Robert J. McKee

Pulsations in Centrifugal Compressor Installations

Compressor stations where both centrifugal and reciprocating compressors operate can exhibit operational problems due to low frequency pulsations. Avoiding these problems requires an understanding of the mechanisms that generate the pulsations, the transmission and attenuation of the pulsation, as well as their effect on the operation of the centrifugal compressor and the accuracy and reliability of flow measurements. The pulsation frequencies in question are typically lower than vortex shedding frequencies and blade passing frequencies. This paper will provide insight into the mechanisms and effects of pulsations and will describe approaches to avoid them.Copyright

Acoustics In Pumping Systems

This tutorial will explain the pumping system speed of sound concept (how to account for acoustic velocity changes due to pipe wall flexibility and liquid properties), the definition of quarter-wave, half-wave, and higher order acoustic mode shapes, the importance of each mode shape, and examples of how to estimate these mode shapes and frequencies. The use of basic acoustics to identify and resolve pulsation and vibration issues in liquid pumping systems using acoustic filters will be described in this tutorial. Some simple examples of acoustic pulsation problems with the corresponding mode shapes and frequencies will be presented. Examples of pulsation control in liquid pumping systems using acoustic filters will be given. This tutorial will conclude with some recommended practices and guidelines for the application of acoustic theory and practice to the design, review, and problem avoidance or resolution in liquid pumping systems.

An Internal Flow Measurement That Senses a Pre-Cursor to Surge in Centrifugal Compressors

Centrifugal compressors are critical for moving large volumes of gas in the natural gas pipeline, hydrocarbon processing, and general energy industries. The operation of centrifugal compressors is limited by a number of factors including the occurrence of surge at low flows. The exact low flow conditions at which surge occurs cannot be determined from external measurements. The precise flow and head at which surge occurs is effected by factors including the installed piping configuration, the dynamic impedance of the piping system, the pulsating pressures in the piping, and the gas properties such as compressibility among other variables. The results of the many factors affected when surge occurs and the sudden nature of surge onset are such that there are no previously identified reliable ways to detect an approaching surge. Current surge control methods rely on external measurements of head or speed and flow to estimate the conditions at which surge will occur and then recycle flow at some margin above the expected surge conditions in order to avoid surge. However, this type of surge control is inefficient and frequently leads to recycling more flow than necessary with the result that fuel and energy are wasted. If a means were available to measure a fundamental pre-cursor to surge then a reliable indication of the operating margin above surge could be developed and the amount of recycle flow and the loss of efficiency could be minimized. Such a pre-cursor has been found and a sensor technique base on drag probe technology has been developed. This paper presents some of the background on pre-surge detection in centrifugal compressors and then describes the known behavior of the impeller inlet outer wall re-circulating flow that develops as surge approaches. In addition, this paper reports on the development of a drag probe sensor to measure these internal flow components. Evidence from direct surge control testing is presented to support the finding that these internal flows are a basic surge pre-cursor and a useful control for centrifugal compressors. The measured flow changes prior to surge are identified and data is shown. The result of this research will lead to an improved surge control system for a significant class of signal stage modern centrifugal compressors and will increase the operating range and overall efficiency of such machines.Copyright

A Complex 3D Acoustic Induced Vibration of a Low NOx Refinery Boiler

An increasing number of refinery and other large process steam boilers are being installed with or converted to low NOx combustion. The primary result of low NOx combustion is reduced emissions; however, this type of combustion can also lead to unsteady pressures (pulsation), rumbles, and vibrations of boiler components and walls. This paper reports on an investigation of vibrations that were observed on the walls of a new low NOx boiler during its commissioning. Dynamic measurements and 3D acoustic modeling were used to identify and define the fluid force (acoustic) induced vibration problem and to design baffles to eliminate the specific complex acoustic mode that was causing the vibrations. Vibration measurements were recorded at numerous locations on the exterior walls of the boiler so that variations in magnitude and phase of the wall motions at different locations could be determined. The mode shape of the boiler motion was mapped from this data. Pulsation measurements were made on pressure taps at several locations around the boiler. The pulsation data defined the amplitude and phase relationships of the combustion and flow induced acoustic responses within the boiler. A 3D acoustic model of the boiler was created with internal gas properties including the speed of sound based on temperature distribution within the boiler. The model results were compared to measured data and the complex mode of the troublesome acoustic response was identified. Simple symmetric centerline baffles had been attempted by the boiler supplier without eliminating or reducing the vibrations and noise of the boiler. Based on the complex acoustic mode shape of the pulsation within the boiler as identified by the model results that agreed with field data, specific extended baffles were designed and installed to eliminate the vibrations that had been observed. Smooth operation of the low NOx boiler over a wide range of normal operation not previously obtained was achieved with the new baffles installed. The process of dynamic measurements and 3D acoustic modeling to match field data and confirm the mode shape of acoustic responses within a boiler, as described in this paper, can be used to resolve vibration problems including complex situations that are not solvable by other means.Copyright

Prototype Development of a Novel Radial Flow Gas Turbine

A prototype of a novel gas turbine concept is being developed to demonstrate the technical feasibility of a gas turbine design based on a straight radial flow with no axial flow turning. The prototype gas turbine consists of only two structural elements — a rotor disk and a stator shroud. The rotor consists of a centrifugal compressor and high impulse radial outward-flow turbine connected to an electric generator. The stator shroud contains the combustor and nozzles. The difference between this novel design and conventional radial gas turbine is that the compressor and turbine section are installed on the same side of the rotating wheel, while the combustor and nozzle are mounted on the stationary shroud. Thus, the entire assembly consists of two components. Technical advantages are: • Single Rotating Disk; • Compact Two-Piece Construction; • Ease of Repair and Maintenance; • High Power to Weight Ratio. This paper discusses the test set-up, instrumentation, and initial mechanical testing of the radial gas turbine. Performance predictions, rotordynamics analysis, and aerodynamic component verification are also discussed.Copyright

INCREASED FLEXIBILITY OF TURBO-COMPRESSORS IN NATURAL GAS TRANSMISSION THROUGH DIRECT SURGE CONTROL

This annual progress report describes the third years technical progress in a three-year program. This report introduces the benefits of improved surge detection and summarizes what is known about internal flows as surge precursors in centrifugal compressors. Early research results and findings concerning surge in centrifugal compressors and possible precursors to surge are presented. Laboratory test results in modern compressors with 3D impellers are described in detail and used to show the changes in internal flow patterns that occur as a compressor approaches surge. It was found that older compressors with recessed impeller blading (2D geometry) do not have the same accessible flow patterns. The laboratory test results indicate a large increase in potential operating range for modern compressors. This annual report also presents results from the field testing conducted during the course of this third year. The field test results showed similar changes in the surge probe strain signals and the same type, although of less magnitude, of indication that the compressor is approaching surge. An algorithm for identifying the nearness of surge has been proposed and evaluated with the available data. This project is co-funded by the Gas Machinery Research Council (GMRC) and by Siemens Energy and Automation (Siemens). The results of the project include a step-by-step process for design, sizing, and installation of surge detection probes and for implementation of the direct surge control in centrifugal compressor controllers. This work is considered a step towards the successful implementation of direct surge control for improved flexibility and efficiency in natural gas transmission compressors.

A Novel Centrifugal Flow Gas Turbine Design

A centrifugal gas turbine is developed based on a straight radial flow design with no axial flow turning. The geometry contains only two major elements — a rotor disk and a stator shroud. The rotor consists of a centrifugal compressor and high impulse radial outward-flow turbine connected to an electric generator. The stator shroud contains the combustor and nozzles. Fuel lines are attached to the shroud, ducted directly into the combustor. The difference between this novel design and conventional radial gas turbine is that the compressor and turbine section are installed on the same side of the rotating wheel, while the combustor and nozzle are mounted on the stationary shroud. Thus, the entire assembly consists of two components. Technical advantages are: • Single Rotating Component; • Short Axial Span; • Compact Two-Piece Construction; • Ease of Maintenance/Repair Access. Aerodynamic design, performance, and structural analysis of this design are discussed.Copyright

Mapping and Predicting Air Flows in Gas Turbine Axial Compressors

Determining the airflow through a gas turbine’s axial compressor is not a simple or one step process as many factors affect flow and there is seldom a flow meter or a means to directly measure airflow rate. Speed of the compressor, inlet pressure and temperature, and resistance or backpressure at the compressor’s outlet all affect the amount of airflow. The type of gas turbine, single or twin spool, the magnitude of power produced, the use of bleed or bypass valves, the power turbine speed, and operating conditions all have influences on the amount of airflow. Despite this, there are several reasons why an estimate of airflow is useful for understanding and describing the behavior and performance of gas turbines. The amount of airflow compared to fuel flow determines the composition and condition of the exhaust gases and is directly related to the turbine’s power output, heat rate, and waste heat recovery potential. A predicted airflow rate and the corresponding axial compressor discharge pressure can be used to identify deterioration in performance and to estimate emission characteristics of a unit. This paper presents an approach based on easily obtained gas turbine data, such as the design point data, test stand data, or manufacturer’s curves for the compressor. Compressor performance curves may be obtained from the manufacturer or by mapping compressor output during normal operations. A great deal of information has been presented in the literature about the performance of gas turbines and axial compressors but this paper focuses on methods that are sufficiently simple and direct that users can obtain an estimate of their unit’s airflow, References 1, 2, and 3. Some manufacturers provide computer data bases or on-line control panel estimates of gas turbine airflow but in these cases, the user has no idea what causes a change. Detailed performance curves for axial compressors are usually not available, however, through the methods presented in this paper, a reasonable approximation of the operating curves can be developed and used to estimate axial compressor airflow over the full range of normal operations. The methods described are based on tracking and mapping a compressor’s operations over a period of time and relating compressor output to other performance parameters and known conditions (design point) in order to establish a normally expected airflow rate.Copyright

Methods for and Benefits of Centrifugal Compressor Design Audits

As gas pipeline and industrial compressors become more powerful and more complex, it has become beneficial to conduct technical audits of these machines in the design stage. Detailed analysis of critical or advanced compressors by independent evaluators have identified operating limitations, resonant responses, potential vibrations, weak components, the onset of stall, and other instabilities, and have recommended ways to eliminate a variety of potential problems before the compressor is placed in operation. The suitability of a compressor and its driver for the planned service should be thoroughly evaluated, so that each component and the system not only satisfy the design conditions, but also extreme operating conditions. This paper presents a description of the tools available for design audits and gives examples of benefits that have resulted from recent audits. The rotordynamics of any large high-speed compressor should be carefully evaluated to identify potential instabilities, high vibration levels, and even destructive responses of the machine. Powerful rotordynamic analysis tools and specific knowledge exists to accurately predict bearing and seal stiffness and damping, lateral critical speeds, and damped forced responses. Some examples of significant results obtained from rotordynamic evaluations are presented, and typical problems that have been identified and eliminated are highlighted. Torsional vibration analyses for compressor trains are an essential aspect of a design audit that have identified vibration problems and weak components. Examples of torsional vibration responses and problems that can be identified and corrected are included in this paper. The aerodynamics of a compressor is a design audit topic to which attention should be paid. Thermophysical properties of the process gas, as it is compressed, are important quantities which can be accurately determined by modern equations of state. The internal velocity distribution and pressure rise per impeller and diffuser can be evaluated to identify areas of excess loss, poor work transfer, or restrictions within a compressor. Flow angles such as at the impeller and diffuser entrances can be predicted and evaluated. The diffuser inlet flow angle is a critical indicator of the onset of rotating stall. This type of aerodynamic analysis also provides important input for performance test planning and evaluation. This paper concludes with a summary of benefits of design audits for pipeline and industrial compressors.Copyright