Characteristics of Centrifugal Pumps
Pumps are generally grouped into two broad categories—positive displacement pumps and dynamic (centrifugal) pumps. Positive displacement pumps use a mechanical means to vary the size of (or move) the fluid chamber to cause the fluid to flow. On the other hand, centrifugal pumps impart momentum to the fluid by rotating impellers that are immersed in the fluid. The momentum produces an increase in pressure or flow at the pump outlet.
Positive displacement pumps have a constant torque characteristic, whereas centrifugal pumps demonstrate variable torque characteristics. This article will discuss only centrifugal pumps.
A centrifugal pump converts driver energy to kinetic energy in a liquid by accelerating the fluid to the outer rim of an impeller. The amount of energy given to the liquid corresponds to the velocity at the edge or vane tip of the impeller. The faster the impeller revolves or the bigger the impeller, then the higher the velocity of the liquid at the vane tip and the greater the energy imparted to the liquid.
Characteristics
Creating a resistance to the flow controls the kinetic energy of a liquid coming out of an impeller. The first resistance is created by the pump volute (casing), which catches the liquid and slows it down. When the liquid slows down in the pump casing, some of the kinetic energy is converted to pressure energy. It is the resistance to the pump’s flow that is read on a pressure gauge attached to the discharge line. A pump does not create pressure, it only creates flow. Pressure is a measurement of the resistance to flow.
Head—Resistance to Flow
In Newtonian (true) fluids (non-viscous liquids, such as water or gasoline), the term head is the measurement of the kinetic energy that a centrifugal pump creates. Imagine a pipe shooting a jet of water straight into the air. The height that the water reaches is the head. Head measures the height of a liquid column, which the pump could create resulting from the kinetic energy the centrifugal pump gives to the liquid. The main reason for using head instead of pressure to measure a centrifugal pump’s energy is that the pressure from a pump will change if the specific gravity (weight) of the liquid changes, but the head will not change. End users can always describe a pump’s performance on any Newtonian fluid, whether it is heavy (sulfuric acid) or light (gasoline), by using head. Head is related to the velocity that the liquid gains when going through the pump.
All the forms of energy involved in a liquid flow system can be expressed in terms of feet of liquid. The total of these heads determines the total system head or the work that a pump must perform in the system. The different types of head—friction, velocity and pressure—are defined in this section.
Gear Pump Operation and Maintenance
1. Introduction
A gear pump uses two meshing, toothed cogs to force water from the inlet of the pump through to the outlet. Figure No. 1 shows a simplified drawing of an external teeth gear pump on the left along with the alternate arrangement of internally pointing teeth.
2. Gear pump Design
Gear pumps use toothed gears turning inside a close tolerance housing to draw-in liquid and then squeezing it out ahead of them. Paddle steamers used the same principle of operation. These pumps are positive displacement pumps and anything drawn into them will be forced out. As a consequence they can generate very high discharge pressures. Materials of construction vary from metals of various types and hardness through to plastics of various types and hardness.
Maintaining the close tolerances between the housing and the cogs is critical to efficient operation. The clearance between the edges of the teeth and the housing and the ends of the cogs and the back and front walls of the housing are very small. Between the teeth and housing it is in the order of 0.1 mm (0.004”) while the clearances between the front and back faces of the gears and the ends of the housing are only 0.025 mm (0.001”). The fine clearances reduce liquid re-circulation back from the high-pressure discharge to the low-pressure suction side and make these pumps one of the most efficient available.
Gear pumps usually have one shaft penetration through the housing for connection to the drive. The gear shafts on the smaller pumps can be supported in journal bearings within the ends of the housing and are lubricated by the product. On larger pumps rolling element bearings mounted in bearing housings are used. To prevent surface to surface contact wear of teeth the product does the lubrication.
3. Gear pump Uses
The design of a gear pump lends itself to use with clean liquids. Insure they draw liquid from well above the bottom of the supply tank in clear liquid space. Both low and high viscosity liquids can be pumped. If food grade products sensitive to shear (i.e. where the churning action of the pump breaks cells and fibres) are to be pumped the size of the pump will need to be increased and the speed reduced.
The design also produces good suction characteristics and they can be used to draw clean, low viscosity liquids from a good depth or distance. Where high viscosity liquids are pumped, or if drawing from a depth or distance, make it easy for the liquid to flow into the pump. Install large diameter suction lines, keep them short and where possible always put the pump lower than the supply tank so the suction is under positive head pressure from the stored liquid.
The very fine tolerances prevent pumping anything with a solid or particulate, as it would be squashed between the teeth and destroy the pump. If there is risk of solids being drawn into the pump it is necessary to install a suction line strainer that can be easily cleaned. Use as fine mesh screen as is possible without greatly increasing the suction pressure loses else the pump will cavitate. If the particulate is so fine that it passes through the screen it is better to choose a different design of pump.
Being a positive displacement pump there deliver very precise quantities for each revolution and this means they have good dosing characteristics regardless of their speed. Gear pumps make good chemical additive dosing pumps provided material compatibility issues are addressed.