National Electrical Code (NEC or NFPA-70)
The National Fire Protection Association publishes NFPA-70 also called the National Electrical Code or NEC. This publication is the "law of the land" in the United States for electrical design, manufacturing, and installation. The Florida Building Code has no electrical section other than NEC, to show how complete this code is regarding commercial and residential electrical practice.
National Electrical Manufacturer's Association (NEMA)
This Trade Association establishes standards for manufacturing and performance of electrical components. They are well known for their MG1 Standard for electric motors. They also establish standards for Circuit Breakers, Motor Contactors, Electrical Enclosures, and more.
Net Positive Suction Head (NPSH )
NPSHA (Net Positive Suction Head Available)
Concept Definition
The amount of energy contained in the liquid absolute, as measured at the pump intake nozzle, less the vapor pressure of the liquid.
Therefore, NPSHA is the amount of pressure contained in the liquid, which is available to satisfy NPSHR. If liquid enters the pump with insufficient margin of NPSHA over NPSHR, then cavitation occurring inside the pump will be excessive, resulting in poor performance and the pump may be damaged.
The amount of margin required, NPSHA over NPSHR, varies widely. This topic is always hotly and continuously debated because the requirements are known to vary widely depending on the type of pump, the type and conditions of the pumpage, and other variables. The solution ultimately may end up being part of "Tribal Knowledge", where experience is the only possible solution.
NPSH can be calculated or measured as either Gauge or Absolute pressure. HI Centrifugal Pump Test standards specify Absolute pressure. Therefore, pump manufacturer supplied NPSHR values are to be considered absolute unless otherwise stated.
Gauge pressure is convenient because that is the most common type of pressure gauge available in the field. The drawback to using Gauge is that the calculations become more complex and people can become confused easily while attempting to make calculations.
Absolute pressure is simpler because the calculations are straight forward. But few people working in the field have ever seen an Absolute pressure gauge while NPSH calculations are often done on site. For simplicity, clarity, and to avoid errors, pump designers and engineers might prefer the Absolute measurement scale.
NPSHR (Net Positive Suction Head Required)
Also called NPSH3% referring to the Hydraulic Institute standard method (ANSI/HI 1.6-2000 Centrifugal Pump Tests), a common test standard used by manufacturers to determine the value for NPSHR.
The NPSHR value is determined by the pump manufacturer based on actual hydraulic testing done according to a standard test method such as the ANSI/HI method. The information is then supplied by the manufacturer most often as a curve plotted against the Head Capacity Characteristic curve of the pump. HI standard testing specifies NPSHR values be reported as "Absolute" pressure. Therefore, unless otherwise stated, NPSHr values should be considered Absolute pressure.
In practice NPSHA must exceed NPSHR by some margin to avoid levels of cavitation that noticeably decrease pump efficiency or are harmful to the pump. This is because the HI standard test procedure sets the NPSHR as that point where known head differential across the pump falls by 3% as suction head is reduced. This reduction in head differential is assumed to be due to cavitation occurring inside the pump at a level sufficient to affect pump performance by the 3% value. Therefore, if NPSHA equals NPSHR then the pump already has cavitation levels sufficient to cause a 3% head drop.
The real and most important question then becomes, how much cavitation does that 3% head loss value represent? The answer is variable depending on the type of pump. In general, low Ns and low Nss pumps are easily affected by cavitation at the 3% level, so cavitation levels are not high enough at the 3% point to cause immediate damage. However, high Ns and high Nss pumps in general handle large amounts of cavitation easily, so the 3% head loss level means that significant cavitation is occurring, sufficient to cause severe cavitation damage to those pumps.
Margin Required NPSHA > NPSHR
Low suction energy pumps (Nss < 9500) - NPSHA should exceed NPSHR by a margin of 1-10 feet for best results. Additional margin may be acceptable but also the pump may have increased wear due to incipient cavitation, especially when working with heavy weight liquids such as water, when working at temperatures below 150 degrees F, and when pressure differential across the pump is high.
High suction energy pumps (Nss >9500) - NPSHA should exceed NPSHR by a larger margin (perhaps 3-20 times or more) to suppress incipient cavitation. For this reason high Nss pumps may have inducers. Only knowledgeable persons working with the pump manufacturer should specify and supervise installation of high suction energy pumps.
High Suction Energy pumps have impellers that are "opened up" to flow to such a large degree that cavitation inside the pump does not impede the flow as much as low suction energy pumps. Therefore, when the standard HI method of NPSHR testing is performed on a high suction energy pump, a very large amount of cavitation must occur before the 3% drop in head differential occurs, (NPSHR ). That large amount of cavitation inside the impeller can quickly damage those pumps.
The red line on the graph below gives NPSHR at any given flow rate.
Related Subjects on this Website:
Net Positive Suction Head Inception (NPSHi)
That point where NPSH is sufficiently high to suppress all incipient cavitation within a pump.
Related Subjects on this Website:
Newtonian & Non-Newtonian Fluids
Newtonian fluids are non-viscous (not sticky), have little resistance deformation at low velocities, and they flow freely. The exact definition is a fluid exhibiting little or no change in viscosity in relation to the shear rate at a constant temperature and pressure.
Examples of Newtonian fluids are gases such as air, and also liquids such as water, gasoline, mineral spirits, and light oils.
Non-Newtonian fluids exhibit traits such as viscous or sticky, they resist deformation even at low velocities, and they do not flow freely. The exact definition is a fluid exhibiting variable velocities at variable shear rates at a constant temperature and pressure.
Examples of Non-Newtonian fluids are molasses, syrups, ketchup, heavy oils and greases, and also non-homogenous or non-heterogeneous fluids such as slurries and mud.
Related Subjects on this Website:
Glossary - Compressible and Non-Compressible Fluids
Glossary - Newtonian and Non-Newtonian Fluids
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