NPSH Explained: NPSHa vs NPSHr, Flooded Suction and Suction Piping Design
NPSH (net positive suction head) is the pressure margin at the pump inlet above the liquid's vapor pressure, in feet of liquid. NPSHa is what your piping delivers; NPSHr is what the pump needs at a given flow, from its curve. Keep NPSHa above NPSHr with margin — Finish Thompson recommends at least 2 ft for its MSDB mag-drive pumps — or the pump cavitates. NPSHr rises with flow, so a system that is fine at one flow can cavitate at a higher one.
How to calculate NPSH available
Add what pushes liquid into the pump, subtract what takes pressure away, all in feet of the liquid you are pumping:
- hatm: atmospheric pressure on the liquid surface. 14.7 psia at sea level is 34 ft of water; it falls with altitude.
- hstatic: liquid level above the pump centerline (positive, flooded suction) or below it (negative, suction lift). Use the lowest level the tank will reach.
- hfriction: losses in the suction pipe, fittings, valves and strainer at the design flow. They rise with roughly the square of flow.
- hvapor: the liquid's vapor pressure at its highest operating temperature. Hot liquid flashes more easily.
NPSHr comes from the pump curve and is a minimum at each flow. It is not fixed for a pump: it climbs as flow increases.
Worked example: a sealless mag-drive pump on a hypochlorite day tank
A Finish Thompson MSDB2 (2-stage sealless mag-drive, 1,750 rpm, impeller 1) pulls 12.5% sodium hypochlorite (SG ≈ 1.2) from a day tank at sea level. Lowest tank level is 2 ft above the pump centerline. Vapor pressure is taken as water's at about 70°F, 0.36 psia; hypochlorite also off-gasses, which eats into margin further.
At 30 gpm
hvapor = 0.36 × 2.31 ÷ 1.2 = 0.7 ft
NPSHa = 28.3 + 2.0 − 3.0 − 0.7 = 26.6 ft
The MSDB curve book gives NPSHr ≈ 9.6 ft at 30 gpm. With the 2 ft margin the pump needs 11.6 ft. 26.6 ft available: comfortable.
Same system, opened up to 50 gpm with a loaded strainer
Friction grows with flow squared (3.0 × (50/30)² ≈ 8.3 ft) and a half-clogged strainer adds about 4 ft:
At 50 gpm the curve gives NPSHr ≈ 22.9 ft, so the pump needs 24.9 ft. 17.3 ft available: the pump cavitates. Nothing about the pump changed. The flow went up, which raised both the friction and the NPSH the pump requires, while a strainer nobody counted took the rest. If yours is already in this state, see NPSHa less than NPSHr: troubleshooting.
NPSHr from Finish Thompson's MSDB curve book (MSDB2, 1,750 rpm, 60 Hz, impeller 1, p.10), read on water. Friction figures are illustrative; calculate your own line.
Suction piping design: the rules that protect NPSH
Most cavitation problems are built into the suction piping on day one. These are Finish Thompson's installation rules for the MSDB, and they apply to any centrifugal or mag-drive pump on chemical service:
| Rule | What it means |
|---|---|
| Pump close to the suction source | Keep the suction line as short and straight as possible to minimize friction. |
| No reducers on the suction | The suction line should be at least as large as the inlet port, or one size larger. Do not reduce it. |
| No high spots | High spots trap air. Run the suction level, or slope it slightly upward to the pump. |
| Bends and valves ten diameters away | Keep bends and valves at least ten pipe diameters from the suction and discharge. |
| Suction valve fully open | Never throttle flow with the suction valve; regulate with the discharge valve. |
| Count the strainer | Filters, strainers and fittings in the suction line all reduce NPSH available and belong in the calculation. |
| Support the piping independently | Support pipe near the pump so it puts no strain on the casing. |
| Margin | Provide NPSH available at least 2 ft (61 cm) above NPSH required. |
Source: Finish Thompson MSDB Series manual, P/N 110989 R5, installation section.
Flooded suction vs suction lift
A flooded suction puts the liquid above the pump so it fills by gravity; a suction lift makes the pump pull liquid up. The MSDB is not self-priming: it has to be filled from a flooded suction tank or primed from an outside source, and Finish Thompson says to install priming piping on the discharge for any suction-lift system. If the pump must lift from a sump or tote, use a self-priming design instead — Finish Thompson's SP series mag-drive is rated for lifts up to 25 ft, and an air-operated diaphragm pump lifts 28 ft wet.
Can a mag-drive pump run dry?
It depends on the bushing, and even then only for a while. On the MSDB, Finish Thompson's manual says:
| Bushing material | Dry running |
|---|---|
| Carbon | Can operate without liquid in the casing; how long varies with operating conditions and environment. |
| PTFE, ceramic, silicon carbide | Cannot be run dry without damaging the pump. |
The same manual says the pump should never be operated without liquid in the casing and recommends run-dry protection, such as an electronic power monitor. Run-dry tolerance is insurance, not a design basis — it does not replace NPSH margin. See mag-drive dry-run bearing failure and mag-drive pump protection.
Get the Mag-Drive Run-Dry & Suction Checklist
The Finish Thompson MSDB pump-review checklist: bushing run-dry limits, the 2 ft NPSH margin, and every suction-piping rule above, on one page you can hand to the installer.
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The Finish Thompson MSDB pump-review checklist: carbon-bushing run-dry limits, NPSH margin and the suction-piping rules that decide pump life.
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NPSH questions
What is NPSH?
Net positive suction head is the pressure margin, in feet of the liquid being pumped, between the pressure at the pump inlet and the liquid's vapor pressure. If the inlet pressure drops close to vapor pressure, the liquid flashes to vapor inside the pump and the pump cavitates.
What is the difference between NPSHa and NPSHr?
NPSHa (available) is what your system delivers to the pump inlet: atmospheric pressure plus static suction head, minus friction losses and vapor pressure. NPSHr (required) is what the pump needs at a given flow, read from the manufacturer's curve. NPSHa must exceed NPSHr with margin; Finish Thompson recommends at least 2 ft above NPSHr for its MSDB pumps.
How do you calculate NPSH available?
NPSHa = atmospheric head + static suction head − suction friction losses − vapor pressure head, all in feet of the liquid. Convert pressures with feet = psi × 2.31 ÷ specific gravity. Use the lowest tank level, the highest flow and the hottest fluid temperature the system will see.
How do you increase NPSH available?
Raise the minimum liquid level or lower the pump, shorten and straighten the suction line, go up a pipe size on the suction, remove or clean strainers and restrictive fittings, keep the suction valve fully open, and keep the fluid cooler. Running the pump at lower flow also lowers the NPSH it requires.
What is a flooded suction pump?
A pump installed below the liquid source, so liquid flows into the casing by gravity and the pump never has to lift it. Non-self-priming centrifugal pumps, including the Finish Thompson MSDB, are installed this way or primed from an outside source.
Can a mag-drive pump run dry?
Only with the right bushing, and only briefly. Finish Thompson's MSDB manual says a pump with a carbon bushing can operate without liquid for a time that varies with operating conditions, while PTFE, ceramic and silicon-carbide bushings cannot be run dry without damage. Finish Thompson still recommends run-dry protection such as a power monitor.
What type of pump can run dry?
Air-operated diaphragm pumps and peristaltic pumps tolerate dry running best because they have no seal or close-clearance bearing running in the fluid. Some sealless mag-drive pumps tolerate short dry periods with carbon bushings. Most centrifugal pumps with mechanical seals are damaged quickly when run dry.
Send us the suction side before you buy the pump
Tank levels, suction pipe run, fluid, temperature and flow range. James checks NPSH margin across the whole operating range and recommends the pump, bushing and protection to match — no charge.
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