Instrument Air system design consideration:
The standard Instrument air line pressure for the utility industry should be ~690 kPa (100 psi) for most utility apps.
Since most industrial plants work with IA water at approximately 7 barg to 8 barg, the fixed relief valve (RV) pressure must be set greater accordingly but must not exceed the design pressure of the ship.
It is necessary to size the IA distribution lines so that the line pressure drop between the air dryer outlet and the farthest IA user is not more than 1 bar. Using 0.015 m3 (0.5 SCF) of air per minute can typically be regarded to be a user.
Air regeneration:
There are two types of Air regeneration, Air purge regeneration & Heater regeneration.,
- Air purge regeneration:
Air purge regeneration is a widely used technique for drying the packed column of molecular sieves by diverting a portion of dry air from the active air dryer vessel to allow moisture adsorption and expulsion via a purge line.
- Heater regeneration:
In techniques of heater regeneration, a heater-blower system is mounted in the regeneration line that heats ambient air and routes heated air through the regenerating dryer. The regenerated dryer is heated by the warm air until the moisture reaches the boiling point and is then expelled through the purge line.
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The regeneration time of each air dryer shall not be more than 6 hours as per IPS–G-IN-200(2). The recommended cycle time between regeneration cycles for normal operation is 6 hours for regeneration and 2 hours of standby hours for regeneration and 2 hours of standby.
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The highest permissible cycle time between cycles of regeneration is 6 hours for regeneration and 4 hours for standby. The air dryers must therefore be intended to dry without increasing the dew point for at least 10 hours.
Air dryer:
The operation of the air dryer adsorption is exothermic and causes the dried air to reach up to 600C. If its temperature is not anticipated to cool down to ~400C, the air dryer outlet would require extra after-coolers.
After cooler:
Air-cooled type or water-cooled type may be available after coolers. Water-cooled aftercoolers are usually sized to cool outlet air at temperatures between ~5 ° C and 8 ° C inlet. Whereas air-cooled aftercoolers are generally sized to cool outlet air to the ambient air temperature between 14 ° C and 17 ° C.
Compression:
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Compression raises the water vapor’s partial pressure. If the partial pressure of the water vapor increases to the pressure of the saturation water vapor, condensation happens.
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The removal of humidity is a significant consideration of air treatment systems for instruments. Airborne water droplets may trigger the development of rust or other corrosion products that block inner passages from electrical to pneumatic converters resulting in moving components being sticked and/or binded.
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Therefore, to dispose of the condensate, an automatic drain (e.g. timer drain, float drain or electronic drain) with a manual bypass should be placed near the bottom of the air receiver.
Vessel design:
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ASME Boiler and Pressure Vessel Code, Section VIII, Div, for the design and construction of the ships. 1 Or any other equivalent authority authorized standard is acceptable.
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For design fabrication, erection and testing of piping ASME B16.5 and ASME B31.3 are acceptable.
Use of valves in Instrument air system:
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In Instrument Air distribution systems, commonly used types of valves are globe valves, gate valves & ball valves.
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Globe valves provide the benefit of controlling system flow rates and providing tight shutdown. They cause lower flow rates on the down hand, enhanced pressure loss, and enable locations for particulates to accumulate causing leakage of the valve.
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Gate valves & Globe valves are used for on / off insulation and provide complete line-size air flow port with minimal pressure drop.
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The disadvantage of ball valves is that they are more costly than globe or door valves of comparable size and their sealing surfaces are prone to particle scoring leakage.