Pressure relief is the layer of protection that only matters on the day everything else has already failed. A blocked outlet, a control valve stuck open, a cooling water failure, a runaway reaction, a fire under a vessel: the relief device is what stands between an abnormal condition and a loss of containment. It is also, on a lot of older plant, the layer nobody has recalculated since the process changed.
We design, size and document pressure relief systems for process manufacturers across the UK, and we review existing ones where the basis has been lost or the duty has moved on. The output is a set of calculations somebody else can follow, check and defend, because that is what a relief case is for.
What the work involves
A pressure relief system is more than the device itself. It is the relief valve, rupture disc or vent, the discharge piping that takes the relieved fluid somewhere safe, and where required the collection or disposal arrangement that handles a toxic, flammable or high-pressure discharge. The system has to open at the right pressure, pass the required flow, and reseat when conditions return to normal, without creating a second hazard on the way out.
The engineering starts with scenarios, not devices. For each item of equipment we identify the credible overpressure cases: blocked outlet, control failure, utility loss, thermal expansion, external fire, and the others that apply to the duty. We then establish the worst credible relief load, size the device against recognised standards including API 520 and API 521 and ISO 4126, and select materials, set pressures and discharge configuration to suit the fluid and the location.
Where the relief case interacts with instrumented protection, we say so. A relief device and a safety instrumented function are different answers to the same question, and the sensible design often uses both. Every assumption is written down, because a relief calculation whose basis is undocumented is not much better than no calculation at all.
Where relief sizing usually goes wrong
Two failure modes dominate. The first is a relief case that was correct for the original duty and has never been revisited, despite a change of feedstock, a debottlenecking, a new heat exchanger or a higher throughput. The device is still there, still tested, and no longer sized for the plant it is protecting.
The second is a relief case that exists as a number with no visible working. Somebody sized it, the answer went on a data sheet, and the scenarios, assumptions and fluid properties behind it were never recorded. It cannot be checked, so at the next HAZOP or insurer review it has to be redone from scratch anyway.
Both are avoidable, and both are cheaper to fix before an assessor finds them. We will also tell you where a system is over-engineered. Oversizing is not the safe option it looks like: an oversized valve can chatter, and unnecessary relief capacity carries cost through the discharge system and the flare.
What you get
- A documented set of credible overpressure scenarios for each protected item
- Relief load calculations for the worst credible case, with assumptions and fluid data recorded
- Device sizing to recognised standards, including API 520, API 521 and ISO 4126
- Specification of relief device type, set pressure, materials and discharge configuration
- Discharge and vent routing assessed rather than assumed
- A view on where instrumented protection should carry part of the case
- Documentation suitable for HAZOP, insurer submissions and regulatory audit
- Review and revalidation of existing relief cases where the design basis has been lost
When clients typically call us
- A process change, uprate or new feedstock has invalidated the original relief basis
- A HAZOP has raised an overpressure scenario with no relief case behind it
- Relief device sizing exists as a number on a data sheet with no supporting calculation
- An insurer, an auditor or a regulator has asked to see the relief study and it cannot be produced
- A new plant or unit is in detailed design and the relief system needs specifying
- An existing installation is suspected of being oversized and the discharge system is a constraint
Frequently asked questions
How often should relief cases be reviewed?
Whenever the basis changes. A new material, a different operating pressure or temperature, a change in throughput, a modified heat input, a revised layout: any of those can move the governing scenario. Relief cases held outside change control stop being reliable, which is the point at which they become a liability rather than a protection.
Can a safety instrumented function replace a relief valve?
Sometimes, and sometimes not. It depends on the scenario, the required risk reduction and what your own standards allow. Where an instrumented function is credited in place of mechanical relief it has to be specified, verified and maintained to match, which is a real commitment rather than a shortcut. We will set out both options honestly.
Do you work on existing plant or only new design?
Both, and revalidation of existing systems is the larger part of it. We reconstruct the design basis from the plant as it stands rather than from documents that describe how it used to be.
Who signs off the calculations?
Experienced process and mechanical engineers, working under our ISO 9001:2015 quality system, with input from control and instrumentation and piping where the case needs it.
Related: Part of our engineering design services. See also P&IDs and process design, HAZOP facilitation and SIL determination and functional safety.
If your relief cases predate the plant you are running now, or exist as numbers nobody can reproduce, that is worth putting right before someone else asks. Talk to us about your plant. Request a call