A General Guide to Design Pressure and Temperature - WittyWriter

A General Guide to Design Pressure and Temperature

1. Introduction

Establishing the correct mechanical design conditions is a critical safety and economic activity. The design pressure and temperature must be selected to cover the most stringent conditions the equipment will experience during its lifetime, including startup, shutdown, and process upsets.

A properly defined design pressure allows for a safe operating margin without significant over-design, which saves on capital cost. This guide outlines the fundamental principles for selecting design pressure and temperature for common process equipment.

2. Internal Design Pressure (DP)

The internal design pressure is based on the maximum operating pressure (MOP) plus a specified margin. This margin ensures that normal process fluctuations do not cause spurious trips or relief valve activation.

General Vessels and Columns

For a pressurized system protected by a relief device, the design pressure is set by the following rules:

Maximum Operating Pressure (MOP) Range	Mechanical Design Pressure (DP) shall be the Maximum of:
≤ 70 barg (approx. 1000 psig)	MOP × 1.1 MOP + 2 bar (approx. 30 psi) A minimum of 3.5 barg (approx. 50 psig)
> 70 barg (approx. 1000 psig)	MOP × 1.05

Special Cases for Vessels:

Flare Knock-Out Drums: Equipment "floating" on the flare system must have a design pressure equal to the flare KO drum's DP. This is typically a minimum of 3.5 barg (50 psig), or 7 barg (100 psig) if no seal vessel is used, to withstand potential deflagration.
Column Bottoms: The DP calculated above applies to the *top* of the column. The design pressure at the bottom must also account for the column's pressure drop (ΔP) and the maximum hydrostatic head of the liquid in the sump.
DP_Bottom = DP_Top + ΔP_Column + P_{Static Head}

Pumps and Compressors

Piping and equipment downstream of a pump or compressor must be designed to withstand the maximum potential discharge pressure.

Centrifugal Pumps

The downstream system design pressure must be set to the pump's shut-off pressure.

P_design = P_{max_suction} + ΔP_max

Where:

P_{max_suction} = The design pressure of the suction vessel plus any static head from the liquid level.
ΔP_max = The pump's differential head at zero flow (shut-off). This value depends on the driver type:
- Constant Speed Motor: ΔP_max ≈ 1.25 × ΔP_Rated
- Variable Speed Motor: ΔP_max ≈ (1.05)² × 1.25 × ΔP_Rated (accounts for 5% over-speed)
- Turbine Driver: ΔP_max ≈ (1.15)² × 1.25 × ΔP_Rated (accounts for 15% over-speed trip)

Positive Displacement (PD) Pumps

The downstream design pressure must be the higher of:

P_design = 1.1 × P_{rated_discharge}
or
P_design = P_{rated_discharge} + 2 barg (30 psi)

Compressor Suction Systems

The suction system (e.g., KO Drum, inlet exchangers) must be designed to withstand the settle-out pressure of the entire compressor loop (suction + discharge) in a shutdown scenario, plus a margin (e.g., + 1 bar).

Heat Exchangers (Tube Rupture)

To avoid needing a relief device for a tube rupture scenario, the low-pressure (LP) side is often designed to contain the high-pressure (HP) fluid. As a minimum, the LP side design pressure should be 10/13ths (or ~77%) of the HP side design pressure. If this is not practical, a formal tube rupture relief case must be calculated.

3. External Design Pressure (Vacuum)

Vessels that can be exposed to an internal pressure lower than atmospheric pressure must be designed to withstand the external pressure to prevent collapse. A "Full Vacuum" (FV) rating is required for any of the following scenarios:

Vessels that operate under vacuum (e.g., vacuum columns).
Vessels where vapor can rapidly condense, such as those being steam-out for cleaning and then cooled, or those in condensing service.
Distillation columns that can lose heat input, causing vapors to collapse.
Elevated exchangers or vessels that can be gravity-drained while blocked in.

4. Design Temperature (DT)

Maximum Design Temperature

The maximum design temperature is the highest operating temperature the metal will experience, plus a safety margin.

General Rule: Max. Operating Temperature + 15°C (approx. 30°F)

Key Exceptions (use the higher temperature):

Solar Radiation: Equipment exposed to the sun must have a minimum design temperature of 65°C, even if it operates cooler.
Steam-Out: Any equipment specified for "steam-out" must have a design temperature at least as high as the utility steam (e.g., 180°C for LP Steam).
Air Cooler (Fin-Fan): Equipment *downstream* of an air cooler must be designed for the cooler's *inlet* temperature, to account for a fan failure.
Compressor Discharge: Must be designed for the highest predicted temperature, which typically occurs at surge conditions.

Minimum Design Metal Temperature (MDMT)

The MDMT is critical for selecting a material that will not become brittle and fracture at low temperatures. It is the lowest temperature the metal is expected to see.

General Rule: Min. Operating Temperature - 5°C (approx. 9°F)

This temperature is often dictated by abnormal or transient scenarios, including:

The lowest ambient site temperature.
Auto-refrigeration: The rapid cooling caused by the flashing of liquefied gases (e.g., C3/C4) during depressuring or a relief event (blowdown).
Process upsets or cryogenic services.

5. Corrosion Allowance (CA)

Corrosion Allowance is a sacrificial thickness of metal added to the calculated required thickness to account for expected metal loss over the life of the plant.

Material	Typical Minimum Corrosion Allowance
Carbon Steel	1.5 mm (or 3.0 mm for general refinery/petrochemical services)
Alloy Steel	1.5 mm
Stainless Steel	0 mm (NIL)
Aluminium	0 mm (NIL)
9% Nickel	0 mm (NIL)

Severe Service: For severe corrosive environments, such as "Wet H₂S" (sour) service, a much higher corrosion allowance (e.g., 6.0 mm) may be required for carbon steel equipment.