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?
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
Two that pop up are:
although a "Google" search would no doubt uncover more.
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.)
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
A positive displacement pump will work the same way except that it will
maintain a constant flow up to its maximum pressure.
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.
Click here to return to the Engineering Archives
Update: June 2012