Nov-2013

Digital comms protect compressors better

Digital communications technologies are aiding condition monitoring of reciprocating compressors by reducing installation costs and increasing reliability.

Klaus Stachel and Tomas Locken
Hoerbiger Compression Technology

Article Summary

Real-time condition monitoring is a valuable tool for optimising the performance of reciprocating compressors 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, for instance, 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 analogue 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 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 now changed, however, with purpose-designed digital communications now available for the most demanding compressor monitoring applications. This article explains how digital compressor monitoring works and its benefits over older analogue technology.

In the past: digital, lower cost but not fast enough
Compared to traditional analogue communications technologies, digital systems cost less, offer greater flexibility, and are more reliable thanks to their resistance to interference and built-in self-checking. 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 analogue 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 shows that properly implemented digital systems can 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 – which is about 100 times more.
 
Analogue: issues with cost and length
Traditional dedicated compressor monitoring systems therefore use analogue sensors and cables. Since the compressor itself is generally mounted in an area of the plant where there is a risk of an explosive atmosphere, 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 an instrument room (Figure 1:).

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. With so many instruments, cabling 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 4–20 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. Accelerometers of the integrated circuit piezoelectric (ICP) type 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: the new approach
Hoerbiger has overcome the speed disadvantage of conventional digital systems by designing a new type of distributed digital system especially adapted for reciprocating compressors. Up to eight analogue transmitters are connected to an intrinsically safe Fast Transmitter Interface Module (FTIM2, Figure 2:) located close to the compressor.

The FTIM2 pre-processes the sampled data, packages it and sends it on to the system’s Central Interface Unit (CIU2, Figure 3:).
The FTIM2 and CIU2 are linked by Industrial Ethernet over a single CAT 7/7A cable (Figure 4:).
The FTIM2 gets its power supply directly from the CIU2 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 advantages of the diagnostic abilities of the CIU2.
The FTIM2 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 of both the FTIM2 and the CIU2, and the power supply to the FTIM2, are continuously monitored for malfunctions.

The CIU2 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 fulfils the requirements of IEC 61508/61511 (SIL) for machinery protection and is certified by TÜV Rheinland, Germany.