Steam utilization system

Find a way to overcome the constraint while still maintaining the areas. This is often possible by using indirect heat transfer between the two areas. The simplest option is via the existing utility system. For example, rather than have a direct match between two streams, one can perhaps generate steam to be fed into the steam mains and the other use steam from the same mains. The utility system then acts as a buffer between the two areas. Another possibility might be to use a heat transfer medium such as a hot oil which circulates between the two streams being matched. To maintain operational independence, a standby heater and cooler supplied by utilities is needed in the hot oil circuit such that if either area is not operational, utilities could substitute heat recovery for short periods. 

Utility systems as sources of waste. The principal sources of utility waste are associated with hot utilities (including cogeneration systems) and cold utilities. Furnaces, steam boilers, gas turbines, and diesel engines all produce waste from products of combustion. The principal problem here is the emission of carbon dioxide, oxides of sulfur and nitrogen, and particulates (metal oxides, unbumt... 

On chemical plants and oil refineries, steam, nitrogen, compressed air. lubricating oil, and other utility systems are responsible for a disproportionately large number of accidents. Flammable oils are recognized as a hazard, but services are given less attention. If the modification to the lubricating system had been systematically studied before it was made, as recommended in Chapter 2, a larger vent could have been installed, or a pipe-break and funnel could have been installed at the inlet to the sump. 

This simplistic approach to costing steam is often unsatisfactory, especially if the utility system already exists. Steam costs will be considered in more detail in Chapter 23. 

Figure 23.2 shows a schematic representation of a boiler feedwater treatment system. Raw water from a reservoir, river, lake, borehole or a seawater desalination plant is fed to the steam system. However, it needs to be treated before it can be used for steam generation. The treatment required depends both on the quality of the raw water and the requirements of the utility system. The principal problems with raw water are1,2 ... 

While this basic definition of cogeneration efficiency seems straightforward, complications are created by the process steam generated from waste heat recovery that can be used for power generation or process heating and that does not require any fuel to be fired in the utility system. The heat supply can be defined as the sum of the heat from fuel (both in the utility boilers and fired heaters) and steam generation from the waste heat recovery (see Figure 23.44)17 ... 

Consider an example involving a site where there are four steam mains with their steam usage and generation detailed in Table 23.3. The utility system can use a gas turbine with a power output of 25 MW, if appropriate. The gas turbine is integrated with a HRSG17. 

Process machines can, in some cases, be equipped with both a steam turbine and an electric motor, allowing the drive to be switched between the two according to steam demands in the utility system and operating costs. It should be remembered that operating costs might vary significantly as a result of different electricity tariffs, according to the time of day, the day of the week and the season of the year. 

Figure 23.52 shows the maximum, minimum and current flows in each part of the system. Ancillary data for the utility system in Figure 23.52 are given in Table 23,49. Data for the fuels used by the utility system in Figure 23.52 are given in Table 23.59. Data for the steam turbines are given in Table 23.6. 

Table 23.6 Steam Figure 23.529. turbine data for the utility system in...

Figure 23.54 An existing utility system with existing steam loads. (From Varbanov P, Perry S, Makwana Y, Zhu XX and Smith, 2004, Trans IChemE, 82A 784, reproduced by permission of the Institution of Chemical Engineers.)...

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... 

The only way to reconcile the true cost implications of a reduction in steam demand created by an energy reduction project is to use the optimization techniques described in the previous section. An optimization model of the existing utility system must first be set up. Starting with the steam load on the main with the most expensive steam (generally the highest pressure), this is gradually reduced and the utility system reoptimized at each setting of the steam load. The steam load can only be reduced to the point where the flowrate constraints are not violated. 

The concept of steam marginal cost is used as an indicator in the analysis. It is defined as the change in utility system operating cost for unit change in steam demand for a given steam main (change in steam main balance)17 ... 

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