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Name: Gary
Status: other
Grade: other
Location: OK 
Country: N/A
Date: 9/21/2005


Question:
If a pump will make 80 psi of water pressure at full restriction, how much water pressure does it make at no restriction, or free flow? I say it still makes 80 psi, and my co-workers say the pressure it makes is minimal. Who is right?


Replies:
The pressure drop, by itself, is insufficient information to answer the question. Other information needed is: pipe diameter and length, roughness, type of flow (laminar vs. turbulent), considering/not considering viscosity of the water. There are a number of on-line calculators that let you input the various factors and compute the flow rate etc.

Two that pop up are:

http://www.engineeringtoolbox.com/pressure-drop-calculator-6_500qframed.html

http://www.efunda.com/formulae/fluids/calc_pipe_friction.cfm

although a "Google" search would no doubt uncover more.

Vince Calder


Well, if it is truly under no restriction, the water would not be building up pressure. It would instead just flow out of the pump as fast as the pump can move it.

Now there will be some pressure, just because moving water does not like coming to an immediate stop without exerting some force. However, your coworkers are closer tot he truth here, in the pressure will be minimal. (well, minimal unless you have one VERY high volume pump.)

Ryan Belscamper


Gary,

A pump will only "make" as much pressure as is needed to move the fluid through the pipe. If you measure pressure at the discharge of the pump, the only pressure you will measure will be the line pressure due to elevation, friction, or any restrictions. As you increase any of these variables, the pressure in the line will also increase.

A centrifugal pump will continue to pump with the increasing pressure until it reaches is maximum at full restriction. Of course as the pressure increases, the flow of the pump will decrease based on the pump curves.

A positive displacement pump will work the same way except that it will maintain a constant flow up to its maximum pressure.

Bob Hartwell


Sorry, but your co-workers. If you look at literature from pump manufacturers, you will see a pump curve associated with a specific make and model and horsepower. The curve has flow on the x-axis and pressure (or head) on the y-axis. Head is usually defined as the amount of elevation between where the liquid is sucked in and the maximum height the discharge can be from that intake. There is a relationship between head (units of feet) and pressure (units of psi or pounds per square inch). So, if you look at the curves, you will see a downward slope of the curve as you go out in increasing flow. That is, the pressure drops as the flow increases. Likewise, if you look at zero flow, you will find the maximum head discharge. Just think of that value as the largest column of water that a pump can push up without moving any of the water. As the flow increases, less resistance to flow occurs and thus less pressure. Pressure is the resistance to flow.

Thanks for using Newton.

Christopher Murphy, P.E.
Mechanical Engineer



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