FM Global DS 3-7 Fire Pumps: A Field Guide

On a European-owned logistics warehouse project, the third-party insurance survey arrives with an unexpected finding: the pump room sits inside the main building, separated only by one-hour construction, and is not even sprinkler protected. The mechanical team had designed to NFPA 20, and national regulation was satisfied. The surveyor cited FM Global DS 3-7: the pump room must either be a detached building at least 15 m away, or be separated by two-hour fire-resisting construction with sprinkler protection. The design was revised at significant cost. This article summarises DS 3-7 in practical terms — the rules that make sure a fire pump is in the right place, with the right connections and the right control logic.

Pump Room Location and Construction

FM Global's strictest requirement concerns pump room separation, given at three levels:

• Preferred: a detached building of noncombustible construction at least 15 m from the protected building.
• Alternative: two-hour fire-resisting construction with a separate door opening directly to the outside.
• Minimum acceptable: where the pump room and adjacent areas are sprinkler protected, the separation may reduce to one hour.

A pump room may not adjoin an unprotected building. Sprinklers are required above engine-driven pumps in all cases. The pump room may not be used for storage — the most frequently violated clause on site, where material stacked in a corner produces an immediate non-conformance at survey.

Alarm Requirements

DS 3-7 requires the alarm set to report to a constantly attended location. For an electric pump, four alarms as a minimum:

• Pump (driver) running
• Loss of power
• Controller transferred to an alternative power source
• Failure to start

For a diesel pump, four further alarms are added: overspeed, low oil pressure, high coolant temperature and engine trouble. These four need not be monitored remotely, but visual and audible indication must operate at the controller.

Suction Piping — The Red Lines

Suction piping is the section most often rejected at survey. The key rules:

• Positive pressure: suction pressure must remain positive across the whole flow range. Suction lift is not permitted; a vertical turbine pump is used instead.
• Friction limit: friction loss between the suction tank and the pump inlet must not exceed about 0.4 bar (6 psi) at 150% of rated flow. This prevents inadequate NPSH so the pump does not cavitate even at low tank level.
• Eccentric reducer: where the suction pipe is larger than the pump flange, an eccentric tapered reducer is used so no air pocket can form — never a concentric one. Flat-side-up orientation is preferred on horizontal split-case pumps.
• Elbow clearance: on a horizontal split-case pump, no elbow or tee in the horizontal plane within 10 pipe diameters of the suction flange. Otherwise the inlet flow is asymmetric and the impeller is unbalanced.
• Velocity limit: velocity must not exceed about 4.6 m/s at 150% of rated flow.
• Prohibited devices: nothing that could restrict pump operation or starting may be fitted on the suction side — no backflow preventer, low-suction cut-off or suction regulating valve. Where a backflow preventer is unavoidable, it goes on the discharge side.

Discharge Piping and Relief Valves

• When a relief valve is required: if the pump shutoff (churn) pressure plus the maximum static suction pressure could exceed the rated pressure of the system components, typically 12 bar (175 psi), a main relief valve is mandatory. On high-rise projects this is usually the case.
• Installation: the relief valve is fitted between the pump and the discharge check valve and must be removable for maintenance without disturbing the pipework. Discharge is directed to a visible and audible point; it may not be returned to the pump suction or the supply line.
• PRV prohibition: pressure reducing valves may not be fitted on the pump suction or discharge. PRVs belong in the distribution system for zone pressure management, per DS 3-11.

Pump Sizing and Selection

The governing rule is that the pump must deliver at least 65% of rated pressure at 150% of rated flow, and no more than 140% of rated pressure at churn. So a pump rated 5,000 L/min at 10 bar must still produce at least 6.5 bar at 7,500 L/min.

• Split case, end suction and in-line centrifugal: may not be used where suction lift is required. An air release valve of at least 12.7 mm vents to atmosphere.
• Vertical shaft turbine: mandatory where the water source lies below the discharge flange or where supply pressure is inadequate — typical for tanks, wells and reservoirs.
• Positive displacement: used for water mist and foam concentrate, not for standard sprinkler service.

Mechanical seals are used only on FM Approved pumps with clean water sources. On pond, lake or wet pit supplies a packed gland is preferred.

Controllers, Sequence Starting and Weekly Testing

An FM Approved fire pump controller is mandatory. The key control rules:

• Automatic start, manual stop: the pump starts automatically on pressure or flow but is stopped manually. Someone attends the pump house, confirms the set is running correctly and that the emergency is fully over, before it is stopped.
• Jockey pump required: a jockey pump makes up normal leakage and small pressure drops. The fire pump may never serve as pressure maintenance, or it wears out through repeated short runs.
• Sequence starting: where two or more pumps operate in parallel, simultaneous starting is prevented. If one pump fails to start, the next is not prevented from starting, so redundancy survives a failure.
• Weekly test: where the controller has an automatic weekly test feature, it may not run unattended. A technician is present in the pump room, watches the test data and can intervene.

Practical Differences From NFPA 20

• Equipment approval: NFPA 20 accepts listed components; DS 3-7 accepts only FM Approved pump, driver, controller and relief valve.
• Pump room separation: NFPA 20 accepts one-hour construction; FM requires two hours where the room is not sprinkler protected.
• Separation distance: NFPA 20 does not give a figure; FM prefers a detached building at least 15 m away.
• Test regime: NFPA 25 governs the annual flow test; FM adds weekly run test monitoring through DS 2-81.
• Variable speed pumps: NFPA 20 permits them under defined conditions; FM accepts only FM Approved packages.

A Recurring Surprise at Survey

A common pattern on industrial sites: a diesel fire pump is installed to a design approved under NFPA 20 and accepted by the local authority. Months later a customer supplier audit or an insurer survey reports that the installation does not comply with DS 3-7, typically for three reasons:

• The pump room is unsprinklered, with only one-hour separation where FM requires two
• A backflow preventer sits in the suction line, which FM prohibits — it belongs on the discharge
• Only the running alarm is wired out; overspeed, oil pressure and coolant alarms terminate at the controller

The remedy is straightforward but disruptive: sprinklers added over the pump room, the backflow preventer relocated, and the alarm panel connected to the building management system. Taking DS 3-7 as the baseline at design stage would have covered all three at a fraction of the cost.

When FM Compliance Is Required in Practice

National regulation accepts NFPA 20 or EN 12845 for fire pumps and does not reference FM data sheets. FM compliance becomes binding through other routes:

• European-owned logistics warehouses operating to a group standard
• Industrial facilities inside an international insurance portfolio
• Automotive and electronics plants subject to OEM supplier audit
• Data centres pursuing certification, where FM compliance is expected alongside

The practical advice is to ask the question explicitly at design stage: will FM compliance be required? Even if the insurer has not been selected, the owner may move the portfolio within a few years, and designing FM compliant from the start avoids a costly retrofit later.

Quick Checklist

• Pump room detached at 15 m, or two-hour rated with sprinkler protection
• No storage in the pump room; sprinklers above engine-driven pumps
• Four alarms for electric, eight for diesel, reporting to a constantly attended location
• Positive suction pressure with friction below 0.4 bar at 150% flow
• Eccentric reducer, 10 diameters clear of elbows, velocity below 4.6 m/s
• No backflow preventer, cut-off or regulating valve on the suction side
• Relief valve where required, FM Approved, not returned to suction
• Pump curve: at least 65% pressure at 150% flow, no more than 140% at churn
• Automatic start, manual stop, with a separate jockey pump
• Sequence starting for parallel pumps
• Weekly test never run unattended
• All components FM Approved

Frequently Asked Questions

Why does FM require a detached pump room or two-hour separation?

Because the pump must survive the fire it is there to fight. A pump room inside the protected building, separated by one hour and unsprinklered, can be lost early in an incident, taking the water supply with it. A detached building at 15 m, or two-hour construction with sprinklers, keeps the pump available for the duration.

Why is a backflow preventer prohibited on the pump suction?

Anything on the suction side that can restrict flow threatens the pump's ability to start and run at full demand, and a backflow preventer adds significant friction loss exactly where NPSH margin is scarce. Where one is required for water authority reasons, it goes on the discharge side instead.

Why an eccentric rather than a concentric reducer at the suction?

A concentric reducer creates a high point at the top of the pipe where air collects, and that air pocket is drawn into the pump when it starts, causing loss of prime. An eccentric reducer fitted flat side up keeps the top of the pipe level so no pocket can form.

Why must the fire pump be stopped manually?

So that someone confirms the emergency is genuinely over before the water supply is removed. An automatic stop at churn pressure can shut the pump down while the fire is still developing, and it also causes repeated short-cycle running that wears the pump. Automatic stopping is only used where specifically approved, with minimum run timers.

What does the 150% at 65% rule actually mean?

It defines the required shape of the pump curve. At 150% of rated flow the pump must still produce at least 65% of rated pressure, and at churn no more than 140%. This guarantees the pump still delivers useful pressure if actual fire demand exceeds the design figure, rather than collapsing off the end of its curve.

How does FM DS 3-7 differ most from NFPA 20 in practice?

Equipment approval and pump room separation. FM accepts only FM Approved pumps, drivers, controllers and relief valves, where NFPA accepts listed components. And FM requires two-hour separation or a detached building, where NFPA 20 accepts one hour. Those two differences account for most of the findings raised at survey.

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