Figure 16: Return temperature variation in chilled water systems 26 Figure 17: Pressure distance diagram 27 Figure 18: Pressure distance diagram 28 Figure 19: Pressure-distance diagram 29 Figure 20: Bodle-Orchard circuit for a variable-volume heating system 34 Figure 21: Bodle-Orchard circuit for a primary/secondary variable-volume heating system 35 Figure 22: Schematic of sub-circuits 36 Figure 23: Schematic of injection sub-circuit 37 Figure 24: Performance of a radiator as a function of the return temperature of the radiator 39 Figure 25: Control of minimum flow through boilers 42 Figure 26: Installation of self acting differential pressure control valve 50 Figure 27: Differential pressure control valve located at the bottom of a riser 50 Figure 28: Differential pressure control valve on floor branch 51 Figure 29: Use of reverse acting bypass valve 51 Figure 30: Two-port isolating valve in branch with differential pressure control valve 52 Figure 31: Installation of a combination valve 52 Figure 32: Least favoured and most favoured circuits 59 Figure 33: Schematic of sub-circuit 60 Figure 34: Commissioning of a differential pressure control valve with single load in sub-circuit 63 Figure 35: The Commissioning of a differential pressure control valve with several loads in a sub-circuit 64 Figure 36: A secondary variable-volume circuit with a constant primary circuit 65 Figure 37: A secondary constant-volume circuit with a variable primary circuit 66 Figure 38: A secondary variable-volume circuit with a variable primary circuit 67 Figure 39: A pump arrangement in a secondary circuit 68 Figure 40: The arrangement of a differential pressure control valve across a control valve 69 Figure 41: Commissioning a combination valve 70 Figure 42: A system diagram detailing the configurations for three typical circuits 73 Figure 43: A commissioning network diagram 79 VARIABLE-FLOW WATER SYSTEMS

6 INTRODUCTION INTRODUCTION A1 PART A: THE DESIGN OF COMMISSIONABLE SYSTEMS 1.1 GENERAL 1 INTRODUCTION As soon as possible, and no later than upon entering the scheme design stage, the designer should pay attention to the strategy for commissioning the scheme, and the level of commissioning expertise required. The designer should ask themself the following questions: Is external commissioning expertise required in support of the design role? Will the services sub-contractor have the capability in-house to carry out commissioning? Will the services sub-contractor have commissioning expertise available during the installation? In consideration of these aspects, the designer should advise if a commissioning specialist is required, and when that specialist should be appointed. The degree to which the designer influences decisions on these matters and advises the project management team will greatly affect the commissionability of the built systems. 1.2 COMMISSIONING INFORMATION To enable a water distribution system to be successfully commissioned, the designer must provide adequate information, documented in the form of drawings, schedules and specifications. The technical requirements of the commissioning works should be developed by the designer to define clearly: The scope of the works, in other words the systems to be commissioned, their functions and duration of operation and an explanation of their operational inter-relationships with other engineering services systems. The setting out of the responsibilities of the various parties (client, design team, main or managing contractor, installation contractor and commissioning specialist). BSRIA Technical Memorandum TM 1/88: Commissioning HVAC Systems, Division of Responsibilities 2, gives guidance on this subject. The technical specification of the commissioning work, for example: the standards (for example relevant parts of CIBSE Codes and BSRIA guides) to which the works should be carried out the tolerances for regulation and test results the reporting procedures required the witnessing procedures to be observed. Design drawings showing the layout of the system in relation to the building form and, if required, other engineering services. VARIABLE-FLOW WATER SYSTEMS 1

7 A1 INTRODUCTION Schematic diagrams clearly illustrating the design intent and including all the design information required to commission the system. For example: Water flow rates and system pressures Sizes Locations Reference Identification Terminal branches Branches Mains Pipes Valves Flow measurement devices Isolating valves Regulating valves Differential pressure control valves Control valves Flow measurement devices Differential pressure sensors Constant flow controllers Automatic flow controller Terminal units Isolating valves Regulating valves Differential pressure control valves Control valves Flow measurement devices Constant flow controllers Automatic flow controller The reference identification should be unique to each individual component to permit cross referencing and to enable a component to be identified in correspondence and telephone discussions. If the schematics are prepared at the same time as the design drawings, potential difficulties in regulation may often be revealed and rectified prior to installation. Schedules of major plant, equipment and components, crossreferenced to the design drawings and schematic diagrams. Additional design information required for commissioning which may not be available until after the appointment of the building services installer may include: electrical wiring diagrams of associated plant and equipment control system diagrams pump performance curves regulating valve calibration data, based upon calculated pipework head losses. 2 VARIABLE-FLOW WATER SYSTEMS

8 INTRODUCTION A1 The specification of management requirements must be related to specific contractual conditions. Although the designer will strongly influence whether the commissioning specialist is to be employed by the main, managing or installing contractor, it is the latter who will specify: Forms of contract programme constraints resource levels method statements quality assurance procedures insurance requirements site establishment details compliance with site safety and industrial relations protocol bonds, warranties and guarantees. The different sources of project information required to fully specify commissioning should not, however, preclude the early appointment of a commissioning specialist. The specialist could be appointed on a consultant basis during the design phase, converting to a contractor later. VARIABLE-FLOW WATER SYSTEMS 3

9 MANAGEMENT MANAGEMENT B1 PART B: THE INSTALLATION OF COMMISSIONABLE SYSTEMS MANAGEMENT GENERAL The installer s responsibility is to provide a water installation which meets the specified requirements. To achieve this, properly managed resources must be allocated to the process of constructing a commissionable system. The tendering or appointed installer must carefully study the enquiry/contract documents to determine precisely the project requirements. The installer should ask himself the following questions: Is the installer called upon to install only? Does the installer have to commission the system and, if so, has the installer got the resources in-house or will a specialist need to be appointed? Has a commissioning specialist been appointed by the designer or client? Is the installer aware of the commissioning specialist s duties and requirements during the system construction? The designer/project team will have been responsible for establishing the commissioning strategy. The system installer must understand the strategy in order to meet the designer s requirements and enable the commissioning process to proceed. 1.2 ORGANISATION AND PLANNING Where the installer is responsible for commissioning, the installer should select and instruct the commissioning specialist at the earliest possible stage to ensure that commissioning expertise is available in the planning and programming of the commissioning tasks. Together, they should: Establish effective lines of communication between the commissioning specialist and other parties involved Produce a set of working drawings that show the detailed provisions for incorporating the commissioning facilities. These drawings must also show the details of required temporary facilities Review the contract documents to determine the requirements for commissioning, taking nothing for granted and seeking clarification where necessary Produce a realistic programme which incorporates the commissioning activities, phased and fully integrated with the main installation and construction programmes Regularly review the programme during installation to establish the effect of modifications and delays on the planned static completion and power-on dates and any other dates critical to the commissioning activities Acquire all the information specified in Section A1.2 from the designer VARIABLE-FLOW WATER SYSTEMS 45

10 B1 MANAGEMENT Obtain from equipment suppliers and manufacturers their latest information for all items supplied. Standard details which are not modified to suit the particular project should always be treated with caution. Manufacturer s literature should be checked for any installation requirements additional to those specified Progressively record as-installed information on at least two sets of installation drawings one clean set to facilitate the production of the record drawings and operating/maintenance documentation, and one site set for use by the commissioning specialist Establish systematic site control procedures to assist the progressive monitoring of the standard of the pipework installation practices maintained on site (see Section B2) Prepare co-ordinated ceiling plans illustrating access panel requirements Establish an equipment and materials procurement procedure which incorporates an effective means of checking each and every delivered item against specified requirements Retain all documents and literature provided with each delivered item of equipment for use by the commissioning engineer (and for inclusion in the operating and maintenance manuals). 46 VARIABLE-FLOW WATER SYSTEMS

11 INTRODUCTION INTRODUCTION C1 PART C: COMMISSIONING OF VARIABLE FLOW HYDRONIC SYSTEMS 1 INTRODUCTION Effective commissioning of variable flow hydronic systems for heating and chilled water distribution is essential if they are to provide good controllability for the heating and cooling loads they serve. Traditionally the majority of building services applications, systems have been designed with fixed speed primary/secondary pumps to provide constant-volume flow with the application of three-port valves for controlling the loads in the sub-circuits. Arrangement of these valves in the sub-circuits can be such that the circuit is either a diverting, mixing or an injection circuit. For these circuits, operation of the circulating pumps is always at constant speed irrespective of whether the system is operating at full load or part load conditions. The main commissioning procedure for constant volume systems is therefore generally one of water balancing where the flows are proportionally balanced in the respective branches to satisfy the particular requirements of the system. Control and balancing are maintained at part load, but there are no savings from reduced pumping unless the heating is switched off in summer or the designer had provided smaller pumps for the summer duty. In variable-flow hydronic systems, reducing pump speed or staging of pumps with boilers and chillers to suit part-load conditions is an energy efficient method of control. These systems were developed as an alternative to constant volume systems because of the potential pump energy savings to be derived by varying the flows to match the diversity on load requirements. There is also the additional benefit from reduced pipe heat losses due to low return water temperatures that two-way valve systems give on part load compared to the higher return temperatures with three-way valve control. For such systems, components such as differential pressure controllers, inverter driven pumps, two-port control valves, combination valves, automatic variable volume limiter, constant flow controllers, orifice plates and pressure-tapped double regulating valves etc must be correctly installed and specified as an integral part of the design to maintain dynamic control and flow stability and to allow easy commissioning. In two-port valve systems one method of balancing and commissioning that makes life easy is to use differential pressure control valves, as these valves can perform a dual duty of maximum volume control and differential pressure control. Once a sub-circuit has been commissioned the differential pressure control effectively prevents the flow and balance in the sub-circuit being affected by other sub-circuits. The main purpose of the commissioning procedure will therefore be to ensure that the system is proportionally balanced to give design flows under design (full load) conditions in the same way as constant volume systems are balanced and that the components provide the desired control in response to changes in the system demand. The following sections of this document provide guidance on commissioning procedures for variable flow hydronic systems and their components. VARIABLE-FLOW WATER SYSTEMS 57

12 C1 INTRODUCTION Section 2 briefly describes the basic principles of conventional commissioning procedures for constant volume systems. These have been fully described in other publications to which the reader is referred and only the main points are summarised here. These include: basics of proportional balancing, procedures for identifying the least favoured and most favoured circuits and balancing of branches and risers. Section 3 gives guidance on commissioning of individual system components such as pumps, differential pressure controllers in subcircuits controlling single and multiple loads, combination valves, twoport valves, thermostatic radiator valves and variable speed drives. For the purpose of describing the commissioning procedure for variable flow hydronic systems, a case study is given in section 4 which describes the commissioning process on an actual project. A diagram is included to detail briefly the system layout and a brief description of controls operation is also included prior to a detailed step by step commissioning procedure. The commissioning procedure employed is then translated into a generic commissioning flowchart for variable flow hydronic systems. Section 5 of the document begins by highlighting typical commissioning problems associated with variable-speed pumping systems and the corrective measures that can be taken. 58 VARIABLE-FLOW WATER SYSTEMS