Flare Header Over-pressure Protective System using HIPS
In the chemical process industry, a key safety consideration is the control and response to over-pressure situations. Traditionally, pressure relief valves and flares were used to handle the relieving of vessels from over-pressure in the worst case scenario. When units are expanded, modified, or when a new unit is being integrated into a plant, existing flare capacity may be inadequate. Flare capacity, an essential safety design feature, is normally sized on the basis of handling the largest release resulting from a single contingency for a unit. Conventional design of over-pressure protection systems require additional flare capacity either by installing another flare system or reducing contingencies of existing flare systems.
An alternative is to apply High Integrity Protective System (HIPS) to reduce some single contingencies to double contingencies, thereby allowing continued operation without compromising safety, or requiring additional expansion or investment in the flare system.
A properly designed and applied High Integrity Protective Systems (HIPS) may be used to reduce loads to existing flare systems or provide additional safeguards where conventional pressure relief devices have proven to be unreliable. The use of HIPS also conforms to ISA S84 "Application of Safety Instrumented Systems for the Process Industries" and the Draft International Electrotechnical Commission (IEC) 61508 Standard "Functional Safety: safety-related systems", Parts 1 through 7.
A simplified typical distillation column and flare design is shown in the Figure.
During normal operation, the distillation column/tower receives feed, which is heated and vaporized in one or more reboilers, then rises through trays, where one or more distillates are drawn off. Thermal energy is introduced by controlling (as a function of product analyzer and/or column temperature and/or pressure) steam or other heat carrying media to the reboiler. Energy is removed in an overhead condenser (not shown).
Should column pressure or temperature rise above the normal operating range, the control system will reduce heat addition by closing the steam control valve. Should column pressure continue to rise and approach the pressure rating of the column, a self actuated Pressure Relief Valve (PRV) opens, discharging to the flare system thereby reducing column energy and pressure. The PRV discharge capacity exceeds the capacity of the heat source to add energy.
The conventional practice for addressing over-pressure is to install one or more PRVs on every vessel. Depending on the material, the PRV discharges to atmosphere or closed system. The PRV is sized to provide an energy discharge path from the system that is greater than the ability to add energy to the system. Column damage will probably result if the PRV fails to open.
The flare system is designed to conservatively manage the simultaneous discharge of all four columns. If one or more column PRVs fail to open on process demand, then the pressure excursion will likely rise to excessive levels in those columns. A hazard analysis indicates the greatest risk of column over-pressure damage is caused by PRV failure and the most frequent PRV demand initiator is loss of cooling to the overhead condenser.
Where temperature excursions may result in overstressing a vessel, pressure relief devices cannot preclude vessel damage. In these cases, a basis for appropriate protection in the form of high or low-temperature alarms/cut-outs, control instrumentation, isolation, depressurizing, quenching, material selection, and/or other means must be developed.
An alternative method for addressing over-pressure is to install
a protective system that automatically isolates the energy source when over-pressure
is detected. Similar systems are currently used where PRVs provide inadequate
protection, i.e. reactor high temperature and/or pressure, boiler and furnace
fuel isolation systems. Instrumentation may have to be configured as a HIPS
consisting of a series of redundant sensors, redundant logic control, and redundant
isolation devices. (voting 2 out of 3, etc.).
To address safety and reliability, HIPS criteria include:
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