The correct design and installation of a power supply system is essential to ensure plant operation and reliability. Errors in the installation of a power supply system can render the most well‑designed control system partly or totally ineffective.
Non‑electrical power supply systems generally used for instrumentation systems and regulating devices fall into two main categories:
pneumatic;
•
hydraulic.
•
Guidance on electrical supply systems is given in Clause 17.
In general, the word “pneumatic” used in this clause refers to air;
however, if other operating media, e.g. process gases which might be flammable or toxic, are used, special precautions will be necessary (see IEC/TR 61081).
11.1
General
all pneumatic and hydraulic supply systems supporting safety functions should be assessed to ensure that the appropriate level of integrity is maintained for the safety system.
11.2
Pneumatic supply systems
11.2.1
Quality and operating pressure
Compressed air or gas used for supplies to instrument and control systems should be dry, clean and oil‑free and, in the case of air, should conform to at least the following recommendations, together with any additional recommendations supplied by the manufacturer.
the dew point at operating pressure should be at least 10 °C a)
lower than the minimum anticipated temperature.
the air should not contain dust particles larger than 3 µm.
b)
the air should not contain more than 1 ppm (part per million) c)
mass/mass of oil at 20 °C with the system pressure at 6 bar 3). instrument air should also be free of all corrosive contaminants and hazardous gases, flammable or toxic, which could be drawn into the compressor air intake. if the possibility of contamination exists, the air should either be taken from a remote or elevated clean location or be suitably processed.
the supply pressure in the plant instrument air header should be suitable to meet the requirements of the control system, usually 6 bar to 7 bar, or should be at least 2 bar above the maximum operating pressure required by any item of instrumentation equipment.
the air supply system should be designed to maintain an
uninterrupted supply for a predetermined time period (see 11.2.4).
11.2.2
System duplication
air compressors supplying instrument air should have back‑up facilities for reliability, by one of the following:
a dual compressor system; or a)
a standby compressor powered from a different source, e.g.
b)
steam or diesel power; or
a cross‑over link with the plant air system, which should be c)
suitably conditioned to conform to 11.2.1a) to 11.2.1c) but which will not permit instrument air to be lost to the plant air system.
Where two compressors are used, the system should be designed such that either compressor can be isolated for maintenance whilst the other is operating.
11.2.3
Instrument air compressors
the output capacity of a compressor should be at least 150% of the total instrument air consumption.
Compressors should be suitable for continuous operation, but the design of the compressor/receiver system should ideally be such that the compressor is on load for approximately 50% of its time in order to increase the long‑term reliability of the system and reduce the maintenance costs.
the compressor should be capable of delivering air at
approximately 2 bar 4) above the air header normal working pressure.
Compressors should preferably be of the oil‑free type, but the lubricated type may be used (sometimes necessary on larger systems) provided that the final air quality conforms to 11.2.1a) to 11.2.1c).
a compressor should be supplied with an after cooler and separator to remove most of the free water from the compressed air, and thereby reduce the burden on the dryers.
3) 1 bar = 105 n/m2 = 100 kPa.
4) 1 bar = 105 n/m2 = 100 kPa.
NOTE 1 This is necessary to allow for the pressure drop through the conditioning system, i.e. coolers, dryers, filters, etc.
On stop/start compressor systems, a pressure sensor should be fitted that controls the duty cycle and hence the receiver pressure. a further pressure sensor on the receiver should be included to initiate automatic starting of the standby compressor. Pressure sensors used for automatic control purposes should not be used for alarm indication. Separate sensors, each with its own isolating valves, should be provided for alarm purposes when required.
Facilities should be provided to ensure that cooling water does not flow unnecessarily when compressors are on standby or are shut‑down.
the noise generated by the compressors should be taken into account, and acoustic treatment applied if necessary to meet the environmental requirements of the plant.
11.2.4
Air receivers
For small systems the air receiver is usually integral with the compressor, whereas on larger systems it is usual to employ one receiver served by two compressors. Facilities should be provided to ensure that the receiver remains in service when either compressor is removed for service.
the total air receiver capacity should be sufficient to provide air at an adequately high pressure to ensure continuous operation of the instrumentation for a period of at least 30 min after compressor failure, or after failure of both compressors on a dual compressor system.
11.2.5
Air dryers
air dryers should be of the adsorptive type, except for very mild climates where the lowest ambient temperature is not likely to be below 10 °C, when a refrigerant type may be used.
adsorptive type dryers may be of the heat‑less or heat‑regenerated type, and facilities should be provided for automatic regeneration of the desiccant without interruption of the dry air supply.
11.2.6
Filters
air filters, which should be provided at both compressor inlet (intake filters) and after the dryers (after filters), should:
have a high separating capacity;
a)
have a good accumulating ability, i.e. they should be able to b)
collect a large quantity of impurities without any significant decrease in performance;
have a low resistance to air flow.
c)
intake filters are primarily to remove foreign particles and moisture from the incoming air. they should be carefully positioned to give maximum protection from dust intake and weather conditions, e.g. wind, rain and snow. they should also be easily accessible for maintenance and inspection.
the after filters should be capable of removing dust down to a particle size of 3 µm.
Oil adsorbers should be fitted if the valves of the compressor are lubricated, and should be fitted after the compressor and before the dryer.
NOTE 2 Continuously running compressors are usually controlled by automatic unloading systems.
NOTE Most systems are designed for automatic regeneration on a time‑cycle basis, usually 8 h, but may be automatically initiated by a dew‑point detecting device.
11.2.7
Cooling water
Where compressors might be exposed to freezing winter conditions, the cooling water circuits should be heat traced and lagged, especially in systems with 100% standby, i.e. at least one compressor normally not running.
attention should be paid to the integrity and reliability of the complete cooling water circuit since its loss is the most likely cause of losing all instrument air pressure.
Filters should be installed at some point in the cooling water circuit to prevent the deposition of scale, etc. in the compressor cooling surfaces.
they should be of twin design and should be readily accessible for changeover and cleaning.
11.2.8
Valves
isolating valves should be provided in the installation to ensure that all sections of the air supply system can be isolated for maintenance without interrupting the plant operation. in addition, individual isolating valves should be provided for each instrument requiring an air supply.
all valves should be easily accessible and capable of tight shut‑off, and should have a low flow resistance.
Changeover valves for diverting air services from one dryer to another should preferably be of the ball‑valve type.
Where ambient temperatures below freezing are expected,
consideration should be given to heat tracing the valves and pipework up to the dryer inlet.
11.2.9
Pipework
the design of air supply piping and the sizing of air headers should be in accordance with 14.2. after fabrication, air supply piping should be degreased, pressure tested, blown out and sealed until required for service.
11.2.10
Pre‑commissioning checks
11.2.10.1 Compressors
the manufacturer’s pre‑start checks and the start‑up procedures should be followed. Small‑bore pipes and capillaries, etc., associated with compressors should be checked for leaks or physical damage.
Measuring instruments, pressure and flow switches, etc., should be checked for calibration and settings in accordance with Clause 19 and the compressor manufacturer’s instructions.
all control systems should be checked for correct operation.
11.2.10.2 Cooling water circuits
the action and setting of pressure and flow switches and interlocks should be checked for correct operation.
Visual flow indicators should be checked to ensure that they function correctly.
the complete circuits should be checked for leaks.
COMMENTARY ON 11.2.7