BACK TO BASICS CENTRIFUGAL PUMPS

Before we get into the details of operation, we need to define some terms. At a fundamental level, pumps pressurize a fluid in order to create flow. The pressure a pump develops is commonly termed as “head.” Head is really just another term for pressure with the units of ft or psi. The units “feet” come from one of the earliest applications of pumps: lifting water out of a well. Head can be related to pressure with the following:
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Intermediate Ammonia Refrigeration
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Process Safety Management Audits for Compliance and Continuous Safety Improvement
PUMP DEFINITIONS
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• Back to Basics: Centrifugal Pumps • Upcoming Ammonia Classes • Noteworthy 1-7 2 2
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Design of NH3 Refrigeration Systems for Peak Performance and Efficiency
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Introduction to Ammonia Refrigeration
The Electronic Newsletter of The Industrial Refrigeration Consortium Vol. 9 No. 4, 2009
BACK
TO
BASICS: CENTRIFUGAL PUMPS
Introduction
Pumps, all industrial refrigeration systems have at least one. Not every refrigeration system will use a pump to circulate refrigerant but all pump water for evaporative condensers. Many industrial refrigeration systems pump secondary fluids such as water or glycol to serve process or space conditioning loads. The main class of prime movers for fluid circulation duty is the centrifugal pump. In this edition, we will look at the basics of centrifugal pumps. The “basics” includes definitions, the pump affinity laws and reviewing the key performance parameters of pumps commonly found on a graphical performance map called a pump curve.
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Vol. 9 No. 4, 2009
2. The static height of the fluid above the pump centerline 3. The pressure losses (frictional and form) due to fluid flowing through the suction piping and pump suction. As an example, consider a water pumping application. The pressure above the water surface is atmospheric pressure and the temperature of the water is 80°F corresponding to a saturation pressure of 0.5 psia resulting in a pressure difference of 14.2 psi or 32.75 ft. Assuming the water height above the pump centerline is 4 ft and there is a 3 psi flow loss (6.9 ft) the NPSHa would be 32.75 ft + 4 ft – 6.9 ft = 29.85 ft.
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Vol. 9 No. 4, 2009
h=
144 ⋅ ∆p
ρ
where h is head (ft), ∆p is the fluid pressure developed (psi) and ρ is the fluid density (lb/ft3). The constant, 144, is used to convert units from ft2 to in2 and there is a factor of g/gc that makes all of the English units work (g = 32.2 ft/s2 and gc = 32.2 lb-ft/(s2-lbf). The fundamental equation relating pressure to head is ∆p = ρ gh . Power, P, is a measure of the work input to turn the pump shaft. Efficiency, η, is ratio of the power imparted to the fluid stream to the input pump shaft power. NPSH stands for Net Positive Suction Head, and is the pressure (i.e. head) on the suction side of the pump. “Net positive” accounts for the balance of positive pressures (static, or height, and absolute pressure above the vapor pressure of the fluid) and negative pressure (losses) attributable to fluid flow. A NPSH of zero (0) indicates the fluid is vaporizing (boiling). Attempting to pump a fluid that vaporizes or boils results in a phenomenon called cavitation. A cavitating pump is immediately recognizable by its acoustic signature – it sounds like the pump is moving gravel because of the collapsing of the vapor as it moves through the impeller and volute. There are two (2) types of NPSH, the required (r) and the available (a). NPSHr is the minimum required net positive suction head in order to keep the pump from cavitating and is given by the manufacturer. NPSHa is the available net head at the pump suction considering all of the positive and negative pressures. In order to keep the pump from cavitating, the NPSH available must be greater than the minimum required.