June 18, 2020
Thriving under Harsh Conditions Pumping Asphalt, Part 1
Three screw pumps and non-timed twin-screw pumps have replaced traditional centrifugal and timed twin-screw pumps in refinery asphalt services.
Pumping asphalt from its origin at the refinery to a terminal has been attempted by several different technologies over the years. Throughout its transition from refinery to terminal, asphalt experiences several temperature changes that present an insurmountable challenge for most pumping technologies.
Beginning at the refinery, asphalt is typically hot (above 350° F) and pumped with centrifugal pumps or timed twin-screw pumps. The asphalt is then stored in heated tanks until it is loaded into railcars, barges or trucks.
Once the asphalt is allowed to cool slightly, centrifugal pumps become very inefficient and unreliable due to the increased viscosity. Many of the timed twin-screw pumps utilized were first installed 35+ years ago. They have product-lubricated timing gears and bearings and up to four mechanical seals. Since the gears and bearings are lubricated by asphalt, these components tend to wear relatively quickly and lead to other more expensive repairs, such as new mechanical seals and rotor sets.
The evolution of three screw pumps over the years has enabled them to replace traditional centrifugal and timed twin-screw pumps in refinery asphalt services.
Three screw pumps have one power rotor and two driven rotors. The rotors are gas nitrided (hardened to approximately 62-Rc) and torque is transmitted from the power rotor to the driven rotors via a rolling contact. The pumped product creates a liquid film that prevents metal-to-metal contact of the rotors.
The rotors run in a liner that acts as a journal bearing that provides support for the rotors throughout the entire length of the pumping chamber. During operation, a liquid film supports the rotors and prevents contact between the rotors and the liner.
Three screw pumps do not require product-lubricated timing gears, as torque transmission is achieved through rolling contact of the rotors. This simplified torque transmission system eliminates a significant maintenance item on timed twin-screw pumps with product lubricated timing gears.
Hydraulically balanced three screw pumps do not require thrust bearings or thrust plates, meaning only one bearing is required for positioning of the rotor set. Bearings are typically re-greaseable and should never rely on the pumped product for lubrication.
Since three screw pumps have only one shaft penetration, they require only one mechanical seal. The mechanical seal is normally a bellows type with a steam quench to prevent the faces from cementing together during down periods.
By design, three screw pumps are relatively simple machines with only a few rotating elements: rotor set, bearing and mechanical seal. This proven technology – when compared to a timed twin-screw pump (product lubricated) that has a rotor set, multiple bearings and seals and timing gears– offers a significantly lower installed cost, as well as a lower overall cost of ownership.
Three screw pumps are positive displacement machines (conforming to API 676) that are not affected by viscosity swings. These same viscosity swings normally force centrifugal pumps to operate at some point on the curve other than their best efficiency point, thus lowering the overall reliability of centrifugal pumps in asphalt service.
Since all of the rotating elements of a three-screw pump are normally contained in a cartridge, the casings are normally fabricated of welded steel, which can accommodate a built-in heating jacket. The flexibility of fabricating the casings allows operators to replace misapplied asphalt pumps with a drop-in replacement that does not require any pipe or baseplate modifications.