A Surrey flange is a fitting on a hot water cylinder which prevents a device, such as a shower pump, from being damaged by air. It does this by drawing off water via a tube which dips down into the main body of hot water.
When water in the cylinder is heated by the coil or the immersion heater, air bubbles form. The top of the cylinder is shaped like a bell in order to prevent these air bubbles from being trapped. This allows them to rise to the highest point of the cylinder and out at the top.
Hot water drawn from below this point with a Surrey flange will be devoid of these air bubbles, protecting a shower pump from damage. The tube which dips down into the cylinder may also have a component on the end which deflects any air bubbles that rise up towards it from directly beneath it.
A Surrey flange will have two outlets. Non-aerated water exits at right angles to the dip tube. Aerated water rises to the top of the cylinder, and out of the top of the flange. This allows air bubbles to rise up and out of the system via the expansion pipe as normal, with the hot water carried off to the rest of the domestic services.