Safety Relief Valve vs Rupture Disc: 5 Key Engi Insights

Introduction – Safety Devices Compared: Safety Relief Valve vs Rupture Disc

When designing or optimizing a chemical plant, one of the most critical safety decisions is choosing between a safety relief valve (SRV) and a rupture disc. While both devices are intended to protect equipment, pipelines, and process units from overpressure conditions, they differ significantly in terms of function, construction, and operational behavior.

In this article, we will examine the pros and cons of each solution, combining theoretical definitions with practical insights based on real-world experience. The aim is to provide a grounded and useful perspective for those involved in plant design, Management of Change (MoC), or HAZOP reviews, following the guidance of the Perry’s Chemical Engineers’ Handbook, API 520/521 standards, and ISO 4126 specifications.

This content is intended not only for junior and experienced chemical engineers but also for project engineers, process safety professionals, and technicians working on turnkey gas distribution systems. In many practical cases, these professionals must respond to customization requests from clients: replacing a safety valve with a rupture disc, or integrating a high-pressure switch (PSH) downstream of a SRV and connecting it to the DCS, allowing real-time detection of overpressure events.

Construction Materials: How Rupture Disc and Pressure Safety Relief Valve Differ

The choice of construction materials plays a crucial role when selecting between a rupture disc and a safety relief valve (SRV), as it directly affects corrosion resistance, operational longevity, fluid compatibility, and maintenance requirements.

Rupture discs are available in a wide variety of metallic and non-metallic materials. Common metals include stainless steel, Hastelloy, Inconel, Monel, and pure nickel, selected for their performance under extreme temperatures and corrosive environments. For highly aggressive or reactive substances, non-metallic materials such as graphite, PTFE, or composite layers are often used. Thanks to their simple design and absence of moving parts, rupture discs can be manufactured with minimal thickness, ensuring rapid response and perfect sealing under normal operating conditions until rupture occurs.

Pressure safety relief valves, on the other hand, have a more complex mechanical structure, consisting of moving components such as springs, seals, seats, and plugs. These parts must be built from materials capable of withstanding repeated pressure cycles, particle-laden flows, and mechanical wear. Valve bodies are typically constructed from carbon steel, stainless steel, or special alloys, while internal parts may include bronze, brass, or high-performance polymers, depending on the process conditions and fluid properties. However, due to the presence of gaskets and contact surfaces, these valves are more vulnerable to localized corrosion, fouling, or mechanical degradation over time.

In summary, rupture discs offer greater flexibility in material selection based on chemical compatibility, while SRVs require more detailed evaluation of internal components to ensure long-term reliability. Selecting the right materials impacts not only the durability of each device but also their effectiveness in relieving pressure safely when needed.

Operation: How Safety Relief Valves and Rupture Discs Work

In any industrial process, choosing the right overpressure protection device means understanding how each option behaves under stress. Both the safety relief valve and the rupture disc serve the same purpose — protecting equipment from excessive pressure — but their mechanisms of action are fundamentally different.

Rupture Disc

A rupture disc is a non-reclosing device designed to burst instantly once the internal pressure exceeds a pre-set burst pressure. The membrane, typically made of metal or graphite, fractures completely and opens the flow path instantly and irreversibly. This rapid response makes rupture discs ideal for scenarios requiring immediate and total pressure relief, especially where toxic, flammable, or corrosive fluids are involved.

There are various disc types depending on the application:

• Graphite or composite discs, used for highly corrosive or extreme temperature environments
• Forward-acting discs, which rupture as pressure stretches the material
• Reverse buckling discs, which are pre-compressed and invert when the threshold is exceeded

Safety Relief Valve

A SRV is a mechanical device designed to open gradually when the system pressure exceeds a predefined set point. Once opened, it allows excess pressure to be released in a controlled manner and then automatically reseats itself when conditions return to normal.

This reusability is a major advantage in systems that must continue operating after a pressure relief event, such as compressors, steam networks, or chemical reactors. Unlike rupture discs—which must be replaced after activation—a safety relief valve can be reset and reused, minimizing process downtime and reducing replacement costs.

However, a key limitation is that a standard safety relief valve does not generate an automatic signal when it activates. Without additional instrumentation, an overpressure event may go undetected—an issue in facilities where real-time event tracking and diagnostics are essential.

If the HazOp study identifies the need to detect pressure relief events, there are options to consider:

• One common approach is to install a pressure switch (PSH) near the valve, connected to the DCS or PLC system. This setup provides a high-pressure alarm as the system approaches the set point, allowing preventive action before the valve opens.
• In some cases, it may be possible to install external indicators or switches to detect valve opening. However, this is not a standard feature of most SRV models. Always verify with the supplier whether the selected valve supports such instrumentation. It is equally important to consult the project engineering team to assess whether this type of signal is technically feasible, cost-effective, and aligned with the plant’s overall safety and automation strategy.

Operation Pressure Considerations

Rupture Discs and Operating Pressure

Rupture discs are highly sensitive to pressure fluctuations. If the normal operating pressure is too close to the burst pressure, even minor pressure spikes can trigger an unintentional rupture. This results in an unnecessary process shutdown and replacement cost — especially problematic in continuous operations.

To prevent this, industry best practices and guidelines like those in the Perry’s Chemical Engineers’ Handbook and API 520/521 recommend that the burst pressure be set so that continuous operating pressure does not exceed 70–90 % of the marked burst pressure, i.e. a margin of roughly 10–30 % in line with ASME BPVC VIII-1 UG-127 and ISO 4126-6.

However, too large a margin can also be dangerous — it may cause the disc not to rupture in time during a fast pressure rise. That’s why correct sizing and material selection are essential.

Safety Relief Valves and Operating Pressure

Relief valves are more tolerant to proximity between operating pressure and set pressure. But if the system pressure is too close to the valve’s opening pressure, a phenomenon called “chatter” or “flutter” may occur — the valve repeatedly opens and closes in rapid succession. This can lead to accelerated wear, seat damage, and unreliable performance over time.

To avoid this, maintain at least a 10 % (or 3 psi, whichever is greater) gap for spring-loaded SRVs as per ASME BPVC VIII-1 Appendix M; in pilot-operated or liquid services a 5 % gap is often acceptable. Some systems also incorporate back pressure compensators or modulating valves to improve stability.

Unlike rupture discs, relief valves can relieve pressure partially, which helps control spikes without necessarily shutting down the process. But this flexibility comes with a trade-off: potential leakage and more maintenance.

Maintenance and Replacement

Maintenance is often treated as a secondary concern when choosing a pressure relief device — until the day something fails. In reality, understanding the upkeep requirements of rupture discs and relief valves is crucial for designing a reliable and cost-effective system.

Rupture Discs: Low Maintenance, But One-Time Use

Rupture discs are praised for their simplicity. With no moving parts, they require virtually no routine maintenance under normal operating conditions. Their sealed design reduces the risk of buildup, fouling, or internal corrosion — especially when selected with the right material for the process.

However, once activated, they must be replaced entirely, and this replacement involves:

• Shutting down the affected line
• Installing a new calibrated disc
• Performing a leak test (in many regulated sectors)

This means that while routine upkeep is minimal, the replacement cost and downtime after activation can be significant. It’s also essential to store spare discs on site and check their shelf life, as some thin metal or graphite types can degrade over time.

Safety Relief Valve: Reusable, But Require Regular Attention

Relief valves offer the advantage of being reclosable and reusable, but this comes at a price: they need regular maintenance and inspection to stay compliant and functional.

Common maintenance tasks include:

• Visual inspection of springs and seals
• Cleaning of internal passages
• Recalibration of the set pressure
• Full bench testing after servicing

In corrosive or dirty service, relief valves may clog, corrode, or stick, especially if not isolated by a rupture disc. Over time, wear on the seat or moving components can lead to leakage or delayed opening, compromising the entire protection system.

Many industries adopt a preventive maintenance schedule, testing valves every 6 to 24 months depending on the process risk level and regulatory framework (API, ISO, or local codes).

Valve or Disc? Choosing Relief Devices in Plants

In the chemical industry, rupture discs and relief safety valves can be configured in different ways depending on the process, safety requirements, and maintenance strategy. In this article, we focus on general single-device installations — either a rupture disc or a relief valve — to highlight the core differences and guide your selection process.

However, one widely used and technically valid setup is to combine both devices, with the rupture disc installed upstream of the relief valve.

Why Combine the Two?

This configuration offers multiple technical and operational benefits:

🔹 Protection against corrosive or contaminated media
The disc acts as a physical barrier between the process fluid and the valve. This prevents aggressive vapors or dirty fluids from reaching the valve’s internal components, reducing the risk of fouling, seal damage, or corrosion.

🔹 Lower maintenance needs
Valves exposed to salts, particulates, or organic residues can lose performance or fail to reseat. Using a rupture disc as a passive shield helps extend the valve’s service life and ensures correct operation when pressure relief is required.

🔹 Perfect tightness under normal conditions
Unlike some valves that may leak slightly over time, rupture discs offer zero leakage until their burst pressure is reached. This is especially critical when handling toxic, flammable, or environmentally hazardous materials.

Other configurations do exist — such as placing a disc downstream of a valve, sandwich setups, or parallel arrangements — but these are more specialized and depend heavily on plant-specific considerations.

To explore these configurations in depth, consult the following references:

• Perry’s Chemical Engineers’ Handbook, Chapter 23: Safety Equipment and Relief Systems
  (Available through McGraw-Hill – 9th Edition or later)
• API Standard 521 – Pressure-Relieving and Depressuring Systems
• ISO 4126-3:2020 – Safety Devices for Protection Against Excessive Pressure – Part 3
  
  

Design and Sizing Criteria: How SRVs and Rupture Discs Differ in Engineering Calculations

When it comes to protecting pressurized equipment, proper sizing and selection are essential for both safety relief valves (SRVs) and rupture discs. However, their design principles and engineering calculations differ significantly.

SRV sizing is based on detailed fluid dynamic analysis, taking into account:

• Set pressure and allowable overpressure
• Flow characteristics (gas, liquid, or two-phase)
• Required relief capacity under worst-case scenarios
• Backpressure effects
• Valve coefficient (Kᵥ or Kᵈ)

The goal is to ensure that the valve can relieve the required mass flow rate while remaining stable and avoiding issues like chatter or flutter. Standards like API 520 – Sizing, Selection, and Installation of Pressure-Relieving Devices and API 521 – Pressure-Relieving and Depressuring Systems are typically used as the basis for calculation.

Rupture disc design, on the other hand, is simpler in terms of fluid dynamics but requires careful consideration of:

• Burst pressure at a specified temperature
• Tolerance and manufacturing range (typically ±5% to ±10%)
• Operating pressure ratio (usually operating pressure must be ≤ 70–90% of burst pressure)
• Fragmentation (non-fragmenting discs for clean service vs. fragmenting for rapid decompression)
• Disc type (forward-acting, reverse buckling, scored, or composite)

While rupture discs do not require spring selection or valve lift calculations, they demand tight control over process conditions to prevent premature failure or delayed activation.

Strategic Considerations: Where and How to Vent Safely

When designing a pressure relief system, choosing between a rupture disc and a safety relief valve is only part of the decision-making process. Equally important is determining where and how the relieved fluid will be discharged. This aspect directly affects plant safety, regulatory compliance, and long-term operational reliability.

Evaluating Discharge Paths

Every relief device must be assessed not only for its set pressure but also for the discharge location and the consequences of releasing the process fluid:

Catch Tanks
These can be suitable for certain liquid services. However, if the relieved substance is volatile, flammable, or reactive, a catch tank may create secondary hazards such as vapor buildup, uncontrolled reactions, or explosive atmospheres.

Atmospheric Venting
A common approach, especially for non-toxic, non-flammable gases. However, it requires careful design to ensure proper dispersion, prevent re-ingestion, and remain compliant with environmental and safety regulations. Elevated vent stacks are often required.

Flare Systems
These systems are used to safely combust flammable or toxic gases. While they provide effective risk mitigation, they also introduce significant design complexity, cost, and operational requirements.

Special Focus: Hydrogen Venting

Hydrogen poses unique challenges in pressure relief systems due to its specific physical and chemical properties:

• High flammability
• Extremely low molecular weight, leading to rapid diffusion and escape through even the smallest leaks
• Broad flammable range in air, from 4% to 75%

Because of these factors, using a rupture disc to vent hydrogen into a closed system or a catch tank can be extremely hazardous. It may cause the formation of an explosive atmosphere, which in turn triggers ATEX compliance for all downstream equipment, including:

• Storage tanks
• Instrumentation
• Piping and enclosures

What is ATEX?

ATEX refers to two European directives for controlling explosive atmospheres:

• Directive 2014/34/EU: Applies to equipment and protective systems intended for use in potentially explosive atmospheres.
• Directive 1999/92/EC: Relates to the safety of workers potentially at risk from explosive atmospheres.

You can find official information here:

👉 According to EU ATEX guidelines – European Commission, the vent stack is certified, designed for rapid dispersion, and positioned away from ignition sources and personnel zones.

Best Practice: Vent to Atmosphere

In many industrial scenarios, the safest option is to vent hydrogen vertically into the atmosphere using a dedicated vent stack, designed for:

• Fast dispersion
• Location away from ignition sources
• Clearance from personnel and equipment zones

To ensure safety and regulatory compliance, engineers must:

• Simulate hydrogen dispersion (CFD or other validated models)
• Determine the impact zone
• Adjust the vent height or orientation based on the results

This engineering evaluation is essential to minimize the risk of delayed ignition or flammable cloud formation.

Recommendation: Case-by-Case Evaluation with HazOp

Each process fluid and plant environment is unique. For this reason, it is strongly recommended to conduct a Hazard and Operability Study (HazOp) to evaluate:

• The most suitable venting strategy
• The potential consequences of different discharge paths
• Interactions with other equipment and safety systems

Thanks for Reading

Ing. Ivet Miranda

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