Real-time condition monitoring is a valuable tool for optimizing reciprocating compressor performance and reducing unplanned downtime. The ability to detect dangerous failures at an early stage and bring the compressor to a halt in just two or three revolutions has saved many machines from serious damage. For issues that develop more slowly, reliable monitoring avoids unnecessary work and allows maintenance to be carried out during planned shutdowns.
A traditional compressor monitoring system uses analog connections to link sensors on the compressor to an electronic processing unit located in a safe area. Although many other areas of the plant will typically use digital communications, standard remote input/output (I/O) systems and fieldbuses such as Profibus, Foundation Fieldbus and Modbus are not fast enough for the real-time vibration monitoring that compressors require.
This situation has changed, however, with purpose-designed digital communications available for the most demanding compressor monitoring applications. Older analog technology is now obsolete, as digital compressor monitoring has substantial benefits over the analog technology of the previous generation.
Compared to traditional analog communications technologies, digital systems cost less, offer greater flexibility, and are more reliable thanks to built-in self-checking systems and resistance to interference. By allowing multiple devices (and multiple signals from each device) to share a single set of wires, digital communications cut cabling costs.
In the past, these advantages did not extend to safety-critical systems, which were still required to be hard-wired and based on analog technology. This changed around 2008 with the advent of the IEC 61508 standard, which allows the use of digital systems in applications related to safety. Experience in the automotive and aircraft industries showed that properly implemented digital systems provide high levels of safety protection.
However, even when they are approved for safety-critical applications, general-purpose digital systems are not fast enough for real-time vibration and rod position monitoring. Profibus PA, for instance, has a maximum data rate of 31 kbit/s. This is fine for process control, but too slow for compressor monitoring, which requires data rates in the Mbit/s range (approximately 100 times more than Profibus PA).
Analog: Cost and length
Traditional dedicated compressor monitoring systems, therefore, use analog sensors and cables. Since the compressor itself is generally mounted in a plant area where there is an explosive atmosphere risk, the usual arrangement is to wire the sensors through intrinsically safe (IS) barriers to rack-mounted monitoring equipment located in a safe area such as the instrument room (Fig. 1).
| Fig. 1. A traditional |
system uses analog
sensors individually wired
to the control unit.
The field instruments typically installed on reciprocating compressors are accelerometers and velocity meters for vibration, indicator pressure transmitters for performance and rod-load and rod-drop transmitters for rider ring wear and piston rod runout monitoring. With so many instruments, implementation costs can be considerable.
There may also be issues with plant layout and electrical interference. Process plants and compressor stations are full of electrical noise, and, even with careful shielding, there is always a risk of interference, which can degrade accuracy and even cause spurious compressor trips. The risk is greatest with accelerometers and rod-drop transmitters, which deliver their output signals in the form of voltages rather than the more robust 4 mA20 mA current signals used by other field instruments.
The maximum signal frequency that can be transmitted reliably depends on the length of the loop, the cable impedance per unit length, and the ratio of the peak signal voltage to the current available from the signal conditioner. Integrated circuit piezoelectric accelerometers provide a high-voltage, low-impedance output that reduces the effects of long cables and electrical interference, but even these devices are limited to a maximum cable length of around 300 m.
Fast digital is the new approach
To overcome the speed disadvantage facing conventional digital systems, a new type of distributed digital system has been designed and especially adapted for reciprocating compressors. The new design allows up to eight analog transmitters to be connected to an intrinsically safe fast transmitter interface module (FTIM) located close to the compressor.
The FTIM pre-processes the sampled data, packages it and sends it to the systems central interface unit (CIU). Fig. 2 shows that FTIM and CIU are linked by industrial Ethernet over a single CAT 7/7A cable.
| Fig. 2. Digital |
only a single Ethernet cable
between the field devices
and the cabinet.
The FTIM gets its power supply directly from the CIU using power over Ethernet (PoE), as defined by IEEE 802.3af. This well-proven arrangement avoids the need for separate power supplies in the hazardous area and takes advantage of the CIUs diagnostic abilities.
The FTIM continuously monitors every sensor for loss of signal (sensor unreachable), signals that fall outside the allowable range and signals that show no variation (stuck or failed sensor). Similarly, the key hardware components for both the FTIM and the CIU, and the power supply to the FTIM, are continuously monitored for malfunctions.
The CIU uses this information to produce a trust signal confirming the reliability of the system. As soon as any safety-related failures are detected, the loss of the trust signal is communicated to the user. The system fulfills the IEC 61508/61511 (SIL) requirements for machinery protection and is certified by TÜV Rheinland, Germany.
Digital interface and protocol
Data transmission between the FTIM and the CIU is via a point-to-point link based on an extended version of the RS-485 interface. Extended means that a special hardware component is used to boost the transmission range to 500 m without repeaters. The same component also offers a high level of protection against electrostatic discharge and ensures failsafe operation.
The data transmission protocol is based on a digital interconnect format, as defined in IEC 60958. It supports an encoding system known as biphase mark code, which combines data and clock information to produce a self-synchronized data stream operating up to several Mbit/s.
The actual data flow has two main parts. The CIU first sends timing information and requests the sampled sensor data, after which the FTIM sends the corresponding response. Fig. 3 shows how the data stream from the sensors is divided into packages known as segments. Each compressor revolution produces several sampled data segments.
| Fig. 3. Data is divided into segments before |
being sent to the FTIM. A second repeat
channel maintains communication in the
event of errors or congestion on the main channel.
All data sent and received is carried within frames to minimize data loss and allow any corruption to be detected. To further increase integrity, each data segment is split across four frames (Fig. 4). Each frame consists of a preamble describing the message type, the data itself and a cyclic redundancy checksum to confirm data integrity. After every four frames, a marker indicates the end of each segment.
| Fig. 4. Each segment of data is divided |
across four frames to increase reliability.
For safety, the data link has two separate transmission channels (Fig. 5). If the checksum calculated and sent by the FTIM does not match that calculated by the CIU after the data has been received, the repeat channel is activated and the faulty segment is retransmitted at the same time as the next segment is being sent on the normal channel.
| Fig. 5. Main and communication redundant |
channel between FTIM remote I/O and
base unit CIU.
Data storage and alarms
The CIU stores all the sensor data it receives (both sensor status information and the actual readings) in a buffer that holds a minimum of 500 revolutions worth of data.
After initial checking to ensure that the data is complete and error-free, the systems event manager compares each signal against two user-configured levels: alarm (lower level) and safety alarm (higher level), as shown in Fig. 6. To cover different operating conditions, a total of 16 limit sets can be configured for each sensor.
| Fig. 6. Built into the CIU is a dynamic signal |
buffer that stores all data from the last
For time-wave vibration signals that vary over the course of a single revolution, each crank angle degree can be automatically assigned its own alarm level, with up to 360 separate values per rotation. This allows the system to detect dangerous vibrations reliably during quiet parts of the cycle, without being swamped by high vibration levels during noisy events such as valve movement and rod load reversal.
Fig. 7 shows the actual vibration reading over one complete revolution and the envelope of the vibration readings since the last machine overhaul. In addition, the so-called persistence envelope displays the vibration readings of several previous revolutions for better visualization of this highly dynamic signal. With such high-resolution alarm levels, emerging problems can be detected immediately and the compressor shut down before major damage can occur. This is not possible with conventional monitoring systems.
| Fig. 7. Time-wave vibration signal over one |
revolution with alarm limits referenced to the
In the event of a limit violation resulting in an alarm or a safety alarm, the CIU copies the sensor data from the latest 500 revolutions into a separate event buffer. The maintenance supervisor or reliability engineer can download the contents of the event buffer at any time for offline analysis.
The digital advantage
Performance monitoring and machinery protection systems allow the compressor condition to be assessed while it runs, supporting maintenance decisions and protecting valuable assets. Repair work can be better planned, and spare parts organized in good time. Most importantly, condition monitoring can cut downtime and increase production.
Digital compressor condition monitoring systems have many advantages over their analog predecessors. During design, installation and commissioning, they offer:
- Greater layout flexibility, thanks to increased maximum cable lengths
- Lower cabling costs, thanks to fewer cables, cable trays, terminal blocks and marshalling boards
- Quicker installation
- Easier project engineering, fewer engineering hours and less documentation
- Less opportunity for error
- More room in cabinets, because barriers are incorporated into the FTIM
- Extra sensors are easy to add at a later stage.
During operation and maintenance, the advantages of digital systems include:
- Higher reliability, thanks to the inherent robustness of digital signals, built-in checking mechanisms and the use of redundant channels
- Continuous sensor status recording, as well as actual measured values
- Less training required, since the system has a simpler layout and fewer parts
- Quicker problem identification. HP
Klaus Stachel works in product management and Tomas Locken works in research and development at Hoerbiger Compression Technology in Vienna, Austria.