At a Moment’s Notice

SwRI field service professionals travel the globe to provide quick response solutions to machinery and piping system dynamics problems

Justin R. Hollingsworth     image of PDF button

Photo of Justin R. Hollingsworth

Justin Hollingsworth is program manager of Plant Engineering in SwRI’s Fluids and Machinery Engineering Department. Hollingsworth, who specializes in machinery dynamics and computer-based analysis techniques, supervises a team of engineers and scientists dedicated to troubleshooting machinery and piping systems around the world.


photo of fatigue-induced cracks in a motor shaft

Dynamic failures are commonly related to high cycle fatigue. This photograph provides graphic evidence of fatigue-induced cracks in an industrial motor shaft (yellow lines), which were caused by excitation of a torsional critical speed.


image of  finite element analysis

SwRI researchers use finite element analysis to develop modifications to shift a structural resonance identified in the field. This case involved a motor drive bracket resonance for a large fin-fan cooler.

photo of plant with a motor drive bracket resonance  
photo of novel instrumentation

Novel instrumentation is often required to obtain data necessary to diagnose certain types of behavior. Here, strain gages were installed inside a reciprocating compressor in natural gas pipeline service to document rod loads during various operating conditions.

 
photo of velocity probes mounted on the drive shaft bearing housings

Velocity probes were mounted on the drive shaft bearing housings of an industrial fan to investigate the lateral rotordynamic behavior of the system.

 
photo of field engineers performing on-site diagnostics

Field engineers perform on-site diagnostics using a variety of equipment and analysis techniques.

 
photo of a refinery

SwRI conducts field testing for many types of installations, including refineries.

It could happen a few miles away or several thousand, and it could happen at any time, setting off a chain reaction. A natural gas compressor station fails, shutting down the fuel source that feeds the utility plant that powers your home. The failure and the resultant downtime are more than just inconvenient, it’s expensive — for the gas distributor, the utility and the consumer.

Field service engineers and scientists in Southwest Research Institute’s (SwRI) Mechanical Engineering Division have been troubleshooting dynamics problems in machinery and piping systems worldwide for nearly 60 years. The group got its start supporting the rapid growth of the natural gas pipeline industry in the 1950s. It grew to include work in many segments of the oil and gas industries including production, refinery, storage, transport and delivery, as well as several other businesses involving industrial machinery such as power plants, paper production, medical, brewery, water supply/treatment, shipboard and aerospace. Because system downtime can result in substantial financial loss — thousands, and in some cases, millions of dollars per day — the group has maintained a “ready to travel” status for decades that allows them to respond literally overnight to critical needs around the globe. Customers value the impact of accurate and timely diagnoses to avoid safety concerns, equipment downtime and damage, repair and replacement costs, adverse environmental impacts and loss-of-delivery penalties.

The group gathers pulsation, vibration, noise, pressure, temperature and strain data using portable sensors, data acquisition systems, analyzers and other devices. A combination of novel analysis techniques, correlation of cause and effect, and significant experience are used to provide effective solutions.

SwRI field service professionals are experienced in troubleshooting problems in many types of machinery, including reciprocating compressors, centrifugal compressors, gearboxes, couplings, piping systems, fin-fan coolers; pumps, induction and synchronous motors, variable frequency drives, reciprocating engines, gas and steam turbines, turboexpanders, large industrial fans, recycle, control and anti-surge valves, boilers, offshore platform structures, piping racks, foundations, skids, scrubbers and chemical, process and refinery vessels.

Frequently, field projects result in acoustics, finite element, computational fluid dynamics, metallurgical, thermal or rotordynamic analyses. The accuracy and value of these analyses are often enhanced and verified with the availability of quality field data. Another byproduct of field work is that it identifies industry trends and opportunities for research to resolve common problems. The plant engineering group works closely with three other groups in the Mechanical Engineering Division — machinery structural dynamics, rotating machinery dynamics, and fluid machinery systems — to develop effective solutions.

SwRI engineers have conducted more than 5,000 field studies in the United States. Other projects have involved travel to numerous locations worldwide, from Abu Dhabi, Azerbaijan and Australia to Taiwan, Turkey, Venezuela and Wales, among many others. Although these destinations can be interesting for the personnel involved, significant challenges also exist with this type of travel. Safety regulations frequently require completion of site- specific training in advance of the trip, and some clients require extensive medical evaluations before contractors are allowed on-site.

The projects usually involve traveling with two to five oversized equipment boxes, each weighing approximately 70 pounds. Appropriate documentation, including customs registration forms, visas, and letters of invitation and guarantee, must be prepared and presented to the airlines and local authorities. Modes of travel can include helicopters and boats, which present additional challenges for transporting equipment. SwRI representatives must be aware of local events and customs, and often receive briefings from SwRI’s Security Department before departure. Significant lead time may be required in some cases to obtain the proper entry visas required to perform this type of work.

Accommodations can vary widely, particularly in remote or offshore locations. The work takes place in all types of climates and weather conditions, which take a toll on equipment and personnel. In addition, return travel plans are frequently changed as a result of the client’s needs and initial testing results.

SwRI recently received a call from a client in the Middle East, and a scientist was enroute the next day. The short- notice nature of this type of work places significant personal demands on the staff members involved. For example, maintaining continuity on projects under way in San Antonio can be challenging for those on extended foreign travel because of time zone differences.

Quick response — The key to avoiding significant damage

Although many projects have some level of lead time, clients appreciate the availability of an extremely rapid response if the situation warrants it.

Many failures experienced in the field are the result of high-cycle fatigue, and it is sometimes possible to avoid a failure if the problem is resolved quickly. This is particularly true with recently installed equipment. Damage can accumulate rapidly, and timely response is often the key to mitigation. Mechanical and acoustic resonances are common causes of excessive vibration, pulsation, strain and noise in the field. Shifting these resonances is a primary method for improving the longevity of the machinery and structures involved. Occasionally, this may be performed temporarily on site as a proof of concept. The data obtained during such a process may be used to justify developing long- term solutions.

Data captured during initial commissioning of new equipment can provide an important baseline for judging future behavior as components wear. Occasionally, behavior that appears to be suspect may be deemed tolerable with slight changes in the operating conditions. Personnel annoyance issues involving noise or vibration are also characterized, and methods to improve the behavior are developed.

Pipeline system pulsation control

The plant engineering group gathers dynamic pulsation data in piping systems at a multitude of operating conditions to ensure that pulsation envelopes are fully defined with respect to suction and discharge pressures, horsepower ranges and speed. Acoustic resonances are defined, and methods identified for shifting or otherwise controlling these resonances are developed. Traditional solutions include installing orifices, redesigning bottles and piping layout, and integrating side branch resonators. Several novel pulsation control concepts are also being investigated and tested in the field.

In addition to pulsation-driven vibration, another common cause of excessive vibration in piping systems is flow-induced turbulence. When necessary, the group gathers specialized pulsation data using insertion probes to fully document the cross-sectional dynamic characteristics for comparison with computational fluid dynamics predictions. Strouhal calculations — a method for determining vortex shedding frequencies in fluid systems — are conducted to determine prevalent excitation frequencies for known flow disturbance geometries.

Mechanical vibration issues

Common vibration issues involve mechanical resonances in skid, platform, pipe rack and cooler structures, inadequate piping supports, clamps, and wedges, manifold or piping span resonance, mechanical shell modes of piping, or lateral and torsional critical speeds in shafting. Typical data collection when a mechanical resonance is suspected of playing a major role in structural vibration (such as a large suction bottle or manifold attached to a reciprocating compressor) includes gathering operating deflection shapes. This type of study involves characterizing the amplitude and phase relationships among several vibration sensors on a structure to define the differential motion observed and any relationship to predicted mode shapes. This information is particularly useful for comparison with finite element analysis results.

Impact testing is often used to determine the mechanical natural frequency of structures, with the unit off- line and background noise minimized. This determines separation margins between excitation frequencies and the natural frequencies involved. It provides a measure of damping and allows for testing of changes to the support stiffness, which can cause a beneficial frequency shift of the modes involved.

Dynamic strain data is often gathered when vibration is considered marginal or when failures have occurred and the mechanisms involved must be identified. These data are analyzed to document complex wave strain values and frequency content to determine the excitation source and cumulative fatigue implications. Where thermal loads or other mechanisms induce significant mean stress, static strain levels may also be recorded. This type of measurement is important, for example, when an attached piping system heats significantly, expands and contributes to the nozzle loads being placed on a compressor.

Shifting a mechanical resonance is often accomplished by adding stiffness or mass, or by relocating or redesigning supports. Of particular importance is ensuring that acoustic and mechanical resonances do not coincide, which greatly increases the damage potential. In typical piping systems, valve operators and the attached tubing are often excited by prevalent pulsation or mechanically transmitted energy, requiring modifications of the support structure to avoid mechanical resonance. The addition of damping, for example by adding elastomeric material between piping and restraints, may also be tested in the field to determine effectiveness.

SwRI also has a rich history of confirming rotordynamic behavior in the field. SwRI teams have routinely documented and investigated lateral critical speeds, separation margins, damping levels, stability, bearing performance, casing vibration and balance behavior in the field for many types of machinery. The unusual thermal effects of turboexpanders, which can cause significant nozzle loads and casing movement during the operational envelope, and the resultant effects on vibration, have been studied on several occasions.

Torsional studies have also been conducted using strain gage telemetry systems and encoders to determine the steady state and transient stress levels present in machinery shafts during typical operations and to document critical speeds, damping levels and separation margins.

Noise surveys

Dynamic pulsation or vibration can create significant noise levels in machinery and the attached piping systems or structures. Sound pressure and sound intensity signatures are obtained and analyzed to diagnose the energy generation mechanisms or machine faults involved. The effects of the noise in terms of potential mechanical damage or personnel annoyance are examined. Sound suppression techniques, including silencers and acoustic piping insulation, along with methods to affect the generation mechanism of the noise, are typically evaluated in the field.

Performance diagnostics

The performance of reciprocating compressors has traditionally been documented by analyzing pressure-volume cards developed with SwRI proprietary software using dynamic pressure data obtained from cylinder taps. In addition to determining the power and efficiency of the compressor, the P-V cards may also be used to determine the presence of valve faults and acoustic resonance effects, among other phenomena. Rod load, web deflection and other specialized measurements are also available as needed.

Centrifugal compressor performance curve verification, surge-line determination, stall and stonewall investigations have been conducted for several clients. For centrifugal pumps, a common cause of operational problems involves cavitation, and SwRI has been involved in diagnosing this behavior on many occasions.

Performance assessments have been conducted for turbines using heat rate testing to determine engine efficiency. These data are typically used to ensure contractual compliance with energy production schedules and to develop effective maintenance strategies.

Conclusion

SwRI’s mechanical engineering teams have a wide variety of experience to apply in resolving critical machinery and piping dynamics issues. These skills are routinely used to provide timely and effective solutions to clients worldwide. The group also provides an outlet for field-testing of novel SwRI-developed concepts, and allowing for determination of future research needs for industry.

Questions about this article? Contact Hollingsworth at (210) 522-2537 or justin.hollingsworth@swri.org.

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Southwest Research Institute® (SwRI®), headquartered in San Antonio, Texas, is a multidisciplinary, independent, nonprofit, applied engineering and physical sciences research and development organization with 10 technical divisions.
04/15/14