Since the 1970s, many oil and gas plants have been built with a large fleet of reciprocating compressors installed. These types of compressors have not received the same high-priority monitoring given to centrifugal machines. The reasons for this are partially based on the higher number of centrifugal and axial machines in comparison to reciprocating compressors.
Operators also have not encountered severe damages due to the lower kinetic energy of these relatively slow-running machines. Recently, however, reciprocating compressors are experiencing more failures while still being process-critical.
At all times, operators, engineering procurement and construction companies (EPCs) and original equipment manufacturers (OEMs) have followed the existing, applicable guidelines and standards during the final engineering stage. Those standards were effective at the time of construction and to a large extent are still valid today.
However, upon studying the age of the reciprocating compressor population, one will recognize that, in many cases, these large, critical machines have never been replaced and have been in operation since their initial startup date decades ago. To understand why, even after several catastrophic failures, you still find inadequate machinery protection and monitoring on most of these machines, a little history of the applicable standards may help.
From the first edition published in 1976 to the fourth edition in 2000, the API 670 standard only focused on the technical requirements of centrifugal and axial compressors. Only in November 2014 when the fifth edition of API 670 was released were special guidelines included for the monitoring and protection of reciprocating compressors. Therefore, there is now an emergent need for new and existing compressors in the plant to comply with these requirements and fill the gap to increase availability, detectability and monitoring of these types of compressors.
Reciprocating Compressors in Industry
Reciprocating compressors are some of the most critical assets in a plant. These types of machines can provide higher compression ratios than similar axial or centrifugal compressors. Reciprocating compressors have been in service within industry for decades and are among the oldest compressor designs. They play an essential role in numerous industrial processes, including oil and gas production, refining, enhanced oil recovery, corrosive gas production and cryogenic applications.
Reciprocating compressors in these industries often have different characteristics such as unique construction materials, lubrication system designs, driver unit types, capacity control systems and process features. The most interest in condition monitoring solutions is in the downstream segment. In these processes, profit is linked directly to reciprocating compressor availability, so plants often categorize these compressors as critical machines.
Compressors designed for service in refineries are typically built to the API 618 standard. Refineries use reciprocating compressors primarily for two purposes: In hydrocracker units, process gas hydrogen is utilized, which is a low molecular weight gas that can be compressed more efficiently by a reciprocating compressor. In flare gas and vapor recovery services, a wide variety of molecular weights are encountered, which a reciprocating compressor can handle better compared to centrifugal compressors.
Challenges in Monitoring Reciprocating Compressors
In spite of the criticality and importance of reciprocating compressors, they are sometimes unobserved by condition monitoring specialists. A portable vibration analyzer, which is routinely used on rotating equipment, is not well suited for reciprocating machines and has been unsuccessfully monitoring reciprocating compressors for years. Therefore, overall machine health is frequently ignored and not diagnosed correctly until damage occurs. Then, it may be too late to save the machine from failure.
Portable vibration analyzers can be useful for non-critical machines that have long periods between failures. Online monitors are most beneficial for critical machines and units with a short point-to-failure time frame. These traditional vibration analyzers are suitable for early fault detection of rotating (centrifugal and axial) machines but have limitations with reciprocating compressors.
Reciprocating machines generate uneven forces, and the process normally is intermittent. Therefore, the overall vibration amplitude is relatively unaffected and provides little warning of serious faults while serious damage may exist. In addition, impact caused by mechanical looseness produces very short duration waves and high amplitude spikes, which require continuous monitoring (online systems) on crossheads for early detection.
Common valve problems such as sticky valves (delayed opening impact) and lost valve springs (earlier opening impacts) demand a crank-angle tracking system for fault detection. Also, wear in compressor piston rings and rider bands, gas condensation and other operational malfunctions can be difficult to detect by traditional analyzers, calling for special continuous monitoring using internal sensors.
Unique Forces and Loads in Reciprocating Compressors
When comparing the working principle of a reciprocating compressor with a centrifugal machine, it becomes apparent that reciprocating compressors require a more dedicated monitoring approach designed to handle all the special challenges these machines bear.
The normal operation of a reciprocating compressor reveals a very different behavior. Pistons are driven back and forth by a crosshead drivetrain involving the reversal of piston rod forces from tension to compression and vice versa, which causes the entire frame with all its components to shake and bend. Suction and discharge valves create opening and closing impacts, producing vibration on the entire machine.
In examining the working of reciprocating machines, the crosshead is clearly the focal point where the rotating movement of the crankshaft is transformed into a reciprocating (linear) movement of the piston rod. It’s the central component where all major forces are transferred via a sophisticated crosshead to the piston rod.
Reciprocating forces are typically much larger than any rotational forces associated directly with the crankshaft (misalignment, rubs, unbalance, etc.). These reciprocating forces are transmitted through the piston rod. Equal and opposite reaction forces are transmitted through the distance piece back to the frame.
When you evaluate all the components that transmit reciprocating forces in the compressor, the crosshead pin tends to be the weakest link. Forces on the crosshead pin can be affected by the gas load, inertial load and combined load.
These are just some of the components that are susceptible to abnormal motion when undesirable conditions exist. Normal component wear is a large contributor to excessive clearance, so abnormal component motion may be expected to become more severe over time.
Since abnormal motion contributes to poor performance and accelerated component wear, there can often be an increased rate of wear once the components have started to degrade. This affects the slope of the P-F curve as time goes by.
High maintenance costs are associated with reciprocating compressors and generally are attributed to expensive components such as valves. However, outage costs can easily exceed the maintenance costs to replace a valve. While a valve in an early stage of failure may not have a significant effect on the machine’s condition, advanced valve failure can lead to excessive loading and machine wear.
With an advanced compressor monitoring system, it is possible to evaluate possible safety concerns, assess the effect of the valve leak on the machine’s condition and make an economic evaluation of the cost of operating with faulty components.
Although trending vibration measurements is an excellent tool for monitoring the health of rotating machinery, it generally is not effective for monitoring reciprocating machinery because common faults present themselves as impacts with little effect on the overall vibration level. As a result, abnormalities are not diagnosed until damage has occurred and it is too late to take simple corrective measures. Several reciprocating machinery faults do not significantly increase a machine’s overall vibration level until damage has reached a severe level.
The main intent of installing an online monitoring system is to ensure the safety of personnel, assets and environment. It also provides critical data that operators and engineers can use to assess the equipment’s condition, determine machine availability and make informed decisions. In some cases, this data may even be used to maximize operating efficiency and machine health.
Common compressor problems produce recognizable vibration symptoms. By using appropriate monitoring techniques, you can gain a much better understanding of the compressor’s condition.
In reciprocating machinery, accelerometers, due to their higher frequency response characteristics, should be used to measure impact and gas leak malfunctions which have high-frequency signals. Velocity transducers, with their lower frequency response characteristics, are intended to measure rotation-related malfunctions, such as overloading, foundation degradation or unbalanced forces. These two transducer types work together to provide a reasonable picture of the machine’s condition.
These measurements can be useful for evaluating the forces caused by piston rod loading (gas and inertial forces) and moment forces imparted to the main bearings by the crankshaft because of its asymmetric design. Velocity sensors can be installed on the frame to measure overall machine vibration. Recommended transducer locations are aligned with frame webs to efficiently capture the transmitted vibration from the main bearings.
The magnitude for a frame velocity signal is usually low (less than 0.3 inches per second). However, at low frequencies, even small amplitudes of measured velocity correspond to large amounts of displacement.
These measurements can be used to detect load reversal and valve events. Acceleration sensors are recommended to be installed on each crosshead to monitor compressor impulse events. Proper surface preparation is critical for accurate vibration measurements.
As parts wear over time, the mechanical clearances increase. The extra clearance gives the parts more distance to gain velocity, and the impacts between the components become more energetic.
The mechanical impacts produce high-frequency structural vibration. The frequency band of 3 hertz to 2 kilohertz depicts mechanical looseness, and this signal is used for protection. Higher frequency components up to 30 kilohertz carry energy from impacts associated with valve operation and gas leaks. This unfiltered waveform is used to capture malfunctions mainly related to valves.
This vibration measurement deals with valve operation and movement, acoustic effects and the analysis of various leak scenarios. Acceleration sensors can be installed on the cylinder to detect impulse events associated with the compressor cylinder valves. It is especially important to ensure that recommended sensor mounting practices are used for accelerometers.
Crank Angle Measurements
A traditional phase reference signal (used for vibration analysis) is triggered by a detectable feature that provides one signal pulse per rotation of the machine rotor shaft. Sensors that are commonly used for this purpose include eddy current displacement, inductive (magnetic) and optical transducers.
The reciprocating machine crank angle signal should be triggered by a specially designed multi-event wheel. This wheel incorporates a once-per-turn reference pulse (triggered by the double notch developed by the indexing tooth) along with multiple synchronizing pulses that enable correcting the crank-angle measurement for torsional error during every rotation of the crankshaft.
The multiple notches on the special recip multi-event wheel allow the data-collection process to be re-synchronized 12 times per crank rotation (every 30 degrees). The divided notch is used to identify the top dead center location of the primary reference throw. Correlating different measurements referenced to the crank angle enables a more thorough diagnosis of observed symptoms.
Both the rider bands and piston rings contact the cylinder liner. However, the rider bands are designed to carry the full weight of the piston, while the piston rings are intended only to provide sealing between the piston and bore. The piston rings and their mounting grooves are purposely engineered with clearance to allow the rings to “float” without carrying any of the vertical load of the piston’s weight. The rider bands are intentionally designed to seat fully within their mounting grooves so they can support the piston’s weight.
Rider bands are constructed to slowly wear away over time. In good operating conditions, the rate of wear can be very low, especially in lubricated cylinder applications. However, adverse conditions such as improper cylinder lubrication, component misalignment, or abrasive particles or moisture in the process gas can increase the wear rate.
This measurement focuses on the DC voltage signal from the proximity probes and calculates a displacement value inside the cylinder using the principle of similar triangles. Trending this displacement measurement allows one to get an indication of potential rider band wear over time. Some operating conditions may make the rod drop measurement unreliable, so care must be taken before applying this monitoring method.
It is important that the proximity probe is calibrated for the actual piston rod that it will be measuring. The standard scale factor is 200 millivolts per millimeter, but several factors may cause the as-installed scale factor to be different. These factors include composition of the piston rod casting alloy, surface treatments, coatings or contamination, as well as the roundness of the cylindrical rod.
The main purpose of a rod position monitor is to constantly measure both the vertical and horizontal position of the piston rod during operation and to monitor rod flexing in both axes. Knowing the crosshead clearance and the maximum movement expected at the piston, you can find the maximum displacement expected at the pressure-packing case proximity probe and can create alarm setpoints accordingly.
The probe position changes continuously as the piston travels through its stroke. Peak-to-peak vibration levels in both planes can provide indications of potential problems, such as impending rod breakage or damage from rubbing on pressure packings.
Investing in Monitoring
Advancements in technology and enhancements in industry standards and best practices are encouraging top management to invest in close monitoring of their existing reciprocating compressors and implement new monitoring techniques. Start by asking your condition monitoring and reliability teams if they are as confident with their current systems for monitoring reciprocating compressors as they are with their centrifugal ones. You may be shocked that your recips are not detected and monitored well.
Steven M. Schultheis, Charles A. Lickteig, and Robert Parchewsky. (2007). “Reciprocating Compressor Condition Monitoring.” Proceedings of the 36th TurboMachinery Symposium
Bryan R. Long and David N. Schuh. “Fault Detection and Diagnosis in Reciprocating Machinery.” Provided by Beta Machinery Analysis
European Forum for Reciprocating Compressors. (2012). “Guidelines for Vibrations in Reciprocating Compressor Systems.” Third Edition
Steven M. Schultheis, Charles A. Lickteig and Robert Partchewsky. (2007). “Reciprocating Compressor Condition Monitoring.” 36th TurboMachinery Symposium
G. Zusman, J. Palm. (2002). “Impact Measurement for Reciprocating Compressors.” Vibration Institute 26th Annual Meeting
“Reciprocating Compressor Operation and Maintenance.” Heinz P. Bloch and John Johoefner
API 670 5th Edition. (2014). Machinery Protection Systems. Annex P.
API 618 5th Edition. Reciprocating Compressors for Petroleum, Chemical, and Gas Industry Services
Compressor Handbook (McGraw-Hill Handbooks) 1st Edition by Paul Hanlon
Troubleshooting Rotating Machinery: Including Centrifugal Pumps and Compressors, Reciprocating Pumps and Compressors, Fans, Steam Turbines, Electric Motors, and More. 1st Edition
This article was previously published in the Reliable Plant 2019 Conference Proceedings.
Written By Amir Basyouni and Stephen Plaisance