EDA, Incorporated

Sizing a Pump

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To Size a Pump

To size a pump or determine its total head, a calculation of the amount of fluid friction is required. Viscosity is the fluid property responsible for friction. The following is a brief discussion of viscosity.

VISCOSITY

The Definition of Absolute (or Dynamic) Viscosity.

Many years ago, Isaac Newton investigated the fluid property called viscosity. He set up an experiment designed to measure viscosity. The experiment consisted in the interaction of two plates separated by a short distance, one fixed and one moving with fluid between. He theorized that it required a force F to move the top plate at a given velocity v. If the top plate was moving at velocity v, he deduced that the fluid layer just underneath the plate must be moving at the same velocity or else the plate would be skipping over the fluid surface. In addition, force F can only be produced if the top layer of fluid is attached to the top plate and the bottom layer to the bottom plate. Since the bottom plate is not moving, the bottom fluid layer is also not moving or has zero velocity. Therefore, the fluid layers between the moving surface and the fixed one have different velocities. Each layer of fluid is traveling at a different speed. It is this speed variation (or velocity gradient dv/dy, see Figure A-l) that is the cause of viscosity and is responsible for shearing the fluid internally. Newton's assumption was that the velocity gradient is independent of viscosity. In other words, a force twice as large would be required to move the fluid twice as fast, meaning there must be a constant relationship between the force F required to move the fluid and the rate of shear. Alternatively, in mathematical form, the force F is proportional to the velocity gradient dv/dy:.

 
(A-1)

The constant A/K is called the viscosity of the fluid and is represented by the Greek letter m . The value of m will determine the magnitude of the shearing force F. Fluids with higher viscosities will require a greater shearing force for the same velocity differential. Since, the experiment should be valid for fluid bodies of any size, then the tangential stress t = F/A is a more appropriate parameter to relate to viscosity.

The term m is known as the absolute viscosity of the fluid (see equation A-l). The velocity gradient dv/dy is known as the rate of shear. Newton could not test his hypothesis because of experimental difficulties. Many years later, Poiseuille (1849) developed an experimental method that consisted in measuring the flow of liquid in a small tube and relating the pressure driving the fluid through the end of the tube to the flow and viscosity. Poiseuille's experimental apparatus verified the correctness of Newton's hypothesis. Newton's viscosity equation describes a class of fluids that came to be known as Newtonian fluids. Many fluids behave in this fashion (see Table A-2). The unit of absolute viscosity is the Poise (or centipoises), in honor of Poiseuille. One (1) centipoises (the unit symbol is cP) is the viscosity of water at 68 0F making it easy to compare the viscosity of various fluids to that of water.

Newtonian vs. non-Newtonian fluids

Many fluids do not behave in the well-ordered fashion of Newtonian fluids. These are known as non-Newtonian fluids. These fall in several categories (see Table A-2) depending on what shape the tangential stress vs. velocity gradient takes. For these fluids, the velocity gradient is dependent on the viscosity. That is, the velocity affects the viscosity resulting in a much higher (or in some cases lower) rate of shear than for a Newtonian fluid. A typical household product will help illustrate this point, try the following experiment. Make a solution of cornstarch and water, approximately 1 part water to 2 parts cornstarch. Mix well into a large shallow bowl. Try moving this fluid rapidly around with your fingers. The faster you try to move through the fluid, the higher the resistance. If you move your fingers fast enough they will skip over the surface. At that rate of shear, the solution almost behaves as a solid, when the fingers are moved slowly, the solution behaves as expected offering little resistance. Compare this behavior to another fluid that seems equally thick, such as molasses (molasses is not considered a Newtonian fluid, however it is much closer to being Newtonian than a starch solution). The molasses flows readily no matter how fast the movement. This is what is meant by viscosity being dependent on rate of shear.

This explains why centrifugal pumps with their high rate of shear are not suitable for non-Newtonian fluids (i.e. yield pseudoplastic or pseudoplastic). A pump operating at low speed, of the fixed displacement type is more appropriate.

Kinematic viscosity

A term frequently used in fluid mechanics (for example in the definition of the Reynolds number) is the Kinematic viscosity n . The relationship between the absolute and Kinematic viscosity is:

   

The Kinematic viscosity of water at 68 0F is 1 centistokes (cSt), and was named in honor of G.G. Stokes of the Navier-Stokes equation fame.

NEWTONIAN
NON NEWTONIAN
  Non Newtonian Bingham plastic

Yield pseudoplastic

Yield dilatant

Pseudoplastic Dilatants Thixotropic

Rheopectic

 

 

Viscosity decreases with the rate of shear.

Viscosity increases with the rate of shear.

Thix.: decrease viscosity with time.

Rheop.: increase viscosity with time.

- water

- high viscosity fuel

- some motor oils

- most mineral oils

- gasoline

- kerosene

- most salt solutions in water

- light suspensions of dye stuff

- kaolin (clay slurry)

oils containing polymeric thickeners, viscosity index improvers and waxy or soot particles - thermoplastic polymer solutions

- sewage sludge’s

- digested sewage

- clay

- mud

- ketchup

- chewing gum

- tar

- high concentrations of asbestine in oil

- GRS latex solutions

- sewage sludge’s

- grease

- molasses

- paint

- starch

- soap

- most emulsions

- printer’s ink

- paper pulp

- starch in water

- beach sand

- quicksand

- feldspar

- mica

- clay

- candy compounds

- peanut butter

- most paints (thixo.)

- silica gel

- greases

- inks

- milk

- mayonnaise

- carboxymethyl cellulose

- bentonite (rheop.)

- gypsum in water

- asphalt

- glues

- molasses

- starch

- lard

- fruit juice concentrates

Table A-2 Rheological properties of Fluids

 

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For free calculations go to the Calculator page.  The calculators should be used with caution.  Many of these calculators are not verified and are only intended to provide a starting point in performing a design calculations.  All final design calculations should  be performed using verified and validated programs or calculators that have documented evidence of the verification and validation.

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Company Profile

EDA, Incorporated provides quality-engineering services on time, on schedule and within budget.  EDA, Inc. is able to do this by performing the work correctly the first time. We accept the most challenging problems and look forward to working with the client as a team member.  EDA believes that the client should be an active participant in the work process to ensure that the product is commensurate with client expectations and is delivered within schedule and budget constraints.

EDA, Inc. belongs to the American Society of Mechanical Engineers (ASME), the National Society of Professional Engineers (NSPE), the Society of Instrument Control Engineers, Society of Professional Engineers (ISA) and the American Nuclear Society (ANS).


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For more information on EDA, Incorporated services, please contact Client Service Manager at:

Client Service Manager

EDA, Inc.

6397 True Lane

Springfield, VA 22150

 

or email  the Client Service Manager at SiteManager@edasolutions.com .

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Contact Information

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6397 True Lane
          Springfield, Va 22150
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General Information:  Site Manager@edasolutions.com