Training programe

Pumps can be classified in the two basic types:

- Centrifugal pumps

- Positive displacement pumps

The centrifugal pump produce a head differential and a flow by increasing the velocity of the liquid through the machine with a rotating vane impeller.

The positive displacement pump operates by alternating of filling a cavity and then displacing a given volume of liquid. The positive displacement pump delivers a constant volume of liquid against a varying discharge head.

When to use a Centrifugal Pump (CP) or a Positive Displacement Pump (PDP) is not always a clear choice. The two types of pumps behave very differently. The CP has varying flow depending on pressure or head, where the PDP has more or less a constant flow regardless of pressure.

Another major difference between the pump types is the effect of viscosity on the capacity of the pump.

· The CP losses flow when the viscosity goes up

· The PDP pump actually increases flow.

Higher viscosity liquids fill in the clearances of a PDP causing a higher volumetric efficiency.

The pumps behaves different considering mechanical efficiency as well. Changing the pressure have little or no effect on the volume flow in the PDP, but has a dramatic one in the CP.

Another consideration is Net Positive Suction Head NPSH. In a CP the NPSH varies as a function of flow, determined by pressure. In a PDP NPSH varies as a function of flow determined by speed. The lower the speed of a PDP pump, the lower the NPSH.

A PDP is better suited for high viscosity applications. A CP becomes very inefficient at even modest viscosity.

PDP pumps generally gives more pressure than CP's.

Net Positive Suction Head - NPSH

A definition and an introduction to Net Positive Suction Head - NPSH.

Low pressure at the suction side of a pump can encounter the fluid to start boiling with

• reduced efficiency
• cavitation
• damage

of the pump as a result. Boiling starts when the pressure in the liquid is reduced to the vapor pressure of the fluid at the actual temperature.

To characterize the potential for boiling and cavitation, the difference between the total head on the suction side of the pump - close to the impeller, and the liquid vapor pressure at the actual temperature may be used.

Suction Head

Based on the Energy Equation - the suction head in the fluid close to the impeller can be expressed as the sum of the static and the velocity head:

h_s = p_s / + v_s² / 2 g (1)

where

h_s = suction head close to the impeller

p_s = static pressure in the fluid close to the impeller

= specific weight of the fluid

v_s = velocity of fluid

g = acceleration of gravity

Liquids Vapor Head

The liquids vapor head at the actual temperature can be expressed as:

h_v = p_v / (2)

where

h_v = vapor head

p_v = vapor pressure

Note! The vapor pressure in a fluid depends on temperature. Water, our most common fluid, starts boiling at 20 ^oC if the absolute pressure in the fluid is 2,3 kN/m². For a temperature of 80 ^oC the boiling starts at absolute pressure 47,5 N/m², and of course at 101.3 kN/m² (normal atmosphere) boiling starts at 100 ^oC.

Net Positive Suction Head - NPSH

The Net Positive Suction Head - NPSH - can be expressed as the difference between the Suction Head and the Liquids Vapor Head:

NPSH = h_s - h_v (3)

or

NPSH = p_s / + v_s² / 2 g - p_v / (3b)

Required NPSH - NPSH_r

The required NPSH - NPSH_r - is the NPSH that must be exceeded to avoid vaporization and cavitation in the impellers eye. The NPSH_r is always higher than the theoretical NPSH due to head loss in the suction pipe and the pump casing, and local velocity acceleration and pressure decrease on the impeller surface.

NPSH_r is in general determined experimentally by the pump manufacturer and a part of the pump performance curves documentation.

The required NPSH_r increases with the square of increased capacity.

Available NPSH - NPSH_a

The available NPSH - HPSH_a - is the NPSH available for a particular system and must be determined during design and construction of the system, or determined experimentally on the actual physical system.

The available NPSH_a can be calculated with the Energy Equation. For a common application - where the pump lifts a fluid from an open tank at one level to an other, the energy or head at the surface of the tank is the same as the energy or head before the pump impeller and can be expressed as:

h₀ = h_s + h_l (4)

where

h₀ = head at surface

h_s = head before the impeller

h_l = head loss from the surface to impeller - major and minor loss in the suction pipe

In an open tank the head at surface can be expressed as:

h₀ = p₀ / = p_atm / (4b)

For a closed pressurized tank the absolute static pressure inside the tank must be used.

The head before the impeller can be expressed as:

h_s = p_s / + v_s² / 2 g + h_e (4c)

where

h_e = elevation from surface to pump - positive if pump is above the tank, negative if the pump is below the tank

Transforming (4) with (4b) and (4c):

p_atm / = p_s / + v_s² / 2 g + h_e + h_l (4d)

The head available before the impeller can be expressed as:

p_s / + v_s² / 2 g = p_atm / - h_e - h_l (4e)

or as the available NPSH_a:

NPSH_a = p_atm / - h_e - h_l - p_v / (4f)

Available NPSH_a - the Pump is above the Tank

If the pump is positioned above the tank, the elevation - h_e - is positive and the NPSH_a decreases when the elevation of the pump increases.

At some level the NPSH_a will be reduced to zero and the fluid starts to evaporate.

Available NPSH_a - the Pump is below the Tank

If the pump is positioned below the tank, the elevation - h_e - is negative and the NPSH_a increases when the elevation of the pump decreases (lowering the pump).

It's always possible to increase the NPSH_a by lowering the pump (as long as the major and minor head loss due to a longer pipe don't increase it more). This is important and it is common to lower the pump when pumping fluids close to evaporation temperature.