Electricity and Control March 2023

MEASUREMENT + INSTRUMENTATION : PRODUCTS + SERVICES

Enabling early leak detection in boiler tubes

The Inspecta Automatic Purge System, designed, engineered and manufactured in South Africa, is a cost effective, ambient-air purge system to remove the fly ash build-up within Inspecta pipe-sets, for leak detection in power stations. And it does not require either compressed air or an additional power source to operate. Instrotech designed the successful Acoustic Steam Leak Detection System (Inspecta ASLD) and has devel oped a fully automatic purging system that can be eas ily retrofitted to existing Inspecta installations. The new automatic system eliminates routine manual purging of the pipe-sets and maintains the leak detection system at peak performance, without manual intervention. Installed in most Eskom power stations, Inspecta ASLD systems use acoustic detection technology which detects tube leaks less than 2 mm in diameter in the large boiler structures. The coal used by Eskom has a very high ash content, so a considerable amount of fly ash re mains after combustion. The fly ash swirls around inside the boiler and quickly blocks the ‘listening’ pipe-sets that contain the sensitive leak-detection microphones. Un less the systems are cleaned frequently, either manually or automatically, their ability to detect the leaks early is significantly degraded. Cost savings case study The Inspecta ASLD has been shown to detect steam tube leaks reliably and accurately. To achieve this, the system must be correctly calibrated and the listening pipe-sets kept clear of any obstruction that would re duce their sensitivity. Experience has demonstrated that if there is any obstruction that reduces the sensitivity by more than 10%, it will result in a measurable degradation of the Inspecta ASLD and a delay in the detection of a tube leak. Figure 1 illustrates that in order to optimise the per formance of the leak detection system, the pipe-sets, particularly those located in the combustion areas of the boiler, should be cleared of fly ash build-up every one and a half to two hours. It is not practical to do this con sistently and reliably using a manual purging system. If the fly ash is allowed to build up over a period of 5

The Inspecta Automatic Purge System in application. to 6 hours, this will result in a serious degradation in the sensitivity and effectiveness of the leak detection system. Under such degraded conditions, tube leaks may only be detected after progressing for some time. Such de lays may well result in extensive secondary tube damage and, in turn, extend the downtime required to repair the damaged boiler. Comparative cases of early detection and delayed detection of tube leaks highlight the positive impact that an effective leak detection system can have in reducing the consequent extent of damage and the costs of down time for repairs. Early detection of a tube leak In the case where a leak was detected promptly and the boiler shut down before significant secondary tube dam age had occurred, an outage time of two days was need ed before the boiler was brought back online. Delayed detection of a tube leak In the case where detection of the tube leak was de layed and the boiler shut down two hours after the boiler cited in the first case, it is assumed that the tube leak happened quickly and created significant secondary damage, so more than 40 tubes were damaged. EPRI [1] information (EPRI 2003) confirms that the boiler outage time increases steeply as secondary damage increases. In this case, it took 11 days to repair the boiler – which resulted in 11 days of lost electricity production. Retrofitting the automatic air purge system The Inspecta APS system can be installed into an exist ing Inspecta ASLD configuration simply by replacing the existing Inspecta pipe-set with an Inspecta APS-enabled pipe set. All existing plant wiring remains the same. If there is a delay in the detection of a fast-progressing tube leak by just a few hours, the downtime to repair the boiler can increase by more than five times compared to a leak that is detected earlier. Installing Inspecta APS on a boiler ensures the Continued on page 21

Figure 1 shows the rate at which sound level deteriorates over time due to fly ash build-up. The pipe sets need to be cleared at regular intervals to maintain effective leak detection.

20 Electricity + Control MARCH 2023