SPECIAL TOPIC: EMISSION CONTROL IN TANKS
very low and tight tank pressure setpoint of one to two inches WC. There is a broad range of options for these regulators to cover various incoming inert gas pressures and flow capacities( Figure 3).
Larger capacity regulators are often pilot-operated regulators with external pressure sensing tubes, and these are used for higher flows that elevate pressure at the regulator outlet. Blanketing regulators are specif- ically designed for this application, and key design features include very tight pressure control at low flow, and no leakage to the atmosphere during operation to reduce inert gas usage. High capacity or high inlet pressure will often require multistage, pilot-operated regulators.
Vapour recovery regulators
These back-pressure regulators vent tank vapours to a scrubber, vapour recovery system, or a flare so they
can be processed in an environmentally acceptable manner. These devices typically provide control to a tank pressure setpoint of a few inches of water above the tank blanketing regulator, but a few inches below the tank vent set pressure. Precise control with tight shut-off is required to avoid needlessly venting product vapours and / or nitrogen, and to prevent tank pressure from rising high enough to begin venting to the atmosphere through the tank pressure vent.
Key design features for this equipment include very precise control within a narrow range of extremely low pressures and exceedingly tight shutoff when not in operation. Regulator seals and soft goods must be carefully specified for compatibility with the tank vapours. Note that downstream equipment and pressure conditions can impact the sizing and selection of this equipment. For example, a vapour recovery system may be operating at a vacuum, and its detonation arrestors can create significant pressure drops at higher flows.
Tank pressure / vacuum vents
Figure 3: There are a variety of tank blanketing regulator designs available to handle varying incoming gas pressure and flow capacities( Emerson’ s Fisher blanketing regulators shown). Very tight pressure control at low flow and fast speed of response are key features.
Tank pressure / vacuum vents( Figure 4) should only come into operation during atypical conditions where the required flow rates exceed that of
14 FUGITIVE EMISSIONS JOURNAL • APRIL 2025