Steam purging

The effectiveness of steam purging depends on heat loss from the stack. Thus, weather conditions are a factor. With purge steam in the flare stack, isothermal conditions exist from the bottom flange of the stack upward to an elevation determined by the weather and purge rate. Above this elevation, the stack wall temperature decreases with height. 

The isothermal zone is air-free. Air concentration rises with declining temperatures above the isothermal zone. The appearance of a steam plume from the stack exit is no guarantee of nonflammable conditions. 

The relationship between oxygen concentration and temperature suggests that a stack can be kept nonflammable by temperature control. The steam rate to the stack would be controlled by a differential thermocouple with one junction in the isothermal zone and the other in the declining temperature zone. The latter junction could be placed at the elevation below which non-flammable conditions are to be maintained. 

With thermocouples located in the stack wall, the temperature differential for the 6% oxygen concentration for stacks of 10 to 24 inches in diameter varies 17 to 30° F. For thermocouples located axially within the stack so that the water 

Actual requirements of a specific installation are governed by the sum of the heat losses from the stack drum and collecting lines. Steam purge tests of a refinery installation indicated an additional 

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Water addition to vessels that have been steam-purged If vacuum conditions can develop, then either the equipment must be designed for vacuum conditions or a vacuum relief system must be instiled. 

Shutting off steam at the completion of steam purging without admitting a noncondensible vapor (e.g., air at shutdown, fuel gas at startup). 

Improperly designed, eroded, or even missing restriction orifices used for steam purge or aeration nozzles could cause catalyst attrition. Catalyst attrition is also caused by broken air and stripping steam distributors. 

J.2 Minimize Use of Aeration and Purge Steam. Excess fines will be generated by overuse of aeration and steam purges during cracker operation. 

A rupture disk is a thin sheet of metal installed below the valve, intended to protect the relief valve from plugging. The rupture disk ruptures at the relief-valve-set pressure. A better approach to retard this plugging problem is to maintain a steam purge, or inert-gas bleed, below the relief valve to prevent the accumulation of solids below the valve. 

The Farmer-Norton steam-purge system is extremely effective in promoting a thorough saturation of the fabric with chemical pre-treatment liquors. 

Vertical steam-heated exchangers must also be vented to remove carbon dioxide and other gases which can accumulate under the top tubesheet. Corrosion of the shell, tubes, and tubesheet, especially in the area just opposite the steam inlet nozzle, may result if adequate venting is not provided. Sometimes a continuous steam purge or intermittent venting is recommended. 

Figure 6-20. Thermal profiles of stack wall at a steam purge rate of 10 Ibs./min.

When it is not practical or possible to use thermocouples in a stack, the data of Figure 6-20 offer an estimate of steam purge requirements. Assuming that the 6% oxygen level can be located by a 30°F difference in wall temperature, the 10 pound per minute steam rate in the 24-inch diameter stack affords only mai inal protection on very cold windy days and days of heavy rain. However, the rate is excessive on warmer days. 

Because safe steam purge rate is governed largely by heat loss from the stack, safe rates can be estimated on the basis of stack surface area. Thus, a rate of 7.5 pounds per minute per foot of diameter is a safe minimum for stacks of comparable height, if the collecting lines and drum are insulated and the lines are not long,... 

Once steaming is completed, the column should be opened to let air or inert gas in on shutdown, or placed under gas pressure (inert gas such as nitrogen or fuel gas, or process gas) at startup. The alternative of leaving small steam purges to keep the column pressure up at the conclusion of steaming is often unsatisfactory and resulted in creation of vacuum in some instances (3). Unless the column is fully open to the atmosphere, so that no vacuum can be created, a pressure of at least 5 to 10 psig should be kept in it (3). 

Adequately locate steam-purged relief valves, install a conductivity probe in feed line. 

When a steam purge is applied at the base of the column relief valve (to prevent the valve and its inlet line from plugging), the relief valve should be located so that any condensed water drains into sections of the system that can tolerate it (e.g., the reflux drum). In one case (239), this condensed steam dripped back into the column during short outages, repeatedly causing pressure surges upon restart. 

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