PLANT MAINTENANCE
these environments there is an acknowledgement that the entirety of the standby system should be regularly tested to ensure that it all functions correctly, while at the same time there is an operational prerogative not to put fully functioning business systems at risk. In this scenario the idea ‘off load testing’ was born, and its practice widely adopted. When all of the above elements come together, and are combined with a range other application-driven needs, it is clear to see how a significant number of installations across the UK are running lightly loaded generators for many hours each, eventually leading to issues with wet stacking.
Figure 3: The author says the ‘much finer tolerances’ now being achieved in newer pistons and piston rings in modern generators, compared with those encountered in older engines, bring a range of benefits.
and increasing the chances of failure to meet the required emissions target set for the installation. While it is unlikely that wet stacking will cause engine damage in the short term, over time it will lead to poor performance, and reduced engine life, and expose the user to higher maintenance costs. There is then the issue of a build-up of unburnt or partially burnt fuel and other carbon deposits in the exhaust/flue system. In extreme cases, usually under rapid application of the high exhaust temperatures, the exhaust gas stream can combust soot in the system.
Ways to address the issue The well-trodden path to solving the problems of wet stacking has, for years, been to regularly run the generator(s) at load levels well above the minimum 30% of rated capacity. Depending on the hours run, this can be monthly or annually. This has been done to ensure that the engine gets to its optimum operating temperature; a temperature at which the piston rings expand appropriately, and burn off any cylinder glazing or partially burnt fuel that has built up in the exhaust system. Recent advances in technology are making the need for this costly procedure less demanding. The problem of wet stacking has been with us for many
years, and occurs in most types of standby generator applications. The main cause is the oversizing of the generator(s) at project design stage. The many reasons for this include: n A lack of clarity of client load requirements at the design stage.
n Client loads never reach those that are expected. n Energy efficiency improvements over time. n The inclusion of a clause in the specification for ‘an additional 25% capacity to cover any future business expansion’.
n Application-related factors such as the requirement for generators being sized to accommodate locked rotor starting on life safety sets.
n Confusion over compatibility between generators and static UPS equipment.
n Multi set ‘rolling’ redundancy typical in mission critical/ data centre applications.
By their very nature, standby generators are installed to protect critical loads of all types be they hospitals, commercial businesses, or critical infrastructure. In many of
92 Health Estate Journal October 2025
It is worth pointing out that when generator manufacturers, generator dealers, and specifying engineers work more closely together, specifically at the design stage, these issues can be can largely resolved. Generator dealers with a wide range of application knowledge that also have access to today’s much- improved generator sizing tools generator sizing tools can provide more accurate solutions. The wet stacking problems prevalent in the market today caused by oversizing should be a lesser concern in the future, confining the problem to more of an issue associated with monthly or quarterly off load testing.
Load bank testing The easiest way to avoid wet stacking during the monthly exercise is to run the generator at the manufacturer’s recommended minimum load level (this will vary from engine to engine, and manufacturer to manufacturer). Many of today’s infrastructure operators do not wish to ‘risk’ operational integrity by transferring the building load onto the generator at critical times. With many ‘operations’ today running ‘24/7’, time to undertake key preventive maintenance works is at an absolute premium, ensuring that any generator testing is either off load or with a load bank. It is often the case that load bank testing is undertaken annually. In many operational environments such as – for example – in a data centre, monthly load testing might be mandatory. The use of a load-bank artificially boosts the amount of load available to be placed on the generator itself – ideally ensuring that the engine reaches optimal temperature, enabling it to burn any accumulated build up. Most generator operators recommend that load-bank testing be done at least annually for a minimum of 30 minutes runtime. Of course, needs will vary depending on overall hours run in the period; your generator maintainer will be happy to advise on this. In addition to running time, the operator or maintenance company will need to consider the time it takes to set up the load bank. For a typical large hospital or data centre facility using between five and 10 generators, that can add up to a significant time commitment for Operations personnel, not to mention fuel consumption. The push and pull of market forces, government legislation, and a growing environmental awareness within the populations in general has, over recent years, seen significant changes to the modern diesel engine. Modern engine designs now incorporate a number of new technologies that enhance operating efficiency. These include: n More precision engineering in cylinder blocks and heads, which enables a reduction in the gaps between pistons and rings that previously allowed lube oil from the sump to transit so easily into the combustion chamber, or ‘blow-by’ – which is when unburnt fuel escapes the cylinder.
n The introduction of high pressure common rail
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