Reciprocating Pumps

Reciprocating Pumps

Introduction

There are two main types of pumps namely the dynamic and positive displacement pumps. Dynamic pumps consist of centrifugal, axial and mixed flow pumps. In these cases pressure is developed by the dynamic action of the impeller on the fluid.

Momentum is imparted to the fluid by dynamic action. This type was discussed in the previous chapter. Positive displacement pumps consist of reciprocating and rotary types. These types of pumps are discussed in this chapter. In these types a certain volume of fluid is taken in an enclosed volume and then it is forced out against pressure to the required application.

1 Comparison

Dynamic pumps

1. Simple in construction.

2. Can operate at high speed and hence compact.

3. Suitable for large volumes of discharge at moderate pressures in a single stage.

4. Lower maintenance requirements.

5. Delivery is smooth and continuous.

Positive displacement pumps

1. More complex, consists of several moving parts.

2. Speed is limited by the higher inertia of the moving parts and the fluid.

3. Suitable for fairly low volumes of flow at high pressures.

4. Higher maintenance cost.

5. Fluctuating flow.

2.Description And Working

The main components are:

1. Cylinder with suitable valves at inlet and delivery.

2. Plunger or piston with piston rings.

3. Connecting rod and crank mechanism.

4. Suction pipe with one way valve.

5. Delivery pipe.  

6. Supporting frame.

7. Air vessels to reduce flow fluctuation and reduction of acceleration head and friction head.

A diagrammatic sketch is shown in Fig

The action is similar to that of reciprocating engines. As the crank moves outwards, the piston moves out creating suction in the cylinder. Due to the suction water/fluid is drawn into the cylinder through the inlet valve. The delivery valve will be closed during this outward stroke.

During the return stroke as the fluid is incompressible pressure will developed immediately which opens the delivery valve and closes the inlet valve. During the return stroke fluid will be pushed out of the cylinder against the delivery side pressure. The functions of the air vessels will be discussed in a later section. The volume delivered per stroke will be the product of the piston area and the stroke length.

In a single acting type of pump there will be only one delivery stroke per revolution. Suction takes place during half revolution and delivery takes place during the other half. As the piston speed is not uniform (crank speed is uniform) the discharge will vary with the position of the crank. The discharge variation is shown in figure.

In a single acting pump the flow will be fluctuating because of this operation.

Fluctuation can be reduced to some extent by double acting pump or multicylinder pump. The diagrammatic sketch of a double acting pump is shown in figure In this case the piston cannot be connected directly with the connecting rod.

A gland and packing and piston rod and cross- head and guide are additional components. There will be nearly double the discharge per revolution as compared to single acting pump.When one side of the piston is under suction the other side will be delivering the fluid under pressure. As can be noted, the construction is more complex

.

3.Flow Rate and Power

Theoretical flow rate per second for single acting pump is given by,

Compared to the piston area, the piston rod area is very small and neglecting this will lead to an error less than 1%.

4.Slip

There can be leakage along the valves, piston rings, gland and packing which will reduce the discharge to some extent. This is accounted for by the term slip.

Percentage o Slip = Qa -Qn / Qa   x 100

Where Qth is the theoretical discharge given by equation and Qac is the measured discharge. If actual discharge is greater than theoretical discharge negative value is found

this negative value is called negative slip.

5.Coefficient of discharge

It has been found in some cases that Qac > Qth, due to operating conditions. In this case the slip is called negative slip. When the delivery pipe is short or the delivery head is small and the accelerating head in the suction side is high, the delivery valve is found to open before the end of suction stroke and the water passes directly into the delivery pipe. Such a situation leads to negative slip.

Theoretical power = mg(hs + hd ) W where m is given by Q × ?.

Problem.1 A single acting reciprocating pump has a bore of 200 mm and a stroke of 350 mm and runs at 45 rpm. The suction head is 8 m and the delivery head is 20 m. Determine the theoretical discharge of water and power required. If slip is 10%, what is the actual flow rate ?

6.Indicator Diagram

The pressure variation in the cylinder during a cycle consisting of one revolution of the crank. When represented in a diagram is termed as indicator diagram. The same is shown in figure.

Figure represents an ideal diagram, assuming no other effects are involved except the suction and delivery pressures. Modifications due to other effects will be discussed later in the section. Point 1 represents the condition as the piston has just started moving during the suction stroke.

1-2 represents the suction stroke and the pressure in the cylinder is the suction pressure below the atmospheric pressure. The point 3 represents the condition just as the piston has started moving  when the pressure  increases  to   the  delivery  pressure.  Along 3-4  representing  the delivery stroke     the  pressure  remains    constant.  The  area  enclosed  represents the work done during a crank revolution to some scale.

7.Acceleration Head

The piston in the reciprocating pump has to move from rest when it starts the suction stroke. Hence it has to accelerate. The water in the suction pipe which is also not flowing at this point has to be accelerated. Such acceleration results in a force which when divided by area results as pressure.

When the piston passes the mid point, the velocity gets reduced and so there is retardation of the piston together with the water in the cylinder and the pipe. This again results in a pressure. These pressures are called acceleration pressure and is denoted as head of fluid (h = P/?g) for convenience.

Referring to the figure shown below the following equations are written.

This is the acceleration in the cylinder of area A. The acceleration in the pipe of area a is=A/a w2rcosot.This head is imposed on the piston in addition to the static head at that condition.This results in the modification of the indicator diagram as shown in figure.

The effect of acceleration head are:

No change in the work done. pressure at 1?is around 2.5 m of head of water (absolute). Which is directly related to speed, the speed of operation of reciprocating pumps is limited. Later it will be shown than the installation of an air vessel alleviates this problem to some extent.

8.Work done by the Pump

For single acting

W= ?gALN(hs+hd+0.67hfs+0.67hfd)/60

For Double acting

W=2?gALN(hs+hd+0.67hfs+0.67hfd)/60

Where hfs, hfd =loss of head due to acceleration in the suction and delivery Pipe.