In short
- Filling a tank displaces its headspace, so fume leaves the vent at the delivery rate.
- Pneumatic offloading adds the blow-down air on top of the displaced volume.
- A tank vent scrubber has to be sized for the delivery peak, not the daily average.
Bulk deliveries of hydrochloric acid, nitric acid or ammonia solution are routine at many sites. The fume they release is easy to overlook because it only happens for an hour or so at a time. It is also the moment a visible plume, a strong smell or an exposed driver is most likely.
Where the fume comes from
A storage tank is never empty of gas. Above the liquid is a headspace full of air saturated with vapour from the stored chemical. When a tanker fills the tank, that headspace is pushed out through the vent at the same rate the liquid goes in.
Supplier guidance on scrubbers for tank vents lists the design factors: whether transfer is pumped or pneumatic, the transfer rate, and whether there is tank breathing between deliveries.
| Source of vent flow | When it happens | Effect on sizing |
|---|---|---|
| Displaced headspace | Throughout filling | Equal to the liquid delivery rate |
| Pneumatic blow-down air | During and at the end of a pneumatic offload | Can exceed the displaced volume |
| Tank breathing | Temperature changes between deliveries | Small, but continuous |
| Thermal effects | Warm product into a cool tank, or the reverse | Adds fume concentration |
Have a delivery causing fume?
Tell us the chemical, tank size and how it is offloaded. We will size the vent scrubber.
Which chemicals need a vent scrubber
Concentrated hydrochloric acid, nitric acid, ammonia solution and some other fuming chemicals give off enough vapour to cause visible fume and odour at the vent. HSE guidance sets a short-term workplace exposure limit for hydrogen chloride of 5 ppm and for ammonia of 35 ppm, so the area around a delivery point can exceed safe levels quickly if the vent discharges at ground level.
Engineer's note
The figure most often missing at enquiry stage is how the tanker is offloaded. Ask your chemical supplier whether deliveries are pumped or blown off with compressed air, and at what rate, because that sets the scrubber size.
Sizing for the peak
The design flow is the highest rate air leaves the vent, which is usually at the end of a pneumatic offload when the tanker is blown clear. A scrubber sized for the average delivery rate will be overwhelmed at that moment.
- Find the maximum liquid delivery rate.
- Add the compressed air flow if the tanker is offloaded pneumatically.
- Allow for any tanks that share the vent and can fill at the same time.
- Check the pressure drop through the scrubber is within what the tank can tolerate.
Siting and interlocks
A scrubber on the tank vent sits close to the tank so the vent line is short. Many are designed to run only during deliveries, with the fan and pump started from the delivery point. Interlocks can confirm the scrubber is running before the delivery valve opens, so a delivery cannot start without fume control.
Planning a new bulk tank or fixing an existing one? Our tanker unloading page covers active and passive vent scrubbers.
See tanker unloading scrubbersOur tanker unloading scrubbers are designed around the delivery peak and the chemicals you store.
Common questions
Do I need a scrubber if deliveries are only monthly?
Frequency does not change the peak. If each delivery produces visible fume or odour, the vent needs treatment regardless of how often it happens.
Can several tanks share one vent scrubber?
Yes, if the chemicals are compatible and the scrubber is sized for the combined peak flow.
What liquor does a scrubber on the tank vent use?
It depends on the stored chemical: water or dilute caustic for HCl, dilute acid for ammonia. Some small units are once-through; larger ones recirculate with dosing.