GB2410055A

GB2410055A - Fluid distribution system

Info

Publication number: GB2410055A
Application number: GB0415396A
Authority: GB
Inventors: Geoffrey Taylor
Original assignee: VIRWATT Ltd
Current assignee: VIRWATT Ltd
Priority date: 2004-07-09
Filing date: 2004-07-09
Publication date: 2005-07-20
Anticipated expiration: 2024-07-09
Also published as: GB2410055B; WO2006005896A1; GB0415396D0

Abstract

Methods of constructing and operating fluid distribution systems, particularly those for the distribution of water, are characterised by first determining the hydrodynamic resistance, capacitance and resulting time constant for each possible control action within the network and then using those determinations to dictate the selection of the control actions taken to minimise the overall hydrodynamic time constant of the network e.g. by opening/closing/throttling valves or installation of intelligent valve systems to reroute fluid flow.

Description

24 1 0055 - 1

FLUID DISTRIBUTION SYSTEM

Field of the Invention

The invention relates to methods of constructing and operating fluid distribution networks. It is particularly applicable to water distribution networks, for distribution of

potable water.

Background and Prior Art known to the Applicant

In the United Kingdom, and other countries, the water utility industry is subject to government regulatory bodies (e.g. The Environment Agency and OFWAT in the UK). In the UK, these bodies have developed standards of industry performance into a water balance table which is now accepted internationally. This table is used to account for real and apparent losses within the water supply network and each UK water utility's performance in the supply of water is monitored to ensure the UK industry supplies water in a cost efficient manner in the face of increasing demand and limited supplies.

Jo Since privatisation of the UK water industry in 1989 the industry is very much focussed on leakage, believing this to be the main cause of excessively high water-into-supply and night time flow levels - the 'nightlife' (the lowest average night time flow over a period of one hour - NL). In the early years of privatization, water companies made significant advances in leakage detection and "find & fix" methodologies and made significant reductions in real leakage levels. In more recent years, however, some water utilities have experienced a deterioration in their measured performance as water-into-supply and night time flow levels are on the increase in spite of continual increased spend on "find & fix"; This has led to increased regulatory pressures.

Water supply and distribution systems, especially in urban areas, started off as relatively simple networks but have evolved into highly complex configurations of underground assets. They have not been designed for economical deployment of potable water and lo with this continual add-on after add-on due to urban development has made these networks highly inefficient in many cases. Until recently there was very little visibility of how these networks are performing due to a lack of telemetry systems, flow meters and pressure measurement. Therefore, network performance was simulated using hydraulic computer modelling techniques using text book theory. Many assumptions are made in this approach to network performance assessment but it has served the industry well over the years. This has been the only method available, until recently, to track how water moves through the "invisible" underground network. In the last few years, due to pressure from the regulators, many of the larger water companies have installed more comprehensive metering systems and have started to collect much more flow and pressure so data on these sophisticated water supply and distribution systems. Despite this continued investment, little improvement in the measured performance has resulted.

It is an object of the present invention to attempt a solution to these problems, and to provide methods of operating and constructing water distribution systems to reduce water losses, to increase the reliability of water supply and to enable its distribution for lower energy input.

Summary of the Invention

so In a first aspect, the invention provides a method of operating a water distribution network which includes the steps of: determining the hydrodynamic resistance (as defined herein) of each element of the network; determining the hydrodynamic capacitance (as defined herein) of each element of the network; and adjusting the connectivity of the network to change the hydrodynamic resistance and capacitance values (e.g. by opening, closing or throttling valves or by installation of intelligent valve systems to reroute the fluid flow) so as to reduce the overall hydrodynamic time constant (as defined herein) of the network.

In a second aspect, the invention provides a method of constructing a water distribution network wherein the overall hydrodynamic time constant of the network is minimised by configuring the distribution network to favour elements being connected in series rather than in parallel, so maintaining low values of hydrodynamic capacitance and resistance under all water supply conditions.

In a third aspect, the invention provides a method of constructing a water distribution network characterised by the following steps: determining the elevation of each demand point in the network; dividing the area over which the network is to operate into a number of contour bands, each band spanning a range of elevations; assigning each demand point t5 to one of the contour bands, according to its elevation; connecting demand points within each contour band by supply pipes, said supply pipes being positioned substantially within the contour band; connecting each of said supply pipes to at least one water source (such as a pumping station or reservoir) via flow throttling means; adjusting said flow throttling means to deliver water preferentially to supply pipes at higher elevations.

Typically, each contour band will span an elevation of approximately 10 metres. This range of elevations is particularly appropriate, as it has been found to be narrow enough to enable to advantages of the system to be realised, whilst being wide enough to give design flexibility.

In a fourth aspect, the invention provides a method of operating a variable-demand fluid distribution network, comprising at least one fluid source and at least one fluid sink, said method characterised by the steps of first determining the hydrodynamic resistance, capacitance and resulting hydrodynamic time constant for each possible control action Jo within the network and then using those determinations to dictate the selection of the control actions taken to minimise the overall hydrodynamic time constant of the network.

Included within the scope of the invention is a method for operating or constructing a water distribution network substantially as described herein, with reference to and as illustrated by any appropriate combination of the accompanying drawings.

Whilst the preceding and following description describes the operation of a water distribution network, it will be appreciated that that the same methods of construction and operation may be applied to other fluid distribution networks carrying liquids which, by virtue of dissolved or suspended gases, demonstrate a degree of compressibility.

Brief Description of the Drawings

The invention will be described with reference to the accompanying drawings in which: Figure 1 is a schematic illustration of a pumping station and two DMAs (District Metered Areas) configured in series.

Figure 2 is a schematic illustration of a pumping station and two DMAs (District Metered Areas) configured in parallel.

Figure 3 is a schematic illustration of a pumping station, a DMA and a reservoir, configured in series.

Figure 4 is a schematic illustration of a pumping station, a DMA and a reservoir, configured in parallel.

Figures 5 and 6 are schematic diagrams showing the capacitative and resistive elements of Figures 3 and 4 respectively.

Figures 7 and 9 illustrate how single water distribution network elements may be composed of multiple elements.

The Underlying Inventive Concept Through its work with water companies to install, maintain and calibrate flow meters the applicant has acquired expertise in analysing flow meter data and has increasingly provided assistance to water utility companies in devising strategies to provide lower night time flow levels. Data has been collected to learn how potable water moves through the pipe system and how the networks are performing overall. During this process it was noticed that the water was not passing through the system as is generally understood by the industry and the more networks that were analysed the more examples were found.

For example, it was found that significant reductions in the nightline level (NL) could be achieved by changing how water was pumped through the pipe system during the night; this cannot be predicted using current software simulation tools, and it has achieved greater success rates than the leakage "find & fix" activities.

It appears that there are some discrepancies and gaps in current and generally accepted assumptions within the water industry. Findings indicate that more detailed behaviour of lo the fluid and the piping system (e.g. the effects of free air, pipe work elasticity, etc) has to be taken into account when designing and operating networks - these are not currently considered by the industry and simulation tools.

Approaching the operation and construction of water distribution networks in the way disclosed by the present invention allows them to be operated in a significantly improved manner, avoiding the negative consequences of a "badly tuned" network such as inefficient water deployment leading to erratic pressures, sporadic network performance, high Distribution Input (DI), high Night Line (NL), frequent pipe bursts, and high operational costs. The technical benefits that accrue from the present invention are analogous to the benefits achieved from tuning a "badly tuned" engine.

The typical current approach to design, construction and operation of water distribution networks is based on based on a number of underlying premises: calculations are performed on a steady-state basis, assuming that time constants of the distribution system are small, with the result that changes to network configurations, demands etc are almost immediately reflected in a new set of steady-state system parameters (flow rate, pressure etc). It is also assumed that the water being distributed is incompressible.

However, the present inventor has found that, hydrodynamically, potable water is a highly complex fluid: the presence of dissolved and dispersed gases in particular lead it to behave in a far from "ideal" manner such that the incompressibility assumption breaks down.

It is this lack of understanding of the true nature of water distribution systems that leads current practice to misinterpret measured system parameters. This has a number of consequences: firstly, much of the water loss that is measured as part of the standard "water balance" techniques is not real, and is merely a result of the complex nature of the water and distribution system; secondly the misinterpretation leads to a failure to design, construct and operate the distribution systems in a manner that reduces real water losses (e.g. by removing large pressure transients from the system); thirdly, the resulting "ill- tuned" systems are highly sub-optimal in terms of energy requirements for pumping.

lo Table I (below) shows a summary of the current world-view of "water balance", in terms of the International Water Association (lWA) Water Balance Table. The table illustrates the categories of consumption and losses that make up the system input volume. This total volume is taken to be composed of water that is consumed with the authorization of the supplier, and that which is lost from the system ("water losses"). Of the authorised t consumption, there are four categories of authorised use, depending on whether the water is metered and billed. It is only the billed water -- both metered and unmetered - that goes to form the socalled "Revenue Water".

Within the category of Water Losses, there are the "apparent losses" due to unauthorized consumption and metering inaccuracies, as well as the "real losses" from the various forms of leakage.

Current thinking, and more importantly, practice within the water supply industry (and utilities) is aimed at reducing the so-called "real losses" by leak detection and repair- the so-called "find and fix": any disparity between measured supply and measured (or estimated) consumption is assumed to be "real loss", and thus capable of elimination by detection and repair.

so The underlying inventive concept behind the present invention is based upon the introduction of a further category in this table, as illustrated in Table 11. Within the "Apparent Losses" in the system, are those losses that merely result from an improper analysis of the distribution network, and which are the subject of the present invention: this discrepancy is referred to as "Virtual Water".

The underlying inventive concept of the invention is based upon a hitherto unrecognized analogy between water distribution networks and resistive-capacitative electrical networks. in electrical circuits, the primary operating parameters are the voltage (V, in units of Volts); the current (T. in Amperes); the resistance (R. in Ohms); the power (W. in Watts) the capacitance (C, in Farads) and the charge (Q. in Coulombs).

The key equations linking these parameters are as follows: Ohm's Law: V = I R (Eqn. l) Power: W = V I (Eqn. 2) Addition of resistances: For two resistors in series with values Rat and R2, the total resistance Rat is given by: R7. = R] + R2 (Eqn 3) For more than two resistors, the equation applies mutatis mutandis.

For two resistors in parallel, the total resistance is given by: 1 1 1 R + R (Eqn. 4) RT I 2 or, rearranging: R7' = I 2 (Eqn. 5) Again, the equation applies mutatis mutands for more than two resistors.

Adding Capacitances: For two capacitors, of value C' and C2 in series, the total capacitance C r is given by: 1 1 1 CT Cl + C (Eqn. 6) or, rearranging: Cl + C2 The equation applies mastitis mutandis for more than two capacitors.

For two capacitors in parallel, the following equation applies: CT = Cl + C2 (Eon 8) Capacitors are, as is well known, storage devices that hold charge. In electrical terns, the charge, Q. held by a capacitor is given by: Q = I t (Eqn 9) or Q = C V (Eqn. 10) where t is the time over which the current, I, has flowed into the capacitor.

The equations governing the transient charging of a capacitative circuit are, for the voltage and current: Vc = V(1 - e t/T) (Eqn. 1 1) and IR = I e t/T (Eqn 12) During the discharge phase, the voltage is given by: Vc = Ve t/T (Eqn. 13) where t is the elapsed time, and T iS a time constant given by: T = C R (Eqn. 14) For fluid flows in a water distribution network, the following relationships hold, due to the compressibility of the water: For the pressure, P: P = F LT (Eqil 15) where P is the local gauge pressure in metros of water head (i.e.the pressure difference between the pipe and atmospheric pressure), F is the flow rate (litres/second). LT is the Hydrodynamic Resistance; this is not a parameter used in classical hydrodynamics, and we give its derived unit the name "Langtays".

The hydrodynamic resistance may thus be calculated (by rearrangement of equation 15) as: LT = F (Eqn. 16) On a practical stance, this equation may be modified for the situation of zero flow, as to follows: LT= (Eqn. 16a) where k is a small constant, used to prevent the indeterminacy caused by having a zero divisor; a value of k=0. 1 is an appropriate figure for the above measurement units.

This is a point measurement, which may be made by dividing pressure by the flow rate.

The appropriate pressure to use for this calculation is, again, the local gauge pressure.

These relationships are analogous to Ohm's Law in electrical circuits.

There are also analogous equations relating the flow and capacity of water distribution system elements. In electrical systems, capacitors hold charge; in hydrodynamic systems, pipes and reservoirs are the storage devices: they also hold the equivalent of charge. The quantity is, again, not a parameter used in classical hydrodynamics, and we give its derived unit the name "Tayjons". The equations governing these relationships are as follows: Q = Ft (Eqn. 17) or, alternatively: Q = CP (Eqn 18) where Q is the charge (Tayjons), F is the flow (litres/second), t is the time (seconds), C is the capacity (cubic metros) and P is the local gauge pressure (metros of water head). It is necessary to take into account the local land levels when calculating the overall effective l charge of the network under consideration. The Hydrodynamic Capacitances (or capacity, C) add in a way entirely analogous to that of electrical systems, i.e. according to Equations 6, 7 and 8.

In an analogous way to electrical charging curves, hydrodynamic systems in a C-Lr circuit also have a charging pressure curve, as follows: Pc = P(1e) (Eqn. 19) and FR =Fe (Eqn20) the corresponding discharge pressure curve is given by: Pc = Pe (Eqn 21) where, again, t is the elapsed time (s) and T iS the Hydrodynamic Time Constant given by: T = C LT (Eqn. 22) where C is the Hydrodynamic Capacitance (cubic metros) and LT is the Hydrodynamic Resistance in Langtays. The hydrodynamic resistances (LT) add in a way entirely analogous to that of electrical systems, i.e. according to Equations 3, 4 and 5.

The inventor has made the unexpected finding that the observed flow rate through sections of a water distribution network changes proportionally to any change in capacitance or charge levels. The observed percentage change in flow, dF, is given by: dF (parallelcapacitance-series capacitance) (E 2 series capacitance or, alternatively: (parallel charge - series charge) dF = (Eqn. 23a) series charge

Description of Preferred Embodiments

The application of the invention will now be described by means of a number of

examples.

Example I - Feeding Multiple DMAs As an illustrative example, we may consider a situation where two sub-regions of a distribution network need to be supplied with water. These sub-regions may, e.g. correspond to two sections of a housing estate. In the terminology of the UK water industry, these are referred to as DMAs (District Metered Areas, or alternatively Demand Managed Areas).

Figure l is a schematic illustration of one way of configuring the supply, wherein a pumping station 1 supplies a first DMA 2, followed by a second DMA 3. The pumping station!, and the two DMAs 2,3 are shown connected by pipework 4. A further section of ts pipework 5 leads to other sections of the distribution network.

Figure 2 illustrates an alternative configuration. Here the pumping station 1 feeds the two DMAs 2,3 in parallel. Again, the connections between the elements is by means of pipework 4, 5.

In each of these cases, the interconnecting pipework 4 would also be included in the analysis that follows, but are omitted in this example for the sake of clarity.

If we say, for example, that these two DMAs have a capacity of 1000 cubic metres each, and consider how best to supply water to them both, we first calculate the overall hydrodynamic capacitance of the system when configured in series (as Figure 1), and then in parallel (as Figure 2).

Using equation 8, we calculate: parallel capacitance = lOOO + 1000 = 2000 cubic metros.

and using equation 7 we calculate: series capacitance = (l OOO x 1000) / ( 1000 + l 000) = 500 cubic mctres.

Finally, using Equation 23, we can calculate the expected observed flow reduction by configuring the two DMAs to be fed in series, rather than in parallel as follows: dF = (2000 - 500) /500 = 3% i.e. a reduction in the observed flow of 3%. This quantity forms part of the "Virtual Water" identified in Table II. Thus, for improved performance, we would design, construct, or operate the system to favour the series configuration, as shown in Figure 1.

Example II-DMA and Reservoir As a further example, we may consider a DMA and a reservoir being fed from a single pumped source. When constructing or operating such a configuration, a choice must be made between connecting the DMA and reservoir in series or in parallel.

Figure 3 illustrates a configuration of a pumping station 6, a DMA7, and a reservoir 8.

This could form part of a larger network (not illustrated). These elements are shown connected in a series fashion, with water being pumped from the pumping station to the go DMA through the first section of pipework 11, and then on to the reservoir 8 through the second section of pipework 12. Should the demand of the DMA be greater than can be supplied by the pumping station 1, then the flow through the second section of pipwork 12 would be reversed, i.e. from the reservoir to the DMA.

A third section of pipework 10 is also illustrated in Figure 3, this pipework 10 also having flow control means 9, such as a valve. In this illustration, the third section of pipework 10 is shown with a dashed line to indicate that the flow control means 9 is configured to prevent flow through this third section of pipework 10.

Figure 4 illustrates an alternative configuration of the elements illustrated in Figure 3.

There, the flow control means 9 is configured to allow flow through the third section of the pipework 10. In this network configuration, the DMA7, and the reservoir 8 are thus operating in parallel.

Typical values for the hydrodynamic capacitance (i.e. capacity) of the reservoir and DMA might be 26,500 cubic metros and 560 cubic metres respectively.

Using equation 8, we may calculate: parallel capacitance = 26,500 + 560 = 27,060 cubic metros.

and using equation 7, we may calculate: series capacitance = (27060 - 548. 4)/ 548.8 = 48.3%.

Thus, configuring the system to operate preferentially with the DMA and reservoir in series (as illustrated in Figure 3), rather than in parallel (as illustrated in Figure 4), would lead to a reduction in the observed water flow of almost half In an analogous fashion to Example I, the pipework elements of this network 10, 11, 12, may also have significant hydrodynamic capacitance and resistance. Figures 5 and 6 illustrate a fuller analysis of the network, but indicate that the pipework elements 10, 11, 12 of Figures 3 and 4 may be more properly represented as pipework networks. This is illustrated in Figure 7, where it is shown that one of these pipework elements 13 may in reality be a sub-network composed of pipework 16, 17 and 18, together with e.g. flow control means l 9 and 20. Each of these sub-elements 16-20 may be analysed as above to produce a hydrodynamic capacitance and resistance for use in the wider network analysis, allowing calculation of overall network parameters in a recursive manner.

For example, Figure 8 illustrates that each of the pipework elements 16, 17, 18 may be analysed in a similar fashion as pipework networks 21, 22, 23.

Table I

Standard International Water Balance Table (International Water Association) l Billed Billed metered consumption Authorlsed (mcludmg water exported) Revenue Water Authorlsed Consumption Billed unmetered consumption Consumption Unbllled I Unbllled metered consumption | Authorlsed Vnbllled, unmetered consumption Consumption System Input Apparent Unauthorised consumption Losses Customer metering inaccuracies Volume Non-Revenue Leakage on transmission and/or Water Water distribution mains Losses Real Leakage and overflows at Losses Utllity's storage tanks Leakage on service connections up to pomt of customer metering

Table II

Modified Water Balance Table Billed Billed metered consumption Authorised (including water exported) Revenue Water Authorised Consumption Billed unmetered consumption l Consumpbon Unbilled Unbllled metered consumption Authorlsed Unbilled, unmetered consumption Con sumptl on System Apparent IJnauthorised consumption Input Losses Customer metering inaccuracies Volume "VIRTUAL WATER" Non-Revenue Water Leakage on transmission and/or Water I.osses distribution mains Real Leakage and overflows at Losses Utlty's storage tanks Leakage on service connections up to point of customer metering

Claims

Claims 1. A method of operating a water distribution network which

includes the steps of: determining the hydrodynamic resistance (as defined herein) of each element of the network; determining the hydrodynamic capacitance (as defined herein) of each element of the network; and adjusting the connectivity of the network to change the hydrodynamic resistance and capacitance values (e.g. by opening/closing/throttling valves or installation of to intelligent valve systems to reroute the fluid flow) so as to reduce the overall hydrodynamic time constant (as defined herein) of the network.
2. A method of constructing a water distribution network wherein the overall hydrodynamic time constant of the network is minimised by configuring the distribution network to favour elements being connected in series rather than in parallel, so maintaining low values of hydrodynamic capacitance and resistance under all water supply conditions.
3. A method of operating a variable-demand fluid distribution network, comprising at least one fluid source and at least one fluid sink, said method charactersed lay the steps of first detemmining the hydrodynamic resistance, capacitance and resulting hydrodynamic time constant for each possible control action within the network and then using those determinations to dictate the selection of the control actions taken to minimise the overall hydrodynamic time constant of the network.

3. A method of constructing a water distribution network characterized by the following steps: determining the elevation of each demand point in the network; dividing the area over which the network is to operate into a number of contour bands, each band spanning a range of elevations; assigning each demand point to one of the contour bands, according to its elevation; connecting demand points within each contour band by supply pipes, said supply pipes being positioned substantially within the contour band; connecting each of said supply pipes to at least one water source (such as a pumping station or reservoir) via flow throttling means; so adjusting said flow throttling means to deliver water preferentially to supply pipes at higher elevations.

4. A method of operating a variable-demand fluid distribution network, comprising at least one fluid source and at least one fluid sink, said method characterized by the steps of first determining the hydrodynamic resistance, capacitance and resulting hydrodynamic time constant for each possible control action within the network and then using those determinations to dictate the selection of the control actions taken to minimise the overall hydrodynamic time constant of the network.

Amendments to the claims have been filed as follows 1 A method of operating a water distribution network which includes the steps of.

detemmning the hydrodynamic resistance (as defined herein) of each element of the network; determining the hydrodynamic capacitance (as defined herein) of each element of the network: and adjusting the connectivity of the network to change the hydrodynamic resistance and capacitance values so as to reduce the overall hydrodynamic time constant (as lo defined herem) of the network.

A method of CoilStrUcting a water distribution network wherein the overall hydrodynamic time constant of the network is minimised by configuring the distribution network to favour elements being connected in series rather than in parallel, so maintaining low values of hydrodynamic capacitance and resistance under all water supply conditions.