Steam is a convenient and highly efficient heat transfer medium that can also be used to perform mechanical work or to generate electricity on industrial and commercial sites. The operating costs associated with steam systems vary widely from one site to the next, and even on small sites, I often find that these costs can easily dwarf those of energy carriers such as electricity (I’m of course not including electrode boilers in this comment). South Africa’s upcoming Carbon Tax also places the spotlight on Scope 1 emissions, to which steam generation is a major contributor. It is therefore essential, except for the very smallest of steam systems, that detailed system-level assessments are carried out to identify energy efficiency and cost reduction opportunities. So what does this mean?
There are 4 major components in any steam system:
- Steam generation i.e. the boilers – this is where steam is produced from feed water
- Steam distribution – this is the piping network and associated equipment through which steam is transported to point of use
- Steam users – the most diverse aspect of a steam system, this is where steam is used in production processes and for utilities such as large HVAC systems. The steam can be incorporated into product, injected directly for heating or used to transfer heat in heat exchangers. In the latter case the steam is condensed back to liquid water (called condensate)
- Condensate recovery – condensed steam is a valuable resource containing high-quality water and thermal energy, and should be returned to the boilers for incorporation into boiler feed water, allowing it to be converted back into steam.
Steam systems can also include turbines, which lie between the “distribution” and “user” aspects of the system and are hence mentioned separately here. These can be used to drive equipment (e.g. pumps or compressors or any piece of equipment requiring shaft work) or to generate electricity. Back-pressure turbines have significant system-level efficiency advantages, but require a detailed evaluation of the site concerned to ensure that the preconditions for their use are met. The opportunity for their application in my home country, South Africa, which remains embroiled in a power crisis, is enormous and remains largely untapped.
The steam system itself is embedded within a wider system comprising the site and local environment that it operates in. This adds even more complexity, but cannot be ignored. The steam system interfaces with this wider system at various touch points, all of which have an impact on system efficiency, operating costs and emissions.
In my experience across a wide range of industries, I find that most organisations tend to tackle steam at the component level, dealing with individual aspects of the steam system rather than addressing the system as an integrated whole. Hence there may be projects to address boiler efficiency, or to insulate parts of the distribution system, or perhaps projects aimed at reducing the amount of steam used or the amount of condensate returned to the steam raising plant. In most cases there are benefits. The problems with this piecemeal approach are however two-fold:
- It can lead to unintended consequences
- It can lead to (often very large) missed opportunities
Let me illustrate with just a few examples how system effects impact on the evaluation of individual steam-related energy efficiency opportunities:
- Changes in user pressure: If the pressure at which steam is used is changed, this impacts on the amount of flash steam produced during condensate recovery. A seemingly simple change in pressure by steam users can therefore have a marked impact on the viability of projects such as flash tanks or vent condensers (in the case of a pressure decrease) or increase flash and trap losses (in the case of a pressure increase).
- Changes in steam usage profile: If the nature of operation of low-pressure steam users is changed from a continuous operation to an intermittent operation, this could limit the opportunity for the use of back-pressure turbines, or make existing back-pressure turbines redundant. There can also be significant impacts on boiler efficiency as boilers are made to operate outside of their optimum loading range.
- Fuel switching projects: These can have major impacts on operating costs, in some cases reducing costs by several multiples. The implications of such fundamental change are that all system-related projects require re-evaluation. This includes heat recovery projects, which are very dependent on the costs of the heat being substituted. A heat recovery project (e.g. water heating using heat recovered from an oil-flooded screw compressor) that may have been considered attractive where a site uses fuel oil to generate steam, may become marginal should that site switch to coal as a fuel source. The same applies to heat recovery projects within the steam system itself e.g. economisers, air pre-heaters and blow down heat recovery systems.
The above are only a few limited examples, but illustrate the potential dangers associated with taking a component-level approach to steam systems optimisation. I am not suggesting that micro-level optimisation opportunities should be ignored – they can and must be investigated and implemented. The point though is that these types of opportunities, while important, still require some consideration of potential system-level risks before implementation. They are also not able to unlock the quantum of value available through a systems approach. If you are not taking a systems approach to the management and optimisation of your steam system, chances are you are missing out on the types of opportunities that could be game changers for your system’s efficiency.
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