What Does This Boiler Feed Tank Sizing Calculator Do?
This calculator estimates the gross capacity required for a boiler feed tank when the design feedwater flow and required storage duration are already known. It first calculates the working water volume needed to sustain the selected flow. It then applies a visible capacity margin and divides by the usable fraction of the tank.
The main result is the minimum nominal tank volume. This is larger than the working water inventory because a real tank normally operates between selected low and high level limits rather than from completely full to completely empty.
The tool is limited to storage capacity. It does not calculate feedwater flow, tank heating duty, deaeration performance, vent size, condensate return temperature, pump NPSH or structural tank dimensions.
How Boiler Feed Tank Capacity Is Calculated
The basic storage requirement is the volume withdrawn at the design feedwater flow during the selected storage time. A capacity margin can then be applied before the usable operating fraction is considered.
Base storage volume = Feedwater flow × Storage time ÷ 60Design usable volume = Base storage volume × (1 + Margin % ÷ 100)Nominal tank volume = Design usable volume ÷ (Usable volume % ÷ 100)
Maximum feedwater flow in m³/h or US GPMRequired storage period in minutesAdditional project capacity allowanceVolume between selected low and high operating levelsHow to Use the Calculator
- Select Metric or US Units. Metric mode accepts m³/h. US mode accepts US gallons per minute.
- Enter design feedwater flow. Use the maximum tank outlet flow required by the boiler system.
- Enter the storage time. Use the duration specified by the plant operating philosophy or project standard.
- Add a visible capacity margin. Enter any explicit allowance for uncertainty or future operating flexibility.
- Enter the usable tank fraction. This is the percentage of gross capacity between the chosen low and high working levels.
- Select a practical tank size. Choose the next available nominal size above the calculated minimum.
Understanding the Results
Base storage volume
This is the water volume required to sustain the design feedwater flow for the selected number of minutes. It excludes the additional capacity margin and does not yet account for unusable tank volume.
Design usable volume
This is the base storage requirement after the entered capacity margin is applied. It represents the minimum working inventory that should be available between the selected operating levels.
Minimum nominal tank volume
This is the required gross tank capacity after accounting for the usable percentage. It is the principal sizing result and should be rounded up to an available tank size.
Reserved and ullage volume
This is the part of the gross tank capacity outside the calculated usable inventory. In practice, it can include low-level reserve, space above the normal high level, and volume reserved for temporary condensate-return surges. The actual allocation should be defined by the tank level arrangement.
Worked Example
Consider a boiler feed system requiring 12.27 m³/h. The selected storage time is 60 minutes, the additional capacity margin is 10%, and 70% of the gross tank is available as usable operating volume.
| Stage | Calculation | Approximate result |
|---|---|---|
| Base storage | 12.27 × 60 ÷ 60 | 12.27 m³ |
| Design usable volume | 12.27 × 1.10 | 13.50 m³ |
| Nominal tank volume | 13.50 ÷ 0.70 | 19.28 m³ |
| Reserved / ullage volume | 19.28 − 13.50 | 5.78 m³ |
The preliminary minimum is therefore about 19.28 m³, or 19,281 litres. The selected tank should be the next practical nominal size above this value after the level positions and internal displacement are confirmed.
What Values Should You Enter?
Use one consistent operating case. The flow, storage time, margin and usable fraction should all represent the same design basis.
Design feedwater flow
Enter the highest continuous flow expected from the tank to the operating boiler feed pumps. Do not use average daily consumption when the tank must support a higher duty.
Storage time
Enter the period for which the stored water must support the feedwater demand. Use the project or plant requirement rather than assuming one duration suits every installation.
Capacity margin
Apply a clearly stated allowance only when required. Avoid adding the same contingency in the flow, storage time and margin inputs.
Usable tank volume
Enter the percentage between the selected low and high operating levels. The remaining capacity is not treated as normal working inventory.
Choosing the Storage Time
Storage time represents how long the feed tank can sustain the selected outlet flow without relying on incoming water. It is usually chosen to cover a temporary interruption in make-up water supply or another defined operating event.
Traditional boiler-house practice may use one hour of storage at maximum boiler evaporation as a starting point. Larger plants may use a different arrangement, such as a smaller hot feed tank supported by separate treated-water storage. The correct duration should therefore come from the project design basis.
When the calculator is supplied with feedwater flow rather than steam evaporation, the result directly reflects the water leaving the tank, including any allowances already contained in that flow value.
Usable Volume Versus Gross Tank Capacity
A tank's nameplate or geometric capacity is not normally equal to its usable working inventory. The low-level setting leaves water below the normal operating range, while the high-level setting leaves free space above the normal inventory.
The usable-volume input converts the required working inventory into gross capacity. For example, if only 70% of the tank is available between the selected operating levels, a 14 m³ working requirement needs a nominal tank capacity of 20 m³.
Use actual level elevations when they are known. A percentage is suitable for preliminary work, but final tank volume should be checked from the vessel geometry and the specified high, normal and low levels.
Allowing for Condensate Return Surges
A feed tank may receive short periods of unusually high condensate return, especially during plant start-up or when condensate stored in pipework returns quickly. Adequate free space above the normal level helps prevent useful hot condensate from being lost through the overflow.
The calculator represents this space indirectly through the usable-volume percentage. A lower usable percentage creates more non-working capacity, but the actual high-level and overflow arrangement must still be checked against the expected surge volume.
Do not use a broad percentage as a substitute when a known surge quantity is available. In that case, verify that the selected gross tank has sufficient free volume above the normal operating level to receive the calculated surge.
Selecting the Final Nominal Tank Size
The calculated result is a minimum gross capacity, not an instruction to fabricate a tank to the exact decimal value. Round upward to a practical standard or project-specific size.
Then confirm that the selected tank provides the required usable volume between actual level settings. Internal components, connections, sloping bottoms or other occupied spaces may reduce the true water capacity and should be considered in the final vessel drawing.
Where several boilers share one tank, use the maximum coincident feedwater demand defined by the operating philosophy. Standby boilers should not automatically be added unless they can operate simultaneously.
Common Boiler Feed Tank Sizing Mistakes
- Using average feedwater demand: the tank may be unable to support the maximum operating condition.
- Treating gross volume as fully usable: real tanks operate between selected high and low levels.
- Applying contingency several times: avoid duplicating margins in flow, storage time and capacity allowance.
- Ignoring condensate surges: inadequate upper free space can cause hot condensate to discharge through the overflow.
- Using steam flow when a higher feedwater flow is already available: size from the actual tank outlet duty for a direct storage calculation.
- Adding every boiler nameplate rating: use the maximum coincident operating case for a common tank.
- Selecting the exact calculated size: the final tank should normally be rounded upward to a practical nominal capacity.
- Ignoring occupied internal volume: coils, pipes and geometric features can reduce actual water capacity.
Using the Result for Preliminary Tank Selection
Use the calculated nominal volume to screen available tanks or prepare an early equipment enquiry. The enquiry should state the design feedwater flow, storage duration, usable level range and required gross capacity.
Final selection should confirm the actual internal volume at the specified levels, allowance for condensate surges, overflow level, minimum operating level and any internal displacement. The tank supplier or project engineer should verify that the finished vessel provides the required usable inventory.
Frequently Asked Questions
What is the basic boiler feed tank sizing formula?
Multiply feedwater flow by the required storage time to obtain working storage, apply any explicit capacity margin, and divide by the usable fraction of the tank to obtain gross nominal capacity.
Should feed tank size be based on steam flow or feedwater flow?
Use feedwater flow when it is already known because it represents the actual water leaving the tank. Steam evaporation can be used as an earlier starting point when a separate feedwater-flow calculation has not yet been completed.
How much storage time should I enter?
Use the duration required by the plant or project operating philosophy. One hour is a traditional starting basis in some boiler houses, but it is not a universal requirement.
Why is nominal tank volume larger than working storage?
Only part of the gross tank volume is normally available between the low and high operating levels. Additional space is retained below and above that working range.
Does the calculator include condensate return?
It assumes the stored inventory alone supports the entered feedwater flow during the selected period. Condensate surges should be checked separately against the free volume above the normal operating level.
Can this calculator determine tank dimensions?
No. It estimates required capacity only. Length, width, height, shape, level elevations and internal displacement must be established from the selected tank design.