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:

1. Flow.

2. Level.

3. Temperature.

4. Pressure.

5. Automatic controls.

6. Control valves.

7. Alarms and protective devices.

8. Analyzers.

The various types of measurement instruments can be further classified according to the principle of operation. Some of the principles upon which instrument operation is based are:

1. Differential pressure, created by flow through a restriction in the line.

2. Buoyancy of afloat.

3. Thermal expansion of liquids or gases in a constant volume system.

4. Generation of electricity.

5. Analysis of a characteristic property of a product.

Brief descriptions of various types of instruments and illustrations thereof are presented in this chapter.

The inspection of many instruments includes checking transmission systems, power supplies, seals, purges, panels, and other associated equipment Since the inspection of all of these types of associated equipment may not be covered in other chapters of' this guide, such equipment is illustrated and discussed herein.

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 having separate instrument departments where the instruments are not normally inspected by the regular plant inspector, the instrument inspector and the plant 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, a discussion of installation methods is not included herein. For recommendations on proper installation of instruments, reference should be made to the various sections of API RP 550, Part I. This chapter, therefore, outlines only equipment that requires inspection, develops the methods and frequency of inspection, and discusses records and reports that 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 publications in this field.

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, vibration, and time instruments (including cycle timers), digital systems, and computer equipment.