scope:

The primary purpose of this chapter is to present a guide for instrument inspection. It is not intended as a maintenance manual, although some of the inspection procedures suggested may appear to belong in the category of preventive maintenance. These procedures cover the majority of types of standard commercial instruments and associated equipment used in modem refineries. Particular attention is given those instruments which affect safety and continuity of operation.

Instruments are classified according to type of measurement or function performed. The broad classifications of instruments are: flow, level, temperature, pressure, automatic controls, control valves, and analyzers. The various types of measurement instruments to be further classified according to the principle of aeration. Some of the principles upon which operation of instruments is based are: differential pressure, created by flow through a restriction in the line; buoyancy of a float; thermal expansion of liquids or gases a constant-volume system; generation of electricity; and analysis of a characteristic property of a product. Brief descriptions of the various types of instruments, together with illustrations of typical installations, are outlined in this chapter. The inspection of many instruments also includes checking transmission systems, answer supplies, seals, purges, panels, and other associated equipment. Inasmuch as the inspection of all these types of associated equipment may not be answered in other chapters of this guide, such equipment illustrated and discussed in the following pages.

Refinery operations are, to a great degree, dependent upon accurate and reliable instrumentation. Measurement and control systems can be relied upon only if they are inspected periodically and serviced by competent, specially trained personnel who are familiar with both process requirements and instrument functions. Some instruments are inspected by calibration. A thorough understanding of instrument design and function, knowledge of calibration procedures, and skill with which to make accurate measurements and delicate adjustments are all necessary to assure proper inspection and calibration. Therefore, instruments should be inspected only by qualified personnel. Throughout this chapter, the use of the term "inspector" shall be construed to mean a person or persons qualified to perform the instrument inspection.

In plants which have regular instrument departments and where the instruments are not normally inspected by the regular plant inspector, the instrument inspector and the plaint inspector should collaborate in the inspection of control valves, level-float cages, instrument, air and hydraulic systems, and similar instrument equipment where metal thickness measurements or pressure testing are required.

Inasmuch as this chapter is a guide to instrument inspection procedures, the subject of installation methods has not been included. For recommendations on proper installation of instruments, reference should be made to API RP 550; Manual on Installation of Refinery instruments and Control Systems. This chapter, therefore, outlines only the equipment which requires inspection, develops the methods and frequency of inspection, and discusses records and reports which should be maintained on the various types of instruments and systems. For specific information on various makes of instruments, reference should be made to manufacturers' instruction manuals and to any of several good publications in this field. An excellent book of recent date is the Industrial Instrument Servicing Handbook.*

The types of equipment requiring inspection include:

1. Flow instruments.

2. Level instruments.

3. Temperature instruments.

4. Pressure instruments.

5. Automatic controls.

6. Control valves and positioners.

7. Transmission systems for measurement and control.

8. Seals, purges, and winterization equipment.

9. Instrument air-supply systems.

10. Hydraulic-pressure systems.

11. Electric power systems for instruments.

12. Instrument panels.

13. Analyzers and sampling systems.

14. Alarms and protective devices.

15. Miscellaneous instruments, such as speed, electrical, and time (including cycle timers) instruments and data loggers.

*Grady C. Carroll, Industrial Instrument Servicing Handbook, McGraw-Hill Publishing Co., Inc., New York (1960).