Using a pump to unload a tank or liquid tanker truck is one of the quickest and most efficient methods. But things can get dicey when the liquid level gets low. Minimum submergence becomes a factor and the risk of air entrainment increases.
Oftentimes, when the pump starts to pull air, operators will shut down the pump and use other methods to further empty the tank, taking more time and effort. In this post, we’ll discuss why minimum submergence is important when pumping from tanks, and how you can make adjustments to move more liquid with your pump and reduce manual labor.
Submergence is defined as the distance from the surface of the liquid to the center of the inlet suction pipe. Minimum submergence takes into account two factors, the amount of liquid above the center of the inlet suction pipe, and the speed at which the pump operates.
Both factors are important. If the velocity of the fluid is too fast for the fluid level, vortices will form, pulling air into the centrifugal pump, engulfing the impeller, and causing pump performance to drop.
If too much liquid exists above the center of the inlet suction pipe, a siphoning effect can occur. The pressure and liquid weight end up assisting the pump with the flow and ultimately reducing the system head. The siphoning effect is rare, but something to be aware of.
Large centrifugal pumps and vertical turbines are most susceptible to issues with minimum submergence due to their size and speed. However, all centrifugal pumps can run into trouble with this.
What are the minimum submergence requirements when unloading tanks? The whole point of this operation is removing all the liquid. This obviously will cause the liquid level to drop below the minimum submergence line. How can an operator maximize time spent unloading by using a pump, without leaving product behind as either waste or to be manually unloaded?
Let’s walk through an example. In this case, we're pumping out a tanker truck. As the tanker empties, the pump crosses the line of minimum submergence. It’s constantly air binding the pump and air-entraining the product, yet a significant amount of fluid remains in the tanker truck.
How can the truck be successfully emptied with the same pump?
As the liquid level in the truck decreases, the fluid’s velocity should decrease. Using a VFD, slowing the pump with the fluid level will allow operators to pump significantly more liquid from the tanker truck without crossing the minimum submergence threshold. Eventually, there will be a point of no return. However, more is removed from the tanker truck during the transfer time than if the operator fought the air entrainment and constant minimum submergence condition.
Here’s a video demonstration of using a VFD to prevent crossing the minimum submergence threshold.
If a change in processes caused the minimum submergence threshold to change in the tank, or operators are struggling with unloading tanks and tanker trucks, talk to an expert for help. They can take an overall view of the system and make recommendations that will improve the long-term health of your pump and achieve better pumping results.
Struggling with minimum submergence? Ask us about it! We provide technical assistance to businesses and municipalities in Wisconsin, Minnesota, and upper Michigan.