Kenneth E. Atkins

The New Fifth Edition Of API 618 For Reciprocating Compressors - Which Pulsation And Vibration Control Philosophy Should You Use?

The proposed Fifth Edition of API 618 (“Reciprocating Compressors for Petroleum, Chemical, and Gas Industry Services”) incorporates significant changes in the section concerning pulsation and vibration control. There are still to be three “design approaches,” but the requirements to perform certain analyses that were presented as optional in the Fourth Edition will now be dependent on pressure pulsation and force levels determined from the acoustical simulation. The confusion concerning when piping forced mechanical response calculations should be performed, which originated in the Fourth Edition, has been eliminated; forced response calculations are not required to satisfy API 618 Fifth Edition when pulsation levels are controlled properly. A separate article on pulsation and vibration control is being developed by the API 618 sub-task force on pulsation and vibration control as an appendix (annex) to API 618. This text will be a stand alone “RP” (Recommended Practices) document in the API system, which would then be referenced by API 618 as well as other API standards (e.g., API 674 for Positive Displacement Pumps) for which pulsation and vibration control are an issue. This document, to be issued in 2002, will discuss the different design philosophies inherent to the new edition of the standard. The purpose of this tutorial is to provide the user with a working knowledge of good engineering practices for pulsation and vibration control of reciprocating machinery in relatively high mole weight gases (e.g., natural gas), as well as an indepth understanding of the proposed changes in API 618 and the differing design philosophies. Several case histories are used to illustrate why robust pulsation control is important for reciprocating compressor piping systems. The authors are members of the API 618 sub-task force on pulsation and vibration control and each has over 20 years of experience in this field. INTRODUCTION In the 1950s and 60s, design techniques were developed using analog simulation tools for the control of pulsation in compressor piping systems. Acoustical designs utilizing reactive pulsation control (acoustic filtering), in combination with resistive elements (orifice plates) where necessary, became very successful in controlling pulsation levels transmitted to piping, piping shaking force, and bottle unbalanced force. Over the last 20 years, digital techniques have progressed significantly as the speed and capacity of computers have developed, and today, digital techniques for acoustic simulation are in greater overall use worldwide than analog methods. However, in recent years there has also been a trend in some industry segments away from utilization of effective pulsation control techniques and toward more reliance on mechanical techniques to “control” vibration. There are several reasons for this disturbing trend. First, the basic pulsation control technology has historically been proprietary to certain organizations. Many users of acoustical simulation software do not understand reactive pulsation control and/or their software does not permit them to be cost competitive 183 THE NEW FIFTH EDITION OF API 618 FOR RECIPROCATING COMPRESSORS— WHICH PULSATION AND VIBRATION CONTROL PHILOSOPHY SHOULD YOU USE? by James D. Tison Senior Staff Engineer and Kenneth E. Atkins Senior Staff Engineer Engineering Dynamics Incorporated San Antonio, Texas in designing reactive filter systems; resistive designs require significantly less engineering effort and technical expertise. Another reason for this trend is the proliferation of finite element based structural dynamics software for piping. Virtually every pipe stress analysis package on the market today has some dynamic capabilities. Mechanical natural frequencies and forced vibration levels of complex piping systems, once modeled, can be calculated fairly easily; however, it is the lack of understanding of the limitations on the accuracy of these calculations that leads to serious problems and in some cases disastrous consequences. As will be shown herein, even if the structural dynamics calculations were extremely accurate, there is no justification for the risk involved by designing systems with inadequate pulsation control. However, the new Fifth Edition of the API 618 (2001) standard will continue to include language concerning detailed mechanical response and natural frequency calculations, implying that these calculations are sufficiently accurate to be useful in the design stage. While such calculations can be performed to any degree of accuracy in theory, practical considerations put limits on the accuracy that is actually achievable. It is the goal of this tutorial to illustrate this point, and to present well-established design techniques that can reduce the dependence on expensive and problematic forced response analysis for the qualification of piping system designs. SOURCES OF VIBRATION IN RECIPROCATING COMPRESSORS Pulsation Excitation Mechanism Reciprocating compressors generate flow modulations that in turn generate pressure pulsations. The flow modulations come about as a result of intermittent flow through the suction and discharge valves, as well as geometry effects due to the (finite) length of the connecting rod. Figure 1 shows a schematic of a compressor cylinder. The suction flow (QS) enters the cylinder, and the discharge flow (QD) exits the cylinder. The velocity of the piston, shown in Figure 2, is approximately sinusoidal in shape. The deviation of the actual piston motion from the sinusoidal shape is due to the finite length of the connecting rod. As the ratio of the connecting rod length to the crank radius (L/R) is increased, the shape becomes more closely sinusoidal. The pressure pulsation generated by the compressor is proportional to the flow (QS or QD) modulation. Since the flow is based on the product of the piston velocity and the piston swept area, the shape of the discharge flow at the piston face is of the same shape as the piston velocity curve (Q = Area Velocity). Since the suction and discharge valves of each cylinder end (e.g., the head end) of a compressor are never open simultaneously, the suction and discharge piping systems are isolated acoustically. Therefore, we can look at the flow excitation of either the suction or discharge independently for the purpose of understanding the pulsation excitation mechanism. Figure 1. Reciprocating Compressor Slider Crank Mechanism. Figure 2. Piston Velocity for Slider Crank Mechanism. Figures 3-6 show the effect of the valve action on flow through the discharge valves of a compressor. Figure 3 shows the discharge valve flow versus time for the head end of a cylinder. During compression, the suction and discharge valves are closed. When the pressure in the cylinder reaches the discharge back pressure, the discharge valve opens, and the flow versus time wave through the valve has the shape of a portion of the piston velocity curve shown in Figure 2. As the cylinder reaches top dead center (TDC), the discharge valves close, and the flow returns to zero. Figure 3. Single Acting Compressor Cylinder (L/R = ∞, Ideal Valves). A frequency analysis of the flow wave of Figure 3 is shown in Figure 4. Due to the repetitive action of the compressor cylinder, excitation is generated only at discrete frequencies, which are multiples of the running speed. These frequencies are commonly referred to as harmonics. The highest amplitude occurs at 1 running speed, with the levels generally decreasing at higher harmonics. Figure 4. Flow Frequency Spectrum for Single Acting Cylinder. For a “perfect” double acting cylinder (symmetrical head end and crank end flows, L/R = ∞) the flow versus time contains two identical flow “slugs” 180 degrees apart in time. Therefore, the odd harmonics (in this idealized case) cancel, so that the nonzero PROCEEDINGS OF THE 30TH TURBOMACHINERY SYMPOSIUM 184

Discussion Group T08 Reciprocating Compressors

Bruce McCain graduated from Texas Tech University in 1987 with a BS in Mechanical Engineering. He provides technical support around the world on both rotating and stationary equipment. Bruce consults on many aspects of reciprocating compressor problems including foundation strengthening and grouting, bolting and torquing, installation, maintenance, vendor surveillance, pulsation and vibration, and forensics. He has contributed to various trade publications and industry conferences. He is a Registered Professional Engineer (Texas) and a Certified API 510 Pressure Vessel Inspector and has worked for Amoco, Rohm and Haas, Altura, and Oxy Oil and Gas. His current position at Oxy is Rotating Equipment Engineering Lead.

Discussion Group T12 Reciprocating Compressors

past 40 years, he has been active in the field engineering services, specializing in the analysis of vibration, pulsation, and noise problems with rotating and reciprocating equipment. He has authored and presented several technical papers. Prior to joining EDI, he worked at Southwest Research Institute for 15 years as a Senior Research Scientist, where he was also involved in troubleshooting and failure analysis of piping and machinery. Mr. Smith received his B.S. degree (Physics, 1969) from Trinity University. He is a member of ASME and the Vibration Institute. Bruce McCain graduated from Texas Tech University in 1987 with a BS in Mechanical Engineering. He provides technical support around the world on both rotating and stationary equipment. Bruce consults on many aspects of reciprocating compressor problems including foundation strengthening and grouting, bolting and torquing, installation, maintenance, vendor surveillance, pulsation and vibration, and forensics. He has contributed to various trade publications and industry conferences. Bruce is a Registered Professional Engineer (Texas) and a Certified API 510 Pressure Vessel Inspector. Kenneth Atkins is a Senior Project Engineer with Engineering Dynamics, Incorporated and has experience in performing lateral and torsional critical speed analyses, rotor stability analyses, and the evaluation of structural vibration problems using finite element methods. He has been actively involved in field troubleshooting of a wide variety of rotordynamics, structural, and piping vibration problems. Mr. Atkins earned his. B.S. degree in Engineering Science from Trinity University in 1978. He is a member of ASME and is a Registered Professional Engineer in the State of Texas.

Discussion Group T11 On Reciprocating Compressors

Donald R. (Don) Smith, Coordinator, currently serves as President of Engineering Dynamics Inc. (EDI), in San Antonio, Texas. For the past 40 years, he has been active in the field engineering services, specializing in the analysis of vibration, pulsation, and noise problems with rotating and reciprocating equipment. He has authored and presented several technical papers. Prior to joining EDI, he worked at Southwest Research Institute for 15 years as a Senior Research Scientist, where he was also involved in troubleshooting and failure analysis of piping and machinery. Mr. Smith received his B.S. degree (Physics, 1969) from Trinity University. He is a member of ASME and the Vibration Institute.