Operational Emissions — Filling, Pigging, Roof Landings and Other Events That Change the Picture
Why annual routine-loss calculations can miss the short-duration operating events that drive emissions and damage control equipment.
A tank can have an excellent roof and seal system and still experience high emissions during specific operating events. Filling, emptying, pigging, roof landing, cleaning, gas freeing, mixer operation and abnormal venting can change both vapour generation and mechanical loads.
These events should be treated as part of the emission-control design basis rather than as operational exceptions that engineering does not need to consider.
1. Filling a fixed-roof tank
Incoming liquid reduces the vapour-space volume and displaces vapour through the vent system. If the product flashes as pressure or temperature conditions change, the vapour volume and composition can be significantly different from simple geometric displacement.
The emission-control design should therefore use representative incoming product conditions and peak filling rate, not only annual throughput.
2. Filling beneath a floating roof
When the roof is floating normally, increasing liquid level raises the roof and the principal routine emissions remain associated with seals, fittings and seams. When the roof is on its supports, however, a large vapour space can exist beneath it and the loss mechanism changes until the roof refloats.
The transition between landed and floating condition is therefore important for both emission estimation and operating procedure.
3. Roof landings
Floating-roof landing can create a significant non-routine loss because the liquid surface becomes exposed to a vapour space beneath the roof. Emptying, standing while landed and refilling can each contribute. API MPMS Chapter 19 includes a dedicated part for evaporative loss from floating-roof landings.
Reducing unnecessary landings can be an operational emission-control measure with little capital cost. Where landings are unavoidable, operating sequence, landed duration and refilling strategy should be considered in the emission inventory.
4. Pigging and gas slugs
Pipeline pigging can introduce a volume of gas into a receiving tank. A liquid diffuser designed for normal filling may not safely disperse a large gas slug. Gas can create transient uplift, turbulence and local pressure beneath an IFR and can increase the required vent flow.
Pigging should therefore have a defined operating case: estimated gas volume and rate, receiving tank level, diffuser configuration, vent capacity, roof condition and any restrictions on pig arrival. The objective is to avoid both uncontrolled venting and mechanical upset of the floating roof.
5. High liquid flow and turbulence
High inlet velocity can disturb the product surface and the floating roof, particularly before the roof is fully afloat or where inlet geometry directs flow upward. Diffuser design is intended to reduce local velocity and distribute flow. The relevant criterion is not simply nozzle diameter but the flow field the roof actually experiences.
Outflow can also create transient pressure differences beneath a landed roof if venting is inadequate.
6. Cleaning and gas freeing
Taking a tank out of service intentionally removes the normal vapour-control configuration. Draining, sludge handling, washing, ventilation and gas freeing can release significant VOCs over a short period. Routine annual inventory methods may treat these separately from normal tank losses.
A maintenance plan should therefore include an emission-control plan: product recovery, closed draining where practicable, vapour routing, ventilation strategy, monitoring and the conditions for safe entry.
7. Mixers and changing liquid level
Mixers can interact physically with IFRs and can increase local liquid movement. The design must consider mixer envelope, roof low position, potential contact and operating restrictions. Fittings around mixer penetrations also need to maintain their vapour barrier.
The same principle applies to columns, ladders, gauge poles and other internals: operation and geometry are part of emissions performance.
8. Abnormal and upset events
Blocked vents, rapid pump changes, nitrogen or process gas ingress, damaged seals and roof sticking can create conditions well outside the normal inventory calculation. These cases belong in process hazard analysis, operating procedures and incident learning as well as in emission management.
Repeated abnormal venting should trigger a root-cause review rather than being treated as an unavoidable environmental loss.
9. Build an operational emissions register
| Event | Record | Control question |
|---|---|---|
| Roof landing | Frequency, duration, product, level | Can it be avoided or shortened? |
| Pig receipt | Gas volume/rate, tank level, vent path | Can diffuser and venting manage the transient? |
| Tank cleaning | Method, duration, vapour handling | Can product/vapour be recovered before ventilation? |
| High-rate transfer | Peak rate, inlet arrangement | Are turbulence and vent capacity acceptable? |
| Abnormal venting | Time, cause, operating state | What failed and how is recurrence prevented? |
10. Include operations in the scorecard
A tank with modest routine emissions but frequent landings, cleaning or pigging events can deserve higher priority than a routine calculation suggests. Operational events should therefore be captured explicitly in the tank inventory and the emission-reduction scorecard.
References and technical basis
- US EPA — AP-42 Chapter 7.1, Organic Liquid Storage Tanks (final revision October 2024)
- US EPA — TANKS Emissions Estimation Software, Version 5 (TANKS 5.3 released July 2026)
- American Petroleum Institute — Subcommittee on Evaporation Loss Estimation, MPMS Chapter 19
- American Petroleum Institute — Standards Plan (including API 2000 current-edition information)
- EEMUA 213 — Emission reduction from oil storage tanks and loading operations
Standards and regulations change. Confirm the edition, legal applicability and permit conditions for the tank, product and jurisdiction before design or compliance decisions are made.