Suction Head to PSI Converter

Tank-to-pump elevation design: sizing the minimum liquid level above a pump to prevent cavitation.

Hydrocarbon transfer: verifying adequate suction pressure for volatile liquids with high vapor pressure.

In the design of pump stations for water and wastewater service per ANSI/HI 9.6.1 and the Hydraulic Institute standards, the NPSH calculation is a primary design check that determines the maximum allowable pump flow (because NPSHR increases with flow), the minimum liquid level in the wet well (a 2 MGD submersible pump station typically requires a wet well bottom that keeps the water level above the pump volute by 1–2 feet to ensure flooded suction and adequate NPSH), and the need for a suction side booster pump for low-pressure applications.

In petroleum refinery pump applications, the NPSH calculation for hot hydrocarbon service (crude column bottoms pump at 650°F, vacuum column bottoms pump at 700°F) requires careful accounting for the fluid vapor pressure at the pump centerline, with the conversion from suction head to pressure using the fluid specific gravity at the operating temperature (not the standard 60°F SG).

LNG pump applications (LNG at -162°C) have an extreme case: the vapor pressure of LNG is essentially 0 psig (it's a boiling liquid), so the available NPSH is the static head minus the suction piping friction, and the pump must be located below the LNG tank to ensure flooded suction.

Cryogenic pump design per API 685 and the Linde/Statoil specifications for LNG plants requires very careful NPSH calculation with margin for transient operations (vapor bubble formation during start-up, sudden valve closure, etc.).

Hydrocarbon and chemical pump applications with high vapor pressure (butane, propane, ethylene, propylene, VCM, vinyl chloride) require pressurized suction vessels or suction side pumps to maintain NPSHA above NPSHR, with the conversion at the safety relief valve and the control valve sizing.

Boiler feedwater pump (BFP) selection for utility and industrial boilers requires the NPSH calculation to account for the hot water at the deaerator outlet (typically 220–250°F), with the suction head from the deaerator to the BFP sized to provide 5–10 ft of NPSHA above the pump's NPSHR.

The BFP must be located below the deaerator to provide flooded suction, and the suction piping must be sized for low velocity (3–6 ft/s) to minimize friction loss and maintain NPSH margin.