Road Tanker Design For Fast Unloading And Onshore Pump Drain Tank. To completely empty a road tanker the compartments need to be designed to have a sump that permits total drain-out of the liquid from the lowest point. If a pump is mounted on the trailer, it is best mounted under the trailer. Where a pump is mounted on the tray or fenders it will cavitate at low tanker liquid levels. For an onshore unloading pump installation it is best to install a buffer tank between the tanker and onshore pump to let the road tanker completely drain-out before the pump starts cavitating. Tanker load-out pumps must have a very low net positive suction head requirement.
Keywords: onshore unloading pump arrangement, road tanker unloading, load-out pump, isotainer unloading, tank vortex
Road Tanker Compartment Design
A road tanker liquid compartment ought to have a nearly full-length, deep, sloping sump if you want it to drain out completely at fast rates. The sump collects and funnels the ullage to the lowest point where the outlet nozzle is located. When a road tanker or isotainer is nearly empty the remaining liquid is moving slowly along the bottom. Yet the unloading pump is still drawing out at its full rate. This means the pump will be starved of liquid and it will run dry until enough product trickles in to fill the volute and be flung out as a surge of liquid.
Pumps with mechanical seals will have many seal failures in such situations. Fortunately there is a way to design the road tanker or isotainer to allow rapid drainage of the remaining ullage. The sketch below shows you two arrangements to eliminate trailer or truck mounted load-out pump cavitation and vortexing problems.
Notice that the outlet nozzle is at the very bottom of the elongated sump and faces downward to capture the last drop of liquid. Keep the outlet nozzle at least one pipe size bigger than the pump inlet flange size to allow easy, low-friction flow to the pump. Going two sizes bigger, if possible, is even better. The welds will need to be of high quality so transport vibration and flexing do not start cracks at inclusions and under-cuts. It is also good practice to stiffen the sump with gussets along its length so loads are distributed away from the welds and onto the tank walls.
The pump suction inlet should be set even lower than the compartment outlet nozzle so that the pump always has positive suction right up to the last drop. If you can’t set the pump below the sump nozzle, then set it as low as possible under the tanker barrel and run a full size pipe from the nozzle to the pump inlet and only reduce down right at the pump suction flange.
You can shape the sump to suit where the pump is located under the trailer. Make the slope as steep as is possible so the last of the liquid washes down the sump fast.
On-Shore Unloading Pump With Drain-Out Tank
If you are using an on-shore unloading pump to empty a tanker that does not have a decent sump, you will have slow ullage running along the compartment bottom. In this case install a buffer drain-out tank just before the on-shore pump and let the pump draw from the buffer tank while the ullage from tanker trickles out into the drain-out tank. The drain-out tank protects the pump from surging and running dry when the tanker ullage is low. Make the drain tank volume equal to about two minutes of pumping capacity.
The drain-down tank is first filled from the full road tanker. Once the unloading hoses are connected, the station valves are opened and the drain-down tank starts to fill. The tank vent valve is opened to release trapped gases and air. As the tank gradually fills the liquid level inside rises and also runs through the piping to the unloading pump.
The unloading pump outlet flange needs to be lower than the top fill height of the buffer tank. This insures the pump is also fully flooded before starting. You should be able to hear the gas coming out the vent. When the liquid level rises to the underside of the vent nozzle (notice the vent nozzle extends into the drain-out tank a small distance) you will hear a burbling noise and you shut the vent valve.
The drain-down tank is now full, the load-out pump and piping is flooded and you can start the pump and empty the tanker.
Selecting On-Shore Unloading Pumps
One other useful fact to help you unload road tankers and isotainers trouble-free, is to select pumps that easily handle low liquid level situations. You can tell these pumps by looking at their net positive suction head requirements. Select those that are happy to operate with low liquid levels and/or high liquid temperatures before they cavitate. Those sort of pumps will give you less problems at low ullage levels.
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