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Operation and Performance of Reciprocating Pump

In the world of fluid mechanics, pumps play a vital role in transferring liquids from one place to another. Just as the human heart pumps blood throughout the body, maintaining life and vitality, reciprocating pumps work tirelessly to circulate fluids through industrial systems, keeping them running smoothly and efficiently. Like the rhythmic beating of the heart, reciprocating pumps operate with a steady cadence, using a piston and cylinder arrangement to transfer fluids with precision and reliability. In this post, we'll delve into the operation and performance of reciprocating pumps, exploring the intricacies of their mechanism and what makes them tick.

Theory:-

Imagine a mechanical device that can harness the power of mechanical energy and transform it into hydraulic energy, allowing liquids to flow effortlessly through a pipeline. This device is none other than a pump, a crucial component in various industrial and domestic applications. In essence, a pump is a mechanical workhorse that converts the mechanical energy supplied to it from an external source into hydraulic energy, thereby increasing the energy of the flowing liquid.

Mathematically, this can be represented using vector analysis. Let's consider the velocity vector (v) of the fluid, which can be resolved into two components: the pressure energy vector (P) and the potential energy vector (PE). As the pump imparts energy to the fluid, the pressure energy vector increases, allowing the fluid to flow against gravity and overcome frictional losses. This increase in pressure energy is subsequently converted into potential energy as the fluid is lifted from a lower level to a higher level.

The various pumps can be broadly classified into two types: those that increase the pressure energy of the liquid, such as centrifugal pumps and reciprocating pumps, and those that increase the potential energy of the liquid, such as positive displacement pumps and rotary pumps. Each type of pump has its unique characteristics, advantages, and applications, making them an essential part of various industries, including oil and gas, chemical processing, and power generation.

(i) Positive-Displacement Pumps

Positive-displacement pumps guarantee a precise amount of fluid delivery, regardless of pressure or head. These pumps work by literally pushing or displacing the liquid using a moving member. The reciprocating pump is a classic example, where a piston or plunger moves back and forth, creating a chamber that fills and empties with each stroke.

From a tensor analysis perspective, the flow rate (Q) of a positive-displacement pump is represented by:

Q = (V * N) / (2 * π)

where V is the displacement volume and N is the rotational speed.

(ii) Rotodynamic Pumps

Rotodynamic pumps, also known as dynamic-pressure pumps, rely on angular momentum transfer to increase fluid pressure energy. These high-performance pumps are designed for applications requiring large volumes of fluid at high pressures. Centrifugal pumps are a common example, where an impeller rotates at high speed, imparting energy to the fluid.

From a tensor analysis perspective, the flow rate (Q) of a rotodynamic pump is represented by:

Q = (ρ * g * H * D^2 * N) / (2 * π)

where ρ is fluid density, g is gravitational acceleration, H is head, D is impeller diameter, and N is rotational speed.


Main Components and Working of a Reciprocating Pump:

A reciprocating pump essentially consists of a plunger and a cylinder where the plunger is enclosed by the cylinder. The cylinder is connected to suction and delivery pipes, each of which is provided with a non-return or one-way valve called suction valve and delivery valve respectively. The function of non-return or one-way valve is to admit liquid in one direction only. Thus the suction valve allows the liquid only to enter the cylinder and the delivery valve permits only its discharge from the cylinder. The piston Of the plunger is connected to a crank by means of a connecting rod.

As the crank is rotated at a uniform speed by a driving engine or motor, the piston or plunger moves to and fro (or backward and forward) in the cylinder. When the crank rotates from e =0' to e = 180', the piston or plunger which is initially at its extreme left position (that is, it is completely inside the cylinder), move to its extreme right position, (that is, it moves outwardly from the cylinder). During the outward movement of the piston or plunger, a partial vacuum (pressure below atmospheric) is created in the cylinder, which enables the atmospheric pressure acting on the liquid surface in the well or sump below, to force the liquid up the suction pipe and fill the cylinder by forcing open the suction valve.

Since during this operation of the pump the liquid is sucked from below it is known as its suction stroke. Thus at the end of the suction stroke the piston or plunger is at its extreme right position, the crank is at e = 180' (i.e., at its outer dead centre), the cylinder is full of liquid, the suction valve is closed and the delivery valve is just at the point of opening.
Types of Reciprocating Pumps:

The reciprocating pumps can be classified according to the liquid being in contact with one side or both the sides of the piston or plunger; and according to the number of cylinders provided.

According to the first basis of classification, the reciprocating pumps may be classified as:

(i) Single acting pump.
(ii) Double acting pump.

If the liquid is in contact with one side of the piston or plunger only, it is known as a single acting pump. Thus as shown in Fig. a single acting pump has one suction and one delivery pipe and in one complete revolution of the crank, there are only two strokes-one suction and one delivery stroke. On the other hand, if the liquid is in contact with both the sides of the piston or plunger, it is known as a double acting pump.As shown in Fig. a double acting pump has two suction and two delivery pipes with appropriate valves, so that during each stroke when suction takes place on one side of the piston, the other side delivers the liquid. In this way, in the case of a double-acting pump in one complete revolution of the crank, there are two suction strokes and two delivery strokes.

According to the number of cylinders provided the reciprocating pumps may be classified as :

(i) Single cylinder pump.
(ii) Double cylinder pump.
(iii) Triple cylinder pump.
(iv) Duplex double acting pump.
(v)Quintuplex pump.

The function of air vessel: -
The air vessel is a cast iron chamber, which has an opening at the base, through which water can flow, one chamber is fitted on the suction pipe just near to suction valve and one on the delivery pipe just near the delivery valves. Each channel is converted through a small length of pipe. For efficient working, vacuum vessels should be 3 to 5 times the discharge per stroke and the air vessel on the delivery side 6 to 10 times the discharge per stroke.
During the middle of the delivery stroke, when the pump is facing the water into the delivery pipe at a velocity greater than the average, excess water flows into the air vessel and compresses the trapped air in the upper portion of the chamber. At the end of the stroke when water flows into the delivery pipe at a rate less than the average water flows out of air vessel from the excess amount of water already stored to keep the discharge more uniform. This fluctuating water column causes the acceleration head to be reduced that in between pump cylinder and the air vessel, which allows the pump to run at higher speeds. Thus in this way it saves a large amount of power lost in developing accelerating heads on the suction side, water first collects in the air vessel and then flows in the cylinder on the delivery side. Water first goes to air vessel and then flows with a uniform velocity. An air vessel provided in a reciprocating pump acts like a flywheel of an engine.

Other functions of an air vessel:-
1. Reduces the possibility of separation and cavitation.
2. Allows pump to run at high speed.
3. The suction head can be increased by increasing the length of pipe below air vessel.
4. A large amount of power is saved due to low acceleration head.
5. Uniform discharge.

Operating characteristic curves of reciprocating pump:-
The operating characteristic curves indicate the performance of the reciprocating pump are obtained by plotting discharge, power input and overall efficiency against the head developed by the pump when it is operating at a constant speed under ideal conditions. The discharge of a reciprocating pump operating at constant speed is independent of the head developed by the pump. However, in actual practice, it is observed that the discharge of a reciprocating pump slightly decreases as the head developed by the pump increases almost linearly beyond a certain minimum value with the increase in head developed by the pump. The overall efficiency of this pump also increases with an increase in head developed by the pump.

TO DETERMINE THE PERFORMANCE OF THE RECIPROCATING PUMP:


Learning objectives:-
To calculate the efficiency of the reciprocating pump
To determine the relationship between efficiency and head
Aim:- To study the performance of the reciprocating pump.

Apparatus:- Reciprocating pump test rig, Tachometer, stopwatch.

Specifications:-
1. Reciprocating pump 38.11 mm bore diameter (D) 45.3 mm stroke length
Double acting with air vessel on discharge side Suction pipe 31 mm diameter
Delivery pipe 25 mm diameter
2. AC motor 3 HP – Speed variations controlled by a stepped pulley
3. Measuring tank – 400 x 400 x 450 mm wide
4. Sump tank – 600 x 900 x 600 mm height
5. Piston rod – 9.52 mm
Measurements :
1. Pressure gauge – 0 to 7 kgf/cm2 for discharge pressure.
2. Vacuum pressure – 0 to 760 mm of Hg for the suction pipe.
3. 3 phase energy meter for motor input measurement.
Description of apparatus:-
The apparatus consists of single cylinder, double acting reciprocating pump mounted on the sump tank. The pump is driven by an A.C motor with stepped cone pulley. An energy meter measures electrical input to the motor. Measuring tank is provided to measure the discharge of the pump. The pressure and vacuum gauges provided to measure the delivery pressure and suction vacuum respectively
Procedure:-
1. Fully open the discharge valve.
2. Start the pump, certain discharge is observed.
3. For that particular discharge, note down the discharge head, suction head, time required for 10 lts. Of water collection. Also, note down the time taken for 5 revolutions of energy meter and speed of the pump.
4. Note down the above observations at different discharge pressure and tabulate the values.

Model calculations :-
1. Volume/stroke = D2L/4 + (D2 - d2) L/4
= 103 cc/stroke = 1.03 x 10-4 m3/stroke
2. Theoretical discharge = 1.03 x 10-4 x N/60 m3/sec
3. Suction head = suction vacuum of Hg
hs = suction vacuum in m x 12.6
=
4.Delivery head = discharge pressure in kgf/cm2 x 10 m of water
hd =
5. Total head = hs + hd + 3 m
=
6. Actual discharge = 0.01/t m3/sec
7. Output power of pump = WQactHt
= 1000 x 9.81 x QactHt
8. Input power to pump
Let time required for 10 lts from energy meter be te sec
Input power = 10 x 3600 / te x 240 W
Taking motor efficiency as 80 %
Input shaft power = input power x 0.8
SP = _______ kW
9. o of pump = WQactHt / SP %
Coefficient of discharge = Qact/Qth
10. % Slip = Qth – Qact / Qth

Graphs:-
1. Ht Vs Qact
2. Ht Vs SP
3. Ht Vs o

Precautions:-
(a). Operate all the controls gently.
(b). Never allow raising the discharge pressure above 4kgf/cm2
(c). Always we clean water for the experiment.
(d). Before starting the pump ensure that discharge valve is opened fully.

Result:-
The performance curves are obtained and are compared with standard ones.