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.
Through the years, many pump technologies have been used in the effort to optimize liquid transfer from ship, barge, railcar or tank truck into storage terminals. The one pump technology that has repeatedly been proven the most effective in this application—for all the many products that are handled in this fashion—is the positive displacement sliding vane pump.
Sliding vane technology was invented in 1899 by Robert Blackmer as an alternative to the inefficient gear-type pumps that dominated the market at the time. Because of the nature of their operation, the flow rate and efficiency of gear pumps will erode over time as the pump's gear teeth wear. On the other hand, sliding vane pumps feature vanes that slide out of the pump rotor as they wear, meaning there is no drop in flow rate and volumetric efficiency as the pump ages. Realizing that he had found the solution to the liquid handling needs of a wide variety of industries, he incorporated his company in 1903.
Sliding vane pumps contain a series of vanes that freely slide into or out of slots in the pump rotor. The pump's rotation draws liquid in behind each vane, through the inlet port and into the pumping chamber. As the rotor turns, the liquid is transferred between the vanes to the outlet where it is discharged. Each vane provides a positive mechanical and hydraulic displacement of the liquid.
The vanes are actuated by three forces:
- Centrifugal force from the rotor's rotation
- Push rods that move between opposing pairs of vanes
- The liquid pressure that enters through the vane slots and acts on the bottom of the vanes
Therefore, each revolution of a sliding vane pump displaces a constant volume of fluid with variances in pressure having a minimal effect. This minimizes energy-wasting turbulence and slippage, while the pump's high volumetric efficiency is maintained.
Further since the vanes constantly adjust to accommodate for wear, the pumps maintain near-original and consistent volumetric performance over time. A key consideration for the liquid-terminal storage industry is that sliding vane pumps are able to create a tremendous amount of dry suction.
This suction capability results in a pump that can most effectively strip pipes and hoses while removing as much of the heel as possible from barges, railcars and tanker trucks. Also, the ability of a vane pump to move air allows it to “blow down” the discharge lines. The operational ability of sliding vane pumps also makes them the ideal solution for transferring highly viscous liquids.
Realizing the growth in green operations and the increased concern for the environment, some companies design pumps and compressors to be energy efficient. This enables pump users to gain a competitive business advantage through the deployment of energy-saving positive displacement sliding vane pump technology.
The liquid-terminal industry is one of the most crucial in the world. Every day, millions of gallons of raw materials and finished products in a wide array of industries are transferred into and out of liquid terminals around the globe via delivery vessels. The terminal operator has a large number of challenges:
- Ensure that the product is loaded and unloaded safely, for both terminal personnel and the environment
- Enable the product to be transferred in the most energy- and time-efficient manner possible
- Remove as much of the heel from the barge, railcar or truck as possible
- Guarantee that no product cross-contamination occurs
- Perform these tasks in an environmentally-friendly way
For more than 100 years, positive displacement sliding vane pump technology has met these parameters. That is why more savvy terminal operators are turning to sliding vane pumps as the solution to the product transfer needs of their liquid-storage terminals.
Pumps & Systems, April 2011