Abstract:
A pressure relief valve monitoring device is provided. The monitoring device includes a sensor input module located proximate to a pressure relief valve, a microcontroller located within the sensor input module, and a real time clock/calendar also located within the sensor input module. The monitoring device also includes a number of sensors, including (1) a position sensor mounted on the pressure relief valve for measuring the position of the valve&#39;s closure element relative to the inlet nozzle seat and for generating a lift signal representative of such position; (2) a pressure sensor mounted on the pressure relief valve for measuring the pressure of the pressure system and generating a pressure signal representative of such pressure; and (3) a leakage sensor mounted on the pressure relief valve and positioned in close proximity to the inlet nozzle seat and capable of detecting noise generated by leakage of fluid between the inlet nozzle seat and the closure element when the closure element is engaged with the inlet nozzle seat. The microcontroller is configured to receive and store signals from any or all of the three sensors and correlate the receipt thereof with an indication of time from the real time clock/calendar to determine certain characteristics of valve performance. A method for monitoring the operation of the pressure relief valve is also provided.

Description:
This Application claims the benefit of U.S. Provisional Application No. 60/083,021, filed Apr. 24, 1998. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to pressure relief devices, and more particularly to devices for monitoring their performance. 
     BACKGROUND OF THE INVENTION 
     Pressure lines (e.g., pressure vessels and piping systems) are often designed with pressure relief valves located at various locations to protect the pressure line from excess overpressure. The pressure relief valves are self-actuated devices set to open when the pressure in the pressure line exceeds a specified level. When the pressure in the pressure line exceeds the pressure at which the pressure relief valve is set to open, the closure element of the pressure relief valve moves away from the inlet nozzle seat and fluid is allowed to flow out of the pressure line and through the pressure relief valve. This flow of fluid will continue at a sufficient rate to prevent the pressure in the pressure line from rising above a predetermined level or above a specified overpressure. When the pressure in the pressure line is reduced to a level below the pressure at which the pressure relief valve is set to open, the closure element in the pressure relief valve will return to its closed position, i.e. into contact with the inlet nozzle seat, preventing additional flow from the pressure line. Under normal operating conditions, the closure element of the pressure relief valve is in the closed position. 
     Prior art monitoring devices used in these pressure lines typically employ position transducers mounted on the pressure relief valve to sense the position of the closure element. These position transducers transmit analog signals indicating the position of the closure element with respect to the inlet nozzle. These devices, however, do not store this information and apply the information to determine operating characteristics of the pressure relief valve, such as total flow through the pressure relief valve during a specified time interval when the pressure relief valve is open. 
     Moreover, in the prior art the presence of leakage flow past the closure element of a pressure relief valve (i.e., flow past the closure element when the closure element is in the closed position) could only be determined by physically examining the valve in its installed position, removing the valve from its installed position, and performing a seat leakage test on a test stand, or by isolating the valve (through the use of appropriate valving) in its installed position, but not in active service, and performing a seat leakage test in situ. Such techniques for determining the presence of seat leakage, however, do not allow for continuous monitoring to detect seat leakage past the closure element while the pressure relief valve is both installed and in service. 
     In addition, unstable operation of pressure relief valves, i.e. rapid opening and closing of the closure element, can occur when the system pressure rises just to or slightly above the set pressure and then drops, as a result of fluid flowing from the system through the pressure relief valve, as soon as the closure element lifts off the seat permitting the spring to immediately seat the closure element. Such unstable operation, however, can cause physical damage to components of the pressure relief valve. It is therefore desirable to know when such unstable operation occurs so that corrective action may be taken. The prior art practice has been for personnel to listen for the noise, often referred to as “valve chatter,” generated by the closure element being rapidly and repeatedly forced against its seat. This practice, however, is ineffective if no personnel are near the valve at the time the unstable operation occurs or if the location of the valve is beyond earshot of attending personnel. 
     SUMMARY OF THE INVENTION 
     In light of the above, a pressure relief valve monitoring device is provided. The monitoring device includes a sensor input module located proximate to a pressure relief valve, a microcontroller located within the sensor input module, and a real time clock/calendar also located within the sensor input module. The monitoring device also includes a number of sensors, including ( 1 ) a position sensor mounted on the pressure relief valve for measuring the position of the valve&#39;s closure element relative to the inlet nozzle seat and for generating a lift signal representative of such position; ( 2 ) a pressure sensor mounted on the pressure relief valve for measuring the pressure of the pressure system and generating a pressure signal representative of such pressure; and ( 3 ) a leakage sensor mounted on the pressure relief valve and positioned in close proximity to the inlet nozzle seat and capable of detecting noise generated by leakage of fluid between the inlet nozzle seat and the closure element when the closure element is engaged with the inlet nozzle seat. The microcontroller is configured to receive and store signals from any or all of the three sensors and correlate the receipt thereof with an indication of time from the real time clock/calendar to determine certain characteristics of valve performance. A method for monitoring the operation of the pressure relief valve is also provided. 
     The present invention provides a monitoring device mounted on or near a pressure relief valve and which will continuously monitor the performance of the pressure relief valve while the valve is in active service. The monitoring device will also convert analog signals received from sensors attached to the valve into digital format, will store the digital information, will detect leakage flow through the pressure relief valve while the valve is in active service, will calculate fluid mass flow through the pressure relief valve when the valve is open and allowing fluid to flow from the pressure line, will detect and warn of unstable operation of the pressure relief valve, and will communicate with a host computer to transmit the information stored by the valve monitoring device and receive information regarding that particular valve from the host computer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing objects and other attributes of this invention, and the attendant advantages thereof, may be more fully understood from the following description when read together with the accompanying drawings in which: 
     FIG. 1 is a view of a typical pressure relief valve, shown in vertical section, with a valve monitoring device according to the present invention attached thereto; 
     FIG. 1 a  is an enlarged view of a portion of the pressure relief valve shown in FIG. 1; 
     FIG. 1 b  is a view of the pressure relief valve shown in Fig. 1 a  showing the valve in an open position; 
     FIG. 2 is a view similar to FIG. 1, but showing two pressure relief valves with the valve monitoring devices connected to a host computer; and 
     FIG. 3 is a schematic block diagram of the circuitry incorporated into each valve monitoring device. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a preferred embodiment of the pressure relief valve monitoring device, indicated generally at  8 , according to the present invention. As shown, this embodiment includes a sensor input module or SIM  10  which is connected to and receives analog signals from a position sensor  12 , a leakage sensor  14 , and a pressure sensor  15 , all mounted on a pressure relief valve  13 . The pressure relief valve  13  includes an inlet nozzle  20  with an inlet nozzle seat  25  connected in fluid communication with a pressure system  18 , and a closure element  16  which contacts the inlet nozzle seat  25  when the pressure relief valve  13  is closed and which is able to move away from the inlet nozzle seat  25  to allow fluid from the pressure system  18  to flow through the nozzle  20 , with a compression spring  21  controlling the position of the closure element  16  by opposing the force of the pressure acting on the closure element  16 . A housing  22  supports and contains the aforementioned valve components. A spindle  17  extends through the housing  22  and is held by the spring  21  against, and thus exactly replicates the movement of, the closure element  16 . 
     An enlarged view of the area of the pressure relief valve  13  where the closure element  16  contacts the nozzle seat  25  when the valve  13  is closed is shown in FIG. 1 a . With reference to FIG. 1 a , the portion of the closure element  16  that comes into contact with the inlet nozzle  20  at the inlet nozzle seat  25  (i.e., the surface of the closure element  16  where contact occurs) may be identified as the closure element seat  16   a . Similarly, the portion of the inlet nozzle  20  that comes into contact with the closure element  16  (or, more specifically, with the closure element seat  16   a  of the closure element  16 ) may be identified as the inlet nozzle seat  25 . Both the closure element seat  16   a  and the inlet nozzle seat  25  (and their interaction) may be more clearly seen with reference to FIG. 1 b , which shows the section of the pressure relief valve  13  shown in FIG. 1 a  except that in FIG. 1 b  the valve  13  is open. 
     Referring again to FIG. 1, the pressure relief valve  13  is designed to protect the pressure system  18  from excess pressure. The spring  21  is pre-compressed and applies a force which holds the closure element  16  in contact with the nozzle seat  25 . In this position, the closure element  16  forms a seal with the nozzle seat  25  and thus prevents fluid from the pressure system  18  from flowing through the nozzle  20 . Pressure in the pressure system  18  acts upon the closure element  16  creating a force which opposes the spring force. When pressure in the pressure system  18  reaches a predetermined level, i.e. the set pressure of the pressure relief valve  13 , the force of the pressure acting on the closure element  16  overcomes the force exerted by the pre-compressed spring  21 , thus permitting the closure element  16  to move away from the nozzle seat to allow fluid to flow out of the pressure system  18 , through the nozzle  20 , past the closure element  16 , and out of the pressure relief valve  13 . The flow of fluid out of the pressure system  18  prevents pressure in the pressure system  18  from increasing above an allowable level above the set pressure. 
     A lift or position sensor  12 , which preferably is a high impedance variable resistor (or other means of indicating lift) such as a 50k Ohm potentiometer sold by Betatronix, Inc., is in contact with the spindle  17  which moves with the closure element  16 . The resistance generated by the position sensor  12  is an indication of the position of the closure element  16 . This position information is transmitted to the sensor input module  10 . When the closure element  16  moves, the change in resistance of the sensor  12  indicates a change in the position of the closure element  16  and the magnitude of that change, which information is stored in the sensor input module  10  as a function of real time—i.e., the time at which movement of the position sensor  12  occurred, the extent of that movement, and the elapsed time the closure element  16  remained in that position are recorded. The information so recorded, along with corresponding information from the pressure sensor  15 , permits a determination of the quantity or mass of fluid that has escaped through the pressure relief valve  13  under critical flow conditions when the fluid is compressible. When the fluid flow through the pressure relief valve is compressible and subcritical, both the inlet and the outlet pressure may be used to determine the total mass flow. When the fluid flow through the pressure relief valve is noncompressible, the differential pressure between the inlet and outlet may be used to determine the total mass flow. The pressure sensor  15 , which preferably is a thin film strain gauge such as sold by Strain Measurement Devices, Inc., is connected to the inlet pressure line  18  and provides a signal which is representative of the magnitude of the pressure in the pressure line  18 . This pressure signal is transmitted to the sensor input module  10 , where it is recorded in relation to real time. 
     Additional sensors may be provided which measure outlet pressure, differential pressure, fluid or ambient temperature, or other parameters. Information from such additional sensors may also, in a similar manner, be transmitted to and stored in the sensor input module  10 . 
     When the magnitude of the pressure in the inlet pressure line  18  is reduced to a specified level below the opening pressure of the pressure relief valve  13 , the force of the spring  21  overcomes the pressure force on the closure element  16  and the closure element  16  moves back into contact with the nozzle seat  25  stopping further flow of fluid from the inlet pressure line  18 . When the closure element  16  comes in contact with the nozzle seat  25 , movement of the closure element  16  ceases. When this occurs, storage of additional inlet pressure data and closure element position data in the sensor input module  10  may be discontinued. 
     Under normal operating conditions, the closure element  16  remains in contact with the nozzle seat  25 . This is the closed position of the pressure relief valve  13 . While there should be no flow past the closure element when the closure element  16  is in its closed position, such “leakage flow” can occur and it is often important to know when it does. A leakage sensor  14 , preferably a piezo electric crystal, such as that sold by Massa Products Corp., is attached to the housing  22  of the pressure relief valve  13  and protrudes through the housing  22  in close proximity to the interface between the inlet nozzle seat  25  and the closure element  16 . When the closure element  16  is not in perfect sealing engagement with the nozzle seat  25 , or when either the closure element seat  16   a  or the inlet nozzle seat  25  is damaged, or when solid particles are present on the nozzle seat  25  or on the closure element seat  16   a , there may be leakage flow between the closure element seat  16   a  and the inlet nozzle seat  25 . Such leakage creates a noise having a characteristic frequency, which frequency is a function of the fluid within the system. The leakage sensor  14  is capable of detecting noises in this range of frequencies and, upon detection, sends a signal to the sensor input module  10  indicating the presence of leakage flow. This signal, as a function of time, is stored in the sensor input module  10 . This signal may also be transmitted from the sensor input module  10  to an enunciating device or to a process controller to activate an alarm indicating the presence of leakage flow in the pressure relief valve  13  in installations where the device is connected to such a network. 
     As shown in FIG. 3, which represents a preferred arrangement, the circuitry incorporated into the SIM  10  is provided with a microcontroller  30 , such as Phillips 80CL 580. This particular microcontroller has integrated analog to digital (A/D) conversion capability, but a microcontroller without such capability may be used if separate A/D conversion is provided. Each of the sensors  12 ,  14 , and  15  is connected to the microcontroller  30  through a signal conditioner  32 , which amplifies and conditions the signals from their respective sensors for A/D conversion by the microcontroller  30 , which signals are representative of the magnitude of the parameter being measured by each of the sensors  12 ,  14 , and  15 . A real time clock/calendar  34  is also connected to the microcontroller  30  to provide an accurate indication of the time the signals are generated by each of the sensors  12 ,  14 , and  15 . The input connections for each sensor also provide power to the respective sensors  12  and  15 . 
     The digital signals so generated are correlated with the data from the clock/calendar  34  by the microcontroller  30  and stored in on-board random access memory (RAM)  36 . The microcontroller  30  also converts the digital information into readable information for display on any suitable readout device, such as a liquid crystal display (LCD)  38 , provided on the valve monitoring device  8 . The microcontroller  30  will generate messages for display on the readout device which (i) indicate leakage flow through the pressure relief valve  13  has or is occurring, (ii) warn of unstable operation whenever the opening and closing of the closure element  16  in a given time interval exceeds a predetermined limit, which indicates that valve chatter has occurred, and (iii) indicate that the valve  13  has opened permitting escape of fluid from the system, the pressure at which it opened, the time at which it opened, and the length of time it was open, and may also calculate the mass or volume of fluid that escaped from the system through the valve  13 . The microcontroller  30  may also be programmed to send correlated data to a host computer  19 , as shown in FIG. 2, upon receipt of a command from the host computer  19 . 
     Electric power may be provided to the SIM  10  by means of a battery (e.g., for stand alone applications), an external power source, or a 4-20 mA current loop powered from a process control network or similar source. The 4-20 mA connection also provides a convenient means for transmitting information to a direct connected network host computer. Communication between the SIM  10  and a host computer  19  (shown in FIG. 2) may be provided via a conventional RS-232 port or a modem  40  supporting commonly-used communication protocols, such as HART. 
     With reference again to FIG. 1, in some cases it is desirable to sense the difference in pressure between the inlet pressure system  18  and the pressure in the pressure line at the outlet  26  of the pressure relief valve  13 . In such cases, a differential pressure sensor  24  is connected to the inlet pressure line  18  and the outlet pressure line  26  and the signal from the differential pressure sensor  24  is sent to the sensor input module  10  in addition to, or in lieu of, the signal sent from a pressure sensor  15  in the inlet pressure line  18 . Alternatively, an outlet pressure sensor  23 , which is similar to the pressure sensor  15 , may be connected to the outlet  26  of the pressure relief valve  13 . The signal generated by the sensor  23  is sent to the SIM  10  so that the microcontroller  30  can calculate the pressure difference. 
     FIG. 2 shows two pressure relief valves  13  mounted on a pressure line  18 , each with a valve monitoring device  8  in communication with a host computer  19 , which arrangement is representative of what may be a plurality of valves, each of which may be mounted on separate and independent pressure lines. Communication between the host computer and each of the valve monitoring devices  8  may be provided by means of a permanent connection therebetween or by temporarily connecting the host computer to each of the valve monitoring devices  8  periodically for the purpose of data transfer. The host computer  19  may be arranged to extract stored data from each valve monitoring device  8  and utilize such data to determine present operating characteristics of the pressure relief valve  13 , such as total flow through the pressure relief valve  13  during a specified time interval that the pressure relief valve  13  was open. When the valve monitoring device  8  is continuously connected to a host computer, a real time indication of lift or of the presence of leakage may be communicated as an alarm signal. The host computer  19  is arranged to analyze the information provided to it and to output information useful in determining the operational readiness of each valve, and may be used, when in constant communication with any particular valve monitoring device  8 , to sound an alarm or otherwise indicate when the corresponding pressure relief valve  13  is leaking and/or open. The host computer  19  may also be used to receive, update, and store data such as valve configuration, maintenance history, or other useful information. 
     One of the advantages of the present invention is that it provides a monitoring device that continuously monitors the performance of the pressure relief valve while the valve is in active service, including detecting leakage flow through the pressure relief valve, calculating fluid mass flow through the pressure relief valve when the valve is open and allowing fluid to flow from the pressure line, and detecting and warning of unstable operation of the pressure relief valve. The monitoring device of the present invention may also communicate with a host computer to transmit the information stored by the monitoring device and receive information regarding that particular valve from the host computer. Of course, other objects and advantages of the present invention will become readily apparent to those skilled in this art from the above-recited detailed description. Accordingly, the drawings and descriptions are to be regarded as illustrative in nature, and not as restrictive.