Raising the precision and reliability of process-control instruments

Abstract

ing racks and techniques are described in [2, 3]. These racks serve to measure the tested parameters with a high precision, to maintain strictly specified conditions in studying the instruments' reliability with a minimum expenditure of time and resources. The racks operate automatically in closed cycles. The duration of testing is registered on cycle counters. Testing is accelerated mainly by reducing the duration of the instruments' working cycle and speeding up processes which lead to their failure. Moreover, the operations which are included in a normal working cycle, but do not affect instrument reliability (taking readings, replacing tested components) are dispensed with. The application of racks and accelerated testing techniques provides an annual saving of not less than 85,000 rubles. The PCI operational properties were studied, as a rule, by the process-control laboratories personnel of the consumer enterprises and periodically by representatives of the manufacturing-plant reliability service. All the failures, their causes, and methods of their elimination were entered in special logbooks. In the course of rack and field testing it was found that disruptions of faultless operation; i.e., failures to function are due mainly to various breakages, jamming of moving parts, as well as open and short circuits in electrical units (breakdown of bellows and their wrenching off the distribution block, rupture of cords and pneumatic pipeline). It was found in rack testing at the CIP laboratory that faultless operation of various PCI models amounted to 500,000-1,100,000 cycles. Surpassing of normalized precision boundaries (precision failure) is due to pollution or wear of reference surfaces and moving parts, as well as failure to meet the operational and technical-servicing requirements. Moreover, the malfunctioning of PCI units in transmitting and processing measured information produces gross operational errors (overshoots) which are due to accidental rises in mechanical loading, to electrical interference, etc. In studying the PCI reliability it was revealed that pollution of the nozzle-vane face produces misadjustment and overshoots, and that the measuring leverts prismatic shape does not insure uniform distribution of temperature in their cross section, thus producing bending moments. Instruments test-rack operation without any precision failures amounted to 250,000650,000 cycles and overshoots were in the range of 0.1-0.5%. Misadjustments up to values which led to precision failures occurred after 6-8 h of operation. As a result of testing it was found that intervals between failures are distributed according to the exponential law. It should be noted that the considerable difference obtained between rack testing and operation under utilization conditions is mainly due to the departure from maintenance rules and a great variety of operation conditions (for instance, the pneumatic PCIs are not provided with appropriately cleaned and dried compressed air). Therefore, it is advisable to