Posted By Roger the Dodger Depends what the precise application is: Maybe 'Wear Eye Protection' sign if risk of disturbing debris or blasting into the eyes. 'Caution Injection Risk' if high enough pressure to penetrate the skin. Usually factory ring mains etc should be painted a light blue colour with suitable information/contents warning tape at intervals along the pipe and at bulkheads/wall and/or where valves might need to be fitted. Also shut off point/isolation points might need indicating. Sometines the pressure in the system is indicated.