Les guides techniques EHEDG en Anglais

Today, food process plants incorporate various multifunctional flow paths. Often one piping system is cleaned while another still contains product. This simultaneous cleaning can potentially result in the dangerous situation where product and cleaning liquid are separated by just one single valve seat. Any cleaning liquid that leaks across such a seat will contaminate the product. Therefore, often two or three single seat valves in a "block-and-bleed" arrangement are applied.   Single-body double-seat butterfly valves are used as well, but these cannot be guaranteed to be mix-proof.

This document describes the basic hygienic design and the safe use of single-body double-seat mixproof valves, that ensure that intermixing cannot take place.  Surge valves have been used in brewing, beverage,  food and milk plants in Europe for 20 years.