Managing the energy efficiency performance of any system is not possible without measurement. With the plethora of possible performance measures available, and the fact that measurement requires resources, it is important to be selective. A “shotgun” approach to performance indicators with no clear measurement strategy can be expensive to implement and also ineffective in terms of supporting continuous improvement. In this post I’d like to point out the two critical performance indicators at the heart of steam system monitoring and optimisation. While it is true that further supporting measurements and performance indicators are needed once these measures are in place, they are an excellent place to start in your pursuit of an optimised steam system.
Your steam system needs to generate steam as efficiently as possible and it also needs to use that steam as efficiently as possible. In addition, any condensate produced from heating or power generation processes should be recovered as efficiently as possible, in terms of the quantity recovered as well as the temperature of the condensate. If all of these areas are managed well, you will be operating an efficient steam system. It is true that the drivers of efficiency in these areas are diverse and complex, and can require a range of field measurements to enable robust short-interval control. It is however possible, with just two basic plant measurements, to gain insight into overall steam system energy efficiency performance.
The first measure to put in place would be the quantity of fuel used per unit of steam generated, or a variant of this measure, such as the quantity of fuel energy per unit of steam energy. This measure tells us slightly more than a measure such as boiler efficiency does. Boiler efficiency highlights the efficiency with which the energy in the boiler fuel is added to boiler feed water to produce steam. If feed water temperature increases, say as a result of increasing condensate recovery rates, this will not be reflected in the boiler efficiency measure. The fuel to steam ratio will however capture the impact of both boiler efficiency as well as condensate recovery rates, and will also capture the effectiveness with which the heat contained in the condensate produced is preserved.
The measurements needed to calculate the fuel to steam ratio are the fuel rate and the steam rate. The plant instrumentation required to measure these parameters depends on the type of fuel used and the degree of accuracy required. Many sites use feed water measurements instead of steam measurements – bear in mind that if you use feed water flow as a proxy for steam flow you would need to account for boiler blowdown. If the energy content of the fuel is consistent and the pressure of the steam produced is consistent, the measure can be used in its most simple form. If the fuel characteristics vary, as can be the case with fuels such as coal or biomass, compensations would have to be made to the calculation. This would also be the case should steam generation conditions vary widely. It does however depend on the location of the steam meters used e.g. is the meter installed downstream of a pressure reducing valve?
The second measure to put in place would be the quantity of steam used per unit of product produced. The objective would clearly be to minimise the amount of steam used per unit of product. This measure captures the impacts of a wide range of potential inefficiencies in your steam system. These would include the wastage of steam at individual user points, steam leaks and excessive flash losses. Where a uniform product is manufactured, this measure is straightforward to implement. For sites producing a range of products, this measure can be tricky, as product mix can significantly influence results, and if ignored, can lead to this measure being misleading. In such instances it may be useful to reference all products which use steam to a base product, and to adjust the units produced accordingly. Hence if product A requires twice as much heat energy as product B, and product A is taken as the base product, each unit of product B would be the equivalent of 0.5 units of product A for the purposes of calculating this ratio. Products that do not require steam could be left out of the calculation.
If you measure and trend the amount of fuel required to produce a unit of steam and the amount of steam used to produce a unit of product you will have a good sense of whether your steam system performance is improving or not, and also have an idea of where in your steam system the required improvements may be achieved.
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