Q19Basic Mechanical Engineering

A reciprocating pump is classified as a Positive Displacement Pump. Unlike centrifugal pumps which use kinetic energy, a reciprocating pump operates by physically trapping a fixed volume of liquid and then mechanically forcing (displacing) that trapped volume out into the discharge pipe. Because the displacement is physical and undeniable, these pumps can generate absolutely enormous delivery pressures, limited only by the structural strength of the casing and the motor power.

Main Components: - Cylinder: A perfectly machined hollow cast-iron chamber where the liquid is processed. - Piston or Plunger: A cylindrical block that moves tightly back and forth inside the cylinder, sealed by piston rings to prevent leakage. - Crank and Connecting Rod: A mechanical linkage driven by an electric motor. It converts the continuous rotary motion of the motor into the back-and-forth reciprocating motion of the piston. - Suction and Delivery Pipes: Pipes connecting the water source to the pump, and the pump to the overhead destination. - Check Valves (One-Way Valves): Absolutely critical components. The Suction Valve only allows water to flow from the suction pipe into the cylinder. The Delivery Valve only allows water to flow from the cylinder out into the delivery pipe. They prevent any backflow.

Working Mechanism (Single Acting Pump): The entire pumping cycle is completed in one revolution of the crank, comprising two distinct strokes of the piston:

• 1. Suction Stroke (Outward Stroke): As the crank rotates from $0^{\circ }$ to $180^{\circ }$, the connecting rod pulls the piston backward (outward) within the cylinder. This movement physically increases the volume inside the cylinder, causing the internal pressure to drop significantly below atmospheric pressure, creating a partial vacuum. The higher atmospheric pressure resting on the surface of the water source physically pushes water up the suction pipe. The pressure difference forces the suction check valve to fling open, while the delivery valve is sucked tightly shut. Water rushes in and completely fills the cylinder.
• 2. Delivery Stroke (Inward Stroke): As the crank continues from $180^{\circ }$ back to $360^{\circ }$, it viciously rams the piston forward (inward) into the fluid-filled cylinder. Liquids are virtually incompressible. Therefore, the instant the piston moves forward, the pressure inside the cylinder skyrockets violently. This massive internal pressure slams the suction valve shut, trapping the water. The only escape route is the delivery valve, which is blasted open by the pressure. The piston physically shoves the entire trapped volume of water out through the delivery valve and up the delivery pipe to the required height.

Applications: Reciprocating pumps are bulky and deliver pulsating flow, but they are unparalleled for applications demanding exceptionally high delivery pressure with small quantities of liquid, such as boiler feed pumps, pneumatic pressure systems, and heavy oil pumping.