Continuous Monitoring for Reciprocating Compressors

Traditional compressor monitoring has focused on velocity vibration and rod drop. Hardy provides two additional measurement techniques, Impact and Triangulated Rod Drop, that were developed especially for reciprocating compressors. These two new measurements monitor mechanical looseness and wear on compressor cylinders with a high degree of dependability. Combined with traditional condition monitoring, Hardy provides a complete condition monitoring solution for reciprocating compressors.


What to Monitor
When designing a monitoring system, it is important to have the end result clearly defined. For example, is the intent to have a safety shutdown monitoring system? Or is the purpose to trend the measurements to provide operating condition data that operators can use to make run/don't run decisions?


The table below (Typical Measurements) provides a summary of monitoring choices. It identifies measurement types and the applicable measured components .


Typical Measurements

Monitoring choices and corresponding measured components


Compressor Cylinders

Rod Drop

Compressor Rods


Casing Vibration

Fin Fans


Auxiliary Equipment

Impact or Velocity

Engine Main Bearings


Suction and Discharge Gas

Bearing Temperatures

Electric Motor Windings


Impact Monitoring

Detects mechanical looseness on compressor cylinders

Although reciprocating machinery produces vibration, it is normally not related to the health of the machine. However, looseness can be. Impact monitoring has proven itself to be a sound measurement for detecting early stages of mechanical looseness on compressor cylinders.


Hardy's DI 115 impact sensor is placed on the cross-head or extension piece. Mechanical conditions such as loose rod nuts, loose bolts, excessive slipper clearance, worn pins and liquid in the process can be detected and measured using impact monitoring. An example of an impact signal is shown below.


These short duration "peaks" or events represent impact which may be caused by mechanical looseness


The HI 2652 Impact Card not only detects short duration events, it qualifies them by confirming that they are repetitive before alarming.


Rod Drop Monitoring
Measures wear on rider bands, rings and seals


Rod Drop has been a popular measurement on reciprocating compressors. It is intended to measure the wear on rider bands, rings and seals. The measurement is made with a non-contacting displacement eddy probe.


Challenges in measuring rod drop

Two potential difficulties can make rod drop measurements less sensitive than desired in order to know the running condition of the rod. The first is simply that there is too much mechanical runout during one cycle which makes measuring changes of 3 or even 5 Mils very difficult. Also, if a rod is plated, the surface below the plating is usually irregular. Since the eddy probe penetrates below the surface, it electrically sees this irregular surface. This is called electrical runout and can mask attempts to make accurate displacement measurements.


The Hardy solution

Standard rod drop and rod runout measurements are made continuously during each complete operation cycle. In order to overcome the effects of mechanical and electrical runout, a displacement measurement must be "gated" on and off. By synchronizing this with a tachometer signal on the crank flywheel, the point on the rod where the measurement is made can be designated by the monitoring channel. This gives a basis for a consistent measurement.

This concept can be taken a step further using the HI 2658 Rod Drop Card by offering "triangulated measurement". In this mode, two readings are taken and a third measurement point is projected. The user can elect this third point to represent the rider band area and the monitor will indicate the displacement reading as if the sensor were inside the compressor cylinder and capable of measuring rider band wear directly. The monitoring channel offers other measurement options as well as default conditions to prevent false alarms if the key phasor signal is lost during operation.


Auxiliary Equipment Monitoring

Monitoring impact and rod drop are just the beginning of a complete Hardy compressor monitoring solution. It is easy to add channels to gather other valuable operating data on auxiliary equipment.


Generally velocity vibration is the best type of measurement for auxiliary equipment. The sensor is usually an accelerometer, but the signal is converted to velocity in the monitoring channel. The HI 2654 Accel/Velocity card allows the user the ability to select the bandwidth of the measurement frequency range. This allows a monitoring system to be tailored to the type of machinery and block characteristics that are known to exist but are not critical. The unwanted characteristics are not included in the measurement and will not be the basis for alarms.


The following table provides a summary of the types of measurements that are typically made on machinery and the types of sensors that are used. Notice that the type of sensor does not always determine the type of measurement.


Summary of Measurement Types

Type of Sensor

Type of Measurement


Acceleration, g's

Velocity, ips

Displacement, Mils p-p

Impact Sensor

Impact, g's

Velocity Pickup

Velocity, ips

Displacement, Mils p-p

Eddy Probe & Driver

Displacement, Mils p-p


If your compressor has a motor drive, add velocity measurement points at the coupled end and the outboard end of the motor. These should be in the horizontal direction. If you are operating integral gas fired engine compressors or separates, there are several points that should be monitored. Among these are cooling water fans, turbochargers, water pumps, and oil pumps. By mounting a sensor inside the engine on main bearing supports, detonation and mechanical looseness can be detected.


Measuring suction and discharge gas temperatures is also important. They reveal important cylinder and valve operating information. If your Control Processor is not already monitoring these temperatures, they can be added to the vibration and impact monitoring system. The HI 2664/65 Temperature monitoring system monitors absolute values, average temperatures, and differential temperatures, among a selected group.

Monitoring System Summary

The Hardy compressor monitoring system provides continuous monitoring of current operating conditions. It can be used for safety shutdown, or to notify operators of changing machinery operating conditions.

An important benefit of the operating data is the ability to trend measurements and predict operating conditions. While the current absolute reading is informative to a trained analyst with experience on a particular machine, a trend tells an easy-to-understand story of the changes in the machine over time. Internal trending of measured parameters is a standard feature in the HI 2601 monitoring system.


In summary, a minimum compressor monitoring system should include the following:

  • Suction and discharge valve temperature

  • Impact on each compressor cylinder

  • Casing vibration (velocity)

  • Rod drop on each compressor rod

  • Driver vibration (may vary with driver type)

  • Motor winding temperature (if applicable)

Gas fired engine monitoring systems should also include:

  • Turbocharger vibration

  • Main bearing vibration

  • Water pump vibration

  • Cooling fan vibration


HI 2601 Monitoring System Description by Model Number


HI 2601

Monitoring System, Built-in Display, NEMA 4 Style enclosure, includes Card Cage and Control Card

Must specify power supply and relay types

HI 2652

Impact Card, Dual channel, measures mechanical looseness, provides power for Impact sensors

HI 2654

Accel/Velocity Card, Dual channel, measures Acceleration, Velocity or Displacement, provides power for Accelerometer or Velocity sensors

Must specify Frequency and Gain/Integration header for each channel

HI 2656

Displacement Card, Dual channel, measures radial vibration or axial position (jumper configurable), provides -24 Vdc power for non-contacting displacement transducers

HI 2658

Rod Drop Card, Dual channel, measures "gated" rod drop for multiple measurements, provides -24 Vdc power for non-contacting displacement transducers, uses system tachometer for triggering

HI 2664

Temperature Card, 8 inputs plus Average and Delta, connects to HI 2665 Temperature Interface Module (TIM) via RS-422 link

HI 2665

Temperature Interface Module (TIM), Measures 8 or 16 RTD's or Thermocouples, separate stainless steel enclosure, includes 4-20 mA output per channel

Must specify power supply and 1 or 2 signal conditioning cards



Standard options for the monitoring system include; 4-20 mA output cards, BNC connector panel for analysis instrumentation attachment, hermetically sealed relays, and other relay options.


Hardy provides industrial grade sensors and mounting hardware

DI 100 Series Accelerometers and Impact sensors

DI 200 Series Eddy Probes, non-contacting displacement sensors

DI 5901 Series Probe Drivers


This article is provided by Hardy Instruments,

Click Here for More Information

Did you enjoy this great article?

Check out our free e-newsletters to read more great articles..