Published on 09 Aug 2019 by Arun Patel
There are options to minimize the hazards that may be thermally unstable or have a tendency to react with other chemicals present. Some options include:
- Trays without outlet weirs.
- Proprietary designs and sieve trays.
- Wiped film evaporators.
- An internal baffle in the base section to minimize hold-up.
- Reduced base diameter.
- Vacuum distillation to lower temperatures.
- Smaller reflux accumulators and reboilers.
- Internal reflux condensers and reboilers where practical.
- Column internals that minimize holdup without sacrificing operation efficiency.
- Specify design pressures high enough to contain pressures generated during exothermic reactions and avoid opening the relief valve and/or rupturing the vessel.
- Use physical limits of pipe size; restrictive orifices, and pump sizing to limit excessive flow rates.
- Consider the incident avoidance benefits of reliable equipment when specifying hardware.
- Use inherently safer strategies when establishing company design standards, guidelines, or practices.
- Use gravity flow in plant layout where feasible to minimize the need for pumps or solids handling equipment for hazardous materials. Conduct a hazard review to assess the effect of layout on potential spills.
- Review injection points for erosion concerns. Design for lower velocities.
- Use materials of construction that enhance inherently safer operations. Corrosion leads to leaks; incompatible materials can lead to unwanted reactions.
- Use materials with low corrosion rates for the process.
- Use the right alloy for the job (more expensive is not necessarily better).
- Use materials that are applicable over the full range of operating conditions such as normal, startup, routine shutdown, emergency shutdown, and draining the system. For example, carbon steel may be acceptable for normal operating conditions but may be subject to brittle fracture at low temperatures under abnormal conditions (as in the case of a liquefied gas). Cold water, of less than 60°F, during hydrotest may cause brittle fracture of carbon steel.
- Avoid materials that crackor pit; uniform corrosion is safer than nonuniform corrosion patterns.
- Avoid incompatible materials that could come into contact in abnormal conditions.
- Do not use copper fittings in acetylene service, or titanium in dry chlorine service. These principles also apply to gaskets, lubricants, and instruments.
If possible, eliminate inherently weak equipment like sight glasses, hoses, rotameters, bellows, expansion joints, and most plastic equipment. The spare parts consumption from the shop and warehouse will indicate what is wearing out.
Minimize contamination via fewer cross-connections and fewer hose stations. Minimize the number of hoses required in loading/unloading facilities. Cross-contamination, sometimes even from catalytic amounts of material, can result in undesired hazardous reactions. To prevent contamination due to rainwater and spills, consider storing a material that can react vigorously with water under a roof.
Flexible connections should never be used as a cure for improper piping alignment and piping support concerns. Where expansion joints are required in piping systems containing toxic materials, consider using double-walled expansion joints with pressure indication between the two walls for leak detection. All welded pipe is preferable to flanged piping, and threaded piping should be avoided for flammable and toxic materials.