Reciprocating compressors are positive displacement machines in which the compressing and displacing element is a piston having a reciprocating motion within a cylinder. There are two types of reciprocating compressors- one type requires a combustion gas engine or an AC motor ( high speed separable) which may be connected to a gear box to drive the coupled compressor. Operating speed is typically between 900 and 1,800 rpm.   Separable units can have single or multiple stages. They can be skid mounted, easy to install, and can be easily moved to onshore and offshore locations.

The other type has internal power cylinders (low speed integral) that drive the compressor. These units run at speeds between 200 and 600 rpm. They are commonly used in gas plants and pipeline service where fuel efficiency and long life are critical.   Integral compressors may be equipped with two to ten compressor cylinders.

Many plants rely heavily on mechanical maintenance personnel to keep the reciprocating compressors running, but periodic machine monitoring with vibration data collectors is not the best way to evaluate an important machine because critical vibration information can be missed.

It is ultimately up to the manufacturer or plant owner to decide what levels of vibration monitoring is adequate for protecting and monitoring their plant equipment assets. The table below is intended to point out the commonly used vibration sensor types and their application disadvantages for the protection of plant equipment. The table is for informational purposes only and the empirical evaluation of your vibration device and application is recommended.

Mechanical Vibration Switches

Manufacturers generally do not list the accuracy or repeatability of their mechanical vibration device.   It is important to note that this device has no useful signal outputs, no trending capabilities, no analysis capabilities for condition monitoring, and no advance warnings for a deteriorating machine.

In 1995, API 618, 4th edition, the standard for reciprocating compressors wrote that mechanical vibration switches are “unacceptable” to be specified as a continuous vibration device.  To this date, due to the unreliability of the mechanical vibration switch, they are not specified in any American standard publication for any type of process equipment or machinery.

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Operators prefer to test the functionality of the mechanical switch while it is still mounted on the machine.  The typical approach is to impact the machine casing at or around the location of the switch with a hammer. The operator then repeats this procedure until the switch trips.

Unfortunately, repeating this test over time can dislodge the internal adjustable spring, armature, or some mechanical part or component. The switch may work partially, but, the switch is basically unreliable.

Another possible reason that the switch did not trip initially to the hammer impact is because the internal mechanical components may have become corroded due to ingress of moisture or ambient corrosives.   Therefore, with corroded internal components, lowering the g level setpoint for test purposes will not allow your mechanical switch to trip.

Basically, there are many non-functional mechanical switches in the field and your plant equipment assets are not properly protected against excessive machine forces.

Two-Wire Loop Powered Vibration Transmitters

Traditional 2-wire loop powered vibration transmitters utilize RMS detection circuitry which will not detect any mechanical looseness problem because the impacts do not affect the overall level of the signal output.

Once installed the performance of this device can be verified, but, in the field or at the factory, they cannot be recalibrated. This electronic device is simply a pass/fail and disposable unit. Further, two-wire loop powered devices are especially vulnerable to direct two-way radio interference created in many remote process plants.

There are no fault protocols for problem transmitters, so, discrete fault level cannot be set to 0mA, DC.  This configuration is not suitable for control rooms that require a 0ma, DC level for a fault indication.

Two-Wire Loop Powered Impact Transmitters

The 1st generation impact transmitter has been around for many years. Because there is only one adjustable threshold level, only the impact pk level above the threshold level is detected and counted within a preset time frame. This indicates that all the impact pk levels below the threshold level are not detected and cannot be trended for condition machine analysis.  For example: Assume the adjustable threshold level is set to 6 g, and the impact levels within a specified window are between 2 to 4 g’s.   These low level impact peaks will not be detected nor trended for analysis by reliability professionals who could use this information to assess the deteriorating condition of the reciprocating machine.

Electronic Vibration Switch with Built-in Sensor

Traditional electronic vibration switches w/ internal sensor utilize RMS detection circuitry which will not detect any mechanical looseness problem because the impacts do not affect the overall level of the signal output.

Internal electronic components have a limited temperature range which restricts an electronic vibration switch from hot machine casing temperatures, e.g., combustion gas engine.

Built-in sensor may have a wide band range, but, the actual system frequency response is controlled by the signal conditioning electronics. This affects the ability to accurately measure low speed vibration measurements less than 120 RPM (2 Hz), e.g., low speed process equipment.  Further, for displacement measurements (mils, pk-pk), there is the added measurement errors created during signal conditioning by double integrating  the vibration signal from acceleration to displacement at speeds below 600 RPM (10 Hz).

Many manufacturers assemble their product without encapsulating the internal electronics with epoxy or alternatively, by using resin based conformal coated printed circuit boards. While the real intent for the epoxy or coating is to protect the electronics from moisture and corrosive environments; there is the added benefit of securing the electronic and mechanical components from the destructive forces encountered during the product’s life cycle.

Reciprocating Compressor – Vibration Sensors

In the past, operators have used mechanical and electronic vibration switches for continuous vibration monitoring.  The driver and driven components are accessible, but, legacy vibration devices are only configured as single axis devices that are less likely to detect mechanical looseness, valve closure, or impeding roller bearing failures.  Today, with the latest innovations from Machine Saver, Inc., a new technology can be coupled to a practical approach to reliably detect, monitor, analyze, and protect reciprocating machine assets with a our state of the art digital transmitters with integral cable assemblies, minimal instrumentation wiring, and an industrial computer configured with impact severity and vibration monitoring software. Any instrumentation wiring from the industrial computer to the control room can be avoided by simply using the wireless option.

Advantages of Wireless Vibration Sensor Monitoring

Wireless vibration sensor monitoring can provide early alarms for remote locations so that effective measures and machine reliability can be improved. One advantage of wireless vibration monitoring is that it uses the latest technologies from Machine Saver, Inc., to move vibration and temperature data rather than people. Online vibration analysis can be obtained at any time from any location, thereby minimizing installation related costs.

Trended overall vibration levels can be kept on the cloud for future reference. Early alarms can be set up to provide sufficient time for management to plan for the scheduling and purchasing of parts and tower downtime. Another advantage is the accessibility of many digital protocols and the ease to make our vibration monitoring system all wireless with any third party wireless transmitter and receiver system.

TriVibe-Sensor

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TRIVIBE-Sensor is 3-axis (X,Y, and Z) digital and temperature transmitter with integral 9 foot (3 meter) cable can detect rotational and structural problems (low frequency), e.g., imbalance, misalignment, defective  rolling bearing problems (high frequency) in their early stages.  The sensor can simultaneously detect in three measurement planes (X,Y, and Z) and in all three vibration measurands- acceleration, velocity, and displacement. The embedded temperature sensor has a service range of -40°F to 221°F (-40°C to 105°C). By integrating 3-axis vibration detection and temperature into one digital transmitter, one transmitter can take the place of seven sensors. Figure 1. illustrates the vibration measurement planes detected by TRIVIBE-Sensor.

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Machine Gate System/Condition Machine Monitor

By using TRIVIBE-Sensor and the Machine Gate, sensor computer together, Machine Gate and Web Portal becomes a cost-effective online, vibration system that can be easily installed to detect and protect reciprocating compressors. By integrating TRIVIBE-Sensor, and the Machine Gate, as wellas Machine saver’s Web Portal machine condition monitoring software, an enhanced Machine Gate and Web Portal can take a snapshot of the dynamic signals and to upload the dynamic vibration and temperature data to the cloud where its is automatically analyzed to detect trends and predict future problems with reciprocating equipment assets.

More plant maintenance departments, instrumentation personnel and reliability managers are adding TRIVIBE-Sensor to their machines. It no longer requires a capital expenditure since you can add the TRIVIBE-Sensors one at a time.  You only need to run the communication link back to the nearest and initial TRIVIBE-Sensor. With so many digital protocols available on the Machine Gate and Web Portal system, it is also very easy to make this Machine Gate and Web Portal system all wireless with any third party wireless transmitter and receiver system so it lends itself well to integration with the very latest technology.

Machine Gate and Web Portal system advantages: (1) Low cost; (2) Easy installation; (3) Less parts to fail; (4) Less wire to install; (5) wireless option;  (5) Smaller footprint; (6) More information from one sensor; (7) Lower cost on the computer I/O module; (8) Unique universal sensor mounting feature; (9) Dynamic capture and signal analysis; (10) Zone 1 / Div. 2 certified or only one intrinsic safety barrier is required for many of TRIVIBE-Sensors in Zone 0 / Div. 1 hazardous areas; (11) Dependable with a limited warranty

Practical Vibration Monitoring Guidelines

Machine Saver, Inc., is pleased to provide a new technology that can be coupled to a best practice procedure that can be used for all your process plant vibration monitoring.

In the past, a portable vibration meter was used to determine the highest vibration plane on a machine. Then the permanent vibration sensors were placed on the vertical or horizontal axis that was most sensitive to a machine’s vibrations.

TRIVIBE-Sensor, 3 –Axis Digital Transmitter takes the guess work out of the mounting location and can simultaneously detect in three measurement planes (X,Y, and Z) and in all three vibration measurands- acceleration, velocity, and displacement. The embedded temperature sensor has a service range of -40°F to 221°F (-40°C to 105°C). By integrating 3-axis vibration detection and temperature into one digital transmitter, one transmitter can take the place of seven sensors.

For small reciprocating compressor applications, mount one (1) TRIVIBE-Sensor on machine driver (inboard) and one (1) TRIVIBE-Sensor on the driven machine (inboard) per the following guidelines:

  1. Decide to install what is required instead of installing what is simple or convenient.  As a protection device, a mechanical switch is unreliable and has no useful signal outputs, no trending capabilities, no analysis capabilities for condition monitoring, and no advance warnings for a deteriorating machine.  Some companies have a machine condition monitoring system and they use a mechanical switch for machine protection in case there is a brown out period or a power outage.  TRIVIBE-Net will accurately perform as a condition machine monitor and for machine protection by utilizing a separately priced external relay board to shutdown you machine asset.  To insure continuous vibration monitoring and protection, simply utilize an Uninterruptible Power Supply (UPS) set to a nominal +24 VDC to power the TRIVIBE-Com, Sensor Communication Computer and daisy chained TRIVIBE-Sensors.
  2. Obtain technical expertise from Machine Savers, Inc., with applications involving the use of band pass filters for the diagnosis of machine vibration levels;  or, the use of vibration devices in a hazardous locations or corrosive environments, e.g., sour gas, salt-spray, high or low pH levels.
  3. Verify that the machine rotating shafts are supported by rolling element bearings. TRIVIBE-Sensor is a digital sensor that was designed to detect and monitor low frequency vibrations, e.g., imbalance, misalignment, high frequency roller bearing condition, and temperature.  Mount the TRIVIBE-Sensor in the radial position at or around the roller bearing cap. Do not mount the digital transmitter on a flimsy bracket, or the sheet metal part of the machine.
  4. Reciprocating compressors (air) may have a mix of sleeve bearings and roller bearings.  Simply mount the TRIVIBE-Sensor as close as possible to the roller bearings.
  5. Review the machine’s maintenance records. Mount the TRIVIBE-Sensor as close as possible to source of vibration. Based on the records, this area will have the most wear and is most likely to have problems.
  6. On less expensive motors, reciprocating compressors, consider a budgetary approach and mount at least one TRIVIBE-Sensor per set. Place the TRIVIBE-Sensor where it will monitor the most- on the inboard section of the driven machine. This area will have the most wear, and is most likely to have problems.
  7. Understand what trending the overall vibration levels means.  Overall vibration is the total vibration energy measured within a wide frequency range. Overtime, a higher than normal overall vibration level indicates that some force is causing the machine to vibrate more.  As you increase the speed of the machine that vibration energy becomes more destructive.   The enhanced overall vibration levels provided by TRIVIBE-Sensor will indicate continuously what your overall vibration levels are and this provides time for operators to create a standard baseline vibration level for each machine asset. Once the baseline vibration levels are reached, you can plan and schedule an inspection of the machine components and the roller bearings.
  8. The TRIVIBE-Com can be mounted inside a NEMA 4X, IP65, agency approved enclosure (UL, CSA, IEC, ATEX) with a built-in window.  To avoid any overheating concerns, verify that there is at least a two inch space from the edge of the TRIVIBE-Com and all four sides of the enclosure. The enclosure must be resistant to ambient air corrosives, e.g., hydrogen sulfide (H2S), and hazardous gases. Shelter the enclosure from direct sunlight and verify that the input/output glands, strain reliefs, and instrumentation wiring are rated for the applicable hazardous location. To recommend the best enclosure, glands, strain reliefs for mounting the TRIVIBE-Com, please contact our team to assist you.    
  9. The recommended wiring format is 18 AWG/120 ohm, 4 conductor and shielded twisted pair cable which is only available from Machine Saver, Inc.

Small Reciprocating Compressors

The easiest way to obtain the base line vibration for your machine is to simply measure the vibration levels over a wide frequency range. The TRIVIBE-Sensor must be mounted on the bearing housing or as close as possible to the roller bearings. The vibration measurements can be trended over time and compared with know levels of vibration or alarm and shutdown set points can be set due to changes in the condition of the machine.

Analysis of trended vibration levels combined with experience and familiarity with the machine is essential to monitor the status of your machine. In addition to vibration measurements, temperature is an important parameter for providing information on bearing stress and machine operating conditions. Analysis of vibration and temperature together provides condition monitoring where the condition of the machine is monitored for early signs of deterioration.   The table below provides some common machine vibration and temperature faults for small reciprocating compressors.

Conclusion

With a combination of best practice techniques, correct setting of vibration and temperature alarm settings, and interpretation of vibration spectra, your reciprocating machine assets can be protected against mechanical looseness, increasing frame vibration and elevated valve temperatures.   These machine faults can cause complete machine failure which cause plant processes to stop running.

This technical note has practical suggestions to assist you in your vibration monitoring and protection application.  While our product will not detect every vibration and temperature fault, we understand what others don’t- that every application requires essential machinery vibration expertise and involvement so that we can provide a customer focused solution to your vibration monitoring requirements. We want to support you with a reliable vibration and temperature product that successfully and consistently detects, monitors, analyzes, and protects your equipment investment. Let us know about your application by consulting with the Machine Saver team at sales@machinesaver.net  Our team can provide vibration monitoring solutions and benefits for your present application and extend their vibration expertise and new technology to your entire balance of plant. Product and application information is available at www.machinesaver.com.

References

  1. A. Almasi, “Reciprocating Compressor Optimum Design and Manufacturing with respect to Performance, Reliability, and Cost”, 2009, World Academy of Science, Engineering and Technology.
  2. Bryan R. Long, Ph.D., P. Eng., and David N. Schuh, P. Eng., “Fault Detection and Diagnosis in Reciprocating Machinery”, provided by a machinery consultant company- Beta Machinery Analysis.
  3. Bruce M. Basaraba and James A. Archer, “IPT’s Rotating Equipment Handbook”, 2000, Quebecor Jasper Printing
  4. Claire Soares, “Process Engineering Equipment Handbook”, 2002, McGraw-Hill Handbooks.
  5. European Forum for Reciprocating Compressors, “Guidelines for Vibrations in Reciprocating Compressor Systems”, Third Edition – May 2012.
  6. James E. Berry, P.E., “Vibration Diagnostic Handbook”, 2004, Technical Associates of Charlotte P.C.
  7. Ralph T. Buscarello, “Practical Solutions to Machinery and Maintenance Vibration Problems”, 1991, Update International, Inc.
  8. Steven M. Schultheis, Charles A. Lickteig, and Robert Parchewsky, “Reciprocating Compressor Condition Monitoring”, 2007, Proceedings of the 36th Turbomachinery Symposium

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TriVibe – Low Cost, Practical Approach to Vibration Monitoring for Reciprocating Compressors | Machine Saver – Houston, TX