A Surrey flange is a plumbing fitting installed on a hot water cylinder to provide a separate, air-free hot water supply to a shower pump, preventing air bubbles that can cause noise, surging, and damage. It works by drawing water from a lower, air-free part of the cylinder and has a built-in vent that automatically purges trapped air from the shower circuit, ensuring a consistent and splutter-free shower experience. 
How it works
Provides independent supply: The flange creates a dedicated hot water line for the shower pump, which draws water from a specific, lower-level outlet inside the cylinder.
Prevents air accumulation: A key feature is the automatic air vent, which expels air that gets trapped in the hot water feed as the cylinder heats or when water is replaced.
Protects the pump: By delivering a steady supply of water without air, it prevents a condition called "cavitation" (air bubbles getting into the pump), which can damage the pump's impeller and shorten its lifespan.
Improves performance: This process also eliminates air surging and temperature fluctuations, leading to a more stable and pleasant shower experience. 
Benefits of a Surrey flange
Ensures a splutter-free and consistent shower.
Reduces noise, air surging, and temperature variations.
Protects the shower pump from air-related damage, extending its life.
Provides a professional and tidy finish to the plumbing installation.