Isolating valves and hydrants installed in underground pipelines primarily carrying water intended for human consumption within the United Kingdom are required to be fitted with a stem cap to enable them to be operated by a ring-key and bar or T-shaped key. The stem cap is fitted to the top of the valve stem to convert the manufacturer’s stem configuration to mate with operating keys. Two sizes of stem cap are specified, covering respectively, valves from DN 50 through DN 300 and DN 350 through DN 600. NOTE Stem caps are also commonly used on other waterworks apparatus, e.g. penstocks (specified in BS 7775).