Effective Vibration Monitoring For Reciprocating Compressors

Modern standards require frame sensors and more

Reciprocating compressors show the highest number of damages while being process-critical at the same time. The reasons why are partially based on the higher number of centrifugal machines in comparison to reciprocating compressors, and operators just did not fear severe damages due to the lower kinetic energy of these comparably slow-running machines. Insufficient protection and condition monitoring principles are still being applied on some reciprocating machinery.

Inadequate machinery protection

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.

However, upon reviewing the age of the reciprocating compressor population, one will recognize that, in many cases, these large, critical machines have never been replaced and they have been in operation since their initial startup date many decades ago.

To understand why, even after numerous catastrophic failures, we still find inadequate machinery protection on many of these machines, a view into the history of applicable standards can help to lift the fog.

Crosshead acceleration as a safety shutdown parameter

The API 670 fifth edition, released in November 2014, offers valuable information and guidance on how to effectively protect reciprocating compressors.

Users and machinery protection system vendors have agreed upon the inclusion of applying crosshead acceleration as a safety shutdown parameter, which is a pivotal decision.

API 670 is the central document for machinery protection of reciprocating compressors. Piston rod position measurement as a reliable second layer of protection is also recommended.

Frame vibrations mea­sured as velocity, and piston rod position measurement

Numerous reciprocating com­pressors are equipped with machinery protection systems origi­nally designed for centrifugal machinery. Two of the most widely adopted approaches that can often be found on aged reciprocating machinery are frame vibrations, mea­sured as velocity, and piston rod position measurement.

Many compressor operators confirm the inadequacy of these outdated systems to protect against the most-feared compressor damages, such as breaking piston rods, seiz­ing wrist pins and other failure modes involving loss of con­tainment in some cases. While old-school systems often miss detecting the development of catastrophic damages in time or at all, users regularly report about a history of nui­sance trips due to transient process peaks or single-time, uncritical impacts.

Consequentially, operators often consid­er disarming their outdated protection system and putting their trust in proven maintenance practices and relying on the sturdy machine design. It is importand to understand the difference between a uniform rotating movement of a turbine shaft in comparison to a reciprocating movement. Machines with a uniform rotation typically show almost zero shaft deflection per cycle along with a solid, stiff connection to the ground and virtually no frame vibration detectable.

In contrast, a reciprocating compressor shows a very dif­ferent behavior, requiring a different monitoring approach. Pistons are driven back and forth by crosshead-type drive trains, involving reversal of piston rod forces from tension into compression, making the entire frame with all its com­ponents shake and bend to a good degree. Suction and discharge valves create opening and closing impacts, leav­ing vibration amplitudes on the entire machine – and we call this a normal operating condition.

When comparing the working principles of a reciprocating compressor with a centrifugal machine, it becomes ap­parent that a reciprocating unit requires a more dedicated monitoring approach designed to handle all the special challenges reciprocating machinery bears.

Looking at the working principal of reciprocating ma­chines, the crosshead is clearly the focal point. Here, the rotating movement of the crankshaft is transformed into a reciprocating (linear) movement of the piston rod. It is the central component where all the major forces are trans­ferred via the rather sophisticated crosshead pin/wrist pin to the piston rod. In order to facilitate these forces into the right direction, a solid crosshead guide is an in­tegral part of each reciprocating compressor. The cross­head guide is the most direct connection of the moving drive train to the frame and is the best position to install vibration sensors.

A reciprocating compressor requires a different monitoring approach

As illustrated in Figure 2, reciprocating machinery typically shows very smooth crosshead acceleration characteristics with two distinct impacts around the two rod-load-reversal points. Changes of the acceleration signature and amplitude immediately indicate a different mechanical behavior.

This allows modern machinery protection systems to detect typical failures involving crosshlead wrist pin failure, increased crosshead bearing clearance, loose connections between crosshead, piston rod and piiston, increased con­rod bearing clearance, piston nut failure and liquid slugs, and eliminate consequential damage.

When installing the sensor on the crosshead guide, it is well worth considering the rotating direction (clockwise/ counter-clockwise) of the crankshaft. For best results, it is recommended that the sensor be installed on the top side for up-running crosshead shoes and on the bottom side for down-running crosshead shoes in order to be in-line with the effective direction of forces transmitted to the crosshead.

A brief view into basic physics supports the philosophy of why today many operators rely on crosshead slide ac­celeration as the primary machinery protection parameter.

To explain why acceleration should always be the first pa­rameter detectable, let’s follow this example as an illustra­tion: A car drives from location A to B. At your starting point A, you begin accelerating your vehide mass long enough (acceleration, [m/s2]) until you reach thle desired speed (ve­locity, [m/s]) to finally make enough way (displacement [ml) to location B. Note that before any velocity can be recorded, acceleration must be applied to the masses.

Nevertheless, we want to emphasize that frame vibra­tion (velocity) and especially rod position measurement (displacement) provide some good value when applied and evaluated correctly.

Piston rod po­sition measurement

Frame velocity can reveal slowly developing foundation issues as well as failure modes involving a high number of impacts with high energy agitating the equipment in its natural frequency range, which can develop a dangerous rate of mechanical movement. The installation of frame vi­bration transducers often involves votiing schemes (i.e., two out of three) to reduce nuisance trips, with two groups of three velocity transducers mounted on the drive end and nondrive end of the frame.

Solid reciprocating compressor construction, including the heavy foundation, requires a tremendous amount of kinetic energy provided over mul­tiple strong impacts to reach critical velocity limits. Veloc­ity transducers are typically installed far away from likely failing components where frame velocity is an inaccurate parameter. That should be considered as a second layer of protection only. Please note that modern monitoring sys­tems have the capability to mathematically integrate the acceleration signal over time, delivering a velocity analysis per acceleration sensor location. This finally reduces the value of adding frame velocity transducers to a monitoring sensor scope.

During its early days of implementation, piston rod po­sition measurement can be considenad as meaningful as shaft position measurements on a centrifugal machine, applying the same hardware and the same signal analysis logic. The major difference and monitoring challenge that has led to a long-lasting bad reputation of “rod drop” is the simple fact that a piston rod’s purpose is not to rotate, but to push and pull the piston, which may lead to significant bending of the rod and varying due to different load steps.

Old reciprocating compressors can be upgraded with a modern monitoring system

These effects are not known from monitoring centrifugal machinery to this degree. For its intended purpose – the detection of rider band wear – the signal must be phased and correctly analyzed to keep the rod from bending under different load conditions. Even greater value can be found when analyzing the dynamic component of the rod position signal for machinery protection.

Segmented signal analysis (segmenting the 360° crank. angle into portions of smaller degrees) – such as, for example, an eight-segmented analysis determining safe­ty-critical piston rod bending effects – has proven to be highly reliable at detecting loose connections in the drive train, such as piston rod-crosshead and piston rod-cylinder connections, as well as impending piston rod cracks before the rod completely fails.

As described earlier, the piston rod typically moves and bends even during normal operation, but in case of devel­oping mechanical damages and cracks, the behavior of the piston rod changes significantly. These changes can be detected using an eight segmented analysis based on the dynamic rod position signal as shown in Figure 3.

In many cases, old reciprocating compressors may be satisfactorily upgraded with a modern monitoring system instead of being replaced, whether considering safety, ma­chine rerates or increased load conditions.

In conclusion, looking back at the development of the API standards starting in the 70s, it is understandable from where some of the today’s standards derived. The once state-of-the-art monitoring approach used on centrifugal machines was adopted and applied on reciprocating ma­chinery. That was the time when frame vibration monitoring and rod position monitoring made their way into the moni­toring standards of reciprocating compressors. However, experience has shown that the former standards did not deliver the desired effect in monitoring reciprocating ma­chinery and eventually led to the development of today’s modern monitoring approach.

One of the main aspects is to make use of the working principle of a reciprocating compressor and to focus on the crosshead guide in order to detect developing failures early and reliably. Frame vibration measurement simply is too far away from the main functional components, and the velocity­type measurement leads to missed detection. Nevertheless, frame velocity offers some – but very limited – machinery protection value when looking at common failures experi­enced on reciprocating compressors.

Based on the experience of more than 1500 critical machines equipped, it is recommended to employ cross­head slide acceleration as the prime protection param­eter. In addition, applying dynamic piston rod position measurement as a reliable second layer of protection is also recommended.

If you want to know more about our solutions for effective vibration monitoring for reciprocating compressors: Just contact us via the contact form. We would be happy to hear from you!