Tank Blanketing Valves: Nitrogen Padding and Vapour Control Systems

A tank breathes. As product is pumped out, or as the vapour space cools at night, the pressure inside a fixed-roof tank falls and the tank tends to pull a vacuum. As product is pumped in, or as the tank warms in the day, the pressure rises. Left uncontrolled, this breathing draws in air - bringing oxygen (fire and oxidation risk) and moisture (product degradation) - and vents product vapour to atmosphere. Tank blanketing keeps a cushion of inert gas, usually nitrogen, in the vapour space at a slight positive pressure, so the tank breathes nitrogen instead of air. The valves that manage this are the heart of the system.

The Blanketing Valve (Supply Regulator)

The blanketing valve, or blanket-gas regulator, is a pressure-reducing regulator that senses the tank vapour-space pressure and admits nitrogen whenever it falls below the setpoint - for example, when product is withdrawn or the tank cools. It is a self-actuated, pilot-operated or direct-acting regulator referenced to the tank pressure, typically set at a low positive pressure such as 5 to 15 mbar (a few inches of water column). High sensitivity and tight shut-off are essential, because the setpoints and pressure bands are very small and nitrogen is expensive to waste.

The Blanketing Relief / Vent Valve (Back-Pressure Regulator)

On the pressure side, a back-pressure regulator or blanketing relief valve vents excess gas whenever the tank pressure rises above its setpoint - when product is pumped in or the tank warms. Its setpoint sits just above the supply regulator's, so the two never fight each other. Together the supply regulator and the vent regulator hold the tank within a narrow pressure band around the blanket setpoint.

Pressure-Vacuum Relief Vent (Safety Device)

Independent of the blanketing regulators, every blanketed tank must also have a pressure-vacuum (PV) relief vent as the last-line safety device sized per API 2000. If a blanketing regulator fails or a large upset occurs, the PV vent opens to protect the tank shell and roof from over-pressure or over-vacuum, which can buckle or rupture a thin-wall atmospheric tank. The PV vent is a mechanical, weight- or spring-loaded device, and its set points sit outside the normal blanketing band.

Typical Setpoint Stacking

Sizing the System

Blanketing valves are sized for the worst-case inbreathing and outbreathing flows, following API 2000 (Venting Atmospheric and Low-Pressure Storage Tanks). The inbreathing case combines maximum pump-out rate plus thermal inbreathing from rapid cooling; the outbreathing case combines maximum pump-in rate plus thermal outbreathing from heating and, where relevant, flash vaporisation. Key inputs include:

• Maximum liquid pump-in and pump-out flow rates.
• Tank volume, geometry, and the thermal venting factors from API 2000.
• Product vapour pressure and flash characteristics.
• Blanket-gas supply pressure available at the regulator inlet.
• Allowable tank pressure and vacuum (the mechanical limit of the shell and roof).

Materials and Specification

Blanketing regulators and PV vents are usually aluminium, carbon steel, or stainless steel depending on the product and environment; corrosive or offshore duty calls for SS 316 or higher. Soft seats (FKM, PTFE) give the tight shut-off needed at low pressures. Specify flame arresters integral to or upstream of vents on flammable-product tanks, and confirm the blanketing setpoints are compatible with the tank's rated pressure and vacuum before finalising. For accurate sizing, the API 2000 calculation must be documented and the device curves must match the required flows at the small allowable pressure differentials.

Vajra Industrial Solutions supplies tank blanketing supply regulators, back-pressure vent valves, pressure-vacuum relief vents, flame arresters, and emergency vents sized per API 2000, with material selection for the product and full test and traceability documentation.