Vertical Turbine Pumps and Water Channels

Engineering Wet-Wells to NFPA 20 and HI 9.8 Standards

When designing a vertical turbine fire pump system, it is easy to focus all your attention on the pump itself — the driver, the controller, the baseplate, and the discharge piping. But a fire pump is only as reliable as its water supply. If the wet-well (or wet pit) is improperly designed, even the best turbine pump will suffer from cavitation, vibration, and premature failure when called upon during an emergency.

Designing a reliable wet-well means satisfying two distinct but complementary standards: NFPA 20 (which ensures life-safety compliance) and ANSI/HI 9.8 (which ensures fluid dynamic stability). Here is what you need to know to bridge the gap between the two.

The NFPA 20 Baseline: Compliance and Flow Obstruction

NFPA 20 (Standard for the Installation of Stationary Pumps for Fire Protection) is highly prescriptive regarding the physical constraints of a wet pit to ensure water is always available and free of debris.

• Screening: To prevent clogging, NFPA 20 requires screens for open water sources. You must provide at least 1 square inch (170 mm²) of effective screen opening per 1 GPM at 150% of the pump's rated capacity, using corrosion-resistant materials (brass, copper, Monel, or stainless steel).

• Freezing: Suction pipes and wet pits require protection against freezing. Special attention is necessary if pipes enter ponds or reservoirs.

• Velocity: Per NFPA 20-2019 edition, section A.7.2.2.2, the velocity in the approach channel or intake pipe should not exceed approximately 2 feet/second. And the velocity in the wet pit should not exceed approximately 1 foot/second. For a 500 gpm pump, the 150% flow rate is 750 gpm and the sample calculation is provided below:

Pump Size (GPM)	Minimum Area (sq ft) based upon 1 foot/second	Minimum Area (sq ft) based upon 2 feet/second	Potential Min Nominal Pipe Size (C900)
250 gpm	0.84 sq ft	0.42 sq ft	10 inch
500 gpm	1.67 sq ft	0.84 sq ft	14 inch
750 gpm	2.51 sq ft	1.26 sq ft	16 inch
1000 gpm	3.34 sq ft	1.67 sq ft	18 inch
1500 gpm	5.01 sq ft	2.51 sq ft	24 inch
2000 gpm	6.68 sq ft	3.34 sq ft	30 inch
2500 gpm	8.35 sq ft	4.18 sq ft	30 inch
3000 gpm	10.03 sq ft	5.02 sq ft	36 inch
4000 gpm	13.37 sq ft	6.69 sq ft	36 inch
5000 gpm	16.71 sq ft	8.36 sq ft	42 inch

NFPA 20 dictates what must be present for a compliant fire protection system, but it leaves the intricacies of fluid behavior to the Hydraulic Institute.

HI 9.8: The Fluid Dynamics of Intake Design

While NFPA 20 ensures you have water, ANSI/HI 9.8 (Pump Intake Design) ensures the water behaves correctly before it hits the impeller. HI 9.8 focuses on mitigating six adverse hydraulic effects: pre-swirl, uneven velocity, surface and submerged vortices, air entrainment, and sedimentation.

• Submergence and Froude Number: Minimum submergence isn't just about covering the suction bell. Under HI 9.8, minimum submergence is calculated as a function of the inlet bell design diameter and the Froude number. Failing to meet this dynamic submergence limit is the leading cause of surface vortices, which pull air cores directly into the pump.

• Velocity Control: Intake velocities must be kept low to minimize eddy currents. General guidance limits approach velocities in the wet pit to 1 ft/sec (or roughly 0.3 m/s), while velocities at the suction bell itself shouldn't exceed 2 ft/sec. High velocities cause uneven distribution across the eye of the impeller, resulting in unbalanced loads, noise, and severe bearing wear.

• Sump Geometry and Baffling: The physical shape of the wet-well dictates swirl. HI 9.8 provides exact dimensional ratios for rectangular and circular intakes based on the suction bell diameter. For example, the distance from the bell center to the back wall and floor must be tightly controlled to prevent submerged floor vortices. If the geometry cannot be perfected, baffle walls and floor cones are required to break up swirl before it reaches the pump.

FM Insurance Recommendations

FM Datasheet 3-7 (Fire Protection Pumps) provides the following recommendations regarding suction screening and sump construction. When selecting screen material, consider the prevention of fouling from aquatic growth; this is best accomplished with brass or copper wire. A sound suction screen will be constructed of brass, copper, monel, stainless steel, or other equivalent corrosion-resistant metallic material wire screen of 1⁄2-in. (12.7 mm) mesh and No. 10 gauge wire to a metal frame sliding vertically at the entrance to the intake. The overall area of this screen should be approximately 1.6 times the net screen opening area.

FM copies the dimensions recommended by the Hydraulics Institute Standards for intake design using a sump (suction pit) as follow:

• S is the minimum width of the wet pit.

• B is the suggested maximum dimension of the pump centerline from the back wall. The edge of the bell should be close to the back wall.

• C should be specified by the pump manufacturer.

• H is the minimum value based on minimum water level. Submergence for determining the location of the second impeller from the bottom of the pump bowl assembly is H minus C.

• Y is the minimum recommended distance of the pump centerline to the screen.

Pump Flow	Min Width (S)	Max Back Wall (B)	Min to Screen (Y)
≤ 2,500 gpm	34 in.	13 in.	60 in.
4,000 gpm	40 in.	16 in.	70 in.
5,000 gpm	44 in.	18 in.	75 in.