Short Answer:

An indicator diagram is a graph drawn between the pressure inside the cylinder and the piston movement (stroke) of a reciprocating pump, compressor, or engine. It shows how the pressure varies during suction and delivery strokes. This diagram is recorded by an instrument called an indicator attached to the cylinder.

It helps engineers to study the actual working condition of the pump or engine. From the indicator diagram, we can find out power developed, efficiency, losses, and performance of the machine. It is an important tool for analyzing and improving the operation of reciprocating machines.

Detailed Explanation :

Indicator Diagram

The indicator diagram is a graphical representation of the variation of pressure inside the cylinder of a reciprocating pump (or engine) during one complete cycle of operation. It is drawn on a paper by using an instrument known as an indicator, which records pressure against the piston displacement. The horizontal axis of the diagram represents the stroke length (movement of piston), while the vertical axis represents the pressure inside the cylinder.

This diagram gives valuable information about the suction, compression, and delivery processes happening inside the cylinder. By studying its shape and area, engineers can understand the performance of the pump and find defects like air leakage, valve problems, or frictional losses.

Construction of Indicator Diagram

An indicator diagram is obtained by connecting a small pressure-sensing device (indicator) to the cylinder. The instrument records the change in pressure during piston movement and plots it on a rotating drum that moves in proportion to the piston stroke.

X-axis (horizontal) shows the stroke of the piston (distance moved).
Y-axis (vertical) shows the pressure inside the cylinder.

During one complete cycle, the suction stroke and delivery stroke are traced, forming a closed curve. The shape of this curve helps in understanding how pressure changes throughout the operation.

Indicator Diagram for Reciprocating Pump

In a reciprocating pump, the indicator diagram shows two main parts — suction stroke and delivery stroke.

Suction Stroke:
When the piston moves outward, pressure inside the cylinder drops below atmospheric pressure. This causes the suction valve to open and water to enter the cylinder. The pressure during this period is shown below the atmospheric line in the diagram.
Delivery Stroke:
When the piston moves inward, the pressure inside the cylinder increases above atmospheric pressure. The delivery valve opens, and water is pushed into the delivery pipe. This pressure rise is shown above the atmospheric line in the diagram.

The resulting graph shows the pressure variations during both strokes and helps calculate work done by the pump per cycle.

Theoretical and Actual Indicator Diagrams

Theoretical Indicator Diagram:
This is the ideal diagram assuming no losses. It shows sharp changes in pressure at the beginning and end of strokes. The suction line is straight below atmospheric pressure, and the delivery line is straight above it.
Actual Indicator Diagram:
In real conditions, due to valve inertia, friction, air in the cylinder, and pipe friction, the diagram differs from the theoretical one. The suction and delivery lines are curved instead of straight. The area enclosed by the actual diagram is smaller than the theoretical one, showing losses in the system.

The difference between these two diagrams helps identify performance losses and reasons behind reduced efficiency.

Uses of Indicator Diagram

Performance Analysis:
It helps to study how efficiently the pump or engine operates.
Calculation of Power:
The area under the diagram gives the work done per cycle, which can be used to calculate indicated power.
Detection of Defects:
Any irregularity in the shape of the diagram indicates problems like leakage, valve delay, or air in suction line.
Efficiency Evaluation:
By comparing actual and theoretical diagrams, we can determine mechanical efficiency and energy losses.
Design Improvement:
Engineers use this information to improve pump design and reduce pressure fluctuations.

Shape and Interpretation of Diagram

In the indicator diagram of a reciprocating pump:

The suction line lies below the atmospheric line.
The delivery line lies above the atmospheric line.
The difference in heights between these lines represents the total head (sum of suction and delivery head).
Any curvature or delay at the beginning or end of the stroke shows valve lag or fluid inertia.

By observing these patterns, one can tell whether the pump is working normally or facing problems.

Example of Practical Use

Suppose a pump delivers less water than expected. By drawing the indicator diagram, if the suction line appears more curved than normal, it indicates air leakage or long suction pipe. Similarly, if the delivery line fluctuates, it shows air trapped or valve not seating properly. Thus, the diagram serves as a diagnostic tool.

Conclusion

The indicator diagram is an essential graphical tool used to study the working of reciprocating pumps and engines. It shows the pressure changes in the cylinder during suction and delivery strokes. By analyzing the diagram, engineers can calculate power, detect losses, and improve efficiency. The comparison between theoretical and actual diagrams helps identify mechanical defects and ensures better machine performance. It is therefore a key element in testing and maintenance of reciprocating machines.