High integrity pressure protection systems (HIPPS) are used in the process industries to reduce the risk that a system can exceed its design pressure.
Conventional design standards applied in the oil and gas sector stipulate that the final protection of piping and pressure vessels against over-pressurization shall be provided by means of mechanical relief devices.
Instrumented protection systems provide an alternative solution to this problem in situations where:
- Extremely high pressures or flow rates are processed.
- Environmental constraints or restrictions limit the use of conventional relief systems.
- The economic viability of a development needs improvement.
- The risk profile of the project must be further reduced.
Introduction
HIPPS are a type of safety instrumented system (SIS). In HIPPS, instruments provide the safety function. The safety loop consists of one or more initiators, final elements and a logic solver. Either completely mechanical components or a combination of mechanical and electrical components shall be used. All components shall be failsafe in the de-energized mode. With HIPPS, the protection against over-pressure is obtained by quickly isolating the source causing the over-pressure, reducing the plant’s risk profile. (Refer to Figure 1.)
Figure 1
Results and Observations
Since HIPPS is a type of SIS , the requirements of each specific HIPPS application must be investigated thoroughly. The SIS standards are performance-based with the safety integrity level (SIL) as the primary performance measurement. Each SIL level corresponds with a tolerable probability of failure on demand (PFD) to all HIPPS devices (group) from the sensor to the logic solver.
A HIPPS group consists of (refer to Figure 2 below):
- Initiators/Process Sensors
The elements measuring the process variables (PV) to sense an event are called the initiators. The initiators are typically electro-mechanical pressure switches or electronic pressure transmitters.
- Logic Solver
The programmable electronic system (PES) receives the signals from the initiators, performs the required logics (voting, diagnostics and partial stroke test routine) and drives the final elements.
- Final Elements
The complete valves/actuators/solenoids assemblies are acted on to isolate the over-pressure detected by the initiators. The ball, gate or globe valve is typically designed to close within two seconds.
Figure 2
Conclusion
The justification for HIPPS in the process industries is typically facilitated by an analysis performed by each plant. Many factors must be considered to establish the proper framework for a successful installation.
The HIPPS includes the entire instrument loop from the field sensor through the logic solver to the final elements, along with other possible required devices, such as SIS user interfaces and communications.
References
- International Electrotechnical Commission (IEC), IEC 61511, “Functional Safety: Safety Instrumented Systems for the Process Sector,” Geneva, Switzerland
- International Society of Automation (ISA), ANSI/ISA S84.01, “Application of Safety Instrumented Systems (SIS) for the Process Industry,” Durham, NC
- International Electrotechnical Commission (IEC), IEC 61508, “Functional Safety of Electrical/Electronic/Programmable Electronic Safety Related Systems,” Geneva, Switzerland
