Process steam pressure

An example is shown in Figure 15.14. By raising steam at high pressure (say, 60 bar-absolute and 540°C) and then expanding this through a turbine to the process steam pressure requirements of 3 bar then useful work can be done by the turbine for generation of electrical power. For this example, each kg/s of steam gives 590 kW of electrical power. 

The first parameter to resolve is the process steam pressure or pressures. These will be dictated by the process-heating... 

In the typical case, the maximum pressure and temperature of the boiler are set by the manufacturer and, consequently, the ratio is determined by the turbine exhaust pressure, i.e. the process steam pressure. Typical values of this ratio are given in Table 5.3 and indicate that only low values of power to process heat ratio can be met with this type of cogeneration scheme. 

Thermal Oligomerization. Commercial manufacture of dimer acids began ia 1948 with Emery Industries use of a thermal process involving steam pressure. Patents were issued ia 1949 (45) and 1953 (46) that describe this process. Earlier references to fatty acid oligomerization, antedating the USDA work of 1941—1948, occur ia patents ia 1918 and 1919 (47,48), and ia papers written ia 1929—1941 (49—51). There appears to still be some small use of this approach to making dimer products. 

For most chemical plants, process steam is used at pressures of 1.825 MN/m" (250 psig), saturated or lower. When combined heat and power generation is economically justified, the steam may be generated at about 5.96 MN/m" (850 psig) appropriately superheated and used to drive back-pressure steam turbines passing out process steam at the required pressure level. 

Process Steam Generation. Steam generated in the process sections of the plant may be at the highest plant pressure level or an intermediate level. Also, the process area may have fired boilers, waste heat boilers, or both. There may be crossties between utility and process generated steam levels. Enough controls must be provided to balance far-ranging steam systems and protect the most critical units in the event of boiler feedwater shortage situations. 

One of the first questions the designer must answer concerns which type steam turbine should be used. The back pressure turbine is. selected when process steam demands are greater than the steam required for process drivers such as large compressors. This type turbine is also selected when various steam levels are required by the process. 

The condensing turbine is selected when steam demand for process drivers is greater than the low-pressure process steam requirements. It is also selected when no high pressure steam is available. 

Process steam can be supplied at two or more pressures without having to purchase boilers operating at different pressures or having to throttle steam, which is a waste of useful energy. 

Process steam requirements can be controlled at a suitable pressure and volume required by the process and maintained at these conditions by extraction or induction turbines. 

A mechanic was affected by fumes while working on a steam drum. One of the steam lines from the drum was used for stripping a process column operating at a gauge pressure of 30 psi (2 bar). A valve on the line to the column was closed, but the line was not slip-plated. When the steam pressure was blown off, vapors from the column came back through the leaking valve into the steam lines (Figure 1-3). 

The steam for process heating is generated in either fire or water-tube boilers, using the most economical fuel available. The process temperatures required usually can be obtained with low pressure steam (tyq ically 25 psig), and steam is distributed at a relatively low pressure (typically 100 psig). Higher steam pressures are needed for high process temperatures. 

---