What Is a Pressure Building Coil, and Why Does Your Vessel Have One?

A pressurised liquid nitrogen vessel has no pump. The thing that pushes liquid up the withdrawal line is boil-off, generated deliberately.

How the coil works

A pressure building coil routes a small amount of liquid out of the bottom of the inner vessel through a circuit exposed to ambient temperature. The liquid boils in that circuit and the gas is fed back into the ullage space above the liquid. That raises the head pressure, and the head pressure drives liquid out when you open the dispense valve.

Why this matters operationally

It explains behaviour that otherwise looks like a fault. A vessel that will not dispense often has a closed or frozen pressure building valve rather than an empty tank. A vessel whose pressure climbs when nobody is using it may have a pressure building valve left open.

The safety consequence

Because the vessel is designed to generate its own pressure, the relief devices are not optional extras. The inner vessel needs at least two independent safety devices, typically a relief valve and a separate bursting disc, as set out in BCGA Code of Practice 36.

Know where your pressure building valve is, and know its normal position for your operating pattern.