Design Pressure vs Operating Pressure — Accurate Technical Explanation

Understanding the difference between Design Pressure and Operating Pressure is crucial for ensuring safety, code compliance, and cost-effective design in piping and pressure equipment within industries such as oil & gas 🛢, petrochemical , and power plants .

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🧭 1 Operating Pressure

 Definition:

Operating Pressure is the actual pressure at which the system operates continuously under normal, steady-state conditions.

Determined by process engineers.

Usually lower than the design pressure.

Used for process control, instrumentation, and normal plant operation.

Does not account for abnormal or transient events such as surges or valve closures.

📝 Example:

If a pipeline typically operates at 18 bar, then the Operating Pressure = 18 bar.

>  Important: Operating Pressure reflects real process conditions, not design or testing margins.

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📐 2 Design Pressure

 Definition:

Design Pressure is the maximum internal pressure expected under the most severe operating conditions and is used as the basis for mechanical design of piping systems, pressure vessels, and components.

Determined according to applicable codes (e.g., ASME B31.3, ASME Section VIII).

Includes a safety margin above the operating pressure to cover:

Pressure surges 🚀

Upset or abnormal conditions 

Valve closure or blockages 🔒

Used to calculate pipe wall thickness, flange ratings, valve selection, and test pressure.

📝 Example:

If the operating pressure is 18 bar, the design pressure may be set at around 21 bar or higher, depending on the code and safety factors.

> 📌 Note:

Design Pressure is not the same as Hydrotest Pressure.

Hydrostatic test pressure is typically 1.5 Design Pressure, following ASME requirements.

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🧪 3 Key Differences

Parameter 📝 Operating Pressure  Design Pressure 📐

Definition Actual pressure during normal operation Maximum pressure used for mechanical design

Value Lower Higher (includes safety margin)

Determined By Process engineers Design engineers according to codes

Purpose Operation, control, instrumentation Wall thickness, flange rating, valve selection, testing

Code Reference Not directly defined by design codes Defined in ASME B31.3 / ASME Section VIII

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🌍 4 Why This Distinction Matters

Correctly defining both pressures ensures:

🔸 Safety prevents overstressing or failure.

📏 Optimized design avoids unnecessary over-thickness.

🧰 Proper component selection valves, flanges, fittings.

🧪 Code compliance with ASME, API, and other standards.

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