I have some questions with regards to a diesel pumping application. I am using a vane pump to suck fuel out of a main storage horizontal tank. The layout is such that the the pump is installed about 2 feet above the tank bottom and is connected via. a 2inch SCH 40 pipeline and antispihon valve, other fittings to a suction tube installed on top of the tank with the help of extractor coupling. Because of this arrangement the fuel level varies with the reduction in quantity of fuel remaining in the tank. The tank height is 10 feet and the Antisiphon valve is installed 1 feet above top of this tank. The Antisiphon valve has a setting of 5-12 feet opening head and is currently set at a crack opening head of 5 feet (I suspect this is for pure water with SG = 1).

The pump was selected to suck the fuel out of the tank using this arrangement and the manufacturer has specified a mimimum dry vacuum at pump suction to be about 18 feet of diesel. Now while commissioning the system, the pump is not able to suck fuel out of the tank below 4 feet liquid level ( i.e. only the top 6 feet of fuel is sucked out, remember the tank height/diameter is 10 feet).

I did an analysis and calculated the head loss in friction on the suction side of the pump ,at the set flow rate of 35GPM, to be about 4.5 feet. In addition the Anti Siphon Valve requires 5 feet of head to open. In all the Friction Head (incl. siphon valve crack opening head) is about 9.5 feet in the system.

I fundamentally calculate the static suction head/lift in any pumping system as the difference between the liquid level and the pump suction. In this specific situation the Static Suction Head varies as the liquid level is about +8 feet when the tank is full and the liquid is above the pump suction. The the static suction lift is -2 feet when the tank is just about to be drained. Is this the correct determination of static suction lift even if the highest point in the entire system is about 9 feet above the pump suction. Does the pipeline routing matters in the suction lift calculation or is it always the vertical difference b/w the liquid level and pump suction irrespective of the routing of pipeline b/w the suction pipe inlet and the pump suction?

I want to check if I am using the right suction lift (-2 feet)for this application? I am considering that the pump will always be primed and the system be hydraulically balanced i.e. the Antisiphon valve will hold diesel in the suction tube installed inside the tank.

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