READING ENGINEERING P&IDsDOE-HDBK-1016/1-93Engineering Fluid Diagrams and PrintsREADING ENGINEERING P&IDsStandards and conventions have been developed to provide consistency fromdrawing to drawing. To accurately interpret a drawing, these standards andconventions must be understood.EO 1.9STATE how the following valve conditions are depicted on anengineering P&ID drawing:a.Open valveb.Closed valvec.Throttled valved.Combination valves(3- or 4- way valve)e.Locked-closed valvef.Locked-open valveg.Fail-open valvesh.Fail-closed valvei.Fail-as-is valveStandardsand Conventionsfor ValveStatusBefore a diagram or print can beFigure 17Valve Status Symbolsproperly read and understood, thebasic conventions used by P&IDsto denote valve positions andfailure modes must be understood.The reader must be able todetermine the valve position, knowif this position is normal, knowhow the valve will fail, and insome cases know if the valve isnormally locked in that position.Figure 17 illustrates the symbolsused to indicate valve status.Unless otherwise stated, P&IDsindicate valves in their "normal"position. This is usuallyinterpreted as the normal orprimary flowpath for the system.An exception is safety systems,which are normally shown in theirstandby or non-accident condition.3-way valves are sometimes drawn in the position that they will fail to instead of always beingdrawn in their "normal" position. This will either be defined as the standard by the system ofdrawings or noted in some manner on the individual drawings.PR-02Rev. 0Page 16
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