June 25, 2020
Thriving under Harsh Conditions Pumping Asphalt, Part 2
In our last blog we discussed the pump technology and what is available along with some positives and negatives. To read it, please click here.
This brings us to the asphalt receiving end where terminals have traditionally used gear pumps to unload, transfer and load asphalt. The relatively inexpensive upfront cost and years of experience repairing gear pumps allowed terminal operators to overlook their potential problems.
Since asphalt is routinely delivered at temperatures well below 270° F, terminals had to first heat railcars or barges before pumping with a gear pump. That is because the NPSHa is too low for a gear pump to actually pump the highly viscous product without cavitating. As with any positive displacement pump, lowering the speed improves the NPSHr. However, gear pumps already operate at low speeds via a gear box and typically the speed cannot be lowered enough to compensate for the low NPSHa.
Another potential operational problem encountered with gear pumps occurs when asphalt solidifies in the discharge pipe. Standard practice dictates heating the discharge pipe until the slug is liquefied. If the slug is not liquefied and the gear pump is turned on, there is a great possibility the gear teeth will break, or the shaft will machine out the non-drive end of the pump through the wear plate.
Since gear pumps are unbalanced machines, they require a thrust or wear plate and the seals are typically subjected to discharge pressure, which leads to premature wear of the packing. These maintenance costs are in addition to the amount of extra time, energy and money required to heat the asphalt above 270° F.
Alternatively, terminal operators have begun utilizing non-timed twin-screw pumps over the last ten years. These pumps have one power rotor and one driven rotor. Torque is transmitted from the power to the driven rotor via a rolling contact (the same principle as the three-screw pump). The rotors are supported on each end by product-lubricated bushings, and hydraulic balancing of the rotors is maintained by the flow of product through the bushing/rotor annulus.
Since screw pumps can operate at synchronous motor speeds, they can also be driven with a VFD. This enables the pump to operate at lower speeds and lower NPSHa. This means non-timed twin-screw pumps can typically begin pumping asphalt at 220° F, thus saving tremendous amounts of time and money.
Screw pumps with built-in integral relief valves and hand wheels are a great tool for pushing solidified asphalt out of a discharge pipe. In this scenario, the handwheel is turned to open the relief valve allowing asphalt to internally by-pass some of the liquid while the pump discharge fluid pushes the slug out of the pipe. This method of removing solid asphalt slugs is much safer and efficient than torching the discharge pipe.
Non-timed twin-screw pumps are also very simple machines, with only a few rotating elements: rotor set and bearing. The seal (typically packing) is only subjected to suction pressure and the casings can be supplied with built in heating jackets and relief valves.
Asphalt is only one of many viscous products well-suited for screw pumps. In refineries and terminals, screw pumps are a great selection for vacuum tower bottoms, heavy oils, residuals, flare knockout, pitch, tar, emulsions, hot oils and olefin applications.
One recent example of this is how a Leistritz L2NG untimed twin-screw asphalt pump reduced unloading time from a railcar by over 70%. This facility unloads asphalt with an extremely high viscosity at temperatures around 300°. They replaced their traditional unloading gear pump with the L2NG. The time savings resulted in the pump’s ability to pump colder, higher viscosity product. Bottom line? It took significantly less time and money heating product, greatly improving the facility’s throughput which increased the bottom line.