Pumping Philosophy

Pumping Philosophy

• Pumps transfer liquids from one point to another by converting mechanical energy from a rotating impeller into pressure energy (head)
• The pumping system designer must consider fluid properties, determine end use requirements, and understand environmental condition.
• The fluid properties being pumped can significantly affect the choice of pump, includes;         -Acidity/ Alkalinity (pH) and chemicals composition
         - Operating temperature
          - Solid concentration/particle size
          - Specific gravity
          - Vapor pressure
          - Viscosity

• End use requirement – system flowrate and head: The design pump capacity is needed to accurately size of the piping system, determine friction head loss, construct a system curve, and select a pump and drive motor.

•Process requirement can be met by providing a constant flowrate, or using a throttling valve, or variable speed drive to supply continuously variable flow rate

•The total system head has three (3) components: static head, elevation (potential energy), and velocity (or dynamic) head.

-Static head is pressure of fluid in the system and is measured by conventional pressure gauge
-The height of the fluid level can have a substantial impact on the system head
-The dynamic head is the pressure required by the system to overcome head losses caused by  flow rate  resistance in pipe , valves, fitting and mechanical equipment

 • Environmental considerations, includes ambient temperatures and humidity, elevation above sea level, and whether the pump to be installed indoors or outdoors

 Cavitation
Liquid vaporization within a pump intake is called “cavitation”.
• The pressure on the liquid entering a centrifugal pump is reduced as it moves through the suction eye
• Cavitation occurs when the pressure in the pump inlet drops below the vapor pressure of liquid
• To prevent cavitation the pressure reduction in the suction eye must be compared to the vapor pressure entering the pump to determine whether the liquid will vaporize. The amount of excess pressure above the vaporization pressure is called net positive suction head (NPSH).

Cavitation is often characterized by:
1 )Loud noise
2) Loss of capacity due to bubbles are now taking up space where liquid should be
3) Pitting damage to parts as material is removed by the collapsing bubbles

 

 How to Avoid Cavitation? 

Cavitation can be avoid by increasing NPSHA with following actions,
1) Lower the operating temperature of the pumped liquid
2) Increase the liquid level relative to the pump suction eye
3) Raise the vapor pressure of the liquid by adding chemicals to raise the boiling point
4) Reduce suction pipe friction loss

 NPSHA & NPSHR

 • NPSHA: The absolute pressure at the suction port of the pump
•  NPSHR: The minimum pressure required at the suction port of pump to keep the pump from cavitating
•  NPSHA is a function of system and must be calculated
•  NPSHR is a function of the pump, must be provided by the pump manufacturer
•  NPSHA must be greater than NPSHR for the pump system to operate without cavitating