Safety during transporting bulk LPG by road - SMPV rules

A: Excess flow valves need a high flow rate to trip, so a slow seep looks harmless to them. Hose rupture and drive-away both generate the surge the valve is meant to sense. Mechanical damage that opens the line fast enough also triggers closure. The subtle failure is the one that never reaches the set differential.

A: Solar heat on a full tanker with little vapour space drives liquid expansion fast, which is exactly the scenario SMPV fears. Vapour return issues matter during transfer, not static parking. Residual nitrogen would show up as higher starting pressure, not a slow climb. A bad gauge explains the reading, not the thermodynamic behaviour observed.

A: Fire exposure is the design basis SMPV keeps coming back to, and secondary metal sealing is what survives it. Soft seats seal well until they burn. Gate valves give indication but are slow and bulky for emergency isolation. Butterfly valves are fine elsewhere, but not as primary TT isolation under SMPV expectations.

A: Bench testing establishes set pressure without system variables, then you make sure installation doesn’t defeat it. In-situ pop testing on LPG is rarely practical or safe. Paper review alone ignores installation errors. Hydrotesting with the PSV installed risks damaging it and tells you little about set accuracy.

A: Moisture held against the shell by saddles is a classic road tanker problem. H2S cracking needs sour service conditions that aren’t present. Internal corrosion contradicts the external damage pattern. Galvanic effects exist, but without dissimilar metals the driving force is weak compared to crevice action.

A: The fusible plug sacrifices inventory to save the vessel, venting vapour when fire heats it. Isolation is done by valves, not melting plugs. Cooling is a side effect, not the mechanism. It does nothing to strengthen steel that’s already hot.

A: Marginal flow can make an excess flow valve hunt, choking itself repeatedly. Vapour lock would show pressure instability. Cavitation sits with the pump, not the tanker outlet. Composition affects density, not sudden step changes like this.

A: A verified earth path is the only controllable safeguard here. Tyres vary with wear and temperature. Bonding through the arm assumes perfect connections every time. Higher velocity increases charge generation rather than fixing it.

A: The trend tells you damage is accumulating even if the code box is ticked, and COMAH scrutiny punishes ignoring that signal. Blind acceptance hides risk. Higher pressure accelerates damage. Shorter intervals help only if paired with understanding the mechanism.