Proximity Probe “Rod Drop” Measurements for Reciprocating Compressors

Reciprocating Compressors are critical assets in many different industries including, oil/gas, energy production, petrochemical, refrigeration, and many others. They often serve a critical function in the industrial process and are an indispensable part of the production line. Due to the mechanical behavior of reciprocating compressors, they require condition monitoring systems to ensure efficient operation and to provide early fault indicators that can help prevent critical failures and unplanned downtime.

Unfortunately, in the real world, only a small percentage of compressors present in the industry has adequate monitoring systems, which is often the reason why reciprocating compressors have higher maintenance costs than similar-sized compressors. As the world begins to leverage the power and cost-savings made available by Industry 4.0 and predictive maintenance, asset managers will be driven to new sensor technology paired with innovative monitoring and control systems that will help revolutionize the compressor industry as end users begin to phase out “reactive and periodic” maintenance plans and replace them with more effective predictive and condition-based maintenance programs.

Compressors are machines that are primarily used to move air or gas at high pressure to be stored and/or used for different industrial purposes. A Reciprocating Compressor functions similarly to the engine in a car. These compressors comprise pistons that move inside cylinders. These pistons will suck in air/gas at low atmospheric pressure through the cylinder’s suction valve, then push the air/gas out of the cylinder at an increased pressure through the discharge valve where it can be stored or used for other industrial processes. These pistons are driven by a crankshaft via a connecting rod or piston rod. The crankshaft itself can be driven by an internal combustion engine or by an electric motor.

The piston rod is what drives the movement of the piston while it is moving through the highly pressurized cylinder chamber, the piston is held in position by components called Rider Bands. One of the most important mechanical indicators of machine condition to monitor for horizontal reciprocating compressors is the change in displacement of the piston rod relative to its housing over time. In the reliability industry, this is commonly referred to as “rod drop measuring.”

As these bands degrade, the position relative to the cylinder housing will “drop.” If we know the starting thickness of the rider bands, measuring this drop will give an accurate indication of their condition. The purpose of this measurement is to give the operator time to plan and schedule a shutdown of the machine before the piston comes in contact with its housing causing a catastrophic failure. This allows end users to operate their compressors as efficiently as possible, by maximizing the lifetime of their components and preventing costly damage and unplanned downtime.

Rider Bands (AKA wear bands) are a critical part of a reciprocating compressor; they carry the full weight of the piston while it is moving inside the cylinder. The Rider Band keeps the piston centered to prevent any metal-to-metal contact between the piston and the wall of its housing by carrying the full weight of the piston.

Rider Bands are designed to wear away over time at a very slow rate. However, imperfect conditions and certain mechanical faults can cause the band to deteriorate faster. It is very difficult to measure the vertical displacement of the piston since it is located within a highly pressurized cylinder. Therefore, measuring the change in the position of the piston’s rod is an important mechanical indicator of the condition of the compressor’s rider bands. These measurements are usually taken by Inductive Proximity Probe systems.

As the band wears down and degrades, the piston’s rod will drop further. A reliability professional can set alarm and shutdown levels based on the displacement of the rod from the original zero point. This will indicate to the operators when to schedule maintenance and when they need to shut down the compressor to prevent the damage that would occur if the piston were to make contact with its housing.

As seen in the figure above, the Proximity Probe that is monitoring the rod drop itself must be mounted perpendicular (90° angle) under or above the piston rod. Mounting the Probe above the piston rod is generally preferred as it reduces the chances of contact with the probe itself. (Additionally, a horizontal probe can be mounted 90° away from the vertical probe to measure rod flex).

A Proximity Probe System uses eddy current principles to measure the distance between the face of the probe tip and a conductive metal target object, in this case, the piston rod. As the rod moves toward or away from the sensor the change in relative position is indicated by a dynamic mV signal output from the Probe Driver (Proximitor) which can be displayed on a digital monitor or processed by a DAQ system for recording and trending the rod’s position over time.

Since the piston cylinder is a highly-pressurized environment, it is very difficult to make direct measurements on the piston. Operators should instead monitor the changes in rod position and use those measurements to extrapolate the absolute position of the piston itself. By using the principles of similar triangles, we can assume that the change in the position of the rod is directly proportional to the relative displacement of the piston due to the degradation of the rider bands.

In addition to the Proximity Probe System monitoring displacement, a second proximity probe should be installed at the drive end of the piston to measure the speed and phase of the rotating element. This allows the displacement reading to be taken at the same time and position in the piston rod cycle, which is necessary to be able to accurately trend data over time due to the deflection at various rod cycle stages.

More primitive methods of rod drop monitoring use mechanical systems. These mechanical-based systems typically utilize a mechanism that is placed directly below the piston rod. As the piston’s rider bands wear and the rod begins to drop within its housing it will eventually make contact with this mechanism. When the mechanism comes into contact with the piston rod, it will release an amount of pressurized nitrogen gas. The escaping nitrogen will cause a decrease in supply pressure, which then can trigger an alarm or shutdown.

The main disadvantage of using a mechanical system to protect a compressor’s pistons is that it lacks the ability to perform actual measurements. Mechanical systems can only trigger an alarm right before a catastrophic failure occurs; they are unable to provide any early indications or measurements that can be trended over time.

In the current age of rapid industrial innovation, being able to provide analytics and data on machinery is becoming more and more important. Hyper-competitive market environments and macroeconomic pressures are forcing companies to extract more value out of their assets than ever before. Operators who do not take advantage of these new technologies and processes will have an increasingly difficult time keeping up with their peers who are leveraging them efficiently.

The benefits of Condition-Based Maintenance and Predictive Maintenance have proven to be much more effective and lucrative than reactive and periodic maintenance programs. Having the right hardware and reliability processes in place has never been more important.