Optimal Scan Rates for Automation Projects

Abstract

This paper was prepared for presentation at the 47th Annual Fall Meeting of the Society of Petroleum Engineers held in San Antonio, Tex., Oct. 8–11, 1972. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by who the paper is presented. Publication elsewhere after publication in the JOURNAL paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal provided agreement to give proper credit is made. provided agreement to give proper credit is made. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines. Abstract Statistical methods are presented showing the relationships between precision, confidence level, and scan rate in digital measurement systems. For automation or process control projects these relationships can be used to projects these relationships can be used to evaluate, or to specify, the capabilities of computer and telemetry systems for various combinations of measurement precision and confidence level. Introduction In processes of all kinds the process variables, input, control and output, have traditionally been measured with some type of analog device. One of the characteristics of an analog device is that it provides continuous measurements. Digital measurements, on the other hand, provide a series of samples of the continuous variable through point measurements in time. The sampling rate, or scan rate needed to provide acceptable precision becomes an important design criterion when digital measurement is to be installed. The accelerating use of digital computers for oilfield automation and process control measurement has intensified the need for adequate methods to specify scan rates. Obviously, the precision and timeliness of measurement increases with increasing scan rate. But, so do the cost, sophistication, and ultimately the reliability of the required computer and telemetry system. A trade can be made between the level of precision obtained and the cost to acquire it, or for fixed precision requirements the techniques described in this paper aid in the specification of optimal computer systems. TYPES OF SCAN In scanning for alarm situations, the time that can be allowed to elapse between recognition of an alarm and completion of remedial action can be used to determine the longest feasible time between scans. However, this time includes analysis time at the computer that may depend on priority level in a multipriority system. It also includes any two-way time delay in the telemetry system and mechanical delay in effecting remedial action. The scan rate must be sufficiently rapid to accommodate these time delays and still allow completion of remedial action in the required time. Beyond the preceding paragraph, this paper is not directed toward specification of scan rates for alarm systems. It presents methods to specify scan rates for obtaining a desired level of precision within acceptable confidence limits for measurements of average rates and total quantities during a test period.