Abstract:
A method and apparatus for detecting leakage of flowing liquids from pipes includes an upstream flow-rate sensor positioned between a source of a flowing liquid which is conducted from a source to a destination terminal such as a VAV heat exchanger, and a downstream flow-rate sensor positioned between an outlet port of the destination terminal and a return line for the flowing liquid. The apparatus includes electronic control circuitry which is responsive to a differential flow-rate between upstream and downstream measured flow rates which exceeds a predetermined limit value in removing a valve-opening signal to the upstream shut-off valve, thus closing the valve to interrupt flow of liquid through the valve if the differential flow-rate signifies a leak. Optionally, the apparatus also includes a downstream shut-off valve positioned between the destination terminal and a return line, which is also closed in response to a differential flow-rate exceeding the limit value.

Description:
BACKGROUND OF THE INVENTION 
     A. Field of the Invention 
     The present invention relates to methods and apparatus for detecting and abating leaks of flowing liquids from pipes in buildings and similar structures. More particularly, the invention relates to a method and apparatus which uses differential flow rate sensors for detecting water leaks, particularly in closed loop water circulation systems, and solenoid valves to shut off water flow if a detected leak rate exceeds a predetermined value. 
     B. Description of Background Art 
     Contemporary buildings of various types, and particularly hospitals, institutional buildings, and larger commercial and industrial buildings employ a variety of water distribution piping or plumbing systems. Thus, in addition to plumbing used to supply potable water for consumption by the building&#39; occupants, highly purified water for use in production processing, or lower quality water for other purposes, many larger contemporary buildings also have at least one of the following two additional types of water distribution systems through which water is circulated, but infrequently discharged. One such water circulation or distribution system, which may be referred to as a “closed-loop” or closed-cycle system is used to supply water to ceiling-mounted fire extinguisher sprinkler heads. Obviously, water conveyed through plumbing of a building water supply system to fire sprinkler heads is discharged from the system rarely, that is, only in the event of a fire, or during periodic testing of the fire sprinkler system and sprinkler heads. 
     A second type of closed-loop water circulation system used in many larger contemporary buildings comprises part of the Heating, Ventilating and Air Conditioning (HVAC) system of the building. In particular, some larger buildings including hospitals use hot water as a primary working fluid to heat various regions or zones of the building to different individually controllable temperatures. The hot water is generated by a boiler which is generally located in a basement of the building, or in another structure which houses the “mechanical plant” of the building adjacent to the building. The hot water is typically circulated in a continuously, closed loop cycle, which originates at the hot, discharge side of a boiler heat exchanger. 
     Heated water issued from the discharge side of a boiler heat-exchanger is pumped upwardly through a vertical hot water source (HWS) riser pipe to the highest building floor requiring heating. At each location or zone of a building which requires heating, the hot water is input to a box-like heat exchanger terminal, such as a Variable Air Volume (VAV) terminal. Within the VAV terminal, air from an external source which is moved by an external or internal blower or fan is directed to flow in contact with the exterior surfaces of a coiled length of tubing called a heater coil which has an inlet port fitting which is connected to and receives hot water from the hot water source riser pipe. 
     The heater coil functions as a flowing air to hot water heat exchanger, and heats the air which flows through the heater coil. The flowing air is heated to a temperature which is adjusted by a thermostatically controlled fan and/or a damper valve for varying the volume of temperature controlled air which is discharged from the VAV terminal and conducted through ducts to ceiling diffusers or other outlet ports in various rooms of a building. Cooled water from the discharge side of the heat exchanger coil is conducted back down through a hot water return (HWR) riser line to the cold inlet side of the boiler heat exchanger. Thus, in such a system, a fixed volume of water is continuously circulated through the system, and is not discharged. 
     As can be well imagined, heating systems of the type described above, when used in large buildings with many zones and associated heat exchanger terminals, typically include a substantially large number of individual pipes, tubes and fittings. Thus there is the possibility of a leak developing at many different locations in the closed-loop system, the probability of which is increased in the event of seismic disturbance of the building. Therefore, it is understandable that prudent building maintenance procedures would necessitate monitoring such closed-loop water circulation systems for leaks, and providing an alarm signal to building maintenance personnel in the event of a leak. Also, it would be desirable to provide a method and apparatus for automatically shutting off flow of water if a leak is detected. 
     Regarding first the problem of detecting a water leak, there are of course a large variety of water leak detectors which employ sensors that utilize a supply voltage and a pair of electrodes to detect electrically conductive water which has leaked onto and bridged the sensor electrodes. However, such electrolytic water leak sensors are effective only in detecting water leakage at discrete locations where the sensors are placed. Such point sensors would be for detecting leaks in most closed-loop water circulation systems, such as an HVAC hot water circulation system of the type described above. This is because a water leak detection system using point sensors for systems such as closed-loop water circulating systems which extend over a large area would require an unreasonably large number of individual sensors which were placed near every possible leakage point. 
     That the detection and abatement of water leaks in contemporary buildings is an important problem is evidenced by two recent cases in California, where leakage from broken building hot water circulation systems caused more than one million dollars worth of damage in each of the buildings. Part of the expenses associated with water leaks in buildings results from modern building codes and potential legal liability which require the complete removal and replacement of all drywall that has been subjected to water leaks for more than 72 hours, to prevent the growth of molds which can cause health problems. 
     More important than potential financial losses which can result from water leakage that is not timely detected and abated is the possibility of serious injury or even death which can result if a leak in a hot water circulation system of a hospital building should occur. For example, a hot water leak may allow sufficient water to accumulate, leak through ceilings and scald patients in their beds on lower floors. 
     The foregoing considerations of potential financial losses, bodily injuries and even deaths which may result from water leaks in modern buildings, and the unavailability of an adequate solution to the problem of promptly detecting leaks and shutting off water flow in closed-loop water circulation systems prompted the present invention. 
     OBJECTS OF THE INVENTION 
     An object of the present invention is to provide a method and apparatus for detecting leaks in systems which convey flowing liquids such as water. 
     Another object of the invention is to provide a method and apparatus for detecting leaks from conduits which carry a flowing liquid such as water which uses a pair of differential flow-rate sensors to detect differences between upstream and downstream flow rates resulting from leakage of liquids at a location between upstream and downstream locations of a conduit where the flow rate sensors are located. 
     Another object of the invention is to provide a method and apparatus for detecting leakage of liquid flowing through a piping system including an upstream source line and a downstream return line, the apparatus using a pair of flow-rate sensors for detecting differences between upstream and downstream flow rates which signify leakage of flowing liquid at a location between the upstream and downstream flow-rate sensors. 
     Another object of the invention is to provide a method and apparatus for detecting leakage of liquid from a closed-loop liquid circulation piping system for conducting water to and from a terminal, the apparatus including an upstream flow rate sensor and a downstream flow rate sensor to detect differences in upstream and downstream flow rates, control logic circuitry which outputs an alarm status signal if the difference in upstream and downstream flow rates exceeds a predetermined threshold value, an upstream valve in the upstream line operatively interconnected and responsive to the alarm status signal in shutting off upstream flow of liquid to the terminal, and an optional downstream shut-off valve responsive to the alarm status signal in shutting off downstream flow of liquid from the terminal. 
     Various other objects and advantages of the present invention, and its most novel features, will become apparent to those skilled in the art by perusing the accompanying specification, drawings and claims. 
     It is to be understood that although the invention disclosed herein is fully capable of achieving the objects and providing the advantages described, the characteristics of the invention described herein are merely illustrative of the preferred embodiments. Accordingly, we do not intend that the scope of our exclusive rights and privileges in the invention be limited to details of the embodiments described. We do intend that equivalents, adaptations and modifications of the invention reasonably inferable from the description contained herein be included within the scope of the invention as defined by the appended claims. 
     SUMMARY OF THE INVENTION 
     Briefly stated, the present invention comprehends a method and apparatus for detecting leaks from a conduit such as a pipe carrying a flowing liquid, and for shutting off flow of the liquid if a detected leak rate exceeds a predetermined, preset value. More particularly, the method and apparatus of the present invention provide a means for detecting leakage of water flowing through a pipe, tube or other conduit, providing an alarm status signal if the detected leak rate exceeds a predetermined, preset value, and actuating an upstream shut-off valve and an optional downstream shut-off valve in response to the alarm status signal. 
     The novel leak detection and shut-off method and apparatus according to the present invention have a wide variety of useful applications. However, a primary intended application for the method and apparatus of the present invention is to minimize potential property damage and injuries to humans which could result from leakage of water from a hot water circulation system which comprises part of a Heating, Ventilating and Air Conditioning (HVAC) system of a building. 
     A water leak detection and shut-off apparatus according to the present invention includes a first, upstream flow rate sensor. The upstream flow rate sensor is used to measure the flow rate of a liquid such as hot water at a location between a source of flowing liquid, such as a hot water source (HWS) riser pipe, to an inlet port of a destination terminal for the flowing liquid, such as a Variable Air Volume (VAV) air-to-water heat exchanger box. 
     The leak detection and shut-off apparatus according to the present invention includes a second, downstream flow rate sensor. The downstream flow rate sensor is used to measure the flow rate of a liquid such as hot water at a location between an outlet port of a destination such as a VAV terminal and a return conduit for liquid from the terminal, such as a hot water return (HWR) riser pipe. 
     According to the present invention, the leak detection and shut-off apparatus also includes at least a first, upstream shut-off valve. The upstream shut-off valve is located between a source of flowing liquid, such as a HWS riser pipe, and a terminal such as a VAV box. In a preferred embodiment, the upstream shut-off valve is a solenoid actuated valve which has an inlet port connected to the HWS riser pipe and an outlet port connected to a section of pipe in which is located the first, upstream flow rate sensor. 
     The leak detection and shut-off apparatus according to the present invention includes a control module which contains electronic logic control circuitry that has a first set of input signal or interrupt lines which are electrically connected to signal output terminals of the upstream flow rate sensor. The control logic circuitry processes signals received from the upstream flow rate sensor, and uses scaling circuitry to produce a signal, preferably a digital signal which has a numeric value that has a magnitude which is proportional to the measured, flow rate of liquid through the upstream flow sensor. For example, if the range of normal flow rates through the upstream flow sensor were expected to be between 60 and 80 gallons per Minute (GPM), the upstream flow rate sensor and control circuitry, could be selected to have a full-scale output voltage of 5 volts D.C. for a flow rate of 100 GPM, 4 volts for 80 GPM, 3 volts for 60 GPM, etc. 
     The control module of the leak detection and shut-off apparatus according to the present invention includes a second set of input or interrupt lines which are electrically connected to the signal output terminals of the downstream flow rate sensor. Scaling circuitry within the control module outputs a voltage scaled to the flow rate measured by the downstream flow rate sensor, which has a similar and preferably the same sensitivity or scale factor as that of the upstream flow rate sensor, i.e., 5 volts for a measured downstream flow rate of 100 GPM, 4 volts for a 80 GPM flow rate, 3 volts for a 60 GPM flow rate, etc. 
     According to the invention, the control module also includes a subtractor which is preferably implemented as a software application that resides in a; microprocessor, micro-controller, or other such digital computational circuitry. 
     The subtractor of the control module outputs a signal which is proportional to the difference between upstream and downstream flow rates measured by the upstream and downstream flow rate sensors, respectively. The control module also has a memory location which contains a predetermined differential flow rate limit value, e.g., 0.1 GPM, which is input to the control module as a pre-programmed number, or input by an external data input. 
     If the predetermined differential flow rate limit value is exceeded, comparator circuitry within the control module which has an input connected to the subtractor outputs an alarm status signal, which typically would be a logic TRUE signal. For example, the control module could be programmed to provide an alarm status signal upon detecting a flow rate difference equal to or exceeding 0.1 GPM between upstream and downstream flow rate sensors. Since, in the absence of leakage at any place between upstream and downstream flow rate sensors, the flow rates measured by the two sensors would be substantially identical, save for small frictional losses, it can be inferred that any difference greater than a certain small threshold value between measured upstream and downstream flow rates signifies existence of a leak somewhere in the piping system between the two sensors. 
     When the difference between measured upstream and downstream flow rates exceeds a predetermined threshold limit value and thus causes the alarm status output line of the control module to go to a logic TRUE state, the alarm status signal actuates and energizes a solid state or electro-mechanical valve shut-off relay. In turn, the valve shut-off relay interrupts a 24-volt A.C. power supplied to the upstream solenoid shut-off valve. 
     The solenoid shut-off valve is a normally closed valve which opens to allow flow only when supplied with continuous electrical power. Therefore, if a leak rate which exceeds a predetermined limit value occurs, or if electrical power is interrupted to the building mains which supply power to the control apparatus, according to the present invention, the upstream shut-off valve immediately and automatically closes and interrupts the flow of hot water from the hot water source riser line to a destination which is downstream from the apparatus, such as a VAV terminal. Thus, in the event of a detected water leak or interruption of building power, the apparatus according to the present invention is effective in immediately terminating flow of hot water beyond the upstream shut-off valve. Optionally and desirably, the alarm status signal would also be sent to building maintenance stations to inform maintenance personnel of a detected leak. 
     A preferred embodiment of a leak detection method and apparatus according to the present invention also includes a normally closed, solenoid actuated downstream shut-off valve, which may be identical to the upstream shut-off valve. The downstream shut-off valve is located between the downstream flow rate sensor and the Hot Water Return (HWR) riser pipe. The downstream shut-off valve is operated in unison with the upstream shut-off valve, i.e., it is closed when a leak of a predetermined minimum value occurs, or if electrical power to the building is interrupted. The downstream shut-off valve is provided in addition to the upstream shut-off valve to prevent any water to the riser HWS or HWR lines from flowing back to the area where the leak has been detected. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an apparatus for water leak detection and shut-off method according to the present invention, showing the apparatus installed in a closed-loop water circulation plumbing system. 
         FIG. 2  is a more detailed schematic diagram of a control module comprising part of the apparatus of  FIG. 1 . 
         FIG. 3  is a partly diagrammatic view of a Variable Air Volume (VAV) terminal, one of a variety of different types of terminal equipment interconnectable to the leak detection and shut-off apparatus of  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  illustrates a basic embodiment of water leak detection and shut-off apparatus  10  using differential flow rate sensors according to the present invention.  FIG. 2  is a more detailed schematic view of a control panel  11 A and control module  11  comprising part of the apparatus  10  of  FIG. 1 .  FIG. 3  is a partly diagrammatic view of a Variable Air Volume (VAV) terminal  12  which; typifies a terminal component of a closed-loop water circulation system of a type which apparatus  10  is intended for use with. 
     Referring now to  FIG. 1 , it may be seen that water leak detection and shut-off apparatus  10  according to the present invention includes an upstream inlet port  13  for receiving a flowing liquid such as hot water which is pressurized above ambient atmosphere pressure by a pump and/or a gravity pressure head, i.e., from a pump at any elevation or a tank at a higher elevation than inlet port  13 . Upstream inlet port  13  is connected by a fluid pressure-tight tube or pipe to the inlet port  15  of a first, upstream solenoid valve  14 . 
     As shown in  FIG. 1 , upstream inlet port  13  of apparatus  10  includes an inlet pipe  16  which is continuous with or connected through a fitting (not shown) to a source of pressurized liquid, such as an upper part of a Hot Water Source (HWS) riser pipe  17  which is connected at the lower end to a source of hot water, such as a boiler heat exchanger. As is also shown in  FIG. 1 , inlet port  13  of apparatus  10  may optionally include an upstream inlet manual shut-off valve  18  which has an inlet port connected to inlet pipe  16 , and an outlet pipe  19  which is connected between an outlet port of manual shut-Off valve  18  and inlet port  15  of solenoid valve  14 . 
     Solenoid valve  14  is preferably a normally closed (NC) valve in which an internal spring maintains a gate element, such as a ball or plate, of the valve in a fully closed position unless an electrical power such as 24-volt A.C. power is input to terminals  20 ,  21  of the valve, thus actuating a solenoid within the valve to open the gate against the closing force of the valve&#39;s internal spring. In an example embodiment of apparatus  10  which was designed, built and tested by the present inventors, upstream solenoid valve  14  was a B220+NC/FC+LF24US model manufactured by Belimo Amiricas, 43 Old School Rd., Danbury, Conn. 06810. That valve had the following characteristics spring return normally closed full port ball valve. As shown in  FIG. 1 , valve  14  has an outlet port  22 . 
     Referring still to  FIG. 1 , it may be seen that water leak detection and shut-off apparatus  10  according to the present invention includes a first, upstream flow rate sensor  23 . Flow rate sensor  23  has a tubular body  24  which has disposed longitudinally therethrough a bore  25 . Bore  25  of flow rate sensor  23  preferably has a cross-sectional area which is large enough to provide a negligibly small impedance or restriction to flow of liquid through apparatus  10 . 
     As shown in  FIG. 1 , bore  25  of sensor  23  has an inlet port  26  which is connected in fluid pressure-tight connection through a pipe  27  to outlet port  22  of upstream solenoid valve  14 . Bore  25  of sensor  23  also has an outlet port  28  which is connected in a fluid pressure-tight connection to an upstream outlet pipe  29  of apparatus  10 . Optionally, apparatus  10  may include a manually operable upstream outlet shut-off valve  30  which has an inlet port connected in a fluid pressure-tight connection to pipe  29  and an outlet port connected to upstream outlet port  31 . In an example embodiment of apparatus  10  which was designed, built and tested by the present inventors, upstream flow rate sensor  23  was a 600 model manufactured by Imperial Flange and Fitting Company, P.O. Box 352262, Los Angeles, Calif. 90035. 
     The flow rate sensor  23  is a Pitot tube-type flow meter which has an upstream “high side” Pitot tube probe  32  that protrudes radially inwards through the cylindrical wall  33  of the sensor body  24  into bore  25  of the sensor. The upstream Pitot tube probe  32  has a transversely disposed, upstream pointing face in which is located at least one orifice which has a longitudinally disposed bore for measuring the stagnation pressure of liquid impacting the upstream face of the probe. 
     Pitot tube flow rate sensor  23  also has a second, downstream Pitot tube probe  24  which protrudes radially inwards through wall  33  of sensor body  24  into bore  25  of the sensor  23 . The downstream Pitot tube  34  has a transversely disposed, downstream pointing face in which is located at least one orifice for measuring the hydrostatic pressure of liquid in bore  25  of sensor body  24 . Since the stagnation pressure at the entrance orifice(s) of upstream Pitot tube  34  is proportional to the kinetic energy of fluid impacting the upstream orifice plate face, and hydrostatic pressure at the entrance orifice(s) of downstream Pitot tube is proportional to static pressure of fluid in the sensor bore, the difference between the two pressure measurements is a measure of the velocity and hence mass flow rate of liquid through bore  25  of sensor  23 . 
     Sensor  23  includes a transducer module for converting differences in measured pressures between upstream and downstream Pitot tube probes  32  and  34  into an electrical signal which is proportional to the velocity and hence mass flow rate of liquid through sensor  23 . For example, a sensor  23  having a maximum useable flow rate of 100 gallons per minute (GPM) may have an output signal at output terminal  36  of transducer module  35  of five volts, full scale output level for a flow rate of 100 GPM, 4-volts for a flow rate of 80 GPM, 3-volts for a flow rate of 60 GPM, etc. Signal output terminal  36  of transducer module  35  is connected to interrupt input terminal  97  of control module  11 . 
     As shown in  FIG. 1 , transducer module  35  has input power terminals  37 ,  38  for receiving DC power provided by control panel  11 A, for powering electronics within the transducer module. 
     In a preferred embodiment of apparatus  10 , at least the upstream Pitot tube probe  32  has a plurality of spaced apart orifices positioned on the rear, upstream face of the probe. This arrangement provides a measure of stagnation pressure which is averaged over the velocity profile of liquid flowing through bore  25  of sensor  23 , and thus provides a more accurate measurement of the mass flow rate of liquid through the sensor. 
     Referring still to  FIG. 1 , it may be seen that the components  13  through  38  of apparatus  10  comprise what may be identified as an “upstream leg”  40  of the apparatus Upstream leg  40  of apparatus  10  has an inlet port consisting of an inlet pipe  16  which is connected to a source of pressurized water, which in the present example of an application for apparatus  10 , is a Hot Water Source (HWS) riser pipe. As is also shown in  FIG. 1 , upstream leg  40  of apparatus  10  has an outlet port consisting of an outlet pipe  31 . Outlet pipe  31  is connected to the inlet port of equipment which is supplied with a flowing liquid from apparatus  10 , such as inlet port  41  of VAV terminal  12 , as shown in  FIG. 3 . 
     Referring again to  FIG. 1 , it may be seen that leak detection and shut-off apparatus  10  includes a “downstream leg”  50  which is substantially similar to and may in fact be identical in construction and function to upstream leg  40 . Thus, as will now be described, downstream leg  50  of apparatus  10  has components which are exact counterparts of those in upstream leg  40 , which were previously described in detail above. The foregoing detailed description should be referred to in conjunction with the following abbreviated description of components of the downstream leg  50 . 
     Referring to  FIG. 1 , it may be seen that downstream leg  50  of water leak detection and shut-off apparatus  10  includes a return inlet port  53  for receiving flowing water which has been returned from equipment supplied with hot water from outlet pipe  31  of upstream leg  40  of the apparatus. Downstream return inlet port  53  of apparatus  10  includes a return inlet pipe  56  which is connected to hot water return line of a destination for hot water from outlet pipe  31  of the apparatus, such as outlet pipe  52  of VAV terminal  12  (see  FIG. 3 ). 
     As shown in  FIG. 1 , inlet port  53  of apparatus  10  may optionally include a manual inlet shut-off valve  58  which has an inlet port connected to a return inlet pipe  56 , and an outlet pipe  59  which is connected between an outlet port of manual inlet shut-off valve  58  and an inlet port  66  of a second, downstream flow-rate sensor  63 . 
     In a preferred embodiment of apparatus  10 , downstream flow-rate sensor  63  is identical in construction and function to upstream flow-rate sensor  23 , which was described in detail above. Thus, downstream flow-rate sensor  63  has a tubular body  64  which has disposed longitudinally through its length a cylindrically shaped, circular cross-section bore  65 . Inlet port  65  of downstream flow-rate sensor  63  is connected through pipe  59  to the outlet port of manual shut-off valve  58 . 
     Referring still to  FIG. 1 , it may be seen that downstream flow-rate sensor  63  includes an upstream Pitot tube probe  72  and a downstream Pitot tube probe  74 , both of which protrude radially inwardly through the cylindrical wall  73  of sensor body  64  into bore  65  of the sensor. Pitot tube probes  72 ,  74  are coupled to a pressure transducer transmitter module  75 , which has a signal output terminal  76  that outputs a signal voltage proportional to the pressure difference between the probes, and hence the mass flow-rate of liquid through bore  65  of sensor  63 . Pressure sensor transducer transmitter module  75  is provided with a 24-volt power from control module  11 , which is input to line and ground terminals  77 ,  78  of the module. Signal output terminal  76  is connected to interrupt terminal  99  of control module  11 . 
     Referring still to  FIG. 1 , it may be seen that downstream flow-rate sensor  63  has an outlet port  68  which is connected by an outlet pipe  69  to the inlet port  55  of a normally closed downstream solenoid valve  54 . Downstream solenoid valve  54  may be identical in construction and function to upstream solenoid valve  14 . Thus, as shown in  FIG. 1 , downstream solenoid valve  54  has electrical power input terminals  60 ,  61 , which must be continuously provided with 24-volt AC power from control module  11  for valve  54  to remain open. Solenoid valve also has an outlet port  62  which is connected to a Hot Water Return (HWR) riser pipe  81 . As shown in  FIG. 1 , apparatus  10  optionally includes a manually operable downstream outlet shut-off valve  70  connected between outlet port  62  of solenoid valve  54 , and HWR riser pipe  81 . 
     Referring now to  FIG. 2 , it may be seen that apparatus  10  includes a control panel  11 A on which is mounted control module  11 , along with other components which together comprise a Direct Digital Controller (DDC), of a type which is commonly used in HVAC systems to control parameters such as air temperature and air flow-rate in response to sensed parameters such as ambient temperature and humidity. 
     As shown in  FIG. 2 , the DDC controller module  11  is of conventional design and includes a microprocessor (not shown) and power supply. As shown in  FIG. 2 , DDC controller module  11  has a pair of bidirectional data signal terminals  91 ,  92  connected to network port terminals  93 ,  94  of a first, up-net network port  93 A of DDC control panel  11 A, and network port terminals  95 ,  96 , of a second down-net port  95 A of DDC control panel  11 A. The network port terminals  93 ,  94  and  95 ,  96  of DDC control panel are used to enable interconnection of DDC control module  11  with previous and next DDC controller modules (not shown) which are part of a distributed network such as a Local Area Network (LAN). 
     Referring still to  FIG. 2 , it may be seen that DDC control module  11  has a first interrupt port consisting of a high-side interrupt terminal  97 , and a low side or ground interrupt terminal  98 . High-side input terminal  97  is connected to an input terminal  128  of DDC Control Panel  11 A, which, as shown in  FIG. 1 , is connected to signal output terminal  36  of pressure transducer transmitter module  35  of upstream flow-rate sensor  23 . Microprocessor circuitry within control module  11  converts an analog signal voltage present at the output signal terminal  36  of pressure transducer transmitter module  35 , and hence at interrupt input terminal  97  of the DDC Control Module  11  to a digital value, and stores that digital value in a first memory location of the microprocessor for subsequent processing. 
     As shown in  FIGS. 1 and 2 , Control Module  11  also has a second interrupt input port consisting of a high-side interrupt terminal  99 , and a low-side or ground interrupt terminal  100 . High-side interrupt input terminal  99  is connected to an input terminal  127  of Control Panel  11 A, which is in turn connected to signal output terminal  76  of pressure transducer transmitter module  75  of downstream flow-rate sensor  63 . Microprocessor circuitry within control module  11  converts an analog signal voltage present at the output signal terminal  76  of pressure transducer transmitter  75 , and hence at interrupt input terminal  99  of DDC control module  11  to a digital value, and stores that digital value in a second memory location of the microprocessor for subsequent processing. 
     Microprocessor circuitry within control module  11  also has stored within a third memory location of the microprocessor a digital number representing a maximum allowable difference between upstream and downstream flow-rates measured by upstream and downstream flow-rate sensors  23 ,  63 , respectively. A typical threshold flow-rate difference value might, for example, be in the range of 0.1 to 1.0 gallons per minute (GPM). A selected threshold flow-rate difference value is entered into control module  11  by conventional means, such as via network ports  93 A or  95 A. 
     Microprocessor circuitry within control module  11  cyclically and continuously samples the values of upstream flow-rate and downstream flow-rates stored in the upstream and downstream flow-rate memory locations, and inputs those values into minuend and subtrahend ports of a digital subtractor application. The difference output value of the digital subtractor is input to the second variable input of digital comparator; application of the microprocessor. That application has a first, set point value input into which is input the threshold flow-rate. If the flow-rate difference input to the variable input port of the comparator equals or exceeds the threshold flow-rate, the comparator outputs a digital TRUE alarm status signal. 
     DDC Control Module  11  also contains an electromechanical or solid state relay (not shown) which receives a continuous energization signal from the microprocessor in the control module as long as the alarm status signal is not TRUE. The relay conducts 24-volt AC power input to terminal control module  11  on terminal  103  to output terminal  104  of the control module. Output terminal  104  of control module  11  is connected to switched 24-AC high-side terminals  124 ,  126  of control panel  11 A which are in turn connected to high-side terminals  61 ,  21  of solenoid valves  54 ,  14  respectively. Low-side 24-AC terminals  123 ,  125  of control panel  11 A are connected to low-side terminals  60 ,  20  of solenoid valves  54 ,  14 . The switched 24-volt AC power supplied to solenoid valves  54 ,  14  maintains the valves in a fully open position. However, if the microprocessor in the control module  11  outputs a logic TRUE alarm status signal in response to measured flow-rate difference between upstream flow-rate sensor  23  and downstream flow-rate sensor  63  which exceeds the preprogrammed threshold value, 24-volt AC power supplied to the solenoid valves is immediately interrupted, thus causing the values to close and thus shut off flow of water from apparatus  10 . 
     Referring to  FIG. 2 , it may be seen that control panel  11 A includes a step-down transformer  130  which receives 115-volt AC power input to terminals  119 ,  120 , of the control panel, and a circuit breaker  131  in series with input terminal  119  and the transformer. Transformer  130  supplies 24-volt AC power to terminals  101 ,  102  of DDC control module  11 , as explained above, and to input terminals  134 ,  135  of a 24-volt DC power supply  132 . Power supply  132  has plus and minus 24-volt DC output terminals  136 ,  137  which are connected to output terminals  121 ,  122 , respectively, of DDC control panel  11 A. As shown in  FIG. 1 , 24-volt DC power output on terminals  121 ,  122  of control panel  11 A is input to transducer transmitter modules  35 ,  75  of flow-rate sensors  23 ,  63 , respectively, on input terminal pairs  37 ,  38  and  77 , 78 , respectively. 
       FIG. 3  illustrates a typical terminal  12  apparatus of the type which water leak detection and shutoff apparatus  10  is intended to be used with. As shown in  FIG. 3 , a Variable Air Volume (VAV) terminal  12  includes an elongated box-like heat exchanger duct  141  which has an inlet opening  142  that receives cold air from a cold air inlet duct  143 . Heat exchanger duct  141  also has an air outlet opening  144  which is connected to a number of ceiling-mounted air flow diffusers  145 . 
     As shown in  FIG. 3 , VAV terminal  12  includes an inlet air property sensor module  146  which contains a sensor for measuring properties of cold air inlet through duct  143 , such as temperature, humidity and flow-rate, and inputs the values of the properties to a VAV controller  147 , which may be part of DDC control panel  11 A, or a separate controller. 
     As shown in  FIG. 3 , VAV terminal  12  includes a local or zone thermostat  148  by which a set point for a desired temperature range of a zone or zones services by terminal  12  may be manually or remotely input. 
     VAV terminal  12  also includes a damper valve  149  which has a damper plate  150  which is rotatable by a motor  151  to control the flow-rate of air input into entrance  142  of duct  141  from cold air inlet duct  143 . 
     As shown in  FIG. 3 , VAV terminal  12  includes a heater coil  152 . Heater coil  152  is a flowing air to water heat exchanger which includes an elongated coil of tubing which has high thermal conductivity, such as copper tubing, an inlet filling  153  for receiving in fluid pressure-tight connection a source of hot, flowing water, such as a pipe  154  connected to inlet port  41  of VAV terminal  12 , and an outlet port  155  connected to a discharge or outlet pipe  156 . 
     The heater coil  152  which typically has the shape of a spiral or helix which has a longitudinal axis coincident with the longitudinal axis of heat exchanger duct  152 , provides an efficient means of transferring heat from the heated water input to inlet port  153  of the coil, to cold air flowing longitudinally through the heat exchanger duct and exiting through outlet opening  144  of the duct to ceiling diffusers  145 . 
     As shown in  FIG. 13 , the temperature of air exiting heat exchanger duct  141  and conducted to ceiling diffusers  145  is controlled not only by controlling the air flow-rate via damper valve  149 , but also by controlling the rate of hot water flow through heater coil  152 . Thus, as shown in  FIG. 3 , VAV terminal  12  includes a Normally Closed (NC) proportional control valve  157  which has an inlet port connected to heat exchange coil outlet. discharge pipe  156 , and an outlet port connected to VAV outlet pipe  52 . Valve  157  has an actuator control terminal  158  which is connected to controller  147  and enables the flow-rate of hot water through valve  157  and water coil  152  to be varied over a continuous range from zero to maximum flow-rate.