While routine, closed-loop transfers are inherently safe from internal static ignition, bonding and grounding become critical whenever the system is opened to the atmosphere or when specific maintenance activities generate massive static charges. When we break the closed loop or introduce external energy, the hazard profile shifts from NFPA 58 back to the static mitigation principles found in NFPA 77, Recommended Practice on Static Electricity or API RP 2003, Protection Against Ignitions Arising Out of Static, Lightning, and Stray Currents. Here are the specific scenarios where bonding and grounding are required:
Whenever LPG is intentionally released to the atmosphere—such as blowing down a piping run for maintenance or purging a vessel—the vapor-air mixture at the vent exit will inevitably pass through the flammable range (roughly 2.1% to 9.5% for propane). High-velocity gas flowing through a vent line can carry rust, scale, or condensation. These particulates generate a significant static charge through friction against the pipe walls. Temporary vent stacks, hoses, and flaring equipment MUST be grounded to dissipate this charge so a spark does not arc across the vent exit and ignite the plume.
When preparing an LPG bullet or sphere for internal API 510 inspection, the vessel must be purged of hydrocarbons. This often involves injecting steam or using high-pressure water wands. Steam and high-pressure water create a scenario called mist electrification. The suspended water droplets can accumulate a massive static charge that is highly insulated by the surrounding air. The steam lance or water nozzle must be strictly bonded to the vessel shell, and the vessel itself must be grounded. If an unbonded conductive object (like a dropped tool or an unbonded nozzle) enters the charged mist, it can accumulate energy and release a high-intensity spark into any remaining localized pockets of LPG vapor.