Determination of Design Pressure and Design Temperature: Engineering Standards 2026

The Role of Operational Data in Determination of Design Pressure and Design Temperature

The Determination of Design Pressure and Design Temperature begins with a rigorous analysis of the process simulation. Operational data provides the baseline, but the “Normal Operating Point” is rarely the design point. Engineers must scrutinize Heat and Material Balances (HMB) to identify the “worst-case” scenario. This includes startup, shutdown, regeneration cycles, and emergency depressurization. For instance, a vessel might operate normally at 150°C, but during a catalyst regeneration phase, it could reach 220°C. In such cases, the higher value—plus a safety margin—must dictate the design parameters to ensure the mechanical integrity of the pressure boundary.

Construction Data Requirements for Pressure Equipment

Once the process parameters are established, construction data translates these needs into physical reality. This phase involves selecting materials that can withstand the calculated design envelope without losing structural ductility. According to the ASME Section VIII, the mechanical properties of materials vary significantly with temperature. Therefore, the determination of design temperature directly influences the allowable stress values used in wall thickness calculations. Construction data also accounts for corrosion allowances, joint efficiency, and nozzle loadings, ensuring that the physical asset can endure the design pressure over its intended 20-to-30-year lifecycle.

Integrating Client Design Philosophy and ITB Requirements

Every major operator—from Shell to ExxonMobil—has a unique “Design Philosophy” embedded in their Invitation to Bid (ITB) documents. These internal standards often supersede general codes like ASME. For example, a client philosophy may mandate a minimum design pressure of 3.5 barg (50 psig) for any vessel, even if the process operates at atmospheric pressure, to prevent accidental overpressure during steam cleaning or nitrogen purging. Adherence to these site-specific rules is a non-negotiable step in the Determination of Design Pressure and Design Temperature.

Expected Output Data and Deliverables

The final output of this determination process is the Mechanical Data Sheet. This document serves as the master reference for the fabrication shop. It must clearly state the Design Pressure, Maximum Allowable Working Pressure (MAWP), Design Temperature (Internal/External), and the Minimum Design Metal Temperature (MDMT). These values are used by the manufacturer to perform the final stress analysis and to select the appropriate Welding Procedure Specifications (WPS). You can find more detailed guidance on equipment documentation at the Official ASME Website.

Criteria for Determination of Design Pressure in Pressure Vessels

For a single-phase pressure vessel, the Determination of Design Pressure usually follows a “10% or 25 psi” rule—whichever is greater. If the maximum operating pressure is 100 psi, the design pressure becomes 125 psi. However, for tall towers or liquid-filled vessels, static head must be added to the pressure at the bottom. This ensures that the lowest point of the vessel, which experiences the weight of the fluid column plus the vapor pressure, remains within safe limits. Failure to account for static head is a common error in large-scale refinery column design.

Step-by-Step Calculation: Example 1

Consider a vertical separator with a Maximum Operating Pressure (MOP) of 45 barg at the top nozzle. The liquid height is 10 meters with a density of 800 kg/m³.

• Step 1: Apply 10% Margin: 45 barg * 1.10 = 49.5 barg.
• Step 2: Calculate Static Head: (10m * 800 kg/m³ * 9.81) / 100,000 = 0.78 bar.
• Step 3: Determine Design Pressure at Bottom: 49.5 + 0.78 = 50.28 barg.
• Conclusion: The vessel design pressure is specified as 50.3 barg at the top to satisfy all conditions.

Determination of Design Pressure for Heat Exchangers

The Determination of Design Pressure for heat exchangers is significantly more complex than for static vessels due to the interaction between two independent fluid circuits. According to API Standard 660 and TEMA standards, the shell-side and tube-side must be designed for their respective maximum operating pressures plus a safety margin. However, a critical safety consideration is the “Tube Rupture” scenario. If the high-pressure side operates at a pressure more than 1.3 times the low-pressure side’s design pressure, the low-pressure side must be protected by a relief device or be designed to withstand the higher pressure. This “10/13th Rule” (based on ASME Section VIII Div 1 hydrotest margins) ensures that a single tube failure does not lead to a catastrophic shell-side rupture.

Establishing Equipment Design Pressure Margins

When finalizing the Determination of Design Pressure, engineers must account for the gap between the Pressure Safety Valve (PSV) set point and the operating pressure. A common mistake is setting the design pressure too close to the operating pressure, leading to “simmering” or premature lifting of the relief valve due to minor process fluctuations. API RP 520 recommends a minimum 10% margin to allow the PSV to seat properly. In high-pressure services (above 70 bar), this margin might be narrowed to 5% to reduce wall thickness and cost, provided high-accuracy pilot-operated relief valves are utilized.

Best Practices for Determination of Design Temperature

The Determination of Design Temperature is not merely selecting the highest operating temperature. It must account for “Excursion Conditions” such as loss of cooling water, catalyst runaway, or solar radiation on stagnant lines. For equipment operating above 400°C (750°F), creep-strength-enhanced ferritic steels or austenitic stainless steels are required. Conversely, for cryogenic services, the design temperature must reflect the lowest possible fluid temperature to avoid brittle fracture. Detailed material temperature limits can be verified through the API Standards Portal.

Calculating Minimum Design Metal Temperature (MDMT)

The Minimum Design Metal Temperature (MDMT) is the lowest temperature at which a component can safely withstand its design pressure. Determination of MDMT is critical for carbon steel vessels in cold climates or those containing liquefied gases. If a vessel undergoes rapid depressurization (blowdown), the Joule-Thomson effect can drop the metal temperature to -40°C or lower. If the MDMT is not correctly specified, the steel becomes brittle and can fail via “cleavage fracture” even at pressures below the design limit. ASME Section VIII Div 1 Paragraph UCS-66 provides the governing impact test exemption curves used to establish these limits.

Impact of Steam-out Conditions on Design Temperature

During maintenance turnarounds, vessels are often “steamed out” to remove hydrocarbons. This process usually involves low-pressure steam at approximately 120°C to 150°C. If the normal operating temperature of a vessel is lower (e.g., 60°C), the Determination of Design Temperature must be bumped up to the steam-out temperature. Additionally, engineers must check for “Full Vacuum” conditions. When steam condenses inside a blocked-in vessel, it creates a vacuum that can collapse the shell if it was not designed for external pressure.

Comparison: Design Pressure vs. Maximum Allowable Working Pressure (MAWP)

While often used interchangeably, these terms have distinct legal meanings under ASME Code. The Design Pressure is the value specified by the process engineer. The MAWP is the maximum pressure at which the weakest part of the finished vessel can operate at a specific temperature. MAWP is always equal to or greater than the Design Pressure because it is based on the actual “as-built” thickness (including extra thickness from rounding up plate sizes).

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