Screw Pumps for Asphalt in Tank Terminals and Pipelines

One of the more used and traded commodities in the world is asphalt and similar varieties of asphalt. Way back in time tar pits in northern Alberta, Canada were used by the Athabasca Indians to seal and protect their canoes from water and the environment. Ever since those days asphalt under different names and appearances has widely used in infrastructural and industrial applications. The most typical examples are with paving of roads and driveways where asphalt after mixing with aggregate in a hot-mix plants provide a weather and wear resistant surface. Asphalt is also widely used in construction products from roof shingles and tarpaper to water proofing and sealing compounds.

As the world population keeps growing and people’s standard of living improves it is expected that demand for asphalt products will accelerate from building and facilities construction, and from road and transportation infrastructure improvements The increased demand will come from not just the industrialized world but will grow much faster in developing countries where the starting point is further back and there remains a lot of catch up to do. Additionally, there is an abundant supply available when users of heavy hydrocarbons like fuel in ships and power plants will turn to natural gas or renewables to comply with more restrictive emission standards. 

Production, storage and distribution of asphalt products will follow the increasing demand, leading to more tank terminals, which in turn should encourage manufacturers of tanks, piping, insulation material, heaters and pumps. We will in this article focus on the pumping aspect and high-light the technologies which helps to improve efficient, safe and economical handling of asphalt products in pipelines and tank terminals.

The Challenge of Pumping Asphalt

The generic name Asphalt used in the article should be understood to cover related or overlapping products such as Bitumen, Pitch, Residue, Tar or Asphalt Emulsion. They all have similarities and are derived from petroleum production either as products coming off the crude oil refining process or as raw bitumen direct from extraction or from mining

The density of asphalt products is close to that of water and in most cases behaves as a Newtonian fluid which means its dynamic or flowing viscosity is related to temperature. However, some bitumen can appear as a non-Newtonian fluid which happens in emulsions with water and entrained air. It behaves contrary to a Newtonian fluid and agitation and shearing of the liquids will cause a change in dynamic or flowing viscosity.

The characteristics of asphalt and similar related products require pumps that are designed to efficiently transfer such fluids often under very challenging operating condition. A typical pumping example can be with high viscosity fluids heated to high temperature, often in excess of 200C, with entrained air or gas, solids and impurities, flowing in slugs or starting cold to mention a few of the challenges. Simultaneously to maintain continuous and reliable operation and complying with HSE regulations, with lowest possible energy and maintenance costs, the operator has a very limited choice of pump technology. In some few installations hydrodynamic or centrifugal pumps are used often because of low initial purchase prices. To reach acceptable pumping efficiency the fluids need to be heated excessively to bring the viscosity down for such pumps. The result is high energy costs for both heating and powering the pump and higher CO2 emissions. In addition, due to the limited suction ability of a centrifugal pump, longer stripping time for storage tanks and pipelines which is an added economic disadvantage. To summarize the the higher costs for owning and operating centrifugal pumps in these applications will outweigh their lower purchase price.

Gear, vane and screw pumps

Therefore, it is common is to use positive rotary displacement pumps such as screw pumps, gear pumps or vane pumps. Gear and vane pumps are more economical especially when used in smaller sizes up to 3-inch piping where they can operate with direct drive from an electric motor. However, they have operational limitations and will require a speed reducer for larger sizes in order to reduces internal drag losses and for suction lift (NPSH) reasons. They are also limited in inlet pressure and as the rotors are not hydraulically balanced, wear plates must be used to protect aga

inst axial thrust. These thrust plates are significant maintenance items. Not uncommon are broken gears or vanes due to cold start when the pump is filled with cold asphalt.

The positive displacement technology represented by the screw pump is the best suited pump for handling asphalt and related products. Its pumping principle can be illustrated like a pump having a “piston” with infinite stroke. The “piston” drives the liquid continuously without pulsation, agitation or shear against the backpressure which is coming from the flow resistance in the piping system, static lift and pressure control valves. The screw pump enters the liquids at the suction end of the screw profile and continuously fills the cavities formed by the screw profiles of the rotors. The rotation of the screws moves the liquid trapped in the cavities axially and gently from suction to discharge in a constant flow raising the pressure from suction to discharge pressure. The internal flowing velocity in the screw pump is low and the closing and opening of each cavity is gradual which contributes to a smooth flow without pulsations and a much lower sound level. The low flowing velocity into the screw cavities is the reason for the screw pumps very high suction lift with viscous fluids. It means much faster stripping time when emptying tanks and pipelines also if air or gas is present in the suction line. The pumps can also run in reverse to empty a discharge line against limited back pressure.

The two models widely used in asphalt pumping are shown in the illustrations below.    

Both screw pump models are very simple in design with few moving parts and and hydraulic balancing system which eliminates the needs for service prone wear plates or thrust bearings. The rotors are in rolling contact with each other which is contrary to the sliding contact in a gear or vane pump. It cuts down on internal drag losses and saves on wear and maintenance. The rotors are radially supported by the bushings where a small flow of liquid from the high to the low-pressure side which efficiently lubricates and cools the bushings. The rotors are made carbon steel and are finished ground and hardened using gas nitration for wear protection.

Contrary to twin-screw pumps with external bearings only one shaft seal is necessary compared to four seals in the twin-screw pump. Depending on the installation and application different types of seal arrangements are used. Very common in asphalt applications is the heated and jacketed stuffing box, which is simple to maintain and less stressed at cold start. However, many operators prefer the mechanical seal also with heating jacket and in combination with an injection using diesel oil as solvent to prevent seal hang up. Standard with the pumps is an integral relief valve for over pressure protection and jacketing and heat tracing of the pump body and seal area.

Installation and Operation

The drivers are mostly inverter duty electric motors connected to a variable speed drive control or VFD. The speed of the pump is directly related to the flow, and by using a VFD it helps the operator to control the pumping rate with pumps running in parallel and to reduce the flow during cold start or line emptying and tank stripping. Other options are steam or hot oil jackets for keeping the pump heated when idle. A heat traced pack box or mechanical seal is often used to facilitate cold start up and ease the strain on the seal at start up. Pump monitoring follows traditional practices with temperature and pressure alarm protection. Vibration monitoring is sometimes a good complement to detect an early problem and to give a heads up for a future intervention. An important lesson is to design and install the pipe runs to avoid causing unwanted thermal and mechanical stress on nozzles and the pump casing. It is often done using expanders or flex joints on the suction as well on the discharge piping. The alignment of the driver and flexible shaft coupling is essential to relieve any harmful radial load on the pump and motor bearings. The coupling manufacturer’s permissible misalignment instructions should be followed while doing cold, as well the more important hot, alignment.

Typical Applications

The screw pump can serve in many different applications in the loading racks, tank storage and pipeline systems. Typical services include loading/unloading of rail cars, barges and ships. In the tank terminal the use includes forwarding, heating, circulating, transferring and blending of market grade asphalt as well as of pitch, residue and bitumen in different grades as feed stock or as finished products. Some pump installations are tailored for trunk and lateral export pipelines handling diluted/heated bitumen. The biggest challenge, which the screw pump can overcome, is to get a pipeline flowing again after being shut in or in the case of failure in heating. The screw pump offers the flexibility and ruggedness needed to swiftly resume operation, time after time, year after year