Introduction
The conventional answer to overpressure is mechanical: a pressure safety valve lifts, the excess flows to a flare or vent, and the equipment never exceeds its design pressure. It is simple, passive, and trusted. But there are cases where the relieving load is so large that the flare system needed to handle it becomes absurd — a header the size of a small pipeline and a flare tip you can see from the next field. The classic case is a high-pressure source feeding lower-rated equipment: a well or pipeline that can deliver 200+ barg into topsides or a flowline rated for half that. Size the relief for the full source flow and the flare system dominates the whole facility.
A High Integrity Pressure Protection System (HIPPS) is the instrumented alternative. Instead of relieving the overpressure, it isolates the source — fast-acting valves slam shut before the downstream pressure can reach the design limit. Done properly, it removes the relief case entirely, or shrinks it to something a sane flare can handle. The catch is that you are now relying on an instrumented function to prevent a loss of containment, which means proving it to a high Safety Integrity Level and maintaining that integrity for the life of the field.
This post covers what a HIPPS does, the trade against full relief, the architecture that gets it to SIL 3, and the testing burden that comes with it. HIPPS is the instrumented end of the pressure relief and flare systems spectrum, and its integrity target comes straight out of the process safety lifecycle.
What a HIPPS Actually Does
A HIPPS is a Safety Instrumented Function with one job: detect rising pressure approaching the protected equipment's limit and close isolation valves fast enough to stop the source before the limit is reached. It is governed by IEC 61511 (and IEC 61508 for the devices), with subsea systems also covered by API 17O, and it is almost always required to achieve SIL 3 — a probability of failure on demand (PFDavg) between 10⁻³ and 10⁻⁴.
The defining constraint is time. There is a finite process safety time between the pressure starting to rise and the equipment being overpressured. The HIPPS — detection, logic, and valve closure — must complete well inside that window. A HIPPS valve that strokes in three seconds is useless if the system overpressures in two. The whole design hangs on the relationship between how fast the pressure can rise and how fast the valves can shut.
HIPPS vs Full Relief — the Trade
The decision between conventional relief and a HIPPS is a trade between a large passive system and a high-integrity active one:
- Full relief (API 521) is passive and proven — no power, no logic, no proof testing of a complex function. But for a large high-to-low pressure interface, the relief valves, headers, knockout drum, and flare can be enormous, heavy, and expensive, and on a weight-limited offshore deck they may simply not fit.
- HIPPS removes or drastically reduces that relief case. The hardware is small — transmitters, a logic solver, two valves — but you take on a high-integrity instrumented function that must be designed, verified, and tested for the life of the field, plus the regulatory burden of demonstrating that an instrumented system can stand in for mechanical relief.
A HIPPS is not a way to save money on instruments — it is a way to avoid a flare system that would otherwise dominate the facility. The instrumented cost and the lifetime testing are real and ongoing; the justification is that the alternative is worse. Regulators and the project's own risk process must accept the HIPPS as a credible protection layer before it can offset the relief duty, and that acceptance rests entirely on the SIL verification.
Anatomy of a HIPPS
A SIL 3 HIPPS is built from redundancy at every stage, because no single device is good enough on its own:
- Sensors — 2oo3 pressure transmitters. Three independent transmitters voting two-out-of-three: any two agreeing on high pressure trips the system. This tolerates one failed or spurious transmitter without either failing to act or tripping needlessly — it protects both safety and availability.
- Logic solver. A certified safety PLC (or dedicated HIPPS logic), independent of the basic process control system, executing the voting and trip logic with its own diagnostics.
- Final elements — two valves in series. Two independent fast-closing valves (each with its own actuator and solenoid), so that a single valve failing to close does not defeat the function. Partial-stroke testing (PST) lets each valve be exercised online without a full shutdown, extending the proof-test interval.
The architecture exists to drive down the PFDavg to the SIL 3 band while keeping spurious trips tolerable. Common-cause failure — the same fault disabling redundant channels at once — is the enemy, so the verification explicitly accounts for it through a beta factor; physical and functional separation of the channels is what keeps that factor low.
SIL 3 and Proof Testing
Claiming SIL 3 is not a paper exercise. The PFDavg is calculated from each element's failure rate, the voting architecture (MooN), the diagnostic coverage, the common-cause beta factor, and — critically — the proof-test interval. PFDavg grows with the time between proof tests, so the SIL claim and the test frequency are locked together: a longer interval means a worse PFD, which may bust the SIL band.
That is why partial-stroke testing matters. Fully proof-testing a HIPPS means demonstrating the valves close on a real demand, which is disruptive. PST exercises the valve part-way online, catching the dominant "stuck valve" failure mode between full tests and letting the full-test interval stretch without breaching the SIL. The maintenance and testing regime is therefore part of the design — the SIL claim is only valid if the facility actually executes the tests at the assumed interval, for the life of the field. A HIPPS that is verified to SIL 3 on paper but not tested on schedule is not SIL 3 in practice.
Worked Example — Pipeline Protection at an HP/LP Interface
Scenario: a platform delivers gas into an export pipeline rated for 100 barg. On a control or choke failure, the upstream source can impose up to 200 barg on the pipeline inlet. Two ways to protect it:
Option A — full relief. Size relief valves for the full credible flow at the pipeline inlet. The relieving rate is large; the resulting relief headers, knockout drum, and flare are heavy and costly, and on this platform there is no deck space or weight budget for them.
Option B — HIPPS. A SIL 3 HIPPS monitors the pipeline inlet with 2oo3 transmitters and closes two fast SDVs in series to isolate the source before the pressure reaches 100 barg. The key sum is the time budget: how fast can the pressure climb from the trip setpoint to the 100 barg limit, versus the detection-plus-closure time of the HIPPS. If the valves close in, say, 2 seconds and the system would take longer than that to overpressure from the trip point, the HIPPS protects the pipeline and the large relief case disappears — replaced by a defined testing and integrity obligation for the field life.
The verification: the project then demonstrates PFDavg < 10⁻³ from the transmitter, logic, and valve reliability data, the 2oo3 / 1oo2 architectures, the common-cause beta factor, and the assumed proof-test interval — and shows the regulator that this instrumented layer credibly replaces the mechanical relief.
Common Pitfalls
- Ignoring the process safety time. A HIPPS is only valid if detection plus valve closure completes inside the time the system takes to overpressure. Always check valve stroke time against the rate of pressure rise — a slow valve defeats the whole concept.
- Treating the proof-test interval as flexible. PFDavg and SIL depend directly on the test interval. Stretch the interval in operations beyond the design assumption and the SIL claim is void. Design the testing regime, then actually run it.
- Underestimating common-cause failure. Three transmitters on one impulse line, or two valves sharing an instrument-air header, defeat the redundancy. Separate the channels physically and functionally and account for the beta factor honestly.
- Assuming the HIPPS removes all relief. Regulators may still require a residual relief device or accept the HIPPS only for specific scenarios. Confirm what the HIPPS is credited for before deleting relief from the design.
- Skipping partial-stroke testing. Without PST the full-test interval is short and disruptive, or the SIL band is missed. Design PST in from the start.
- No regulatory engagement. Substituting an instrumented function for mechanical relief is a decision the regulator and the safety case must accept. Leaving that to the end of the project risks a redesign back to full relief.
Conclusion
A HIPPS earns its place where the relief load is too large to flare sensibly — most often a high-pressure source feeding lower-rated equipment, where conventional relief would mean a flare system that dominates the facility. It trades that passive mechanical system for a high-integrity instrumented one: fast valves closing on a 2oo3 vote before the equipment is overpressured.
The price of that trade is rigour. Prove the function to SIL 3 with redundant sensors, an independent logic solver, and dual final elements; verify the PFDavg honestly, including common-cause failure; and commit to the proof-testing and partial-stroke regime for the life of the field. Get all of that right and the HIPPS removes a relief case that would otherwise be unmanageable. Treat it as a box-ticking exercise and you have replaced a trusted mechanical safeguard with an instrumented one you are not actually maintaining — which is worse than the flare you were trying to avoid.
