Instrument Ait pressure is 20-25 psi in the system and Plant (Service) Air pressure is 80-120 psi (about 7 atm). 
Instrument air requires more clean than plant air but both need filter. Their pressure is different sure. plant air changes to instrument air through filter and decreasing pressure i think. Plant air pressure is usually more than instrument air pressure but the most important thing is that instrument air is free from moisture whereas plant air is not. Service air is used in a plant for use with tools and equipment. Instrument air is used to control procces control equipment, such as valves. If you only had the one instrument air system and no service air you could possibly deplete/starve your control instruments and your plant would shut down. The following are a few differences between water and air (there are many more):   Water is a liquid and air is a gas   Water is a compound consisting of 2 parts hydroge … n atoms and 1 part oxygen atoms, and air is about 79% nitrogen gas and 20% oxygen, plus small amounts of many other gases. Above explanation is short and good. I would like to add that in US most of Instrument Ait pressure is 20-25 psi in the system and Plant (Service) Air pressure is 80-120 psi (about 7 atm). Both system require dryers, filters and some other equipment to do air conditioning. Posted. The main difference between plant air and utility air used to be that one was treated more, specifically dried (at CBN a silica gel absorber was used for that purpose) to prevent condensation in pneumatic equipment (such as control valves). The pressure can range from about 50 to 125 psig depending on the system. Plant air can range from air that is simply compressed with no treatment to air that is filtered with at least the free water removed. In some systems it can also be dehydrated but that is not common.
http://control.com/thread/1026157931 http://www.answers.com/Q/Difference_Between_Plant_air_and_instrument_air https://in.answers.yahoo.com/question/index?qid=20070810015932AAvSezf http://www.answers.com/Q/Difference_Between_Plant_air_and_instrument_air http://control.com/thread/1026157931 http://www.cheresources.com/invision/topic/3501-utility-and-instrument-air/ https://sg.answers.yahoo.com/question/index?qid=20090425232714AABuaTB
difference between plant air and instrument air