The Indicator Card

The pressure inside the cylinder can be measured with a pressure transducer, a device that converts a pressure into an electrical output. The pressure transducer screws into the end of the cylinder head. There is a screwed plug in the cylinder head for this purpose. 

The volume of gas inside the cylinder is a function of the piston position. The piston position is a function of the crank shaft position. The crank shaft position can be measured by a magnetic pickup attached to the crank shaft. Anyone who has had an automobile spark plug firing timing adjusted electronically is familiar with this method of determining piston position. 

The output from the magnetic pickup and the output from the pressure transducer are connected to an oscilloscope. After a computer conversion of the data, the resulting pressure/volume plot is printed out. This plot is called an indicator card. The indicator card shown in Fig. 42.3 was generated from a 4000-hp natural-gas compressor near the town of Hebronville, Texas. 

The solid line is the indicator card plot. The dotted line is the Carnot or ideal compression work cycle, which 1 have drawn myself. The piston position, shown on the horizontal axis, is proportional to the volume of gas inside the cylinder. 

The area enclosed by the solid line is the total or actual compression work. The area enclosed by the dotted line is ideal or useful compression work. The area between the dotted line and the solid line represents compression work lost to heat. The area inside the dotted line divided by the area inside the solid line is called adiabatic compressor ficiency. 

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The indicator card is useful in balancing the load per cylinder. The area of the card and the pressure-length scale are used to determine the horsepower. 

P = mean indicated pressure by measurement of the indicator card... 

Take a closer look at the discharge portion of the indicator card shown in Fig. 29.3. Note that the cylinder internal pressure rises well above the discharge-line pressure, before gas is pushed out of the cylinder and into the discharge line. This is a consequence on the spring tension of the discharge valves. 

In order to force these valves open, a certain amount of extra pressure in the cylinder is needed. The extra pressure overcomes the spring tension. Once the valve is open, gas flows out of the cylinder, into the discharge line, and the cylinder pressure falls. But, as shown on the indicator card, the discharge valve apparently opens and closes five times during one cycle. 

I could repeat the same story for the intake or suction valves. Leakage of the intake valves would appear as peaks on the compression portion of the cycle. Pulsation losses for the intake valves would appear as peaks on the indicator card during the suction or intake portion of the cycle. 

Other problems that can be identified by use of the indicator card are... 

The indicator card is the only real way to monitor reciprocating-com-pressor performance. Typically, the equipment and personnel to generate the card can be obtained from a local company specializing in this service. Often, the indicator card is referred to as a beta scan plot. 

The apparent volumetric efficiency is the apparent volume of free air drawn in (as shown by the indicator card) divided by the volume of the piston displacement. This is the term that is commonly used in speaking of volumetric efficiency, and... 

The indicator card would be a true method of measuring the volumetric efficiency if the temperature of the air after being drawn into the cylinder were the same as that of the atmosphere, and if the pressure at the end of the suc-tion stroke were the same as that of the atmosphere. This is never the case. 

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