A General Guide to Overpressure Protection

1.0 Introduction

Properly designing an overpressure protection system is a critical safety task. A well-designed system identifies all credible overpressure scenarios, correctly sizes a relief device for the governing case, and ensures the relieved fluid is disposed of safely. This guide outlines the fundamental principles for identifying scenarios, selecting devices, and understanding correct installation procedures, based on common industry standards (like API 520 & 521).

2.0 Types of Relief Devices

A relief device is designed to open at a predetermined pressure to protect a vessel or system from exceeding its design pressure.

Spring-Loaded Pressure Relief Valves (PRVs)

Conventional Valves

This is the most common, simple, and least expensive type of re-closing relief device. A spring holds the valve disc closed against the process pressure.

Balanced Valves

This valve uses a bellows or piston to "balance" the disc, isolating it from the effects of back-pressure. The bonnet is vented to ensure the bellows area is not pressurized.

Pilot-Operated Relief Valves

This design uses the process pressure itself to keep the main valve sealed. An external "pilot" senses the pressure. At the set point, the pilot triggers, venting the pressure from the top of the main valve's piston, which then opens fully and rapidly.

• Key Advantage: Can be set very close to the system's operating pressure without leaking (simmering).
• Good for: High inlet pressure loss, high back-pressure (up to 70%), and low-pressure applications.
• Warning: The small pilot sensing lines can be prone to plugging in fouling, waxy, or hydrating services.

Liquid Service (Thermal) Relief Valves

These are small, spring-loaded valves designed specifically to relieve non-flashing liquid caused by thermal expansion. They do not "pop" open like gas-service valves; they open proportionally to the pressure increase.

Rupture Disks

Rupture disks (or bursting discs) are non-reclosing devices. They are a one-time-use metal disk designed to burst at a specific pressure.

• Pros: Faster acting than PRVs (good for sudden events like explosions), zero leakage, and can be made from corrosion-resistant materials.
• Cons: The entire system inventory is lost until the system depressurizes. They must be replaced after every event. They can fail from fatigue and are not adjustable.
• Common Use: Often installed *upstream* of a PRV to protect the valve from corrosive or fouling fluids.

Pin Actuated Devices

A non-reclosing alternative to rupture disks. A pin holds a piston or disk in place. At the set pressure, the pin buckles, allowing the valve to open fully. The pin is easily replaced to reset the device. They are often preferred over rupture disks as they are less prone to premature failure from fatigue or pulsating pressure.

3.0 Specific Equipment Requirements

Certain types of equipment have standard overpressure protection requirements.

• Positive Displacement Pumps & Compressors: Must always have a relief device on the discharge, as they can generate unlimited pressure if blocked. Rupture pin devices are often preferred here to avoid valve chatter.
• Centrifugal Pumps: A discharge PRV is only required if the maximum possible pump discharge pressure (at shut-off/no-flow conditions) exceeds the design pressure of the downstream piping.
• Steam Turbines: A PRV is required on the exhaust side if it can be blocked in, unless the casing is designed for the full inlet steam pressure.
• Fired Heaters: A PRV is required on the heater outlet piping. It must be sized to ensure flow through the heater tubes does not drop below ~30% of normal flow during a relief event to prevent tube overheating and rupture.

4.0 Design & Set Pressure Considerations

The allowable set pressure and accumulation (overpressure) are dictated by industry codes. The design pressure (DP) of the vessel is the reference point.

Note: When using multiple valves, the set pressures are "staggered" to prevent all valves from opening at the same time (chattering).

5.0 Identifying Overpressure Scenarios

The relief device must be sized for the "governing scenario," which is the event that produces the largest required relief flow rate.

Blocked Outlet

This common scenario assumes the normal outlet from a vessel is closed, but the inlet flow continues. This can be caused by a closed manual valve, a failed control valve, or a tripped downstream pump.

• Relief Rate Source: The relief load is the maximum inflow from the source, such as the rated flow of an upstream pump (at relief conditions) or the capacity of a compressor.

External Fire (Pool Fire)

This assumes the equipment is engulfed in a fire, causing the fluid inside to boil and expand rapidly.

Wetted Surfaces (Liquid-Filled)

For vessels containing liquid, the fire's heat input is absorbed by the liquid, causing it to boil. The relief rate is the mass of vapor generated, calculated by dividing the total heat absorbed (Q) by the liquid's latent heat of vaporization (H).

Un-wetted Surfaces (Gas-Filled)

For gas-filled vessels, the heat input expands the gas. This is extremely dangerous because the gas is a poor conductor of heat, causing the vessel walls to heat up rapidly.

Utility Failures

Site-wide or local utility failures can trigger multiple simultaneous relief events. These must be evaluated to determine the governing load for the main flare header.

• Electric Power Failure: Often causes loss of pumps and air-fin coolers (loss of condensing duty).
• Cooling Water Failure: Leads to loss of condensing duty on distillation columns, potentially causing massive vapor relief loads.
• Instrument Air Failure: Causes all pneumatic control valves to move to their fail-safe position. The failure position of every valve must be analyzed to determine if it causes overpressure (e.g., a feed valve failing open while a product valve fails closed).
• Steam Failure: Can cause loss of reboiler heat (reducing relief loads) or loss of steam-turbine driven pumps (potentially causing loss of reflux or cooling water).

Gas Blowby

This occurs when a liquid level controller fails, the vessel drains completely, and high-pressure gas "blows through" the liquid outlet into a low-pressure downstream vessel. This can be a catastrophic scenario if the downstream vessel is not designed for the high-pressure gas flow.

To correctly size the relief device, four combinations must be analyzed to find the worst case:

1. Vapor flow through the failed control valve, vapor flow through the PRV.
2. Liquid flow through the failed control valve, vapor flow through the PRV (due to flashing).
3. Two-phase flow through the failed control valve, two-phase flow through the PRV.
4. Vapor flow through the failed control valve, liquid flow through the PRV (if the downstream vessel overfills).

Other Common Scenarios

• Inlet Control Valve Failure: A control valve fails wide open, overpressuring a downstream system.
• Thermal Expansion: Liquid-full piping blocked in and exposed to heat (e.g., solar). Requires TRV.
• Check Valve Failure: High-pressure fluid flows backward through a failed check valve into a low-pressure system.
• Heat Exchanger Tube Rupture: High-pressure fluid enters the low-pressure side via a broken tube. Not typically required if LP design pressure is ≥ 10/13ths (77%) of HP design pressure.

6.0 Vacuum Relief Protection

Vessels not designed for full vacuum must be protected from collapsing due to internal vacuum conditions. Typical causes include liquid pump-out without adequate venting, condensation of steam/vapors (e.g., after a steam-out procedure), or ambient cooling of a closed vessel.

• Vacuum Relief Valves: Admit air into the vessel to break the vacuum.
• Gas Blanketing (Vacuum Breakers): Admit a safe gas (like nitrogen or fuel gas) to maintain positive pressure. This is preferred for vessels containing flammable hydrocarbons to avoid creating an explosive mixture with air.

7.0 Installation & Piping Guidelines

Inlet Piping (Critical)

Discharge Piping

Discharge piping (and headers) must be sized to handle the back-pressure created by the relieving fluid. This is especially important for conventional PRVs, which are limited to 10% back-pressure.

PRV Bonnet Venting

The vent on a PRV bonnet is critical for its correct operation:

• Conventional Valves: The bonnet vent should be plugged. The bonnet vents internally to the discharge side.
• Balanced Bellows Valves: The bonnet must be vented to a safe atmospheric location. It must NOT be plugged or piped to the discharge header. If the bonnet gets pressurized (e.g., due to a torn bellows), the balancing effect is lost, and the valve will act like a conventional valve, potentially failing to open at the correct pressure.

Pilot Sensing Lines

For pilot-operated valves, the pressure sensing line should ideally be connected directly to the protected vessel (remote sensing) rather than the valve inlet pipe. This ensures the pilot reacts to the true vessel pressure, unaffected by inlet piping pressure losses, providing maximum stability.

Isolation (Block) Valves

Isolation valves (typically gate valves) are required upstream and downstream of a PRV to allow for safe removal and maintenance. However, they are a significant hazard if left closed by mistake.

8.0 Discharge System Design

The destination of the relieved fluid depends on its properties. Safe disposal is paramount.

Discharge to Atmosphere

Permissible only for non-hazardous fluids (e.g., steam, air) or for hydrocarbon vapors *if* they can be released safely. For hydrocarbons, the discharge must be at a high velocity (typically >100 m/s) to ensure a jet-momentum release that disperses readily without forming a flammable cloud at grade.

Discharge to Closed System (Flare)

Required for any fluid that is toxic, corrosive, contains liquids (two-phase flow), has a high hydrogen content (>50%), or could form a hazardous mist. Flare systems must be designed to separate liquids (via a Knock-Out Drum) before the gas reaches the flare tip.

9.0 Instrumented Protection Systems (HIPPS)

A High-Integrity Pressure Protection System (HIPPS) is a type of Safety Instrumented System (SIS) that serves as an *alternative* to a pressure relief valve.

A HIPPS uses high-reliability sensors (e.g., pressure transmitters), a logic solver, and final elements (e.g., fast-acting shutdown valves) to rapidly stop the source of high pressure *before* the design pressure is exceeded. This is often used in cases where relieving a fluid is impractical, environmentally hazardous, or economically undesirable (e.g., on offshore platforms).