Safety Considerations for Vault Isolation

Utility employees working in a water distribution system face inherent safety risks while working in confined spaces with distribution pipelines that may be energized. Disassembling pipelines within a vault may create two hazards: (1) excessive leakage through closed valves may flood the vault, and (2) thrust forces that occur when pressure builds up behind a closed valve, either within the vault or at a nearby buried valve, may lead to pipe movement, separation, or unanticipated rapid depressurization. Denver Water is the largest water utility in the Rocky Mountain region, serving over 1.4 million customers. Denver Water partners with 63 nearby distributor districts that maintain their own local distribution networks and to which Denver Water provides treated water via “master meter” connections that range in size from 4- to 12-in. in diameter. In 2019, Denver Water initiated a Master Meter Replacement Program to replace obsolete master meters for which spare parts can no longer be obtained. Over 110 master meters in more than 80 vaults were replaced through this multi-year program. These meter vaults have common design elements, including a bypass pipe within the vault, thrust restraint rods through the vault wall, and isolation gate valves. There are several unknowns that must be considered when developing an isolation plan prior to meter replacement. Such unknowns include the presence and condition of joint restraint in and around the vault and the operability of and expected leakage rate through existing valves. This paper presents case studies that exemplify the methodology that Denver Water has developed for managing these risks, which includes assessing the thrust forces that would result from closed valves that are known to be pressurized or may become pressurized, intentionally leaving valves open to reduce the risk of pressure build-up, performing test isolations, adding valves, and sequencing work such that customer service disruptions are minimized.