Considerations in Sizing your API 650 Tank

100,000 BBL is 100,000 BBL, Right?

Considerations in sizing your API 650 Tank

By: Matt Van Alsburg, Advance Tank Sales Manager


While sizing a tank appears to be an unassuming task, it can quickly become a challenge when all considerations are taken into account. Some questions for consideration when determining tank size include:

  • Do you need nominal capacity, maximum capacity, or networking capacity?
  • Do you need a floating roof?
  • Have you taken into consideration API 2350 overfill protection requirements?
  • Are there size limitations at the location where the tank is to be built?


These are just some of the items that, if answered, can help you successfully define the appropriate sized tank for your application. Let’s look at each of these questions as they would relate to a 120’ x 50’ tank, often described as a 100,000 BBL Tank.



I am personally guilty of looking past this question. A customer will call and request a 100,000 BBL tank. I ask about accessories, foundations and coatings and forget to ask them whether or not the 100,000 BBL is nominal, maximum capacity or working capacity. I hang up the phone and start to move forward with engineering only to realize I do not have what I need to size the tank.


API 650 figure 5.4 depicts the different tank capacities types along with different benchmarks. To better determine what is needed, it is best to understand the definitions involved.

Nominal Capacity –  The nominal volume of the tank is the volume between the top of bottom plate at the shell to the top of shell height. This volume does not take into account any loss from floating roofs, level alarms, overflow slots, or floating suctions.

Nominal Capacity
120’ Diameter by 50’ Tall Tank = 100,659 BBL

This is calculated simply by taking the area of 120’ diameter and multiplying by the full 50’ shell height: 3.14 x 602 x 50’ x 7.48 gallons / 42 barrels


Maximum Capacity – The maximum capacity of the tank is the volume between the top of bottom plate at the shell, to the design liquid level. This may be to the top of shell height if there are no overfill slots or other obstructions, such as internal rafters. Let’s assume our 120’ x 50’ tank does not have overfill slots and does have internal rafters that are 1’ wide.

Maximum Capacity
120' Diameter by 49’ design level = 98,646 BBL 

This is calculated by taking the nominal capacity minus the width of the rafters (1ft); or a 49’ height instead of the full 50’ height of the shell

Net Working Capacity – This is the capacity from the minimum fill level to the normal fill level. We now have two more items to define, minimum fill and normal fill.


  • Minimum fill level is the volume from the top of bottom plate at the shell to the minimum fill level. Most people set the minimum fill level at the top of their outlet pipe.
    • Assume our tank has an 8” Suction that is 13 ¾” to its center (see API 650 Table 5.6b, Regular Type column). Add 4” to get us to the top of the pipe, and this gives us 17 ¾” to the top of the fill line. We will call our minimum fill height 18”.


  • Normal fill level is the highest point where product is anticipated to reach. This elevation takes into account API 2350 overfill requirements.
    • Our tank started at 50’ tall. We reduced it by 1’ due to rafter thickness and now we need to look at overfill protection. For the purpose of this evaluation we are going to assume a High High-Level alarm (HH) and a Critical High-Level alarm (CH). Without going into flow rates, we are going to make the assumption that a 3” rise between the alarms meets the requirements of API 2350. Please consult with API 2350 for determining actual response time requirements. Utilizing our assumption, this will make the Normal fill level 48’6”, as this will account for the 3” between the maximum capacity level and the HH alarm, and another 3” between the HH alarm and CH alarm.


Net Working Capacity 
120’ Diameter by 47’ (Normal Fill 48’6” less Minimum Fill 18”) = 94,619 BBL




Importantly, there is a difference of 6,040 BBL between nominal capacity (the full height of the tank shell) and net-working capacity. This difference might result in the loss of revenue, or worse, it could result in the breach of a contract. While a tank manufacturer can help you determine tank sizing, ultimately it is up to the owner to determine what is needed. Once you determine what you need, that information is best provided to a tank manufacturer by utilizing the tank data sheet provided in API 650 Annex L.



If your tank requires an external floating roof (EFR) or an internal floating roof (IFR) you will need to take into account volume loss associated with this item. The loss affects both the normal fill level and the minimum fill level of a tank.


Effects on Normal fill level – As a general rule floating roofs will reduce your normal fill level by 4 to 6 feet depending on roof type and design criteria.


IFR Losses

  • Floating Roof Outer Rim Height – Generally Range from 24” to 32” in height
  • Secondary Seal and Foam Dam – Generally Range from 18” to 24” in height
  • Total Loss
    • 24” on the “low side” if no secondary seal or foam dam is used
    • 56” on the “high side” if the tank utilizes a secondary seal and/or a foam dam
    • Typical Loss on an IFR is around 48”


EFR Losses

  • Floating Roof Outer Rim Height – Generally Range from 30” to 48” in height
  • Secondary Seal and Foam Dam – Generally Range from 18” to 24” in height
  • Total Loss
    • 48” on the “low side”
    • 62” on the “high side”


Effects on Minimum fill level – While typically the top of the suction line is a good rule for determining the minimum fill level, this rule often changes when a floating roof is included. Conservatively, we would recommend setting your minimum fill level at 36” to 42” to account for a floating roof. This would be a typical range for the low leg setting on a floating roof. It is possible to go lower, but moving lower will require further evaluation from both an engineering and environmental perspective.  We typically set our low leg at 42” which allows for clear operation of all manways and mixers.


Low Leg Loss = 36” to 42”


Back to our example. If we add an IFR, you would be looking at the following net working capacity:

Nominal Capacity.................................................................. 100,659 BBL

Less Minimum Fill (42”)........................................................... 7,046 BBL

Less Loss of Capacity due to roof (48”) .......................................... 8,052 BBL

Less Overfill protection (6”) and Rafter Thickness (12”).......................... 3,019 BBL

Net Working Capacity of the 120’ x 50’ Tank will be............................... 82,542 BBL


Obviously, you cannot hold 100,000 BBL of product in this 100,000 BBL tank. Therefore, when including for a floating roof, consideration must be given to the volume loss associated with it. The actual tank size required to have a 100,000 BBL working capacity tank will be closer to 134’ diameter x 48’ tall.


Compare Exhibit B diagram with Exhibit A to help understand the volume loss that comes with a floating roof.


Exhibit B




I am no expert on API 2350. However, it is important to take this standard into consideration when sizing your tank. API 2350 outlines the requirements for overfill protection. These requirements determine the capacity needed at the top of the tank to allow for alarms to be triggered resulting in the required response time to stop an overfill event from occurring. The volume needed is a balance between what category alarm you utilize (1,2, or 3), the fill rate or your tank, as well as the number of alarms your required by the state in which you operate or company requirements. Typical alarm elevations are High Level, High High Level and Critical High Level. Please consult with your company standards, state/county regulators, and level alarm providers for requirements.





Steel tanks are great because they can be built to accommodate any required size. With this in mind, there are limitations that owners will experience at their construction sites. Here are some items for consideration when determining tank sizing as it relates to the construction site:


  1. Have you taken into account the required dike area for the tank? Traditionally, 110% of the volume of the largest tank in the dike area is required to be contained by the dike. Don’t forget to take into account the width of the dike itself in this evaluation.
  2. How good is your soil? As a rule of thumb 3,000 PSF soil bearing is safe for most tanks. However, if you want to go tall, north of 48’ in height, you may want to get a geotechnical report performed first. This will allow you to make an educated decision on the tank height, as well as determine the cost impacts in relation to the foundation.
  3. Do you want to match the tank height with the other tank you have on site, to allow for interconnecting walkways for example?
  4. Do you have plans for more tanks in this area? Have you accounted for the dike sizing related to adding these tanks?


Every site has its own unique set of challenges and restrictions. However, taking the time up front to look into these questions will save both money and time in the future. If you need assistance with sizing your tank, please call (970) 568-3444 or email Advance Tank is always ready to assist customers with their needs.