Fuel transfer
Size a tanker offload for diesel and gasoline
Last reviewed 21 October 2026.
A pumped tanker offload is a short, lossy system. The product starts above the pump, in the truck, and finishes as a suction lift through a hose full of air and then vapour. A centrifugal liquid end does not clear that hose. A vane-assisted primer on the same shaft moves the air until liquid reaches the impeller, then drops back so it only circulates a little liquid. The centrifugal curve is still the curve you plot. The primer does not add head to it.
The figures are an illustrative Lakeshore study: diesel or gasoline, a 3 in Schedule 40 suction hose, and a 3 in fill line into the receiving tank. Every head and NPSH value comes from CurvePoint’s own evaluation. The catalogue vapour pressures are models for that illustration, not a laboratory assay of a particular load.
| Loss | US | SI |
|---|---|---|
| Pipe friction | 4.35 ft | 1.33 m |
| Fittings on the hose | 14.18 ft | 4.32 m |
| Suction loss, pipe plus fittings | 18.53 ft | 5.65 m |
| Air volume in that hose | 7.68 gal | 29.1 L (0.029 m³) |
Illustrative Lakeshore Process Equipment figures from CurvePoint’s own evaluation. Not a sealed calculation.
| Case | Operating point | NPSH available | Viscosity |
|---|---|---|---|
| Diesel / No. 2 fuel, 20°C, sea level, start | 325 gpm (20.5 L/s) at 78.8 ft (24.01 m) | 22.1 ft (6.73 m) | 3.54 cSt, curve not corrected |
| Diesel / No. 2 fuel, 20°C, sea level, end | 307 gpm (19.4 L/s) at 81.1 ft (24.72 m) | 14.4 ft (4.40 m) | 3.54 cSt, curve not corrected |
| Gasoline, 20°C, sea level, start | 329 gpm (20.8 L/s) at 78.3 ft (23.86 m) | 3.4 ft (1.05 m) | 0.74 cSt, curve not corrected |
| Gasoline, 20°C, sea level, end | 311 gpm (19.6 L/s) at 80.7 ft (24.59 m) | -4.2 ft (-1.29 m), below zero | 0.74 cSt, curve not corrected |
| Gasoline, 35°C, 300 m, end | 311 gpm (19.6 L/s) at 80.6 ft (24.58 m) | -26.3 ft (-8.00 m), below zero | 0.53 cSt, curve not corrected |
Illustrative Lakeshore Process Equipment figures from CurvePoint’s own evaluation. Not a sealed calculation.
| Fluid | psi | kPa |
|---|---|---|
| diesel | 29.0 | 199.8 |
| gasoline | 25.2 | 173.9 |
| water | 34.1 | 235.0 |
Illustrative Lakeshore Process Equipment figures from CurvePoint’s own evaluation. Not a sealed calculation.
Who does the offload, and in what order
Professional petroleum drivers run this work. The Canadian Fuels driver manual is a public procedure for that job, including gravity drops into underground storage tanks at service stations. Those gravity drops usually have no pump in the line at all. This page is the pumped case, where the truck compartment sits below the fill or the hose will not flow by gravity.
Before product moves, bond the truck and the receiving tank and confirm the bond. On a gasoline drop that uses Stage I recovery, connect the vapour hose so vapour displaced from the tank goes back to the truck, then connect the product hose. At the end of the drop, strip the product hose into the receiving tank. One organisation’s loading procedure also stops the fill at 95 percent of tank capacity. API RP 1007, cited here by title, is the loading and unloading practice written for MC 306 and DOT 406 tankers. API 2350, also by title, is overfill guidance for storage tanks, and tank-truck delivery generally sits outside that standard.
The hose, the fill line, and the spill preventer
The suction side in the study is 3 in Schedule 40 hose, 20 ft long, with a sharp inlet, a standard elbow, an API coupler, and a dry-break. The fill line is 3 in by 40 ft, with two standard elbows, a turbine meter, a spill preventer, a swing check, and an outlet. The spill preventer sits on the fill line. It is a system accessory. It is not part of the pump. The first table evaluates that hose at a stated 300 gpm, which is a friction check, and it reports the air volume from the Schedule 40 inside diameter rather than from the nominal size.
Gasoline needs the vapour return. Diesel in the Canadian Fuels manual is treated as a product drop that does not carry the same Stage I hose. Both fluids are in the duty table so you can see the NPSH difference at the same piping. Head in feet stays the head of the curve. The third table converts the diesel start-of-load head into pressure for diesel, gasoline, and water. Pressure and the power that goes with it follow density. Specific gravity does not redraw the head curve.
Start of load and end of load
At the start the suction is flooded by 4 ft and the discharge surface is 12 ft above the pump centerline. At the end the compartment has dropped, the suction is a 6 ft lift, and the discharge static head is unchanged. Read both rows. The end of the load is where vapour and lift spend NPSH. Diesel at 20°C and sea level keeps a positive available NPSH in this illustration, and the viscosity stays under 10 cSt, so CurvePoint leaves the water curve unchanged. Warm gasoline at elevation does not. The head curves can still cross while available NPSH is below zero. That crossing is not a usable suction.
The same ledger is on the NPSH calculator, including the fuel vapour rows. How the primer compares with a wet-prime casing and a vacuum-assisted dewatering pump is on the suction lift guide. Open this diesel start-of-load file in the calculator with the button below, and replace the hose, the fluid, and the elevations with the drop you are actually quoting.