API 14C (1998), API 521 (1997), and Code Case 2211 (1995) of ASME Section VIII, Division 1 and 2, provide alternatives in the design of overpressure protection systems. These alternatives revolve around the use of an instrumented system that achieves a level of safety that meets or exceeds the protection provided by a pressure relief valve and flare system.
Any instrumented system used to provide over-pressure protection is a safety-related system, since its failure would result in the rupture of the pipeline/vessel or in overloading the flare. As a safety-related system, the instrumented system must meet either the United States domestic ANSI/ISA S84.01-1996 (1996) or the international standard Draft IEC 61508 (1998, 1999). Due to the high likelihood that the instrumented system would be needed and the high severity of the consequence should these fail, the SIL assigned per the standards is often 3 (or simply as high as achievable with redundant architecture, high availability devices, and frequent proof testing). Due to the high availability requirements, these over-pressure protection systems are often called “high integrity protection systems” or HIPS.
Industry is increasingly moving towards utilizing HIPS to reduce flare loading. They are becoming the option of choice to help alleviate the need to replace major portions of the flare systems in existing facilities when adding new equipment or units. The relatively low capital cost of HIPS compared to flare system piping upgrades and the ability to install HIPS without incurring significant additional downtime during a turnaround, makes these systems an extremely attractive option.
However, prior to making the choice to install the HIPS, the regulatory and industrial standards pertaining to their design must be well understood. Due to the unique nature of the HIPS application, certain design aspects must be carefully evaluated. Any company considering HIPS is cautioned to do a thorough hazard evaluation prior to the implementation of HIPS.
HIPS do not differ greatly from other trip systems. The systems are composed of field-input devices, a logic solver and final elements. The necessity for high availability and reliability is where the differences truly begin. Redundancy in field devices is utilized to provide a high level of availability while, at the same time, increasing reliability. Typically, the inputs are configured in a two-out-of-three voting basis, the logic solver should have high availability, and the final elements are configured one-out-of-two. The design of any HIPS should be quantitatively verified to ensure it meets the required availability.
Care must be taken in any decision to implement HIPS. The use of HIPS should be generally restricted to the reduction of relief and flare loading in existing facilities. The use of HIPS should not be a justification for reducing the pressure relieving requirements on individual pieces of equipment. The pressure relieving of vessels should be sized for the worst credible scenario for each piece or groups of equipment irrespective of the HIPS design.
Advantages of HIPS:
• Low capital costs compared to upgrading flare systems
• Can be installed without incurring additional downtime during a turnaround
Disadvantages of HIPS:
• HIPS require that many different components work as designed.
• Effectiveness of system is highly dependent on the field design, device testing, and maintenance program.
• Limit of knowledge in the identification of all over-pressure scenarios
• HIPS becomes the “last line of defense”. Failure results in potentially over-stressing of vessel.
