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Mechanical [4]

In our first story, we have already looked at the history of the development of the Pumps and realized that the whole endeavor was to invent a mechanism(s) which can reverse the natural phenomenon of water flow by gravity.

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

A.    Rotodynamic
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.
•    Rotary
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 -

• Gear Pumps

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.

• Twin Lobe Pumps
  The arrangement is similar to gear pump with the difference that liquid is trapped in the rotating lobes as can be seen in the diagram below. Faces of the lobes continuously touch at the center thereby creating a seal between suction and discharge. Lobes are synchronized and driven through a common drive unit and timing gear.

Figure 8 – Typical view of Twin Lobe pump & cross-section

• Vane Pumps
  Vane pump is a single rotor design in which the rotor is placed eccentrically in the pump     housing as shown in Figure 9a so that the rotor keeps a small clearance with the casing on one side. As the rotor rotates, sliding vanes eject out of the rotor due to centrifugal  force and trap the liquid between vane and casing. Trapped liquid rotates with the vane     and gets discharged at the outlet due to a reduction in the trapped volume.

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.

• Screw Pumps
  The concept of screw pump was invented by Archimedes around 200 BC, which says that the angular motion of the screw when rotated in a static body (Figure 10a), can lift water from lower to a higher level. The concept is still used in the transportation of granular material. The design has been modified further to have double and triple screw pumps which are extensively used in oil and viscous fluid pumping.
• 
  Let’s look at the sectional view of a twin screw pump in Figure 10b.  The liquid trapped within screw threads and casing moves along the rotating screws till it reaches at the end. Both the screws are driven by a single prime mover using timing gears.

Figure 10a – Typical Archimedean single screw pump

Figure 10b – Typical view of Twin Screw Pump & cross-section

• Progressive Cavity Pumps
  Progress Cavity Pump uses a single screw rotor installed in a flexible casing. As the rotor rotates within the flexible stator, it pushes the trapped liquid in the direction of the screw. These pumps are used for slurry and high viscous fluids transportation. Typical sectional view of a progressive cavity pump is shown in Figure 11.

Figure 11 – Typical cross-sectional view of a Progressive Cavity Pump

•    Reciprocating
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 -

• Plunger or Piston Type
  Piston or plunger type pumps utilize piston movement as pumping principle, as already explained in Figure 12. Let’s also look at the Figure 13a below. The reciprocating motion of the piston is obtained using a rotating device and cam arrangement. As the piston moves away from piston head, the vacuum created in the cylinder pulls up the ball at inlet check valve and liquid rushes inside. Ball at outlet check valve remains seated preventing backflow from discharge side. With the reversal of piston motion, ball at suction valve goes back to sitting position due to gravity and pressure developed inside the cylinder pushes the liquid through the discharge valve.

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.

• Diaphragm Type
  In a diaphragm pump, movement of a flexible diaphragm using a cam and rotating device creates the changes in the volume of the pumping chamber in a cyclic manner and thereby produce pumping action with the help of suction and discharge valve. See Figure 14a. The capacity of the pumping system is generally low as it is dependent on the size and flexibility of the diaphragm and the rotational speed of the cam. The advantage of this pump is that the wetting of components by the handling fluid is limited to the pumping chamber and the process flow line and therefore useful in handling highly toxic and/or corrosive fluid.
  The design has been further modified to ensure uniform movement of the diaphragm using the plunger and hydraulic mechanism. Reciprocating movement of the plunger by cam arrangement in a confined hydraulic system causes the diaphragm to follow the plunger due to pressure fluctuation in hydraulic fluid and thereby produces pumping action in the process fluid. See Figure 14b.

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 !

Top management introduces systems and practices that define all successful organizations. Equally important is a healthy work culture they inculcate that drives staff at all levels to seeking solutions. This “tech story” is of a group of engineers and workmen that restored the design capacity of the Coal Handling Plant of National Thermal Power Corporation’s Korba Super Thermal Power Project (KSTPP) in the mid-1980s.

It was the fall of 2002. I was at Talcher-Kaniha, Orissa and was working as a Control & Instrumentation (C&I) engineer at the 6X500 MW coal based super thermal power plant of NTPC. The day ahead was either a Sunday or some holiday. I clearly remember the quiet time that I had planned that evening and the day following with my family. Aroma of Mughlai chicken filled the house while I was busy playing with my three-year-old daughter. It was about half-past seven in the evening.