Tanks

Fixed roof:

• vapor pressure < 1.5 psia
• atmospheric and vacuum gas oils, vacuum residue

Floating roof:

• vapor pressure upto 11 psia
• Crude oil, naphtha, kerosene

 API 650

•  Atmospheric tank
•  API 650 tanks can be designed upto 2.5 psig per Appendix F of this standard.
•  Vacuum: 1 inch of water
• Design temperature: Limited to a maximum temperature of 200 deg F. 500 deg F, provided additional requirements of Appendix M are met

API 620

• API 620 tanks can be designed upto 15 psig
• Vacuum: 2-2.5 inch of water
• Design temperature: 250 deg F max
• e.g. light naphtha

VOLUME OF STORED LIQUID

• Tank type typically decided by liquid properties
• Fixed roof tanks are usually the least expensive, followed by floating roof tanks and dome roof tanks, in that order
• For very large sizes, construction of fixed roof tanks becomes more involved with very high cost for supporting the roof
• No limit on diameter by code or universal convention
• Typically fixed roof tank diameter upto 50 m - 65 m

FACTORS LIMITING SIZE OF INDIVIDUAL TANKS

• Local codes that put a cap on maximum volume within a single dyke.
• Soil bearing strength and piling requirements that could cap the maximum height.
• Plot shape and dimensions that could limit maximum diameter.
• Plot elevation profile - slopes could limit maximum diameter.
• Material constraints - maximum available tank plate thickness.
• Construction constraints

Example:

Normal liquid temperature = 40C

Assume temperature rise due to thermal radiation = 10C 

Maximum liquid temperature = 40C + 10C = 50C

Vapor pressure of stored liquid @  50C   = 1.28 kg/cm2

Set pressure of Inbreathing Vent = 1.28 kg/cm2 

Set pressure of Outbreathing Vent = 1.28 + 0.05 =  1.33 kg/cm2a

Set pressure of emergency vent      = 1.33 + 0.05 =  1.38 kg/cm2a 

  Design pressure = 1.38 kg/cm2a

 

Design Vacuum = 2.5 " wc 

Design Temperature = Larger of [Op Temp + 20C] or 65C = 65C