Startup phases

The flowchart on the next page shows an overview of steps for the pre-startup and startup phases of a typical tolling process. 

Some toll processes lend themselves to test runs in the pre-startup phase. Actual materials for the toll may be used in the test or substitute materials, typically with low hazard potential, are often used to simulate the charging, reaction, and physical changes to be accomplished in the toll. Flow control, temperature control, pressure control, mixing and transferring efficiency can be measured. Mechanical integrity can be verified in regard to pumps, seals, vessels, heat exchangers, and safety devices. 

The tolling team has analyzed the hazards, addressed the risks and modifications using a management of change system, revised and written procedures, trained the workers, performed a PSSR, and completed any required test runs. The pre-startup phase is complete at this point. 

Modern nitric acid plants are designed for energy self-sufficiency during normal operation. Except for the startup phase, process heat generated will equal energy consumed by the compressors. Moreover, in many cases surplus energy can be exported in the form of steam, for example. 

Starting up the turbocompressor installation of a nitric acid plant does not present a problem. As mentioned earlier, during the startup phase the expander is not able to contribute any power. Accordingly, electric motor drivers must initially provide power in excess of the nominal operational rating. 

Smooth starts are possible with steam turbine installations that include an auxiliary boiler because the startup phases of the turbocompressor can be matched to different plant operating conditions. Aside from process-related timing issues, the time elapse or machinery startup duration is generally determined by the temperature gradients admissible on both steam turbine and expander. These factors are influenced by the relative expansion rates of the fixed and rotating components in these machines. 

Figure 4-35 shows typical starting times for different train configurations. The different startup phases include ... 

The aeration requirements of the deep shaft (vertical shaft bioreactor using flotation technology) are provided by two, 100 hp rotary screw compressors rated at a pressure of 7 kg/m2 (100 psig). Dissolved oxygen (DO) levels of 4 mg/L are maintained in the head tank, and during the startup phase of the plant a DO meter measured a dissolved oxygen concentration of 25 mg/L at the shaft bottom. 

One of the first operations in the startup phase is the flushing of all lines using air or water. Its purpose is to remove any foreign material that may have gotten into the lines. This includes welding rods, nails, plastic lunch bags, handkerchiefs, and the like. To do this, the water should flow at a velocity exceeding 12 ft/sec (3.6 m/sec) and the air at a velocity above 200 ft/sec (60 m/sec). 

Troyan13 tells how to spot certain symptoms during the startup phase so that problems may be averted. Some examples follow. Noisy equipment or a higher-than-normal temperature may mean poor lubrication. This could be due to plugging because of dirt that accumulated during construction. Vibration may mean that the equipment should be anchored better. Excessive pressures or unexpected vacuums may be due to spray painting that clogged ventilators. In one case, a tank was filled with water and then spray-painted. When the tank was emptied, a vacuum was formed that collapsed it. 

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