LOSS PREVENTION
OUR SERVICES
- Hazardous Area Classification
- Plant Inspection in Hazardous (Classified) Areas
- Investigation of Accidents and Hazardous Events
- Emergency Planning
- Process Plant and Manufacturing Complex Layout Studies
- Evaluation and Specification of Firefighting and Protection Systems
- Firefighting Systems Inspection
- Evaluation and Specification of Secondary Containment
- Relief Systems Specification
- Explosion Protection Systems Specification
Relief Systems Specification
Industrial facilities or process equipment may be subjected to over-pressures or excessive under-pressures in function of:
– Fire on surrounding areas;
– Malfunction or process upsets;
– Equipment or utilities failures;
– Weather conditions changes;
– Exothermic chemical reactions.
To achieve a safer project, and also get the most economical solution, one should always consider the following possibilities:
• Can the risk of over-pressure or under-pressure be eliminated by means of changes in the process or in the plant design?
• Can the risk of over-pressure or under-pressure be reduced through inventories reduction or by changing process conditions?
• Can the over-pressure or under-pressure be contained by equipment designed to withstand the likely maximum pressure?
• Can one consider an alternative safeguard to the relief system?
• Can the relief systems be minimized by mechanical or instrumented systems?
Explosions certainly cause over-pressure, but this subject will be discussed in the next chapter. Pressure relief systems for the present purpose refer to cases when a pressure increasing occurs within the time interval of several seconds or more, and there is no reaction front. In these cases, one can assume that:
• Relief valves or rupture disks are able to open in due course;
• The piping is properly sized to accommodate the relief flow;
• The relief flow can be determined by equations of flow in steady-state regime;
• Flow conditions are approximately equal throughout the phase at any time (two-phase or three-phase flow);
• The additional pressure generation due to reaction in relief piping is negligible.
The following general principles are applicable to an adequate relief system:
• Relief systems must be selected and placed in order to minimize plant upsets and environmental damages;
• Relief systems should not be isolated from the equipment to be protected while the equipment is operating;
• The relief device discharge should be routed to a safe location, such as:
– Catch-pot;
– Upstream the process;
– Storage tank;
– Vessel or cooling tower;
– Drainage system;
– Atmosphere;
– Knock-out drum;
– Absorption column;
– Incinerator;
– Flare stack.
The design basis, as well as the calculation methodology, of all relief systems should be part of the project documents, and their verification must be incorporated into the management of change procedures.