"Online" Monitoring of Reciprocating Compressors...continued 2. Reciprocating Compressor Dynamic Analysis – Basic Concepts The Pressure x Volume (PxV) diagram is the best way to represent the energy cycle in a reciprocating machine. Although the theory is relatively simple, getting actual diagrams online presents technological challenges that only recently were overcome. Solutions to these challenges are addressed in Section 4. For simplification, only one side of the compressor cylinder is shown in Picture 2.1. At A, both suction and discharge valves are closed. During expansion, the piston movement increases the volume of the gas in the clearance volume. This volume increase causes reduction of the cylinder internal pressure. Passing B, cylinder internal pressure below suction line pressure causes the suction valve(s) to open. As the piston moves from B to C, suction line gas at a pressure higher than the cylinder internal pressure is admitted into the cylinder. The portion of the total cylinder volume occupied by the admitted gas is called suction volume [2]. At C, the piston begins to move in the opposite direction. As it begins this movement, the piston reduces the volume of gas contained in the cylinder, increasing its pressure and forcing the suction valves to close. After the suction valves close, the original clearance volume gas and the gas admitted during the suction cycle are reduced in volume by the piston movement. Consequently, the cylinder internal pressure increases until reaching the discharge line pressure in D. A small additional piston movement is enough to raise the cylinder internal pressure above the discharge line pressure causing the discharge valve(s) to open. From D to A, gas in the cylinder at pressures exceeding the discharge line is discharged. The volume of gas discharged is called discharge volume [2]. The shaded zones on Picture 2.1 represent the additional work done to force the gas through the suction and discharge valves. The area of the shaded zones represents (approximately) the valve losses. Theoretical P x V diagrams superimposed on the actual diagrams supply important compressor diagnostic information. Only part of the compressor stroke is used for either suction or discharge. Suction volumetric efficiency is expressed by the equation [3]: Discharge volumetric efficiency is expressed by the equation [3]: All the mass of gas admitted into the cylinder, except that remaining as described above, is discharged. The ratio between the gas mass admitted and the gas mass discharged (Flow Balance), is a function of the ratio of volumetric efficiencies, and ideally is equal to 1. This ratio is one of the most representative parameters of the compressor condition. 3. Reciprocating Compressor Dynamic Analysis – Diagnostic Examples The components that most frequently cause reciprocating compressor shutdowns [4] and the associated maintenance costs distribution [1] are shown in Picture 3.1: Systems that monitor only parameters such as rod drop or valve cover temperature are able to diagnose only a small portion of the failures shown here. Dynamic analysis enables diagnosing the most common defects in reciprocating compressors. In this paper, these capabilities will be exemplified by identification of suction and discharge valve leaks.