One of the more used and traded commodities in the world is asphalt and related varieties of asphalt. The asphalt or bitumen under different names and appearances has wide use in infrastructure, construction and other industrial applications. The typical examples in construction industry are with paving of roads and driveways where the asphalt after mixing with aggregate in a hot-mix plant provides a weather and wear resist surface. It is also widely used in construction products, like roof shingles and tar paper, to water proofing and sealing compounds.
The characteristics of asphalt and related products require pumps that are designed to efficiently transfer and circulate the fluids, often with very challenging operating conditions. The typical pumping scenario can require the pump to handle fluids with high temperature (often in excess of 200oC) high viscosity during cold start, entrainment of air and gas in the pump flow, solids and impurities and slugs of cold product. Simultaneously to safeguard continuous and reliable operation, together with demand for low energy and maintenance costs, limits the choice of pump technology for the application.
Rotodynamic Centrifugal Pumps and Positive Displacement Rotary Pumps are used for handling asphalt and alike products across the various industries but ultimate selection depends on the application and economy of the pump. In some few installations rotodynamic (centrifugal) pumps are used typically because of low purchase price. To get anywhere close to acceptable efficiency the fluids needs to be heated excessively to bring the viscosity down. This results in high cost for energy both for heating and powering the pump. In addition, due to the limited suction ability of the centrifugal pump, longer stripping time of tanks and pipelines is an obvious disadvantage. As a result the higher costs for operating these pumps often outweighs the lower purchase price. Instead, positive displacement rotary pumps such as screw pumps, gear pumps or vane pumps are more commonly used in the industry.
In small scale asphalt production plants like modified bitumen plant and emulsion production plant; and asphalt equipment like asphalt plant and bitumen sprayer, rotary gear pumps are popular for circulating asphalt. Gear pumps use the actions of rotating cogs or gears to transfer fluids. The rotating gears develop a liquid seal with the pump casing and create suction at the pump inlet. Fluid, drawn into the pump, is enclosed within the teeth of the rotating gears and transferred to the discharge. A gear pump delivers a smooth pulse-free flow proportional to the rotational speed of its gears.
There are two basic designs of gear pump: external and internal. An external gear pump consists of two identical, interlocking gears supported by separate shafts. An internal gear pump operates on the same principle but the two interlocking gears are of different sizes with one rotating inside the other. The larger gear (the rotor) is an internal gear i.e. it has the teeth projecting on the inside. Within this is a smaller external gear (the idler – only the rotor is driven) mounted off-centre. This is designed to interlock with the rotor such that the gear teeth engage at one point. A pinion and bushing attached to the pump casing holds the idler in position. A fixed crescent-shaped partition or spacer fills the void created by the off-centre mounting position of the idler and acts as a seal between the inlet and outlet ports.
Advantages of Internal Gear Pump
Internal gear pumps are exceptionally versatile, being capable of operating across a wide range of fluid viscosities and temperatures. While they are often used on thin liquids such as solvents and fuel oil, they excel at efficiently pumping thick liquids like asphalt. The useful viscosity range of an internal gear pump is from 1cPs to over 1,000,000cP. Internal gear pumps have better suction capabilities than external gear designs and are more suited to high viscosity fluids.
In addition to their wide viscosity range, the pump has a wide temperature range as well, handling liquids up to 750oF / 400oC. This is due to the single point of end clearance (the distance between the ends of the rotor gear teeth and the head of the pump). This clearance is adjustable to accommodate high temperature, maximize efficiency for handling high viscosity liquids, and to accommodate for wear.
Since output is directly proportional to rotational speed, internal gear pumps are commonly used for metering and blending operations, making them perfectly suitable for asphalt plants and bitumen sprayers. The low internal volume provides for a reliable measure of liquid passing through a pump and hence accurate flow control.
They are preferred to external gear designs in applications involving higher viscosity fluids, at high temperatures and with fluids containing solids. Typically, internal gear designs operate at lower rotational speeds than external gear designs, have greater clearances and are therefore less susceptible to wear in these applications.
The internal gear pump is non-pulsing, self-priming, and can run dry for short periods. They're also bi-rotational, meaning that the same pump can be used to load and unload vessels. Because internal gear pumps have only two moving parts, they are compact, reliable, simple to operate, and easy to maintain.
Working of Internal Gear Pump
There are three stages in an internal gear pump's working cycle: filling, transfer and delivery
1. As the gears come out of mesh on the inlet side of the pump, they create an expanded volume. Liquid flows into the cavities and is trapped by the gear teeth as the gears continue to rotate against the pump casing and partition.
2. The trapped fluid is moved from the inlet, to the discharge, around the casing.
3. As the teeth of the gears become interlocked on the discharge side of the pump, the volume is reduced and the fluid is forced out under pressure.
Close tolerances between the gears and the casing allow the pump to develop suction at the inlet and prevent fluid from leaking back from the discharge side