Abstract:
A method monitors the gas prefill pressure in hydraulic accumulators. After the pressure supply to the oil side of the accumulator has been interrupted and the contents of the tank have been emptied, the current gas temperature and the current gas pressure are determined, once the temperature has equalized. The data relating to the gas temperature and pressure is telemetrically transmitted to an evaluation unit, which is spatially located at a distance, to calculate an actual value of the gas prefill pressure which corresponds to a reference temperature.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a process for monitoring the gas prefill pressure in hydraulic accumulators. The process comprises the steps of interrupting delivery of pressure to the oil side of the accumulator and emptying of the contents into the tank, measuring the gas temperature on the gas side of the hydraulic accumulator, measuring the gas pressure on the gas side of the hydraulic accumulator, and determining an actual value of the gas prefill pressure based on data relating to gas temperature or gas pressure by means of an evaluation device. 
     BACKGROUND OF THE INVENTION 
     The operating properties of hydraulic accumulators are determined essentially by the prefill pressure selected. For the sake of perfect and dependable operation of accumulators and accumulator assemblies, monitoring must be carried out in the shorter periods between customary maintenance operations to determine if the constancy of the prefill pressure falls within acceptable variation limits. A monitoring process with the four process steps described above to be carried out for this purpose is conventional (cf. “Controle van hydro-accumulator” of the Dutch article “Continue bewaking van de hydro-accumulator bespaart veel volgschade” by T. Van Dale in “Polytechnisch Tijdschrift,” pp. 38-41, March 1992). 
     If the conventional process is performed as customary, protracted interruptions of operation and major difficulties arise, especially in cases in which a large number of accumulators are spatially mounted separate from each other in locations in large plants which are difficult to access. 
     SUMMARY OF THE INVENTION 
     Objects of the present invention are to provide a process which permits substantially simplified monitoring of hydraulic accumulators with a high degree of monitoring precision, even accumulators mounted in locations difficult to access. 
     In the process of the present invention, these objects are attained by measurement of the gas temperature and gas pressure up to the time of temperature equalization reached after emptying and telemetric transmission of data relating to gas temperature or gas pressure to a spatially remote evaluation device. 
     As a result of the telemetric data transmission of the present invention, a system is created allowing monitoring of the prefill pressure from a central measuring station in the shortest possible time and with the lowest possible personnel cost. The process of the present invention makes rapid and simple monitoring possible even in cases in which accumulators are difficult to access inside complex plants. Advantageous areas of application are accumulators and accumulator assemblies in iron and steel plants and rolling mills, in electric power generating plants, or, for example, machinery rooms with several machine assemblies. Simplification of monitoring makes it possible to carry out the monitoring processes in short periods of time, something of particular advantage if the hydraulic accumulators perform security functions. Additionally, the desirable high degree of monitoring precision is assured because data transmission takes into account the reaching of temperature equalization after emptying of the accumulator. 
     Telemetric data transmission may be effected by radio, instrument leads, via the telephone network, for example, or over the electric power supply grid by application of suitable modulation processes. 
     By preference, temperature values and pressure values generated by transducers coordinated with the thermometers and the pressure gauges on the manometer connection of the hydraulic accumulator are transmitted to a modem, one of which is provided on every hydraulic accumulator to be monitored. The temperature value and pressure value signals preferably are transmitted telemetrically by way of the modem of the hydraulic accumulator involved, without further processing, directly to the evaluation unit. On the evaluation unit, the appropriate computer program is run to calculate the actual gas prefill pressure corresponding to a reference temperature from the current temperature and pressure values. Alternatively, this calculation can be carried out at the hydraulic accumulator by an additional computer unit associated with a modem. Then, only the actual value of the gas prefill pressure is transmitted to the evaluation unit for subsequent presentation, for example, as a visual display, and/or for sounding an alarm or the like. 
     Other objects, advantages and salient features of the present invention will become apparent from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring to the drawings which form a part of this disclosure: 
     FIG. 1 is a schematic diagram of a hydraulic accumulator according to the present invention in conjunction with an accompanying evaluation unit; and 
     FIG. 2 is a block diagram of an evaluation unit for application of the process according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a hydraulic accumulator  1  in the form of a bubble accumulator with a gas side  3  and an oil side  5 . The process or method of the present invention can also be carried out in basically the same way with a diaphragm accumulator or piston accumulator. To measure the gas temperature inside the gas side  3  of the hydraulic accumulator  1 , a thermometer  7 , with accompanying transducer  9  for conversion of the temperature values to electric measurement values, is directly connected to the accumulator. A line leads from the gas side  3  of the hydraulic accumulator  1  to a manometer connection  11  coupled to both a manometer  13  and a second thermometer  15 . Manometer  13  and thermometer  15  have accompanying transducers  17  and  19 , respectively, for conversion of measured values to corresponding electric measurement signals. 
     A line  21  connects the oil side  5  of the hydraulic accumulator  1  to a safety and shut-off group  23  forming the connection between the accumulator connection S on the line  21  and the pressure connection P and the tank connection T. In addition to the customary primary shut-off unit  25 , the safety and shut-off group  23  has a pressure limit valve  27  and an electromagnetically controllable two-way valve  29 , which permits connection of the oil side  5  of the hydraulic accumulator  1  through line  21  and the accumulator connection S to the tank connection T of the safety and shut-off group  23 , if the oil valve (not shown) on hydraulic accumulator  1  is in an open state. A monitoring device for monitoring the open position or closed position of this oil valve preferably is in the form of a disk valve, is designated in FIG. 1 as  31  and delivers an appropriate monitoring signal by way of a transducer  33 . 
     The pressure connection P of the safety and shut-off group  23  is connected by an electromagnetically operated distributing valve  35  to the consumer line  37 , to which a motor and pump unit  39  is connected. Lines  43  and  45  lead from the tank connection of the safety and shut-off group  23  and from the distributing valve  35  to the tank  41 , respectively. A pressure gauge  47  for measurement of the pressure on the oil side  5  of the hydraulic accumulator  1  is connected to the manometer connection M of the safety and shut-off group  23  and has a transducer  49  for generation of an appropriate electric test signal. 
     The test signals of the transducers  9 ,  17 ,  19 , and  49 , and the monitoring signal of the transducer  33  are delivered to a modem  50 , which forms part of a telemetry device. The telemetry device transmits the data to an evaluation device  51  illustrated in FIG.  2  and from the evaluation device  51  to the modem  50 . The evaluation device  51  contains a modem  53 , a computer unit  55 , and a display unit  57 . Depending on whether data transmission takes place over a line or is wireless, the modems  50  and  53  have the modulation-demodulation systems suitable for the particular transmission process involved and, optionally, transceiving devices known from conventional telemetry systems. 
     For the purpose of initiating a measurement process, the modem  50  receives by telemetry a control command causing modem  50  to emit control signals S 1  and S 2  to the electromagnets of the distributing valve  35  and to the electromagnets of the distributing valve  29  to interrupt delivery under pressure to the oil side  5  of the hydraulic accumulator  1  and to empty the accumulator over line  21  through the distributing valve  29  into the tank  41 . Temperature and pressure measurements are made on the gas side if emptying has been completed, a situation indicated by the manometer  47  detecting absence of pressure and sending a signal announcing that fact as signal E 5  by way of the transducer  49 . At the same time, the monitoring device  31  sends by way of the transducer the monitoring signal E 4  indicating that the previously open oil valve on the hydraulic accumulator  1  is now closed. In order to determine when temperature equalization has occurred on the gas side  3  of the hydraulic accumulator  1 , that is, when the temperature changes resulting from the process of emptying the accumulator have been offset, temperature measurements are made simultaneously both by the thermometer  7  directly on the gas side  3  and by the thermometer  15  spatially remote from it at the manometer connection  11 , until equalization is signaled by test signals E 3  and E 2  generated by the coordinated transducers  9  and  19  as a result of coincidence of the test values. When this state is reached, the current gas pressure is simultaneously determined by the manometer  13  via manometer connection  11  and is delivered to the modem  50  by way of the pertinent transducer  17  as test signal E 1 . 
     The temperature and data values as thus transmitted are delivered by telemetric means to the modem  53  of the evaluating system  51 . The computer unit  55  of this system now executes a computer program in order to calculate from these data an actual gas prefill pressure value related to a reference temperature. Such calculation may be carried out with very good approximation as an isochoric ideal gas calculation, for example, in relation to a reference temperature of 20° C., in accordance with the equation          p   o     =       293   p     T                            
     in which p o =the temperature-referred actual value of the gas prefill pressure in bar, 
     p=the current measured gas pressure in bar, and 
     T=the current temperature in degrees Kelvin. 
     More particularly, this calculation may be performed by application of the equation of the state for real gases as developed by E. Bender; in this connection, reference is made to S. Rotthäuser, “A Process for Calculation and Investigation of Hydropneumatic Accumulators,” a published dissertation defended at the School of Mechanical Engineering of the Aachen University of Technology. 
     The values calculated by the computer unit  55  of evaluation system  51  as a function of test signals E 1 , E 2 , and E 3  are delivered as display values A to display unit  57  to be visually displayed. Depending on the application case and need, the display valves are subjected to more detailed analysis and documented, for example, by being printed out. 
     While one embodiment has been chosen to illustrate the invention, it will be understood by those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention as defined in the appended claims.