NFPA 22 Fire Water Tank Sizing Guide
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NFPA 22 Fire Water Tank Sizing Guide
The duration is not an NFPA 22 number. It comes from the system design standard and the authority having jurisdiction, and NFPA 22 then requires the tank to provide the net capacity that duration produces once the design flow, the hose allowance and the unusable volume are accounted for. Size the tank to a traceable duration basis, and document that basis so it can be reviewed.
Introduction
Every fire water tank question ends in a number, and the number that decides the cost of the tank is rarely the one people argue about. Diameter, height, shell plate and appurtenances all get discussed. The duration gets inherited.
That is how a tank gets sized to a figure that came from a rule of thumb, a legacy specification or a precedent set before the current code edition was adopted, and how a tank arrives on site with a nominal volume that does not carry the load the authority approved. The duration requirement is not a property of the tank. It arrives from the fire protection system design, and NFPA 22 requires the tank to satisfy whatever that design produces.
This guide sets out where the duration comes from, how it combines with the design flow, where the standard assumptions stop being valid, when a longer duration is imposed, and how the calculation has to be written for an approving authority to accept it. The references are NFPA 22 for the tank and its appurtenances, NFPA 13 for the sprinkler design that generates the demand, and NFPA 20 for the fire pump.
What the duration requirement is
The duration requirement is the length of time the stored water has to remain available at the required rate of flow. NFPA 22 does not create that requirement. It sets out what the tank must be and must deliver, and the duration arrives from outside it, from the fire protection system design and from the authority having jurisdiction. Chapter 4 carries the general requirements, including the capacity basis under 4.1.3 and 4.1.3.1, which is why a tank built perfectly to NFPA 22 can still be the wrong size if the duration fed into that basis was wrong.
Two things follow. The duration has to be traceable to a document: a requirement in the system design standard for the occupancy and hazard, or a written condition imposed by the authority. And changing it after the calculation changes everything downstream, from the demand volume to the shell height and the foundation.
How duration and design flow combine
The volume a tank has to hold is a product of flow and time: V_required = Q_design x t_duration, where V_required is the volume the system must draw, in m3 (ft3); Q_design is the design flow in L/s (gpm); and t_duration is the required duration in minutes. That relationship is simple, and the way it is applied is where most sizing errors are made.
For a sprinkler system the demand is the sum of the sprinkler demand and the hose stream allowance, and each carries its own duration. The correct expression is V_required = sum over i of (Q_i x t_i), not the total flow multiplied by the shortest duration. Where the municipal supply is credited, only the amount it can deliver during the event may be credited, and that has to be demonstrated rather than assumed.
The tank then does not hold the full demand, because part of the stored volume cannot be delivered to the pump. Dead storage below the suction, the anti-vortex clearance at the discharge outlet required by 14.2.13, and the freeboard reduce what is usable. The calculation runs: demand volume, less credited supply, plus unusable volume, equals the net capacity the tank must provide.
Standard duration assumptions and their limits
The durations most often quoted come from the sprinkler system's own design standard rather than from NFPA 22, and they vary with the occupancy hazard classification. What matters is not the number but the conditions attached to it: each figure belongs to a specific code edition, a specific hazard classification and a specific system type. A duration lifted from another edition, or from a standpipe design rather than a sprinkler design, does not carry across.
Those assumptions describe a sprinkler system discharging at its design density over its design area and sustaining that rate. They do not describe a deluge system opening every outlet at once, a process area under a high hazard, or a site whose public water supply cannot be relied on for the length of the event. Nor do they survive a change to the building. Where the authority imposes its own duration, that requirement governs the tank and belongs in the documentation as a written condition.
When a longer duration is required
A longer duration is required when the standard default does not represent the site. The triggers are familiar: the authority having jurisdiction imposes a longer duration as a condition of approval; the occupancy or hazard sits outside the range the default was written for; the water supply is not dependable enough to credit the public source; the site is remote, unstaffed or has a long fire service response; the owner has adopted a resilience objective for fire response; or an insurer has imposed a condition of its own.
The consequence at constant flow is linear and immediate. Double the duration and you double the demand volume and the shell volume, and with them the foundation, the plate weight, the crane picks and the transport route. That is the argument for settling the duration before the tank is tendered, and for giving the tank supplier the duration in writing, with the document it comes from named.
Applying the guide to a sprinkler system
For a sprinkler system the demand is generated by the design procedure in NFPA 13. The occupancy hazard classification fixes the design density and the design area, and the hydraulic calculation that follows produces the design flow at the most demanding point in the system. The duration then comes from the same standard's requirement for that hazard, not from the tank.
The hose stream allowance is a separate demand and is treated separately. It is added to the sprinkler demand rather than folded into the sprinkler flow, because the two are not concurrent in the same way, and it is carried for the duration the standard assigns to it. Folding hose allowance into the sprinkler flow and then applying a single duration to the total usually understates the volume.
Two NFPA 22 requirements sit underneath the number and are satisfied by the tank arrangement rather than by the arithmetic. The tank is kept filled within 4 in. (102 mm) of the fire service level under 14.4.3, and the supply has to restore the volume within 8 hours under 4.2.1.4. A tank that holds the right volume but cannot be refilled in that window does not have a reliably available reserve.
Applying the guide to a hydrant system
A hydrant supply has a different demand profile. The flow is set by the required fire flow for the area and by the number and size of hydrants in service, and the system is normally fed from the public network through a fire pump rather than from a private gravity tank. Where a private tank does form part of the supply, the duration principle still applies, but the credit question comes first: what may be credited is limited by what the tank can deliver and by the reliability of the rest of the network.
Three differences separate the hydrant case from the sprinkler case. The flow is a burst profile rather than a sustained design flow, so the outlet arrangement and the suction geometry carry more of the design. The friction in the path between the tank and the point of use is a larger share of the required pressure, so usable volume has to be assessed against the delivery path rather than the tank alone. And the duration usually comes from the authority's own fire flow criteria, so it has to be obtained in writing.
Where a different duration applies
Three situations routinely put a different number on the table. The first is code adoption. Outside North America the governing framework may be a European or other national standard, and the sprinkler duration table in NFPA 13 is simply not the reference; EN 12845 is the parallel framework for sprinkler systems and their water supplies. The tank may still be built to NFPA 22 and to AWWA D103, but the duration it is sized to comes from the adopted local code, and the two bases have to be reconciled explicitly rather than assumed to be equivalent.
The second is the pump acceptance run. NFPA 20 requires the fire pump to be tested at flows that exceed its rated capacity, and the tank has to supply that test flow. The test is a short event, but it is a genuine draw on the reserve, and it has to be planned so that the tank is refilled and the reserve restored afterwards.
The third is an alteration after approval. A change of occupancy, stored material or protection scheme can change the design density, the design area, and therefore both the flow and the duration. The approved duration basis then has to be revisited rather than carried across, and the tank re-checked against it.
Documenting the sizing for approval
The sizing calculation is the document the authority reviews, and it has to be required. In order, it should state the design flow and where it comes from; the duration and the document that requires it; the hose allowance treated as its own demand; any credited municipal supply with the basis for the credit; the resulting demand volume in the units of the calculation and in the units the tank will be ordered in; the unusable volume and the reason for each element of it; the net capacity the tank must provide, referenced to 4.1.3 and 4.1.3.1; and the refill basis checked against 4.2.1.4.
Attach the system design documents that support the inputs instead of restating them. Where the duration differs from the standard default, attach the authority's written condition and identify the document or clause it comes from.
Be precise about which volume you mean. Net capacity, gross capacity and usable volume are different numbers and they are used interchangeably in conversation. State the one the tank is ordered against and the datum the level measurements refer to. If the reserve is split across more than one tank, the drawing has to show the interconnection that makes the combined reserve real.
Where The Duration Basis Comes From
Source Of The Duration | What It Fixes | Evidence To File With The Calculation |
System design standard, for example NFPA 13 for a sprinkler system | The duration for the occupancy hazard classification and system type | The clause of the adopted code edition that requires it |
Condition imposed by the authority having jurisdiction | A longer or modified duration for this site | The written condition of approval, referenced by date |
Insurer requirement | The duration and any additional water supply obligation | The written requirement, with the schedule or policy it belongs to |
Owner resilience or continuity objective | A duration beyond the code minimum | The signed design basis memorandum and the reviewer's comment |
Municipal supply credit analysis | The part of the demand the public supply may be credited with | The supply test record and the stated availability assumption |
Fire pump acceptance test under NFPA 20 | A short, high-flow draw on the reserve during the commissioning run | The pump test procedure and the refill plan that follows it |
Sizing Calculation Sequence For A Fire Water Tank
Step | Quantity | Input Or Reference | Note |
1 | Design flow for the sprinkler demand | NFPA 13 hydraulic calculation at the most demanding point | At the design density over the design area |
2 | Hose stream allowance | The same system standard, as a separate demand | Not folded into the sprinkler flow |
3 | Duration applied to each demand | The system design standard, or an authority condition | Each demand carries its own duration |
4 | Demand volume | Sum of Q_i multiplied by t_i for every demand | Keep the units of the calculation consistent |
5 | Credited public supply | Supply credit analysis | Only what can be proven as deliverable during the event |
6 | Gross demand volume | Computed from steps 4 and 5 | The volume that must be available over the event |
7 | Unusable volume | Tank geometry, the anti-vortex arrangement under 14.2.13, freeboard | Everything below the usable level at the pump suction |
8 | Net capacity required | Computed, referenced to NFPA 22 4.1.3 and 4.1.3.1 | This is the number the tank is ordered against |
9 | Refill check | Gross demand restored within the 8 hours allowed by 4.2.1.4 | Fixes the fill arrangement and the level controls |
Project Reference
No verified fire water project record in the project library matches the scope of this sizing guide, so the article is presented without a project example. What it carries instead is the part that can be checked: where the duration basis comes from and who owns it, the calculation sequence that turns a flow and a duration into a net capacity, the limits of the standard assumptions, the situations that force a longer duration, and the documentation set an approving authority reads. Each of those is stated against the standard or the clause it comes from, so it can be audited rather than taken on trust.
About Center Enamel
Center Enamel (Shijiazhuang Zhengzhong Technology Co., Ltd.) designs and fabricates bolted steel storage tanks, welded steel tanks, fusion-bonded epoxy tanks, galvanized tanks, stainless steel vessels, pressure vessels, aluminum geodesic dome roofs and floating roofs. The company has more than 30 years of production experience and has supplied equipment to projects in more than 100 countries. Aluminum geodesic dome roofs are designed and fabricated to API 650 Appendix G, AWWA D108, the Aluminum Design Manual, ASCE 7 and IBC, and the company developed and exported a 72 m diameter aluminum dome roof in 2023. Engineering, fabrication, coating, quality control and site service are handled as one scope, which is why the drawings, the calculation basis and the delivered hardware can be reconciled against each other.
Frequently Asked Questions
What duration does NFPA 22 require for a fire water tank?
What duration does NFPA 22 require for a fire water tank?NFPA 22 does not set a duration. It sets the capacity basis the tank has to satisfy under 4.1.3 and 4.1.3.1, requires the tank to be kept filled within 4 in. (102 mm) of the fire service level under 14.4.3, and requires the supply to restore the volume within 8 hours under 4.2.1.4. The duration itself comes from the system design standard for the occupancy and hazard, confirmed by the authority having jurisdiction.
Does NFPA 22 set a fixed tank volume?
Does NFPA 22 set a fixed tank volume?No. It sets the delivery requirements and the capacity basis, not a number of cubic metres. The volume follows from the design flow, the duration and the treatment of the hose allowance, and then has the unusable volume added to it so that the net capacity available at the pump suction meets the requirement.
How is the required duration determined?
How is the required duration determined?From the fire protection system design. For a sprinkler system it is the duration the adopted edition of NFPA 13 requires for the occupancy hazard classification and system type. For a hydrant supply it is usually the authority's own fire flow criteria. In both cases the value needs to be traceable to a document, and any departure from it needs to be a written condition rather than an assumption.
Can the required duration be longer than the standard minimum?
Can the required duration be longer than the standard minimum?Yes. An authority condition of approval, an insurer requirement, a hazard outside the range the default covers, or an owner resilience objective can all require more. At constant flow the demand volume scales directly with the duration, so a longer duration means a larger shell, a heavier foundation and a different delivery route. Settle it before tender, not after manufacture.
What if the system demand changes after the tank is installed?
What if the system demand changes after the tank is installed?The reserve has to be re-derived from the new demand and the existing tank checked against it. The physical volume of an installed tank does not change, so if the new demand is larger, either the supply has to be supplemented, the delivery has to be re-examined, or the non-compliance has to be accepted explicitly by the authority. An alteration of occupancy, storage or protection scheme is the usual trigger.
How do I document a non-standard duration?
How do I document a non-standard duration?Attach the written condition, name the document or clause it comes from, state the design flow it applies to, and show the resulting demand volume, net capacity and the effect on the shell and the foundation. The point of the attachment is that the reviewer can verify the requirement without contacting anyone. An unexplained duration in a calculation sheet is treated as an assumption.
How do you document the sizing calculation?
How do you document the sizing calculation?In a fixed order: design flow and its source, duration and its source, hose allowance as a separate demand, credited public supply and the basis for the credit, gross demand volume, unusable volume with reasons, net capacity against 4.1.3 and 4.1.3.1, and the refill arrangement checked against 4.2.1.4. Name the edition of every standard used, quote volumes in the units the tank will be ordered in, and state the level datum the volume is measured from.
Conclusion
Sizing a fire water tank is short arithmetic wrapped in a long chain of decisions. The arithmetic is flow times duration. The chain is where the value is lost: the duration has to be traced to a document, the hose allowance has to be treated as a separate demand, the credited public supply has to be demonstrable, the unusable volume has to be added back, and the refill arrangement has to restore the volume within the 8 hours that 4.2.1.4 allows.
All of it is easier before the tank is ordered than after it is delivered. A duration settled early is a line in a calculation sheet. A duration settled late is a change to the shell, the foundation and the approval that was already granted.
Send us the system design flow, the duration and the document it comes from, the hose allowance treatment, the site constraints and the governing code edition, and we will return the capacity calculation, the net and gross volume statement, the drawdown geometry and the documentation set the approving authority will ask for.
Talk To Our Engineering Team
Send the duty, the volume basis, the governing code and the site constraints, and our engineering team will come back with a specification that a supplier can price and a reviewer can approve. We will also tell you plainly when a different tank type or a different scope is the better answer.
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