Centrifugal Pumps and Viscosity

[Esam]

When a viscous fluid is handled by a centrifugal pump

• brake horsepower requirement increases
• the head generated is reduced
• capacity is reduced
• efficiency of pump is reduced and the Best Efficiency Point - BEP - is moved

The head, flow and capacity at other viscosities than used in the original documentation can be modifying with coefficients.
[h=Flow]3[/h]

q_v= c_q q (1)
where
q_v = flow compensated for viscosity (m³/h, gpm)
c_q = viscosity flow coefficient
q = original flow according pump curve (m³/h, gpm)

[h=Head]3[/h]

h_v= c_h h (2)
where
h_v = head compensated for viscosity (m, ft)
c_h = viscosity head coefficient
h = original head according pump curve (m, ft)

[h=Efficiency]3[/h]

μ_v= c_μ μ (3)
where
μ_v = effciency compensated for viscosity
c_μ = viscosity efficiency coefficient
μ = original efficiency according pump curve

[h=Power - SI units]3[/h]

P_v= q_v h_v ρ_v g / (3.6 10⁶ μ_v) (4)
where
P_v = power compensated for viscosity (kW)
ρ_v = density of viscous fluid (kg/m³)
g = acceleration of gravity (9.81 m/s²)

[h=Power - Imperial units]3[/h]

P_v= q_v h_v SG / (3960 μ_v) (5)
where
P_v = power compensated for viscosity (bhp)
SG = specific gravity of viscous fluid

[Esam]

[h=Centrifugal Pumps - Minimum Flow]1[/h]
A catastrophic failure of a centrifugal pump can occur if the liquid within the pump casing is allowed to vaporize. To prevent flashing due to overheating of the fluid, a flow must be maintained through the pump to keep the liquid below saturation temperature.

If a temperature rise of 15 ^oF is accepted in the casing - minimum flow through a centrifugal pump can be calculated as

q = P_BHP / 2.95 c_p SG (1)
where
q = minimum flow rate (gpm)
P_BHP = power input (BHP)
c_p = specific heat capacity (Btu/lb ^oF)
SG = specific gravity of the fluid