Monitoring rider band wear

RECIPROCATING COMPRESSORS are critical assets in a wide range of industries such as (petro)chemical, power and oil and gas. Reciprocating compressors are, compared to comparable machinery, very maintenance-demanding. During the reciprocating motion of these machines the piston moves through the compressor cylinder, supported by rider bands.

Rider band wear

A rider band is a critical component that supports the piston while it is moving through the cylinder chamber and prevents it from damaging the cylinder wall. The rider bands make sure there is no metal-to-metal contact between the piston and the cylinder wall. Due to large mechanical forces in their construction and high stress factors, these rider bands wear out over time. Depending on the application this may take weeks or months, but is in any case inevitable.

By monitoring the piston rod displacement (rod drop), it can be determined when the rider bands need to be replaced before damage to the piston and/or cylinder wall occurs. Rod drop monitoring contributes to a condition based maintenance (CBM) or predictive maintenance strategy.

Condition based maintenance (CBM)

Monitoring rider band wear allows maintenance managers to efficiently schedule the replacement of these rider bands during a turnaround/standstill that was already scheduled, instead of having to shut down the machine just to manually inspect or preventively replace the rider bands. Moreover, early detection of rider band wear provides the opportunity to shut down the machine before excessive damage and machine failures occur.

As traditional monitoring methods such as vibration measurements prove to be insufficient to accurately monitor rider band wear, different methods were developed. Rod drop monitoring is the most reliable way to accurately monitor rider band wear. Yet, many companies do not have the means to accurately monitor rider band wear, but rely on periodic manual inspections to assess whether rider bands need replacing.

Rod drop / displacement

Rod drop or rod displacement is the vertical displacement of the piston rod inside a reciprocating compressor. During the wearing process of the rider bands that support the piston inside the cylinder the piston gradually displaces vertically. As the piston is connected to the piston rod, it subsequently causes a vertical displacement of the piston rod, which is known as rod drop.   
To detect wear on rider bands the position (relative to the reference point) of the piston needs to be monitored. However, measuring the actual drop of the piston is difficult due to high pressure in the cylinder chamber. Instead of measuring the actual drop of the piston the drop of the piston rod is measured, which is more accessible. Rod drop monitoring provides information about the condition of the rider bands by monitoring changes in the position of the piston rod.

Rod drop measurement methods

Generally, two primary methods are used for rod drop monitoring:

1. Digital measurements with proximity probes

When using proximity measurements the sensor is mounted vertically above or below the piston rod, as close as possible to the cylinder. A proximity probe works by using eddy current technology, and measures the distance between the sensor tip and a metal object. In the case of a rod drop measurement it measures the distance to the piston rod. When the piston rod displaces vertically, relative to the sensor, the change in distance is detected. These measurements make it possible to determine the vertical displacement of the piston rod. This signal indicates rider band wear and which is used to effectively schedule maintenance.

Phase triggered measurements – A phase trigger or phase reference sensor is used to filter the displacement signal to the desired phase angle for the rod drop measurement. A phase trigger is a proximity sensor that is triggered at the specific position in the cycle; a specific point during the rotating of the shaft. This significantly increases the accuracy of the rod drop measurement, as the measurement point of the cycle influences the position of the piston rod and therefore the output of the measurement. Consistently measuring the position of the piston rod at the same point of the cycle is more accurate than any overall measurement. For accurate and reliable measurements a phase trigger is essential. 

2. Mechanical measurements

Generally, two mechanical methods can be distinguished for rod drop monitoring:

- The first method uses a block of abrasive material with a space inside that contains nitrogen. This block is placed directly under the piston rod. When the piston rod drops as a result of wear on the rider band, the block will wear down until a point where nitrogen escapes. This nitrogen leak will cause a decrease in the pressure supply, leading to an alarm or machine shutdown.

- The second mechanical method for rod drop monitoring involves a type of roller that is mounted under the piston rod. When the rider band wears down, the piston rod drops gradually till a point where it touches the roller. Upon contact between the piston rod and the roller, the roller will start to rotate which allows nitrogen to escape from a reservoir below. This nitrogen leak will cause a decrease in the pressure supply, leading to an alarm or machine shutdown.

Digital versus mechanical measurements

The main disadvantage of mechanical rod drop measurements is the inability to do measurements, and use these measurements for trending. The only thing these mechanical systems provide is an alarm which is triggered when the rider bands are worn out, just to prevent damage. There is no indication of the extent to which the rider band is worn out. A digital measurement can trend the actual displacement of the piston rod, and therefore the extent of the rider band wear. Using the proximity sensor measurements a trend can be monitored over time, allowing for better maintenance planning (CBM).

Rod drop monitoring solutions

There are various solutions to monitoring rod drop and, subsequently, the wear of rider bands. However these solution are often part of complex and expensive monitoring suites which include a wide range of parameters. A solution solely dedicated to rod drop is rather unique. The few solutions that do, often provide overall measurements without the use of a phase trigger.

RecipSys 200

One of the solutions that is dedicated to rod drop monitoring and uses a phase trigger sensor is the RecipSys 200. This system is specifically designed to provide dedicated rod drop monitoring using a phase reference sensor. Its transmitter-based architecture directly opposes complex and expensive rack-based systems. The system can be used both stand-alone for smaller applications and with multiple units connected for larger applications. Modularity allows to integration with systems that monitor other critical parameters, such as vibration.

https://www.istec.com/en/product/recipsys-200/