A bolt preload check based on a solid-state "intelligent" preload monitoring washer was developed by ScanSense (Vear, Norway) and used by SKF of Sweden for crane and flange maintenance. The washer, known as BoltSafe, is said to provide the correct tightness and pre-tension of bolted joints, allowing for increased safety, and more dependable joints.
The system is used as a regular washer but has the added capability of measuring the compressive force created by the nut and its interface surface to determine the pre-tension in the bolted joint. A sensor measures the pre-tension in the bolt as it is tightened, and continues to do so throughout the service life of the bolted joint. Measurements can be read out periodically on a portable instrument or continuously on a PC or any other monitoring station.
UK projects engineer Neil Buck says the system is proving useful for crane operators to meet the new LOLER regulations introduced by the UK Health & Safety Executive. The regulations require all operators to achieve annual certification of lifting equipment. The washer helps monitor bolts without stripping out the whole crane.
Monitoring of joints remotely is achieved by a built-in report generator included in the BoltSafe software, which provides direct printed information on the pre-tension condition of the joint interface. For subsea applications, the topsides monitoring system is linked to sonar scanners which are suspended over the side of the platform. In theory, this means that thousands of joints can be tested at one time, saving operators the expense of using ROVs.
The system comes with two types of sensors - PMS (periodic monitoring system) and CMS (continuous monitoring system). The PMS uses a clearly marked docking area where the sensor's communication probe is inserted for monitoring. The probe is connected to a handheld instrument which measures the joint and transfers the data back to a LCD display. Data captured includes the pre-tension value and the temperature together with the date, time and ID number of the bolt. The CMS links several monitors by a cable in a network so that components can be monitored continuously and provide an early warning signal for critical installations.
Also, the system comes with a multi-function software package consisting of a communication section, an organizer, a database section, a display section and a documentation package which allows the operator to print out certificates of the residual pre-tension obtained during initial assembly as well as any historic data in the system. The whole package provides a complete maintenance, documentation, and monitoring system ensuring the continuing integrity of bolted joints.
For hazardous areas, the system can be obtained with an Exi classification EEx ia IICT4 where it can be used portably. The CMS version requires zener barriers in a safe area and, since it operates as a network, as little as three barriers are required depending on the size of the network.