Sliding Vane Technology Provides the Suction Needed to Optimize Product Transfer for Liquid-Terminal Operations.
Many of us are familiar with the children's relay race that involves a bucket of water. Teams of four or five kids start with a full bucket of water and take turns carrying it to the next person in line before handing it off. The catch is that, invariably, some of the water will slosh out. The winning team is the one whose bucket has the most water remaining in it when it reaches the last person. It is virtually impossible for the bucket not to lose at least a little water.
The shipping and transfer of liquid commodities follows a similar “relay race” type pattern. While there is generally no spillage, not all of a specific shipment makes its way to the final destination. Profitable and responsible transfer relies on speed and the complete transfer of product. Petroleum products are prime examples.
Crude oil is shipped by barge to a refinery feedstock storage tank. After the oil is refined into its marketable forms, those products are shipped to finished goods storage tanks. From there, they are taken by barge, railcar or tanker truck to intermediate bulk-storage facilities. When needed, they are again transferred into railcars or tanker trucks for shipment to end-users.
With each transfer, a small amount of a specific shipment may commonly miss its final destination. This is because a heel is often left in the barge, railcar or tanker. The heel is the amount of product that cannot be lifted out of the delivery vessel by the pumping system that is used at the business end of each link in the supply chain. While getting every last drop out of the delivery vessel rarely happens, some pumping technologies are available that can guarantee that as much of the shipment as humanly possible is removed during delivery.
This article focuses on the technologies that are designed to leave delivery vessels and storage tanks as “dry” as possible after the transfer of a wide array of liquid commodities.
Bang for the Buck
In any transaction, whether buying a pack of gum or a new home, people want to get their money's worth. The same principle holds true in the purchase of bulk petroleum products. If a gasoline-station operator buys 6,000 gallons of premium unleaded, he wants 6,000 gallons of premium unleaded delivered into the underground storage tanks at his site.
The supplier wants to deliver the full 6,000 gallons, too, because he does not want to be known as someone who does not deliver what is promised. He also wants to empty his tanker as completely as possible because he might use it to ship a variety of products. If so, any remaining fuel in his truck or piping could potentially contaminate the next shipment that it is scheduled to complete.
Another concern with product transfer is the discharge hoses and piping that are used as the middlemen when the product is being moved from a storage tank to a delivery vehicle, or vice versa.
Operators need a pumping system that is capable of clearing those discharge lines at the end of the transfer process, to ensure that all the product is delivered and prevent spills, increase safety and ensure that no product cross-contamination can take place. Additionally, no driver wants to have to “walk down” the delivery hoses to ensure that all the product has been cleared from the lines.
When most people think of liquid-storage terminals, the enduring image is one of the giant 500,000-gallon holding tanks that dot the landscape at a petroleum refinery. In reality, refinery storage is just the tip of the iceberg in the liquid-storage terminal universe. Tank farms serve as an essential link in the distribution of a wide variety of other products, including:
- Mainstream and niche chemicals (solvents, fertilizers, pesticides, acids, etc.)
- Alternative fuels (ethanol and biodiesel)
- Vegetable oils for food products
- Animal fats and oils for cosmetics
- Liquefied petroleum gas (LPG)
- Liquefied natural gas (LNG)
In short, any liquid that can be transported in bulk, whether by barge, railcar or tanker truck, at some point in its life is transferred into and out of a storage tank and delivery vehicle, often at numerous points along the supply chain. Popularity is growing in a process known as transloading.
When a product is transloaded, it is directly transferred from one mode of transportation to another, for example, from a railcar to a tanker truck, which eliminates the intermediate stage of transferring the product into a storage tank. The same principles apply in transloading as they do in the transfer of a product from barge to storage terminal:
- Remove as much of the heel as possible
- Suck the lines dry
- Avoid spillage or cross-contamination
The pumping technology used in liquid terminal applications—again, using petroleum production as an example—endures the most stress at the head of the supply chain, where the barge or ship loaded with crude oil arrives at the refinery.
The first large crude carrier (VLCC) was built in 1966. Since then, more than 1,000 of these behemoths—which can carry more than 2 million barrels of oil at one time—have been built to traverse the world's oceans as they ship crude oil to refinery locations around the globe. These types of supertankers were designed to be an economical way to deliver large shipments of oil throughout the world.
When they do arrive at port, however, they need to be unloaded. Again, the same unloading parameters apply—the heel must be minimized; the threat of spillage must be eliminated, and cross-contamination must not occur.