Instrument Air Consumption Calculation

Instrument Air Consumption Calculation Instrument air consumption shall be calculated during detail design design to determine the plant utility air  requirementt by pneumatic operated instrument as well as package which requires air for its utility such as requiremen purging or other purpose. Instrument air consumption consumption calculation will be the basis to size instrument air utility system which consists of air compressor, air drier, and air receiver.  Actuated valve valve such as shutdown shutdown valve, blowdown blowdown valve and control control valve are instruments which which require instrument air for their operation operation.. Instrument air requireme requirement nt of each instrument valves depends on its actuator size and its operation. Shutdown valve and blowdown valve may be considered working intermittently,, while control valve working continuously. It should be noted that the instrument air required by intermittently control valve during steady condition is much lesser than during its transient condition. Assumption To determine the instrument air consumption, some assumptions should be made (please note that these assumptions are for example only, not as standard reference, and may differ on each project):

Normal air consumption demand Control Valve From total number of control valve, 90% of control valve operates in stable condition hence requires steady state air consumption only, while 10% could be in unstable condition hence requires requires transient air  consumption. Shutdown valve / Blowdown valve Shutdown valve and Blowdown valve only require instrument air when they are operating which is predominantly predomin antly during start up after shutdown, so it is considered intermittent consumption consumption and one can assume that only some number of valve are working simultaneously. It could be assumed that 10% of the valve will operate simultaneously for normal air demand calculation. Peak air demand From total number of control valve, 70% of control valve operates in stable condition hence requires steady state air consumption only, while 30% is in unstable condition hence requires transient air consumption. Shutdown valve / Blowdown Valve Let’s say 100% of the valve will operate simultaneously simultaneously for normal air demand calculation. calculation. *Note that the number of percentage state here is assumption only and may differs among each projects Vendor data

The next step is to gather data from vendor / manufacturer m anufacturer catalog which provides provides information of instrument air consumptio consumption. n. The following is data taken from catalog: Control valve in stable condition 0.3 scfm Control valve in unstable condition 7 scfm For actuated valve, the instrument air consumption depends on size of actuator (swept volume) which could be obtained from vendor catalog. The required opening or closing time of the main valve will also influence the air consumption rate. Calculation

For instrument air consumption calculation, calculation, simply multiply the number of valve in the plant with the air  consumption data, then sum for all instruments. Do the calculation tabulation for both normal and peak condition. In addition, other package which requires instrument air shall also be included.