Published in Technology
In our first story, we have seen the evolution of pumps. In the second, we have learnt how centrifugal pump infuses energy into the fluid through the impeller. Besides, there are positive displacement pumps, which also impart energy into the fluid through physical movement of the liquid mass. So how do we decide which type of pump is best suited for a hydraulic system design? Let’s take a glimpse at the classification of pumps and understand the basic working philosophy of each type.
Broad classification of pumps based on working principle can be seen in Figure 1 below –
Figure 1 - Broad Classification of Pumps
Leading by the term, rotodynamic pumps work on the philosophy of continuously adding kinetic energy to the working fluid through a rotating device i.e., the impeller. The category has been further divided into three (3) sub-categories based on impeller construction vis-à-vis the direction of fluid flow exiting the impeller. Figure 2 below provides typical sectional view and characteristics of these categories of impellers.
Figure 2 – Typical impeller construction & characteristics in Rotodynamic Pumps
• The Radial Vane or Centrifugal type
Impellers under this category will force the fluid to exit the impeller in the radial direction by the influence of centrifugal force while the fluid enters the impeller eye in the axial direction. The discharge fluid gets collected at the pump volute casing and guided to the discharge. Please refer to Figure 3a. Technically, this type of pumps has low Specific Speed and is useful for developing high discharge head and can handle the relatively lower quantity of fluid. Impellers can be open / semi-open or shrouded type depending on the process fluid characteristics. Refer to typical sectional details of the pumps in Figure 3b, 3c & 3d below for clarity.
Fig 3a – Working Principle of a centrifugal pump
Fig 3b – Cut away view of a centrifugal pump
|Fig 3c - Sectional view of a double suction pump
||Fig 3d - Typical view of multistage centrifugal pump|
• Mixed Flow type
Impellers under this category and Francis type force the fluid to exit the impeller in an angular direction as can be seen in Figure 4a. Pumps with mixed flow type impellers have medium Specific Speed as can be seen in Figure 2 and is useful for developing medium discharge head. It can handle comparatively higher flow than a centrifugal unit. Typical view of open and closed type mixed flow impellers and sectional view of single stage mixed flow pumps are shown in Figures 4b & 4c below.
Figure 4a – Typical Mixed Flow Pump working principle
Figure 4b – Typical Mixed Flow Impeller construction
Figure 4c – Typical Mixed Flow pump construction
• Axial Flow type
Impellers under this category are sometimes denoted as turbine type and are designed to push the fluid in the axial direction as can be seen in Figure 5a. Pumps with axial flow type impellers can handle large fluid volume as compared to other two categories and develop low discharge head. The impeller is characterized by the highest band of specific speed as can be seen in Figure 2. Typical impeller construction and sectional view of axial flow pumps are indicated in Figure 5b.
Figure 5a – Typical Axial Flow Impeller & pump construction
Figure 5b – Typical view of Axial Flow Impeller & pump cross-section
Rotodynamic impellers have been further developed through researches by the pioneers in the pump industry to satisfy various process needs, handle dirty water & slurry as well as for pumping fluids other than water. For handling slurry or waste water containing solids, semi-open or open type non-clogging impellers are used. Typical view of these impellers can be seen in Figure 6.
Figure 6 – Typical view of closed, semi-open and open type Impellers
B. Positive Displacement Type
As the name implies, this category of pumps physically pushes the fluid from suction to discharge. Positive Displacement Pump has an expanding cavity at the suction side and a decreasing cavity on the discharge side. Liquid flows into the pumps due to fall in pressure as the cavity on the suction side expands and the flows out of the discharge as the cavity collapses. Volume flow through the pump is constant in each cycle of operation.
Positive displacement pumps are classified into two main categories based on working principle.
A rotary pump traps fluid between a rotating element and the closed casing at the suction side and transports the fluid along with the rotary element until it gets discharged at the outlet due to space constraint. It is normally a fixed volume machine and maintains a constant & uniform flow across all discharge pressures. Single or multiple rotor elements can be used in a rotary pump design, as can be seen in the following section. Major distinctive type rotary pump designs are -
Figure 7 – Typical working view of Gear Pump and construction
Pumping activity is achieved by teeth of two rotating gears meshing each other fitted in an enclosed casing. Liquid trapped between the casing and the gear slots at the suction is carried by the teeth and discharges at the pump outlet. The meshing of teeth of two gears at center prohibits liquid backflow from the suction to discharge side. Both the gears are driven by a common driver.
Figure 8 – Typical view of Twin Lobe pump & cross-section
Figure 9a – Typical view of sliding vane pump & cross-section
Figure 9b – Typical view of Flexible vane pump
In the case of flexible vane design, vanes collapse to reduce the trapped volume as they approach closer to casing near discharge point to force the liquid to come out of the pump. Please see Figure 9b.
Figure 10a – Typical Archimedean single screw pump
Figure 10b – Typical view of Twin Screw Pump & cross-section
Figure 11 – Typical cross-sectional view of a Progressive Cavity Pump
As the name implies, to and fro motion of a moving element is used for pumping the fluid. In a reciprocating pump, the liquid is drawn into the cylinder through the suction valve as the piston or plunger moves away from suction point creating a vacuum in the cylinder. Reaching the other end, piston reverses its motion due to crankshaft arrangement and the trapped liquid is discharged through the outlet valve under positive pressure due to a reduction in trapped volume while the suction valve remains closed. See Figure 12. The discharge from a reciprocating pump is therefore pulsating and the discharge volume is fairly constant for a specific pump size and drive speed irrespective of the discharge pressure. The operation of the pump is also independent of the rotational direction of the drive when compared to a rotary or rotodynamic unit.
Figure 12 – Working Principle of a Reciprocating Piston Type Pump
Reciprocating Type Pumps have been further categorized based on their reciprocating mechanism as can be seen below -
Figure 13a – Working Principle of a Reciprocating Piston Type Pump
|Single Acting||Double Acting|
Figure 13b – Working Principle of single and double acting Reciprocating Piston Type Pump
The piston or plunger type pumps can be single or double acting type as can be seen in Figure 13b. In a double acting system, two sets of suction and discharge valves are used, one set at each end of the cylinder so that while piston moves from left to right, the left side of the cylinder is in suction mode and right side goes in discharge mode. The situation reverses as the piston moves from right to left.
Figure 14a – Working Principle of a Diaphragm Pump
Figure 14b – Working of a Diaphragm Pump using hydraulic device
Metering Pumps or the Controlled Volume Pumps are basically a modified version of the diaphragm pump, wherein the stroke or the frequency of the plunger movement is regulated using a stroke control mechanism or a variable frequency drive. Figure 15a below shows how a micrometer adjuster is used to regulate the stroke length of the plunger and thereby discharge volume. Figure 15b shows a cut-away view of the metering pump.
Figure 15a – Working of a Metering Pump using hydraulic
Figure 15b – Cut away view of a Metering Pump
I believe it has been an enjoying session for a beginner to understand pump types and their working principle. Further details may be studied in various literature published on the subject.
In our next story, we will talk about performance characteristics of pumps and their behavior in a hydraulic system. Stay Tuned !