Nitrogen blanketing is used on tanks holding products where oxygen ingress is unwanted, whether for product quality, safety or both. It is a sound practice, but it changes the stream reaching a vapour recovery unit in ways that are easy to miss when a recovery system is specified from a standard template. A unit sized on the assumption that tank vapour is mostly hydrocarbon will behave differently when a substantial part of that vapour is inert gas.
None of these effects makes recovery impractical. They do mean that the sizing basis, the control philosophy and the way performance is judged all need to account for blanketing from the start, rather than being corrected after commissioning.
How blanketing changes the vapour stream
With a nitrogen blanket, the tank vapour space contains nitrogen mixed with hydrocarbon vapour. When liquid is withdrawn, or when the tank warms, the vapour that displaces into the vent or recovery line contains that nitrogen fraction. The hydrocarbon partial pressure, and therefore the concentration at which condensation begins, is lower than in a non-blanketed tank at the same total pressure.
The practical consequence is that a condenser designed around total pressure rather than hydrocarbon partial pressure will recover less than expected, because the hydrocarbon dew point sits at a lower temperature than the designer assumed. Recovery is still possible, but it requires either a lower condensation temperature or an additional stage to concentrate the hydrocarbon before final condensation.
| Effect of blanketing | What changes | Design response |
|---|---|---|
| Lower hydrocarbon partial pressure | Condensation begins at a lower temperature | Review temperature staging, not just total pressure |
| Inert fraction in the vent stream | Blower and piping handle more volumetric flow | Size the vapour path on actual volumetric flow |
| Oxygen excluded from the vapour space | Different safety case, monitoring priorities shift | Confirm the alarm and analysis basis for the site |
| Blanket pressure control | Vent events driven by control, not only temperature | Coordinate blanket regulator with recovery control |
Sizing on the right basis
Volumetric flow is the figure that catches people out. If a significant proportion of the vent stream is nitrogen, the blower or compressor inside a recovery unit is moving more gas for the same amount of hydrocarbon recovered than it would on a non-blanketed tank. Sizing on hydrocarbon load alone produces an undersized vapour path, which shows up as high inlet pressure, excessive run time and, in the worst case, nuisance trips.
The sizing basis should also reflect how the blanket is controlled. A system that maintains a tight blanket pressure will vent in discrete events; a system with a wider band vents in a more continuous pattern. The recovery unit and its control loop need to match the pattern that actually occurs, not an average that smooths both into a figure that matches neither.
Control and monitoring priorities
Because the hydrocarbon fraction is diluted, monitoring based on total pressure or on a single concentration reading is less informative than it would be on a non-blanketed tank. It is more useful to trend the parameters that reveal how the whole system is behaving: blanket pressure, vent line pressure, inlet temperature, and the liquid volume recovered per unit of throughput.
- Trend liquid recovered against throughput, which normalises for trading activity.
- Log blanket pressure and vent events, so recovery behaviour can be matched to them.
- Watch inlet temperature, since it determines how much of the hydrocarbon fraction is already condensed.
- Check the recovery unit is operating within its design flow range rather than in a starved condition.
- Record any nitrogen supply interruption, because the effect on the vapour stream is immediate.
Commissioning points worth extra attention
Commissioning a recovery system on a blanketed tank deserves a deliberate test at different operating conditions, because steady-state behaviour on a mild day is not representative. Testing during a delivery, during a warm afternoon and during a cold morning gives three different pictures of the vent stream, and the differences tell you whether the control loop is behaving or merely appearing to.
It is also worth confirming that blanket control and recovery control do not fight each other. Two control loops acting on the same vapour space can produce oscillation that looks like an equipment fault but is really a control boundary problem, and it is far easier to resolve before the plant is handed over.
Working with the blanketing system, not against it
Recovery equipment and blanket control are usually supplied and commissioned by different parties, and that is where coordination is lost. A blanket regulator set to maintain a narrow pressure band will produce frequent small vent events; one set to a wider band will produce fewer, larger events. The recovery unit should be configured for the pattern the site actually creates, and this is best settled with both parties present rather than by adjusting one side after the other has been signed off.
It also helps to agree who monitors what. If the blanket system and the recovery system each log their own data with no shared reference to time, diagnosing an interaction between them becomes guesswork. A common timestamp on both records costs nothing and turns two separate data sets into one usable picture.
Where blanketed and unblanketed sites differ most
The difference is most visible in how quickly a problem becomes apparent. On an unblanketed tank a decline in recovery usually shows up directly in the recovered volume; on a blanketed tank the same underlying decline can be masked for longer, because the hydrocarbon fraction is smaller and the absolute change is smaller in proportion. Sites that rely on recovered volume alone, without normalising for throughput, tend to detect the change later.
Frequently asked questions
Can a standard VRU be used on a blanketed tank?
It can, provided the sizing basis accounts for the inert fraction and the condensation temperature is reviewed against hydrocarbon partial pressure rather than total pressure. Specifying a standard unit without that review usually means correcting it later.
Does blanketing reduce recovery efficiency?
It reduces the hydrocarbon concentration in the vent stream, which changes the load condition. Recovery can still meet the intent of the system, but the unit will be handling more gas for the same hydrocarbon recovery.
Should the blanket pressure band be widened to reduce vent events?
Sometimes, but it should be assessed together with the recovery control loop rather than changed in isolation. A wider band can reduce event frequency while shifting the problem to a different part of the system.
Luoyang Wohong Petrochemical Equipment designs and builds three-stage vapour recovery equipment for terminals and stations. If you are working on a blanketed tank farm, send us the tank data and blanket control details and we will review the recovery basis with you.