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1. Introduction
Plant safety and continuous effective plant operability are totally dependent upon correct installation and commissioning of the instrumentation systems. Process plants are increasingly becoming dependent upon automatic control systems, owing to the advanced control functions and monitoring facilities that can be provided in order to improve plant efficiency, product throughput, and product quality.
The instrumentation on a process plant represents a significant capital investment, and the importance of careful handling on site and the exactitude of the installation cannot be overstressed. Correct installation is also important in order to ensure long-term reliability and to obtain the best results from instruments which are capable of higher-order accuracies due to advances in technology. Quality control of the completed work is also an important function.
Important principles relevant to installing instrumentation are also discussed in Section 35.
2. General requirements
Installation should be carried out using the best engineering practices by skilled personnel who are fully acquainted with the safety requirements and regulations governing a plant site. Prior to commencement of the work for a specific project, installation design details should be made available which define the scope of work and the extent of material supply and which give detailed installation information related to location, fixing, piping, and wiring. Such design details should have already taken account of established installation recommendations and measuring technology requirements. The details contained in this section are intended to give general installation guidelines.
3. Storage and protection
When instruments are received on a job site it is of the utmost importance that they are unpacked with care, examined for superficial damage, and then placed in a secure store which should be free from dust and suitably heated. In order to minimize handling, large items of equipment, such as control panels, should be programmed to go directly into their intended location, but temporary anti-condensation heaters should be installed if the intended air-conditioning systems have not been commissioned.
Throughout construction, instruments and equipment installed in the field should be fitted with suitable coverings to protect them from mechanical abuse such as paint spraying, etc. Preferably, after an installation has been fabricated, the instrument should be removed from the site and returned to the store for safe keeping until ready for pre-calibration and final loop checking. Again, when instruments are removed, care should be taken to seal the ends of piping, etc., to prevent ingress of foreign matter.
4. Mounting and accessibility
When instruments are mounted in their intended location, either on pipe stands, brackets, or directly connected to vessels, etc., they should be vertically plumbed and firmly secured. Instrument mountings should be vibration free and should be located so that they do not obstruct access ways which may be required for maintenance to other items of equipment. They should also be clear of obvious hazards such as hot surfaces or drainage points from process equipment.
Locations should also be selected to ensure that the instruments are accessible for observation and maintenance.
Where instruments are mounted at higher elevations, it must be ensured that they are accessible either by permanent or temporary means.
Instruments should be located as close as possible to their process tapping points in order to minimize the length of impulse lines, but consideration should be paid to the possibility of expansion of piping or vessels which could take place under operating conditions and which could result in damage if not properly catered for. All brackets and supports should be adequately protected against corrosion by priming and painting.
When installing final control elements such as control valves, again, the requirement for maintenance access must be considered, and clearance should be allowed above and below the valve to facilitate servicing of the valve actuator and the valve internals.
5. Piping systems
All instrument piping or tubing runs should be routed to meet the following requirements:
1. They should be kept as short as possible.
2. They should not cause any obstruction that would prohibit personnel or traffic access.
3. They should not interfere with the accessibility for maintenance of other items of equipment.
4. They should avoid hot environments or potential fire risk areas.
5. They should be located with sufficient clearance to permit lagging which may be required on adjacent pipework.
6. The number of joints should be kept to a minimum consistent with good practice.
7. All piping and tubing should be adequately supported along its entire length from supports attached to firm steelwork or structures (not handrails).
(Note: Tubing can be regarded as thin-walled seamless pipe that cannot be threaded and which is joined by compression fittings, as opposed to piping, which can be threaded or welded.)
5.1 Air supplies
Air supplies to instruments should be clean, dry, and oil free.
Air is normally distributed around a plant from a high-pres sure header (e.g., 6-7 bar g), ideally forming a ring main.
This header, usually of galvanized steel, should be sized to cope with the maximum demand of the instrument air users being serviced, and an allowance should be made for possible future expansion or modifications to its duty.
Branch headers should be provided to supply individual instruments or groups of instruments. Again, adequate spare tappings should be allowed to cater for future expansion.
Branch headers should be self draining and have adequate drainage/blow-off facilities. On small headers this may be achieved by the instrument air filter/regulators.
Each instrument air user should have an individual filter regulator. Piping and fittings installed after filter regulators should be non-ferrous.
5.2 Pneumatic signals
Pneumatic transmission signals are normally in the range of 0.2-1.0 bar (3-15psig), and for these signals copper tubing is most commonly used, preferably with a PVC outer sheath. Other materials are sometimes used, depending on environmental considerations (e.g., alloy tubing or stainless steel). Although expensive, stainless steel tubing is the most durable and will withstand the most arduous service conditions.
Plastic tubing should preferably only be used within control panels. There are several problems to be considered when using plastic tubes on a plant site, as they are very vulnerable to damage unless adequately protected, they generally cannot be installed at subzero temperatures, and they can be considerably weakened by exposure to hot surfaces.
Also, it should be remembered that they can be totally lost in the event of a fire.
Pneumatic tubing should be run on a cable tray or similar supporting steelwork for its entire length and securely clipped at regular intervals. Where a number of pneumatic signals are to be routed to a remote control room they should be marshaled in a remote junction box and the signals conveyed to the control room via multitube bundles.
Such junction boxes should be carefully positioned in the plant in order to minimize the lengths of the individually run tubes. (See FIG. 1 for typical termination of pneumatic multitubes.)
5.3 Impulse lines
These are the lines containing process fluid which run between the instrument impulse connection and the process tapping point, and are usually made up from piping and pipe fittings or tubing and compression fittings. Piping materials must be compatible with the process fluid.
Generally, tubing is easier to install and is capable of handling most service conditions provided that the correct fittings are used for terminating the tubing. Such fittings must be compatible with the tubing being run (i.e., of the same material).
Impulse lines should be designed to be as short as possible, and should be installed so that they are self-draining for liquids and self-venting for vapors or gases. If necessary, vent plugs or valves should be located at high points in liquid-filled lines and, similarly, drain plugs or valves should be fitted at low points in gas or vapor-filled lines. In any case, it should be ensured that there are provisions for isolation and depressurizing of instruments for maintenance purposes. Furthermore, filling plugs should be provided where lines are to be liquid scaled for chemical protection and, on services which are prone to plugging, rodding-out connections should be provided close to the tapping points.
FIG. 1 Typical field termination of pneumatic multitubes.
6. Cabling
6.1 General requirements
Instrument cabling is generally run in multicore cables from the control room to the plant area (either below or above ground) and then from field junction boxes in single pairs to the field measurement or actuating devices.
For distributed microprocessor systems the inter-connection between the field and the control room is usually via duplicate data highways from remote located multiplexers or process interface units. Such duplicate highways would take totally independent routes from each other for plant security reasons.
Junction boxes must meet the hazardous area requirements applicable to their intended location and should be carefully positioned in order to minimize the lengths of individually run cables, always bearing in mind the potential hazards that could be created by fire.
Cable routes should be selected to meet the following requirements:
1. They should be kept as short as possible.
2. They should not cause any obstruction that would prohibit personnel or traffic access.
3. They should not interfere with the accessibility for maintenance of other items of equipment.
4. They should avoid hot environments or potential fire risk areas.
5. They should avoid areas where spillage is liable to occur or where escaping vapors or gases could present a hazard.
Cables should be supported for their whole run length by a cable tray or similar supporting steelwork. Cable trays should preferably be installed with their breadth in a vertical plane. The layout of cable trays on a plant should be carefully selected so that the minimum number of instruments in the immediate vicinity would be affected in the case of a local fire. Cable joints should be avoided other than in approved junction boxes or termination points. Cables entering junction boxes from below ground should be specially protected by fire-resistant ducting or something similar.
6.2 Cable types
There are three types of signal cabling generally under consideration, i.e.,
1. Instrument power supplies (above 50 V).
2. High-level signals (between 6 and 50 V). This includes digital signals, alarm signals, and high-level analog signals (e.g., 4-20 mAdc).
3. Low-level signals (below 5V). This generally covers thermocouple compensating leads and resistance element leads.
Signal wiring should be made up in twisted pairs. Solid conductors are preferable so that there is no degradation of signal due to broken strands that may occur in stranded conductors. Where stranded conductors are used, crimped connectors should be fitted. Cable screens should be pro vided for instrument signals, particularly low-level analog signals, unless the electronic system being used is deemed to have sufficient built-in "noise" rejection. Further mechanical protection should be provided in the form of singlewire armor and PVC outer sheath, especially if the cables are installed in exposed areas, e.g., on open cable trays. Cables routed below ground in sand-filled trenches should also have an overall lead sheath if the area is prone to hydrocarbon or chemical spillage.
6.3 Cable segregation
Only signals of the same type should be contained within any one multicore cable. In addition, conductors forming part of intrinsically safe circuits should be contained in a multicore reserved solely for such circuits.
When installing cables above or below ground they should be separated into groups according to the signal level and segregated with positive spacing between the cables.
As a general rule, low-level signals should be installed furthest apart from instrument power supply cables with the high-level signal cables in between. Long parallel runs of dissimilar signals should be avoided as far as possible, as this is the situation where interference is most likely to occur.
Cables used for high-integrity systems such as emergency shutdown systems or data highways should take totally independent routes or should be positively segregated from other cables. Instrument cables should be run well clear of electrical power cables and should also, as far as possible, avoid noise-generating equipment such as motors. Cable crossings should always be made at right angles.
When cables are run in trenches, the routing of such trenches should be clearly marked with concrete cable markers on both sides of the trench, and the cables should be protected by earthenware or concrete covers.
7. Grounding
7.1 General requirements
Special attention must be paid to instrument grounding, particularly where field instruments are connected to a computer or microprocessor type control system. Where cable screens are used, ground continuity of screens must be maintained throughout the installation with the grounding at one point only, i.e., in the control room. At the field end the cable screen should be cut back and taped so that it is independent from the ground. Intrinsically safe systems should be grounded through their own ground bar in the control room. Static grounding of instrument cases, panel frames, etc., should be connected to the electrical common plant ground. (See FIG. 2 for a typical grounding system.) Instrument grounds should be wired to a common bus bar within the control center, and this should be connected to a remote ground electrode via an independent cable (preferably duplicated for security and test purposes). The resistance to ground, measured in the control room, should usually not exceed 1 O unless otherwise specified by a system manufacturer or by a certifying authority.
8. Testing and pre-commissioning
8.1 General
Before starting up a new installation the completed instrument installation must be fully tested to ensure that the equipment is in full working order. This testing normally falls into three phases, i.e., pre-installation testing; piping and cable testing; loop testing or pre-commissioning.
8.2 Pre-installation testing
This is the testing of each instrument for correct calibration and operation prior to its being installed in the field. Such testing is normally carried out in a workshop which is fully equipped for the purpose and should contain a means of generating the measured variable signals and also a method of accurately measuring the instrument input and output (where applicable). Test instruments should have a standard of accuracy better than the manufacturer's stated accuracy for the instruments being tested and should be regularly certified.
Instruments are normally calibration checked at five points (i.e., 0, 25, 50, 75, and 100 percent) for both rising and falling signals, ensuring that the readings are within the manufacturer's stated tolerance.
After testing, instruments should be drained of any testing fluids that may have been used and, if necessary, blown through with dry air. Electronic instruments should be energized for a 24-hour warm-up period prior to the calibration test being made. Control valves should be tested in situ after the pipework fabrication has been finished and flushing operations completed. Control valves should be checked for correct stroking at 0, 50, and 100 percent open, and at the same time the valves should be checked for correct closure action.
8.3 Piping and cable testing
This is an essential operation prior to loop testing.
8.3.1 Pneumatic Lines
All air lines should be blown through with clean, dry air prior to final connection to instruments, and they should also be pressure tested for a timed interval to ensure that they are leak free. This should be in the form of a continuity test from the field end to its destination (e.g., the control room).
8.3.2 Process Piping
Impulse lines should also be flushed through and hydro statically tested prior to connection of the instruments. All isolation valves or manifold valves should be checked for tight shutoff. On completion of hydrostatic tests, all piping should be drained and thoroughly dried out prior to reconnecting to any instruments.
8.3.3 Instrument Cables
All instrument cables should be checked for continuity and insulation resistance before connection to any instrument or apparatus. The resistance should be checked core to core and core to ground.
Cable screens must also be checked for continuity and insulation. Cable tests should comply with the requirements of Part 6 of the IEE Regulation for Electrical installations (latest edition), or the rules and regulations with which the installation has to comply. Where cables are installed below ground, testing should be carried out before the trenches are back filled. Coaxial cables should be tested using sine-wave reflective testing techniques. As a prerequisite to cable testing it should be ensured that all cables and cable ends are properly identified.
FIG. 2 A typical control center grounding system.
8.4 Loop testing
The purpose of loop testing is to ensure that all instrumentation components in a loop are in full operational order when interconnected and are in a state ready for plant commissioning.
Prior to loop testing, inspection of the whole installation, including piping, wiring, mounting, etc., should be carried out to ensure that the installation is complete and that the work has been carried out in a professional manner. The control room panels or display stations must also be in a fully functional state.
Loop testing is generally a two-person operation, one in the field and one in the control room who should be equipped with some form of communication, e.g., field telephones or radio transceivers. Simulation signals should be injected at the field end equivalent to 0, 50, and 100 percent of the instrument range, and the loop function should be checked for correct operation in both rising and falling modes. All results should be properly documented on calibration or loop check sheets. All ancillary components in the loop should be checked at the same time.
Alarm and shutdown systems must also be systematically tested, and all systems should be checked for "fail-safe" operation, including the checking of "burn-out" features on thermocouple installations. At the loop-checking stage all ancillary work should be completed, such as setting zeros, filling liquid seals, and fitting of accessories such as charts, ink, fuses, etc.
9. Plant commissioning
Commissioning is the bringing "on-stream" of a process plant and the tuning of all instruments and controls to suit the process operational requirements. A plant or section thereof is considered to be ready for commissioning when all instrument installations are mechanically complete and all testing, including loop testing, has been effected.
Before commissioning can be attempted it should be ensured that all air supplies are available and that all power supplies are fully functional, including any emergency standby supplies. It should also be ensured that all ancillary devices are operational, such as protective heating systems, air conditioning, etc. All control valve lubricators (when fitted) should be charged with the correct lubricant.
Commissioning is usually achieved by first commissioning the measuring system with any controller mode overridden. When a satisfactory measured variable is obtained, the responsiveness of a control system can be checked by varying the control valve position using the "manual" control function. Once the system is seen to respond correctly and the required process variable reading is obtained, it is then possible to switch to "auto" in order to bring the controller function into action. The controller responses should then be adjusted to obtain optimum settings to suit the automatic operation of plant.
Alarm and shutdown systems should also be systematically brought into operation, but it is necessary to obtain the strict agreement of the plant operation supervisor before any overriding of trip systems is attempted or shutdown features are operated.
Finally, all instrumentation and control systems would need to be demonstrated to work satisfactorily before formal acceptance by the plant owner.
References
BS 6739, British Standard Code of Practice for instrumentation in Process Control Systems: installation design and Practice (1986).
Regulations for electrical installations 15th ed. (1981) as issued by the institution of Electrical Engineers.
The reader is also referred to the National Electrical Code of the United States (current edition) and relevant ANSi, IEC, and ISA standards.
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