Guideline for Equipment Design Conditions & Margins

Introduction

This document provides general guidelines for establishing consistent design conditions for common process equipment. It covers standard practices for setting design pressure, temperature, corrosion allowance, and over-capacity margins.

These are standard guidelines. Specific client or licensor requirements shall always supersede the recommendations in this document.

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General Definitions

Pressure

• Operating Pressure: The normal pressure at which the process operates.
• Maximum Operating Pressure (MOP): The highest pressure expected during normal operation, including fluctuations from controls, start-up, or transient conditions like a pump's no-flow (shutoff) head. Setting this value correctly helps avoid unnecessary lifting of safety valves.

Temperature

• Operating Temperature: The normal temperature at which the process operates.
• Maximum Operating Temperature: The highest temperature expected during any operating mode, including start-up, operational upsets, or regeneration cycles.

Corrosion Allowance

This is an additional metal thickness applied to shells, heads, and non-removable internals to account for material loss over the equipment's life. It is not applied to surfaces already protected by a corrosion-resistant lining. Typically, no corrosion allowance is needed for 300/400 series stainless steel, aluminum, or 9% nickel components.

Design Margins

A margin added during equipment sizing to determine the "Design Rated Capacity." This accounts for potential errors in estimation, limitations of sizing equations, and other unknown factors or contingencies.

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Storage Tanks

General Design Notes

• Used for storing low-vapor-pressure petroleum, petrochemical, and non-petroleum products.
• Selection of tank type depends on evaporation loss, pollution laws, cost, and climatic conditions.
• Maximum filling is typically assumed to be 95% of the tank's height.
• Tank height is often limited by soil-bearing strength (e.g., maximum of 20 meters is a common limit).
• A preferred height-to-diameter ratio is 1:1 or less.

Common Codes & Standards

• API-650: Used for above-ground vertical cone, dome, and floating roof tanks in non-refrigerated service. Applicable up to 2.5 psig and 260°C.
• API-620: Used for above-ground cone and dome roof tanks. Applicable up to 15 psig, 121°C, and for low-temperature services down to -168°C.

Cone Roof Tanks

These tanks are categorized by their pressure rating:

• Atmospheric: Suitable for storage at atmospheric pressure. Uses an open vent with a gooseneck and bird screen.
• Low Pressure: Designed for low internal pressure/vacuum. Uses pressure/vacuum (P/V) breather valves and is typically blanketed with nitrogen or inert gas.
• Moderate Pressure: Similar to low-pressure tanks but designed for higher P/V settings. Also uses breather valves and blanketing.

Cone Roof Tank Design Philosophy

Type	Design Pressure / Vacuum	Diameter (M)	Accessories
Atmospheric Tank	Full of water/liquid	3 to 48	Open vent
Low-Pressure Tank	+3" WC / -1" WC	3 to 48	Nitrogen/Inert gas blanketing, Breather valves
Low-Pressure Tank	+8" WC / -2.5" WC	3 to 39
Moderate-Pressure Tank	+22.5" WC / -2.5" WC	3 to 20

Dome Roof Tanks

Used to store liquids with vapor pressures up to 15 psig (e.g., NGLs). These are designed per API-620.

Dome Roof Tank Design Considerations

Type	Design Pressure / Vacuum	Diameter (M)	Accessories
Dome Roof Tank	Up to 15 psig / -2.5" WC	15 to 20	Gas blanketing, Breather valve / Vacuum port, Pressure safety valve

Floating Roof Tanks

These tanks operate at atmospheric pressure, with the roof floating on the liquid to minimize vapor space. They are recommended for diameters of 15 meters and above. The True Vapor Pressure (TVP) of the product should generally not exceed 11 to 12.5 psia at the maximum storage temperature. A conical bottom is often used to prevent liquid accumulation and ensure complete drainage.

Floating Roof Tank Design Considerations

Type	Design Pressure	Height (M)	Accessories
Floating Roof Tank	+3" WC + Full of water/liquid	15 to 92	Rim Seal, rolling ladder, vacuum vent, rainwater draining system, emergency vent

Fixed Roof with Internal Floating Head

This is a cone roof tank with an internal floating cover. It is used to reduce vapor loss and air pollution, especially for costly or hazardous materials like Benzene. These tanks require good ventilation in the upper roof space and high-level alarms to prevent the internal cover from hitting the fixed roof.

Tank Design Temperature & Corrosion

Design Temperature: Max. Operating Temperature + 15°C (with a minimum of 65°C).

Corrosion Allowance (Carbon Steel):

Component	Cone Roof	Dome Roof	Floating Roof
Shell	1.6 mm	1.6 mm	1.6 mm
Roof / Floating Deck	Nil	Nil	1.6 mm
Tank Bottom	1.6 mm	1.6 mm	1.6 mm

For services containing acid gases (H₂S, CO₂), an additional 0.5 to 1.5 mm of corrosion allowance may be added.

Tank Design Margins (Hold-Up)

Tanks are sized based on required liquid hold-up time, not a percentage margin. The hold-up volume typically represents 90% of the total tank volume (Total Volume = Hold-up Volume / 0.9).

Description	Minimum Liquid Hold-up Time
Product tanks for export	15 days
Intermediate tanks	8 days
Feed tanks (ex-pipeline)	5 days
Day tanks	1 day

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Pressure Vessels

Design Pressure

• If MOP is < 70 kg/cm²g, Design Pressure = MOP + 10% (minimum 2 kg/cm²g).
• If MOP is > 70 kg/cm²g, Design Pressure = MOP + 5%.
• Must account for pump shutoff pressure.
• Vessels with steam-out connections or handling steam/condensate must be designed for Full Vacuum.
• Minimum design pressure for flare-connected equipment is 3.5 kg/cm²g.
• The minimum design pressure for a flare seal drum should be 7.0 kg/cm²g to withstand potential explosions.
• Vessels storing liquefied products (like LPG) should be designed for the liquid's vapor pressure at 55°C.
• For vessels protected by a rupture disc, the MOP should be no more than 75% of the design (burst) pressure.

Design Temperature

Operating Above 0°C

• Design Temperature = Max. Operating Temperature + 15°C (with a minimum of 65°C).
• The final temperature should be rounded up to the next multiple of 5°C.
• Must be at least equal to the steam-out temperature, if applicable.
• For liquefied products, the lower design temperature is the Adiabatic Flash Temperature at atmospheric pressure (if below 0°C).

Operating Below 0°C

• The design temperature is the lowest of either the minimum operating temperature or the temperature reached during depressurization.

Corrosion Allowance

• Carbon Steel: 3.0 mm (minimum).
• Alloy Steel: 1.5 mm (minimum).
• Stainless Steel, Aluminum, 9% Nickel: No corrosion allowance required.
• Lining: A corrosion-resistant lining shall have a minimum thickness of 2.0 mm.

Design Margins (Sizing)

A 10% design margin on liquid and gas flows is typical for vessel sizing. For sizing separators or drums, the following liquid residence times are common:

• Reflux to tower: 5.0 minutes
• Tower to tower: 5.0 minutes
• Feed to process heater: 10 minutes
• Feed surge drum: 10 to 30 minutes
• Hot oil surge drum: Size based on thermal expansion requirements.
• Very low liquid (manual drain): 8 to 24 hours
• Other general services: 3 to 10 minutes

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Heat Exchangers

Common codes include TEMA/ASME for Shell & Tube exchangers and API-661 for Air-Cooled exchangers.

TEMA Type Selection Guide (Based on Fouling Factor)

Shell Side Fouling Factor (Hr.m².°C/Kcal)	Tube Side Fouling Factor (Hr.m².°C/Kcal)	Preferred TEMA Type
> 0.0002	> 0.0002	Floating Head
≤ 0.0002	> 0.0002	Fixed Tube Sheet
> 0.0002	≤ 0.0002	"U" Tube Bundle
≤ 0.0002	≤ 0.0002	Fixed Tube Sheet / "U" Bundle

Design Pressure

• Plate & Frame / Spiral: Design pressure and temperature are normally the same for both sides.
• Air Cooled: Design pressure is set by the worst-case MOP.
• Shell & Tube:
  • The low-pressure side should have a design pressure of at least 77% of the high-pressure side's design pressure (to avoid needing a tube rupture safety valve).
  • General practice: MOP + 10% or 2 kg/cm² (whichever is greater), with a minimum of 3.5 kg/cm².
  • Any side subject to pump shutoff must be designed for that pressure.
  • Full Vacuum design is required on the respective side for:
    • Vaporizing or condensing services.
    • Steam heating.
    • Cooling water services at high elevation.

Design Temperature

• Operating 0°C to 400°C: Design Temp = Max. Op. Temp + 20°C (min 65°C).
• Operating at or below 0°C: Design Temp = Min. Op. Temp.
• In case of a cooling media failure, the tubes, tube sheets, and floating head must be designed for the maximum temperature of the hotter inlet fluid.

Corrosion Allowance

Corrosion allowance is typically only provided for carbon steel parts. No allowance is needed for stainless steel, copper, or other alloy materials.

Design Margins

Typically, a 10% margin on heat duty (for area) is used. Alternatively, a 10% margin on fluid flows (for pressure drop) can be used. These two margins should not be applied together.