ISA publishes the guidelines for loop diagrams. ISA-5.4-1991 - Instrument Loop Diagrams and ISA-5.1-1984 - (R1992) - Instrumentation Symbols and Identification should be helpful to you. They also publish ISA-5.2-1976 - (R1992) - Binary Logic Diagrams For Process Operations. You should be able to find more information on these at