Steam and Power Balances

Steam and power balances provide the link between the process utility requirements and the utility supply. The 

An energy balance around the deaerator assuming the enthalpy of the vented steam to be saturated at the deaerator pressure gives  

Steam flow and conditions of the inlet steam are known. Outlet conditions are usually fixed by the downstream main conditions, and hence Equation 23.40 can be solved. 

The flash drum pressure will be known as this will be set by the pressure of the main into which the flash steam will be recovered. The flash steam and condensate outlet enthalpies are set by saturation conditions. 

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In assessing the true cost benefits associated with a steam saving, the steam and power balance for the site utility system must be considered, together with the costs of fuel and power (or power credit in an export situation). In general, a surplus of steam resulting from a steam saving in a process demand can be exploited by... 

Modeling of key equipment in the steam and power system is essential for building steam and power balances, which can represent process steam and power demands and respond to variation in production. 

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. 

For tackling this challenge, the method based on marginal analysis can indicate true steam costs at point of use. The reason why the marginal price method (Chapter 17) can provide the true steam cost is that it is based on the last incremental amount of steam saved or generated. Determination of marginal steam prices relies on an overall steam and power balance model, which takes into account the effects from steam balances. 

The above points indicate that the steam and power balance of an ammonia plant is of major importance for the overall efficiency. 

Figs. 6.21, 6.22, and 6.23 show only one possible layout of the steam system in large ammonia plants. Other similar systems are described in [31, 615, 718, 723, 724, 949]. Other types of drivers for ammonia plant compressors than steam turbines are discussed in [398] (electrical motors) and [399, 400] (gas turbines). Steam and power production for ammonia plants in cogeneration units is described in [725-727, 921]. Steam and power balances in ammonia plants and ammonia-urea complexes are discussed in [959] with special reference to the use of gas turbines. 

Additional operations essential to commercial bauxite processing are steam and power generation, heat recovery to minimise energy consumption, process liquor evaporation to maintain a water balance, impurity removal from process liquor streams, classification and washing of ttihydrate, lime caustication of sodium carbonate [497-19-8] to sodium hydroxide [1310-73-2] repair and maintenance of equipment, rehabiUtation of mine and residue disposal sites, and quaUty and process control. Each operation in the process can be carried out in a variety of ways depending upon bauxite properties and optimum economic tradeoffs. 

A combination of small capacity turbines (extraction and backpressure type) may be considered with the balance amount steam sent to a condensing-type turbine for power generation. Cost of steam and power shall also be taken into account before the final decision is taken. 

The purpose is to develop a steam balance for operational supervision as well as for identification of improvement opportunities in the steam system. Models for boilers, turbines, deaerators (DAs), letdown valves, desuperheaters, and steam flash tanks are discussed in the previous chapter. Historian and distributed control system (DCS) data will be coimected to steam balance so that the steam balance is capable of dynamically balancing the steam and power demands due to process variations, units on or off, and weather change. 

The second characteristic of the marginal steam price is that aU incremental steam change must be traced back to the fuel, BFW, and power balances so that the net effects on operating cost can be determined. This is the essence of marginal steam price. 

In the following section, energy balances in ammonia production will be discussed. The difficulties related to the comparison of energy consumption in different process schemes will be dealt with, and finally a description will be given of the main concepts in the integration of ammonia production units and steam and power systems. 

The approach to the problem is to estimate the amotmt of power that can be obtained from the process steam with various initial steam conditions. In this manner a balance between available steam and power demand is determined, and as a preliminary step the appropriate initial steam condition is selected. A check on the enthalpy at the turbine exhaust then indicates possible adjustment of the initial steam temperature to obtain approximately diy steam at the point where the process steam is used. Occasionally, heavy demands for steam in excess of the power load may be provided for by suppl3ung the additional steam throu a reducing valve directly from the boilers. Supplementary power for peak loads may be obtained from an outside source or from a condensing unit. 

There are two ways of presenting steam balance data, schematically or tabulady. For both presentation types, a balance is made at each pressure level. In a schematic balance, such as that shown in Figure 9, horizontal lines are drawn for each pressure. The steam-using equipment is shown between the lines, and individual flows are shown vertically. Table 3 contains the same data as shown in Figure 9. In both cases the steam balance has been simplified to show only mass flows. A separate balance should be developed that identifies energy flows, including heat losses and power extraction from the turbines. 

Often in plant operations condensate at high pressures are let down to lower pressures. In such situations some low-pressure flash steam is produced, and the low-pressure condensate is either sent to a power plant or is cascaded to a lower pressure level. The following analysis solves the mass and heat balances that describe such a system, and can be used as an approximate calculation procedure. Refer to Figure 2 for a simplified view of the system and the basis for developing the mass and energy balances. We consider the condensate to be at pressure Pj and temperature tj, from whence it is let down to pressure 2. The saturation temperature at pressure Pj is tj. The vapor flow is defined as V Ibs/hr, and the condensate quality is defined as L Ibs/hr. The mass balance derived from Figure 2 is ... 

Figure 12-5F. Lubricated and nonlubricated balanced opposed process reciprocating compressors, designed to API 618 code. Fixed- and variable-speed drives using gas or diesel engines, steam or gas turbines, or electric motor. Note power drive to connect to right side of cross-head box in center. (Used by permission Bui. PROM 635/115/95-11. Nuovo Pignone S. P. A., Florence, Italy New York Los Angeles and Houston, Texas. All rights reserved.)...

Having ascertained the process steam flow and developed some ideas on the boiler pressure, the following step is to analyze the power available. Figure 15.23 provides a ready means of determining the approximate relationship between power available and process steam for specific steam conditions. Use of this and similar charts will allow an assessment to be made of the potential of a CHP scheme with a backpressure turbine. The conditions can be changed to give the required balance for heat and power. 

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