Process Vents and Drains

Process vents and drains, including emission control devices, are often overlooked but are important elements in the safety of batch systems. Inadequate attention to these items can result in incompatible chemical mixtures within the 

In addition to the information presented in this chapter, refer to Chapter 3, Equipment Configuration and Layout, for further discussions on shared vent and drain systems. 

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Auxiliary process fluid piping inclndes vent and drain lines, balance lines, product flushing lines, and lines for injection of external fluid. 

The purging steps should he logged, including times at which each vent and drain was opened, and any analytical results (153). Coloring a column process and instrumentation diagram (P ID) at the completion of each pm-ging step is also helpful. 

After purging, and before introducing the process gas, the column is leak-tested. The most common leak-testing technique is pressuring the column up with inert gas, with all vents and drains closed, and monitoring the rate of pressure loss. At the start of the test, the pres-... 

P IDs show all the process lines in a unit, including valves, material specs, and insulation detail. The P ID may or may not include minor piping such as vent and drain lines and tubing. They provide a pictorial representation of all equipment, instmmentation, and piping in a facility. They are crucial documents in almost all types of safety study and analysis. 

Sections of the process that contain large quantities of hazardous chemicals should be contained, i.e., the block valves around them should be closed, but the vent and drain valves may be opened, depending on the contents of the vessels. 

Consider common scrubbers, vents, sumps, drains, off-gas treatment and other opportunities for inadvertently mixing process materials. Cross-contamination potential at transfer stations should not be overlooked. 

Whenever it is necessary to control the process level, pressure, temperature, etc., a control station is installed. A control station may be as simple as a single control valve or it may contain several control vahes, block valves, bypass valves, check valves, and drain or vent valves. 

Process equipment liquid drains should be provided with a sealed drainage system where it is practical and backpressure from the system or containmenation is not a concern. Open drain ports should be avoided and separate sewage and oil water drains provided. Surface drainage should be provided to remove liquid spills immediate and effectively from the process area. Vents on drainage systems should be elevated so as to freely disperse hydrocarbon gases above congested areas that could be released from the system. 

At the end of the washing step, the liquid is drained to the operative tank by the pump, and through the button filter, FB, then the gas-recovery step takes place to recover most of the vapour or gaseous CO2 from the chamber. As the gas-recovery can be assumed to be like an adiabatic process, the fluid in the chamber is pre-heated by an electric resistance heat exchanger, HE2, to contain the reduction in temperature. The CO2 vapour is compressed by the oil-free compressor CV and fed to HE1 to condense before being stored in the operative tank. Finally, the chamber is vented and the door can be opened to recover the cleaned garments. 

Enclosures are another form of double containment that prevent (or delay) a release from reaching the environment until proper action can be taken to stop, and drain or vent, the material in a controlled manner (CCPS, 1988a). Use of enclosures is common where other containment systems would be impractical, as in the case of vapor or volatile liquids. In areas of severely cold weather, where most of a process may already be indoors, a form of enclosure already exists to some extent and certain containment principles already apply. 

Process vents, high point vents, and low point drains ... 

Pressure pipes — water supply and distribution, agricultural irrigation, chemical processing nonpressure pipes — drain, waste and vent pipes, sewer systems, conduits for electrical and telephone eables... 

Liquid drains, manifolded and taken to a heater for vaporizing, process safety valves, vents and thaw lines, together with the major casing vents, all culminate in a vent stack to the atmosphere. This stack should have its exit point well above surrounding equipment to prevent it from being a hazard to personnel in the event of a fire. A continuous flow of nitrogen at a velocity of 1 ft/sec can be supplied at the bottom of the vent stack to blanket all process and casing equipment from air. A check valve placed in this line provides added protection. 

As a by-product of dehairing and tanning process Provide separate sewage lines and cover and vent waste drains add neutraUzing agents (CaCla) as appropriate local exhaust ventilation... 

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