7.2 Control systems Safe and efficient operation depends on the boiler remaining within its safe parameters during operation. A wide range of additional equipment that can be fitted to the boilers is available to help ensure this. This equipment can have a monitoring role or a safety function where it acts in a predetermined manner to prevent a dangerous situation. For example, on an older installation, the “first low” water level alarm may prevent burner operation when the water level is low, but allows an automatic restart and resumption of operation once the water level has risen to a safe level. On the other hand, should the water level continue to fall, the “ second low ” water level alarm shuts the burner down completely and does not allow an automatic re-start. The burner may only be re-started manually once the cause of the low level event is established, the second low water level alarm is cleared, and the water level restored. Control equipment includes the various level sensors, limiters, control devices, relief devices and gauges as well as the communication and alarm systems. The level of control and monitoring will depend on a variety of factors. In general, boilers with automatic control and remote monitoring systems will require more monitoring and control equipment than a locally manned boiler system. New safety-related systems shall be designed, documented and applied according to the requirements of BS EN 61508 so that safety functions are determined, i.e. the Safety Integrity Level (SIL) of each safety function is specified and the measures used to achieve the specified SIL for each safety function are described. BS EN 50156, Electrical Equipment for Furnaces and Ancillary Equipment provides information on the application design and installation of electrical equipment. Every employer shall ensure that, where appropriate, work equipment is provided with one or more readily accessible emergency stop control device (PUWER Reg 16). 7.2.1 Level sensing devices These can be mounted through the boiler shell or in external chambers providing that the system has proven reliability and is inherently fail-safe. Detailed information can be found in BS EN 12953 Part 9, which specifies the following: x External chambers must have, as a minimum, 20 mm diameter boiler shell connections at the steam and water level; x Protection tubes (where fitted) must be designed: o With adequate venting to water and steam space; o To prevent steam bubbles causing undue disturbance to the water level; o To prevent sludge build-up; o With a minimum clearance of 14 mm from the probe; x The two low water level limiters shall be mechanically and electrically independent so as to avoid “ common cause ” failures. Note that while the limiters should be independent of each other they do not have to be independent of other controls. i.e. a controlling probe can also act as a limiter provided it meets all of the other requirements of EN 12953-9. Many existing boilers which were not designed and constructed to BS EN12953 will have a first low level cut-out and alarm (auto reset) and a low level limiter (lock-out). Risk assessment may demonstrate that this is not satisfactory for unmanned (remotely operated) boiler systems.
BG01 Guidance on the safe Operation of Steam Boilers Edition 2 – © 2019
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