NPSHa Calculations

Another very significant area of fluid flow calculations is the calculation for the net positive suction height available. This is basically the fluid pressure (expressed in feet) at the pump suction that is greater than the vapor pressure of the fluid. Every pump comes with a NPSHr (Net positive suction height required) curve. The NPSHa must be greater than the NPSHr or the pump will cavitate. Cavitation is the result of gas bubbles formed in the suction of the pump collapsing as the fluid pressure increases through the pump. The collapsing bubbles can cause a great deal of damage to the pump impeller (eventually leading to its failure). Cavitation is most severe when the pump fluid is a pure fluid Iike water, or a narrow boiling point fluid. This is because the bubbles collapse over a narrow pressure range, and not gradually as pressure builds up.

Normal procedure is to have the NPSHa 2 to 3 feet greater than the NPSHr (depending on the company guidelines). The NPSHa is calculated as follows:

NPSHa = Absolute Pressure above liquid(ft) -Vapor pressure at liquid interface(ft) + Static head above pump (ft) – Frictional losses (ft)

Things that can cause problems in pump suctions are suction strainers. They become plugged and the pressure drop increases until the NPSHa no longer exceeds the NPSHr and the pump begins cavitating. There should be a way to measure the pressure drop across the suction strainer if it is to be left in line. The normal method is to install a dp (differential pressure) cell. Alternatives are to remove the strainers the first time they get plugged. Some companies consider the suction strainers just for initial startup to catch the welding rods, etc and that they should be removed at the first opportunity

We use the following general rules for our initial sizing of lines:

As the project gets better defined, we go back and calculate the pressure drops based on the actual pipe length and fittings.

The fundamental calculation for fluid flow is the Bernoulli Equation:

D P total = DP velocity head + D P friction + D P fitting + DP elevation

D P velocity head

Also known as the Kinetic energy term. The kinetic energy of the fluid is energy of the fluid as a result of its motion.

D P elevation

The potential energy term is a measure of the available energy as a result of its position. Generally only important for liquids. Must be considered when you are pumping into or from tanks.

DP friction

This term is the one we tend to focus on. The frictional term is pressure drop as a result of flow though the pipe.

DP fitting

The fitting loses is pressure drop as a result of fittings (elbows, entrance and exit losses, reducers, floating suction pipe, valves, strainers, etc. Pretty much everything your fluid passes through except for pipe. The fitting losses are estimated by either equating them to “K” values or as equivalent feet. The fitting term is sometimes ignored. Unfortunately there is frequently more pressure drop as a result of fittings than there is as a result of straight pipe.

NPSHA (Net positive suction height available)

Total pressure at pump suction

since Vel 1 = 0.0 (Vel 1 is the flow velocity in the tank, effectively zero)

Total Pressure =

NPSHA = (Total Pressure - Vapor Pressure) x 144g_c/r₂g
NPSHA = Total Pressure at Pump Suction - Vapor Pressure, expressed in feet

Example Calculations

A tank with gasoline in it, is to be pumped at 1100 gpm. The tank will have a minimum liquid level 6 feet above grade, the pump centerline will be 2 feet above grade (use grade as our datum point, ie zero elevation). The vapor pressure of the gasoline will be 9.5 psia at the storage temperature. The pipe absolute roughness is 0.00015 ft (commercial steel).

Based on our liquid sizing criteria for subcooled liquids, 10 ft/sec is the maximum allowable velocity. A 8 inch pipe, with 1100 gpm runs at 7.05 ft/sec. A 6 inch pipe runs at 12.22 feet/sec. We will assume a 8 inch pipe (see line sizing criteria table, subcooled pump suction).

Note: friction factor for fully turbulent flow for 8 inch pipe = 0.014 (see Crane, pg A-26)

25 Feet of straight pipe is used for the length from the tank to the pump suction.

Calculate Reynolds number for 8 inch pipe: