SPONSOR CPD PROGRAMME Continuing professional development (CPD) is the regular maintenance, improvement and broadening of your knowledge and skills, to maintain professional competence. It is a requirement of CIBSE and other professional bodies. This Journal CPD programme can be used to meet your CPD requirements. Study the module and answer the questions on the final page. Each successfully completed module is equivalent to 1.5 hours of CPD. Modules are also available at www.cibsejournal.com/cpd Safety valves for building services systems This module explores the requirements for safety valves in building services applications and the design assessment process to ensure system safety Building services piped systems require protection where, under reasonably foreseeable service conditions, the internal pressure may exceed the maximum allowable pressure. This is a matter of risk assessment to ensure safe and effective operation and, in many cases, to control the risk will require at least one appropriately selected, installed and maintained safety device. The safety system will include the safety devices, the interconnections that are free from potential blockages between the equipment to be protected, and any discharge connection to a safe place. This requires a holistic design assessment to ensure system safety. The term safety valve would typically encompass relief valves, pressure-relief valves and safety relief valves, and this pressure equipment as it is referred to in EU directives is classified by group according to the level of hazard. Fluids used in building services engineering would most likely be those referred to in the standards as being in group 2 (whereas group 1 is for those classified as hazardous). The fluid group would normally be used as part of the assessment of risk category; however, general purpose safety valves will, in any case, fall under the most stringent category 4, which defines the level of design and production quality control to ensure conformity that allows a safety device to bear the legally required CE mark. In the UK, specific requirements for system safety are principally encompassed by the Pressure Equipment (Safety) Regulations 20161 and the Simple Pressure Vessels (Safety) Regulations 20162 that implement Directives 2014/68/EU, the Pressure Equipment Directive (PED), and 2014/29/EU, the Simple Pressure Vessels Directive (SPVD). The regulations specifically come into play when the systems are operating at pressures greater that 50kPa (0.5bar, 5.1m water head). The recently revised part 1 of Safety devices for protection against excessive pressure, BS EN ISO 4126:2019, specifies the general requirements for safety valves and the testing techniques for valves set at pressures of 0.1bar gauge and above. (This standard supersedes the withdrawn, but oft quoted, UK standard BS 6759.3) ISO 4126 has 11 parts that consider the different requirements to provide against excessive pressure. It describes a safety valve as a valve which automatically, without the assistance of any energy other than that of the fluid concerned, discharges a quantity of the fluid so as to prevent a predetermined safe pressure being exceeded, and which is designed to re-close and prevent further flow of fluid after normal pressure conditions of service have been restored. A safety valve may provide protection against hazards arising from a number of scenarios. This could, for example, be a blocked vessel or pipework discharge; excessive heat from external sources, such as the sun, or other equipment that can cause the fluid within the pipework to expand; system thermal expansion from heat fluctuations beyond the expected design conditions; and failure of a pipeline component, such as an ineffective, failed or tampered control valve or other component that may prevent adequate, or any, discharge. When evaluating the expected performance of a valve, some terms are commonly used to describe valve operation, as shown in Table 1 and illustrated in Figure 1. Direct spring safety valves, as illustrated in Figure 2 and Figure 3A, are commonly used www.cibsejournal.com October 2020 65 CIBSE Oct20 pp65-68 CPD168 NABIC v2.indd 65 25/09/2020 17:13