The materials of construction of the pump must be chemically compatible with the fluid being pumped. Once these factors are known or considered and the system's performance curve has been estimated, a pump can be selected.
How Do You Determine NPSHA?Net positive suction head available (NPSHA) is the amount of head available at the pump inlet. Centrifugal pumps must have adequate suction head to perform in accordance with published curves. NPSHA is a characteristic of the system and must be greater than the net positive suction head required (NPSHR) of the pump plus a safety factor of at least 2':
NPSHA is calculated using the following equation:
- NPSHA Basic Equation = A + B +/- C - D - E
A is the pressure on the surface of the liquid, measured on the suction side of the pump, in feet of liquid absolute (or, for open tank suctions, atmospheric pressure [Patm]), which is calcuated by:
(psig + Patm) x 2.31
Specific gravity of the liquid
B is the velocity head in feet of liquid at the point of pressure gauge connection, which is calculated by
Velocity head = V2 ÷ 64.4
V is the velocity of liquid at pressure gauge connection in feet per second (ft/sec).
0.32 x gpm
Area (in2) =
(pi x d2)
d is the diameter (in).
B typically is negligible; as a result, it usually is left out of NPSHA calculations.
C is the static height in feet of liquid that the liquid supply level is above (+) or below (-) the pump centerline.
D is all suction line friction losses in feet between the suction pressure gauge connection and the pump suction, including piping, valves and fittings.
E is the vapor pressure of the liquid at the pumping temperature, in feet of liquid.
What Is Total Dynamic Head (TDH)?Also referred to as pump head and pump total head, total dynamic head is the amount of work done by the pump per unit weight of liquid passing through the pump. A layperson's definition is that TDH is simply the head measured at the pump discharge port minus the head measured at the pump inlet port. In the English system, TDH is normally expressed in units of feet of water.
How Is Head Different Than Pressure?The terms "head" and "pressure" are closely related, but they should not always be used interchangeably. In English units, head is the value used to express the amount of energy in a liquid or work done on a liquid (per pound of liquid) and has units of feet of water. Pressure is the value used to express the force per unit area exerted by a liquid and has units pounds per square inch (psi). Pressure and head are related by the following:
Head (ft) x Specific Gravity
The amount of pressure a pump can develop is directly dependent on the specific gravity of the fluid being pumped. However, the amount of head a pump can develop is the same regardless of the specific gravity of the fluid being pumped. Because of this apples-to-apples comparison, pump performance typically is specified in feet of head instead of psi. Always size and select pumps based on the head required, not pressure.
How Do Specific Gravity and Viscosity Affect Pump Performance?Specific gravity is defined as a liquid's density relative to that of water; i.e., the liquid's density divided by that of water. The water temperature for this purpose is usually 60°F (15.6°C) where its density is 0.9991 g/cm3. The specific gravity of a fluid will affect the brake horsepower (BHP) required and the pressure vs. head relationship as follows:
gal/min x Head (ft) x Specific Gravity
3,960 x Pump Efficiency Expressed As A Decimal
Viscosity is defined as the property of a fluid that offers resistance to flow due to the existence of internal friction within the fluid. Unfortunately, there are no simple equations available to calculate the effect of viscosity on a pump. The general relationship of pump performance to viscosity is that as the viscosity of a fluid increases, the flow, head and efficiency of the pump tend to decrease while the required brake horsepower increases.