Establishing Steam Balances

Building a steam balance is an importent step toward effective energy management because it can give an overview as to how much energy is generated versus how much is actually used. The gap can direct attention to improvement opportunities in operational and capital projects. Inaccurate steam balance could mislead evaluation of energy improvment projects. 

Although energy is used in the processes, the cost is measured and paid at the boiler house for any amount of imported natural gas, power, and steam. As an asset manager or asset supervisor, your ultimate goal in managing the steam and power system is to satisfy the process energy demand in the lowest expense, which is measured by the invoice the boiler house receives on a regular basis. 

It is importent to establish a steam and power balance. To do this, a steam balance must feature high accuracy. A good steam balance can teU how much is used versus generation, and thus can quickly identify opportunities of low hanging fruit. 

We will leam two things from this chapter. The first is how to build a complete steam balance from scratch illustrated by a real example, while the second is to verify the balance to make it more trustworthy. 

Energy and Process Optimization for the Process Industries, First Edition. Frank (Xin X.) Zhu. 

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Although the details of establishing steam balances are discussed in Chapter 16 while steam equipment modeling equations are given in Chapter 15, the intention here is to combine all the related equations to develop a complete simulation and optimization model for the steam system. For specific applications, users can revise the model by providing specific configuration and conditions for the steam system. 

Application Remarks You do not need to feel intimidated with the set of equations above. You will find the task simple when you follow the top-down approach to establish steam balances from boilers to each steam header, to steam turbines and letdown valves, and to deaerator and blowdown flash drum. The top-down approach for steam balance is discussed in detail in Chapter 16. During the process of setting up the steam balances, you could apply some of the equations above for modeling the equipment and subsystems. The steam balances can be conducted readily in a spreadsheet environment. 

When heating with steam, a selection of the proper steam pressure level must be made when designing the evaporator. No definite rules for the selection can be established because of changing plant steam balances and availability. However, it is generally more economical to select the lowest available steam pressure level which offers a saturation temperature above the process temperature required. Some evaporator types require relatively low temperature differences. Some products may require low temperature in order to reduce fouling or product degradation. 

Step 2 Establish the Base Case of Steam Balance... 

Once the highest steam level is set, then intermediate levels must be established. This involves having certain turbines exhaust at intermediate pressures required of lower pressure steam users. These decisions and balances should be done by in-house or contractor personnel having extensive utility experience. People experienced in this work can perform the balances more expeditiously than people with primarily process experience. Utility specialists are experienced in working with boiler manufacturers on the one hand and turbine manufacturers on the other. They have the contacts as well as knowledge of standard procedures and equipment size plateaus to provide commercially workable and optimum systems. At least one company uses a linear program as an aid in steam system optimization. 

The bulk of the samples for this study came from TVA s Allen Steam Plant at Memphis, Tenn. The sampling points (Figure 4) included inlet air, coal, bottom ash, precipitator inlet, and outlet at the 268-ft stack level. During the 2-week sampling period the unit was operated under steady state conditions at 240 MW (12) with a uniform coal supply so that a mass balance might be established for a number of elements. All the coal from southern Illinois was washed and crushed so that 90% was less than 4 mesh. Nominal coal analysis indicated the following composition 9.5% moisture, 34% volatiles, 43% fixed carbon, 13% ash, and 3.4% sulfur. 

Saxena and Pathak (3) made systematic studies of BPH-rice plant interactions, particularly behavioral and physiological responses involved in BPH establishment on rice plants. They found that resistant plants were as suitable as susceptible plants in eliciting some responses. The interaction of all the responses determined the overall susceptibility or resistance to the pest. Saxena and Pathak (3) Saxena and Puma (30), and Saxena and Okech (31) also determined the biochemical basis of suitability of rice varieties to BPH. They found that allelochemics and nutritive balance of rice varieties were important in eliciting optimal or suboptimal responses, thereby affecting BPH ability to establish on rice plants. The steam distillate extracts of resistant varieties and of the barnyard grass were repellent and, when applied topically, caused high mortalities even at low doses. In contrast, extracts of susceptible varieties possessed moderate to high attractance and were relatively nontoxic to the insect. Recently, Obata et al. (32) isolated and identified constituents of BPH attractant in the Japanese rice cultivar Nihonbare . 

There is usually little problem in obtaining capital costs from the vendors, but operating costs cure often more difficult to establish. This is particularly true if the equipment utilizes an unconventional form of "fuel (such as steam, waste heat or a pressure drop) instead of or in conjunction with the more common fuels (such as coal, oil, or electricity). Not only can it be more difficult to price accurately the unconventional fuels, but also variations in the rate at which these fuels cure used may have significant impact on the flows and balances in the rest of the plant, of which the equipment is to be a part. 

The hot water generated is flashed in a separate drum as steam and balance water are returned to the heating coils of the unit for reheating. The water circulation is established through risers and down comers by natural convection instead of a pump. 

By use of the expression for the one-dimensional heat transfer coefficient in Equation (3.26) it is now possible to establish the heat transfer balance for the one-dimensional model for a tubular reformer tube considering only steam reforming of methane. The equation is established from the two-dimensional model in Equation (3.15), the boundary condition in Equation (3.17) and the derived heat transfer coefficient for the one-dimensional model in Equation (3.26) as ... 

The American Boiler Manufacturers Association has established standards for boiler water balances in its standard steam purity guarantees. These are identified as ABMA Limits and are listed in Table 7.2. 

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