considerations Valves are typically rated for maximum temperature and pressure by the manufacturer. The wetted materials in a valve are usually identified also. Some valves rated at very high pressures are available. When a designer, engineer, or user decides to use a valve for an application, he/she should ensure the rated maximum temperature and pressure are never exceeded and that the wetted materials are compatible with the fluid the valve interior is exposed to. In Europe, valve design and pressure ratings are subject to statutory regulation under the Pressure Equipment Directive 97/23/EC (PED). Some fluid system designs, especially in chemical or power plants, are schematically represented in piping and instrumentation diagrams. In such diagrams, different types of valves are represented by certain symbols. Valves in good condition should be leak-free. However, valves may eventually wear out from use and develop a leak, either between the inside and outside of the valve or, when the valve is shut to stop flow, between the disc and the seat. A particle trapped between the seat and disc could also cause such leakage. Images See also , medical References External links ISO-15926-4 - Nearly 500 valve base classifications and definitions from the ISO 15926 standard. Animations showing Internal Function of Various Types of Valve, tlv.com Flow in known Design Types of Shut-off Valves , home.arcor.de Valves: Piping and Instrumentation Diagram Standard Notation, controls.engin.umich.edu Department of Energy Fundamentals Handbook, Mechanical Science, Module 4 Valves Piping Plumbing Water industry