GENERATOR MAINTENANCE
due to the discharge, via the exhaust system, of partially burnt lube oil and additional particulates from the unburnt fuel. The introduction of ever more onerous
emissions targets has, over recent years, seen the introduction of diesel oxidation catalysts (DOC), diesel particulate filters (DPF), and more recently via the medium combustion plant directive, MCPC selective catalytic converters or SCRs. (see Fig 2) The introduction of these additional elements into the exhaust gas stream further add to the problems of wet stacking elevating its importance as an issue to be dealt with. All of these emission treatment devices only function correctly when they reach their optimal operating temperature/operational temperature range. It is heat generated by full and effective combustion in the engine which generates the necessary heat in the exhaust system. The emissions after treatment devices are also susceptible to becoming clogged by unburnt fuel and soot resulting from wet stacking reducing performance and likely failure to meet the required emissions target set for the installation.
Why is wet stacking important? It is unlikely that wet stacking will cause engine damage in the short term, but 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. The well-trodden path to solving the
problems of wet stacking has, for years, been to regularly run generators at load levels well above the minimum 30 per cent of rated capacity. Depending on the hours run, this can be monthly or annually. This has been done to ensure the engine gets to its optimum operating temperature; a temperature at which the piston rings expand appropriately, burn off any cylinder glazing, and burn off unused 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 historic oversizing of standby generators The problem of wet stacking has been with us for many years and occurs in most types of standby generator applications. The main cause of this being down to the oversizing of the generator(s) at project design stage. There are many reasons for this some of which are:
IFHE DIGEST 2025
Carbon deposits
Fuel drip
Standby generators are installed to protect critical loads of all types, be they hospitals, commercial business, or critical infrastructure. In many of these environments, there is an acknowledgement that the entirety of the standby system should be regularly tested to ensure it all functions correctly, while at the same time recognising the 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 Figure 1.
l Lack of clarity of client load requirements at the design stage.
l Client loads never reach those expected.
l Energy efficiency improvements over time.
l The inclusion of a clause in the specification for ‘an additional 25 per cent capacity to cover any future business expansion’.
l Application related such as the requirement for generator being sized to accommodate locked rotor starting on life safety sets.
l Confusion over compatibility between generators and static UPS equipment.
l Multi set ‘rolling’ redundancy typical in mission critical/healthcare applications.
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. In terms of the design phase, it should
be noted that when generator manufacturers, generator dealers, and specifying engineers work more closely together, specifically at the design stage, these issues can be resolved. Generator dealers with a wide range of application knowledge that also have access to today’s much improved generator sizing tools can provide more accurate solutions. The wet stacking problems present 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.
Hydrocarbon
Diesel oxidation catalyst (DOC) Water
Carbon monoxide Soluble organic fraction
Carbon dioxide Particulate matter
Normal exhaust gas flow Empty DPF filter Selective catalytic reduction (SCR) Urea Pressure sensor
Exhaust gases with particulates
Diesel exhaust gas Diesel particulate filter (DPF) Figure 2. 27 Injection Catalytic converter
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96
