Testing and quality

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TESTING AND QUALITY 1 2 3 4 5 6 7 8 1 2 Rogowski coils Local termination box StatorMONITOR signal conditioning unit Signal cable (up to 30m max.) Laptop with data acquisition cards StatorMONITOR analysis software Signal cable RS232 (serial) filter control cable 6 Typical test setup for partial discharge using the StatorMONITOR system. RTD lead, etc. The StatorMONITOR ® online PD measurement system was developed by Sulzer Dowding & Mills to monitor, diagnose, and evaluate the condition of insulating systems used in the stator windings of HV motors and generators. The system is designed to detect insulation problems in their early stages, when remedial action would prove to be most beneficial in terms of the avoidance of unplanned downtime and consequential financial implications. The StatorMONITOR system developed by Sulzer Dowding & Mills uses Rogowski coils as PD detectors. Data is collected online from the three phases simultaneously, which permits the distinction between discharges activity in the main insulation wall (for each phase), between phases, and in the endwindings. The system quantizes and records the basic discharge parameters associated with each individual PD pulse. These include the discharge pulse magnitude, its polarity, and its phase position relative to the power frequency. Pattern recognition algorithms are then applied to these basic quantities to identify the type and severity of problems. They also help in the detection of and removals elimination of regular commonly known interference—such as that due to converter equipment or generator excitation systems—which could reduce the accuracy of analysis. The trending facilities of the system allow the identification of potential problems or degradation processes developing within the insulation. The fundamental requirement of the system is to detect discharge activity that may be harmful to the insulation system and its location. 24 | Sulzer Technical Review 1/2011 4 5 3 6 7 8 Case study—partial discharge in highvoltage rotating machines Three identical two-pole machines with the following specifications were tested for partial discharge using the Stator- MONITOR system: • Motor Size: 4 MW • Voltage: 11 kV • Duty: Induction motor • Location: Offshore oil platform A typical test setup is shown in 6. Two of the three machines tested on site showed normal low-level discharge activity. However, the discharge levels on one machine were measured at 90 000 picocoulomb (pC) and suggested phaseto-phase discharge sites between the RED and YELLOW phases as shown below in figure 7. There was also evidence of slot discharge modulation normally associated with coil movement in the slot, identified as a distinct modulation of the discharge pattern. This effect was pronounced in the blue phase. Coil movement is most commonly associated with the presence of loose wedges. It was recommended that this motor be removed from service and overhauled. The spare motor was scheduled to be installed during a planned shutdown three months later. This exchange was delayed a few months and later the motor suffered a winding failure just prior to the shutdown. An investigation of the winding after removal showed that the bracing had become loose, allowing a phase cable to rub and eventually discharge until a failure occurred in between this phase and another in the end-winding. 8 White deposits in the end-windings as a result of discharge activity. 7 The discharge levels on one of the machines suggested phase-to-phase discharge (RED and YELLOW phases). There was also evidence of slot discharge modulation—most pronounced in the blue phase. Figure 8 shows the photograph of the white powdery deposits in the endwinding as a result of discharge activity. Further investigation showed that there were approximately 10% of the wedges loose—also possibly due to the bracing problem. Conclusions Online condition monitoring using vibration analysis and partial discharge can detect failure mechanisms within a machine before catastrophic failures or unplanned outages occur. Engineers are able to take remedial action to keep the machine in serviceable condition and thus extend the life of the machine. John Allen Sulzer Dowding & Mills Ltd. Camp Hill, Bordesley Birmingham, B12 0JJ United Kingdom Phone +44 121 766 6161 john.allen@sulzer.com 1 2 1 Phase-to-phase discharge 2 Slot discharge modulation (Coil movement in slot)
SULZER INNOVATION The ideal solution for corrosive media The new Sulzer KnitMesh XCOAT™ mist eliminator combines the chemical resistance of fluoropolymers with the strength and form stability of alloys. Thus, the Sulzer KnitMesh XCOAT mist eliminator sets a new dimension in contact with strong acids, especially in applications involving sulfuric and acetic acid. In many applications, droplets need to be efficiently separated from a gas stream in order to avoid problems or damages in downstream processes. For applications with severe process conditions involving corrosive liquids, special resistant materials are required. Suitable materials are characterized by a high corrosion resistance in a wide range of operating conditions and a long lifetime of service. Sulfuric acid production process Sulfuric acid is a chemical compound of sulfur with the formula H2SO4. It is a colorless, oily, very viscous, and hygroscopic liquid. It is one of the strongest acids and is highly corrosive. Sulfuric acid is one of the most important chemicals in general and one of the most-produced commodity chemicals. The new Sulzer KnitMesh XCOAT mist eliminator. Sulfuric acid is produced from sulfur dioxide (SO2), which is oxidized to sulfur trioxide (SO3). This is reacted with water in concentrated sulfuric acid to sulfuric acid. Drying column in the sulfuric acid production The gas stream to and from the reactor is, in direct contact with concentrated sulfuric acid, dehydrated in order to prevent corrosion caused by wet SO2 or SO3 gas as well as to avoid condensation of acid during plant shutdowns, which would damage the catalyst in the reactor. Therefore, to avoid corrosion, the columns are either made of special high alloy steels or lined with acid- and heatresistant bricks. Droplet in the drying column At the top of the drying column, a mist eliminator is installed so that no droplets are entrained to the downstream process. Sulzer Chemtech markets wire mesh mist eliminators with the trademark KnitMesh. A KnitMesh mist eliminator is knitted from wires or fibers. The strengths of the KnitMesh XCOAT mist eliminator Mist eliminators in sulfuric acid drying columns require high-grade materials. Particularly concerned are the knitted mesh pads with their thin wires which are most sensitive for corrosion. Therefore, alloys that are optimized to prevent cor- Close-up image of XCOAT: Thin metal wire coated with 100% pure PTFE (polytetrafluoroethylene). rosion caused by sulfuric acid are applied. Alternatively, synthetic fibers, in some cases in combination with metal wires, are used. However, the above alloys do not completely prevent corrosion, which is why the lifetime is limited. Mist eliminators using fluoropolymer fibers show, indeed, an excellent chemical resistance, but at higher temperatures they tend to shrink, which leads to bypasses in the mesh pad. In the Sulzer KnitMesh XCOAT mist eliminator, the strength and form stability of a metal wire are ideally combined for the first time with the chemical resistance of fluoropolymers. A thin metal wire coated with 100% pure PTFE, the best known chemical- and temperature-resistant plastic, shows a longer service time in hot corrosive media than wires from special alloys or fibers from fluoropolymers. Thus, the Sulzer KnitMesh XCOAT mist eliminator sets a new dimension in contact with strong acids. The Sulzer KnitMesh XCOAT mist eliminator is now exclusively available to our customers from Sulzer Chemtech. Daniel Egger For more information or to express your interest, please contact: Rohit N. Joshi Sulzer Chemtech India, Pune Phone +91 2137 304740 rohitn.joshi@sulzer.com 4334 Sulzer Technical Review 1/2011 | 25
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