A storage tank breathes as its liquid level and temperature change, and if it breathes plain air it draws in oxygen and moisture that can make the vapor space flammable and corrode the tank. Blanket gas, also called pad gas, is the answer: a supply of inert or fuel gas held at a slight positive pressure over the stored liquid so that when the tank needs to breathe in, it breathes gas rather than air. Keeping that pad at the right pressure is a control problem with a floor and a ceiling, and it is usually solved with a split-range scheme that adds gas when the pressure sags and relieves gas when it rises. This guide explains what blanket gas does, how the split-range make-up and relief scheme works alongside a VRU, and what SCADA watches on the tank pressure envelope.
Blanket Gas Pressure Control in one line: Tank blanket gas pressure control maintains a slight positive pressure of inert or fuel gas in the vapor space above stored oil, so the tank never draws in air as it breathes. Excluding air prevents a flammable oxygen-laden vapor space and keeps moisture and oxygen from corroding the tank, while the positive pressure prevents the tank from being pulled into a vacuum and collapsing. It is commonly implemented as a split-range loop that opens a make-up gas valve when pressure falls and relieves gas, often to a VRU, when pressure rises.
A fixed-roof tank has a vapor space above the liquid, and that space grows and shrinks as the level rises and falls and as the temperature swings between day and night. Without a blanket, the tank simply pulls in and pushes out atmospheric air through its vents to accommodate those changes, which is the ordinary breathing of a tank. The trouble is that the incoming air brings oxygen and moisture into the vapor space, and mixing oxygen with hydrocarbon vapor creates a potentially flammable atmosphere, while the moisture and oxygen promote internal corrosion, especially at the vapor-liquid line.
Blanket gas solves this by keeping the vapor space filled with a gas the tank can safely breathe. A supply of inert gas such as nitrogen, or in many oilfield installations fuel gas or produced gas, is fed into the vapor space and held at a slight positive pressure. Because the space is always at least a little above atmospheric, any breathing draws in more blanket gas rather than air, so oxygen is kept out and the vapor space stays fuel-rich and out of the flammable range rather than being diluted toward it with air. The pad both protects against ignition and shields the tank from corrosion.
The positive pressure serves a second purpose beyond excluding air, which is protecting the tank against vacuum. Atmospheric tanks are strong against modest internal pressure but weak against vacuum, and a tank that draws down faster than it can breathe in, for example when it is pumped out quickly or cools rapidly, can be pulled below atmospheric and buckle inward. By actively feeding make-up gas to hold a positive pad, the blanket system supplies gas fast enough to keep the tank from ever going into vacuum, so the pad guards both the flammability and the mechanical integrity of the tank.
Holding the pad pressure means managing both directions: sometimes the tank needs gas added and sometimes it needs gas taken away. When the liquid level falls or the vapor cools, the vapor space wants to shrink and the pressure sags, so gas must be added to hold the pad. When the level rises or the vapor warms, the space swells and the pressure climbs, so gas must be let out. A single controlled pressure with two opposite actions is the classic case for a split-range control scheme, where one measured pressure drives two valves that act over different parts of the controller's output range.
In a split-range blanket loop, as the pressure falls toward the low end of the band the make-up valve opens to admit blanket gas and hold the pad up, and this is the vacuum-protection side of the scheme. As the pressure rises toward the high end of the band the make-up valve is fully shut and a relief path opens to let gas out, and this is the overpressure side. Between the two lies a dead band where neither valve is doing much, so the system is not constantly admitting and venting gas over small ripples. The result is a tank held gently inside a pressure band by whichever action the situation calls for.
Where a vapor recovery unit is present, the relief side of the split range usually routes to the VRU rather than to the atmosphere or the flare, and the two systems work as partners. The blanket make-up defends the low end of the tank's pressure envelope so it never goes into vacuum, and the VRU draws off the high end so rising pressure is captured as recovered vapor rather than vented. In effect the make-up gas sets the floor of the tank pressure envelope and the VRU suction sets the working ceiling, with the flare or a pressure-relief vent left as a backstop above the VRU for surges neither can absorb. The blanket and the VRU together keep the tank inside a tight, safe band with almost nothing lost to atmosphere.
The whole scheme lives or dies on one measurement, the tank vapor-space pressure, and keeping that pressure inside its envelope is exactly the kind of thing SCADA is meant to watch continuously. The system trends the pad pressure against the make-up threshold below it and the relief or VRU threshold above it, so an operator can see the pressure floating gently within its band, and it logs the make-up valve position so the gas being consumed to hold the pad is visible. A healthy tank shows a pressure that ambles quietly inside the band with the make-up valve nudging in occasionally; a tank in trouble shows the pressure pinned at an edge.
Those edges are where the alarms matter most. If the pad pressure falls toward the vacuum limit and stays there, it warns that the make-up gas cannot keep up, perhaps because the supply has failed or a valve has stuck, and that the tank is at risk of being drawn into vacuum, so this is a high-priority alarm. If the pressure climbs toward the relief limit and lingers, it warns that the VRU or relief path is not carrying the vapor away and the tank is heading toward venting. A make-up valve that sits wide open continuously is another quiet warning, often signalling a leak in the vapor space or a lost seal that is bleeding the pad away, which SCADA can surface as a rising, unexplained gas consumption.
A cloud SCADA and monitoring platform such as Merobix suits this well because tanks are usually unattended and blanket problems are slow, dangerous, and invisible until something fails. Merobix trends the pad pressure across the full envelope, logs blanket gas consumption and valve positions, and alarms on approach to either the vacuum floor or the relief ceiling, so a failed make-up supply threatening a vacuum collapse or a leaking pad silently drinking gas becomes an immediate signal rather than a discovery at the next round. Because the blanket often works in concert with a VRU, seeing both the tank pressure and the VRU suction on the same platform lets an operator confirm that the floor and ceiling of the envelope are both being held, and that the tank is protected from both air ingress and vacuum without vapor being lost.
Blanket gas keeps the vapor space above the stored liquid filled with inert or fuel gas at a slight positive pressure, so the tank breathes gas rather than air as its level and temperature change. This excludes oxygen and moisture, which would otherwise create a flammable vapor space and corrode the tank, and the positive pressure prevents the tank from being pulled into a vacuum and buckling. It protects both the flammability and the mechanical integrity of the tank.
One measured pressure, the tank pad pressure, drives two valves over different parts of the controller's output range. As pressure falls toward the low end, a make-up valve opens to admit blanket gas and hold the pad up; as pressure rises toward the high end, the make-up valve shuts and a relief path opens to let gas out. A dead band between the two prevents the system from constantly admitting and venting gas over small pressure ripples.
The blanket make-up gas defends the low end of the tank pressure envelope by adding gas so the tank never goes into vacuum, while the VRU draws off the high end so rising pressure is captured as recovered vapor rather than vented. In effect the make-up sets the floor and the VRU suction sets the working ceiling, with a flare or relief vent left as a backstop above the VRU. Together they keep the tank in a tight, safe band with little lost to atmosphere.
Merobix reads your field devices into a cloud SCADA - the real thing behind these terms, live in days from any browser.