That part of the pump characteristic showing power required to operate the pump at any given flow rate or system resistance. Pumps vary widely in how they consume power in response to changes in flow rate. In general, low specific speed pumps require more power to pump more fluid, while high specific speed pumps require less power to pump more fluid.
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Force per Unit Area, written as: P = F ÷ A
P or p are common abbreviations for pressure.
Force can be expressed in many ways, but pounds is the common US unit of expression.
Unit Area can also be expressed in many ways. Square Inches are a common US unit of expression.
Pounds per Square Inch (PSI) is the most commonly used expression of pressure in the United States. In the English language of mathematics, the word "per" means to divide, therefore PSI = Pounds ÷ Square Inch. Thus, pressure expressed as psi tells us the weight in pounds, pressing upon each square inch of surface area.
Two Different Pressure Calibrations
There are two different calibrations commonly used in industry, and two types of pressure gauges used to measure those different calibrations: Absolute Pressure (PSIA) and Gauge Pressure (PSIG).
Absolute Pressure (PSIA) is calibrated such that 0 indicates an absence of all pressure. Therefore, an absolute pressure gauge indicating pressure in PSI, laying in your hand and exposed simply to the surrounding air, indicates 14.7 psi at sea level, because that is the pressure exerted by the earth's atmosphere at sea level. That same gauge if connected to a vacuum chamber with all the air removed from the chamber, reads 0.
Gauge Pressure (PSIG) is the most common type of pressure gauge. PSIG is calibrated such that a 0 reading really equals 14.7 psi in absolute terms. In other words, gauge pressure ignores the contribution to the pressure made by the earth's atmosphere. Therefore, a "gauge pressure" calibrated pressure gauge, indicating in PSI, laying in your hand and exposed to the surrounding air, indicates 0 psi at sea level, when in fact the absolute or true pressure is 14.7 psi. If that same pressure gauge is of the type called "Combination Gauge", the pressure gauge can indicate vacuum pressures also. In that case, if the gauge is connected to a vacuum chamber with all the air removed from the chamber, the gauge will then read -14.7psi.
Conventions
When pressure is expressed as "psi", assume that pressure to be gauge pressure. A more precise method would be to express that pressure as "psig".
When pressure is expressed in Absolute terms, that measurement must be expressed as "psia", alerting us that the pressure being stated is in absolute terms.
Summary
PSIA = PSIG + Atmospheric Pressure
Example:
A pressure gauge reads 76 psi at sea level.
Question - What is the Absolute Pressure (psia)?
Answer - 76 psi + 14.7 psi = 90.7 psia
Differential Pressure dP or ∆P
The difference in pressure between the inlet and outlet of a hydraulic device or system. The value can be obtained by subtracting outgoing fluid pressure from incoming fluid pressure, the difference being the pressure lost within the device or system.
Filters - dP increases as a filter loads.
Pumps - Pumps produce a dP between the suction and discharge nozzles.
Control Valves - Valves have inherent dP in the full open position based on the flow rate. Valves produce a variable dP when modulated by controls.
Pressure Equivalents
1 psi = 2.31 feet of water
1 psi = 2.03602 inches Hg
1 psi = 6.8947 Kilopascals (kPa)
1 Atmosphere = 14.6956 psi
1 Atmosphere = 27.8164 Inches Hg
1 Bar = 14.538 psi
1 Bar = 0.986923 Atm
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Glossary - Head (A large amount of additional information)
Pressure Control Valve (PCV) - See Control Valve (CV)
Pressure Surge - See Pressure Transient below.
Also Water Hammer and Pressure Surge.
Defined
Pressure Transients, in the context of fluid delivery or conveyance systems, are defined as fast momentary events consisting of one or more pressure waves rapidly moving through the fluid in a confined system (fluid contained in a pipe). Pressure transients are characterized by energy levels above normal system pressures, resulting in the movement of pressure energy through the system at acoustic speeds, rather than movement by diffusion.
Cause
When the total amount of momentum energy contained in a fluid conveyance system is significantly and rapidly reduced, that momentum energy is converted into pressure energy, which in turn is converted into other forms of energy such as heat. When sufficiently high pressure energies are created, we then refer to those events by various names such as water hammer, pressure surge, and pressure transients.
Further Explanation
The normal and correct method for moving fluid through a conveyance system is by diffusion. Diffusion is a slow, orderly, and efficient process involving the movement of molecules from areas of higher pressure to areas of lower pressure. The pumps, valves, piping, and fittings, are all designed and intended to transmit the fluid slowly and efficiently through the system by means of diffusion.
The result of that design intent is a relatively slow buildup of momentum energy in the system. The momentum energy contained in the fluid increases with increasing mass and velocity. Therefore, as the length of pipe increases (therefore containing an increasingly larger mass of fluid), the amount of momentum energy stored in the system also increases.
In an ideal design (usually impractical to achieve), fluid conveyance system designs would properly account for momentum energy contained in the system. In that ideal design, momentum energy would be acted upon in such a manner so as to not cause fast and large conversions of momentum energy into pressure energy. Simply put, an ideal design would slowly develop and then slowly deplete momentum energy.
In real world engineering of fluid delivery systems, momentum energy is seldom directly accounted for due to the following reasons:
Lack of knowledge about the basic physics of pressure transients.
Momentum energy in pipe systems is difficult to analyze and understand, leading to the use of indirect methods for controlling the problem, which then become the standard or accepted practice methods, i.e. the much abused "5-Feet Per Second" rule. Beware, the 5-feet per second design method will not protect you from water hammer.
Standard accepted practice methods can work well enough so that over confidence develops, leading to the false security in thinking that there are no problems to study and solve. No problems, no need to learn more or investigate.
Standard or Accepted practice methods can work well enough such that complacence settles in and dominates.
Because the concepts of momentum energy are never learned, the knowledge underlying the accepted practices is never learned, resulting in mistakes and overextension of the accepted methods.
Water Hammer is a common term for pressure transients describing one symptom of transients that occurs when pipes and system components move in response to pressure transients. The term refers to the noises caused by pipe and system components moving. The term Water Hammer is less informative and less accurate than the terms Pressure Surge or Pressure Transient, because Water Hammer describes only one symptom of the problem, and that symptomatic noise may be totally absent even when there is a serious problem.
Pressure Surge is another term for pressure transient, emphasizing the momentary aspect of pressure transients. Pressure surge describes the momentary increase and then decrease, in system pressure. Pressure Surges occur in most systems, but surges are not harmful until the combined effect of frequency and intensity, attains levels capable of causing immediate damage, or an accelerated aging of system components.
To learn more about pressure transients follow the link below.
Pressure Transients In Depth
The device providing energy to a centrifugal pump. Prime movers are rotating power sources obtaining energy from an outside source such as from electricity (electric motors) and fossil fuels (engines).
Alternative types of prime movers are possible, and some have been used frequently such as wind turbines (windmills), water turbines (watermills), steam turbines, wave action (wave-mills?), tide driven turbines (tide-mills?), steam from earths thermal sources, and a recent arrival is a device using wave action to compress and drive air through a tunnel to spin a turbine, and more.
Performance data for a specific pump. Pump Characteristics include at least the following information:
Head (H) Feet or Meters Of Head Elevation
Flow Rate (Q) - Volumetric Flow Rate - (Gallons, Cubic Feet, or Cubic Meters per second or minute)
Speed (n) - Rotational Speed in Revolutions Per Minute (RPM)
Brake Horsepower (BHP)
Net Positive Suction Head Required (NPSHR) Feet or Meters of Head Elevation
Pump characteristics are determined at least partly by testing, then reported by the manufacturer or testing agency, all under Standard Test Conditions (STC), or at any test condition required, in which case the test conditions should be noted on the documents.
Centrifugal pump characteristics are both dynamic and relational. Each reported characteristic is related to the other characteristics, and all the characteristics may change in response to dynamic system conditions. This complex behavior is best reported and understood by plotting the information on a graph. Graphs are often the best way to report dynamic complex relationships.
Learn In Depth about Pump Characteristics on this Website
The fluid being pumped including entrained gasses or solids carried in the flow, and the condition of that pumpage in terms of temperature, pressure, purity, etc.
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