iwire said:
Hill and Hockey.
I have a question, I was under the impression a typical centrifugal pump draws less current when the inlet is closed down.
Is this not the case?
When an air fans inlet is closed the fan motor will draw less current. (less air moved = less HP required = less current)
That's true for the outlet and generally true for the inlet of a centrifugal pump, although restricting the inlet of a centrifugal pump in order to control the flow is not good.
In submersible pumps the suction is (designed to be) flooded and the flow varies in relation to the back (head) pressure that the pump is pushing against and the suction pressure that floods the impeller (primes) the pump. In submersibles, the suction pressure varies with the level of the water in the sump, but is always positive (gage pressure).
All submersibles have impeller designs and diameters that (normally) require positive suction head (gage) pressure to pump their rated flow.
Gage pressure is a measurement of pressure above atmospheric pressure.
This pressure will always be positive (above atmospheric pressure) while most other above ground centrifugal pumps have a negative suction head which is factored in their design and rated output (flow rate).
The ratio of negative to positive pressure is called the "Net Positive Suction Head" and is referred to by the initials "NPSH". This is the sum of the difference between the total available head pressure available at the pump suction (atmospheric and absolute) and the negative pressure (0 to -14 psi absolute) that the pump will develop when pumping at it's rated flow.
If the NPSH is less than the vapor pressure of the liquid being pumped, trouble is certain.
Think of a vacuum gage (psi) on the intake of a centrifugal pump. It will show a value of negative pressure (unless flooded suction). This pressure will be somewhere between 0 psi gage pressure (which is 14.7 psi absolute) and negative 14.7 psi (which is a perfect vacuum).
If for example the pump requires 6 psi negative pressure to suck the liquid being pumped into it's impeller in order to pump it's rated flow and the availble pressure at it's suction is 1 psi positive (gage). It's NPSH will be the difference between the total available pressure at it's intake (absolute and gage combined) and 6 psi negative required which is 14.7 psi atmospheric + 1 psi gage - 6 psi negative = 9.7psi NPSH.
If the liquid being pumped will boil at less than 9.7 psi absolute pressure, the pump will cavitate.
NPSH can be any value above 0 psi atmospheric pressure (perfect vacuum).
If it goes above 14.7 psi absolute (0 gage) with the pump running, it is called "positive head", as there will be a positive (atmospheric) pressure on the intake, and the intake will be "flooded".
On a centrifugal pump curve, the flow rate is determined by comparing the required flow rate to the (total) operating Head pressure. Normally,
above ground centrifugal pumps will have a negative (suction) pressure on it's intake unless it has a "flooded suction" (such as a submerged pump).
The amount of negative pressure must be added to the (positive) discharge pressure to arrive at the total operating pressure of the pump. The amount of negative pressure allowed on the pump suction is limited by several factors, such as the vapor pressure of the liquid being pumped, it's viscosity
and the discharge pressure on the head. If the negative (suction) pressure is
greater than the vapor pressure of the liquid being pumped, the liquid will boil in the intake (negative) chamber of the pump and will cause "cavitation" in the pump housing. The noise that you hear when a pump is cavitating is the collapse of the created bubbles of air (or other gases depending on the liquid being pumped) when it passes from negative (evaporative pressure) to positive (condensing pressure) inside the pump. It can be pretty severe and will damage the pump if not corrected.
To make that more simple, think of the pressure in your car tire. If the "gage pressure is 32 psi, the absolute pressure is 32 psi (gage) + 14.7 psi (atmospheric) which totals 46.7 psi absolute pressure.
If you go to the bottom of a swimming pool, the pressure that you feel is 14.7 psi atmospheric plus the head pressure of the water (2.31 psi/ft of depth). If you carry a pressure gage to the bottom with you, it will not record this pressure increase. Why? Because it is still atmospheric pressure and the pressure is common to both the inside and outside of the gage, therefore there is no differential pressure for the gage to read.
If you stay at the bottom of the pool long enough, the pressure inside your body (14.7 psi absolute) will equalize with the surrounding increased (atmospheric) pressure and you will no longer feel the increased pressure. Of course, you may not survive.
We live at the bottom of a ocean of air, and the pressure that we live in is called atmospheric and can not be measured with a conventional gage. It is 14.7 pounds per square inch absolute pressure and 0 psi gage pressure (at sea level).
As far as the question of the OP goes, I know that I have seen the situation that I described. Why does the motor overload when the suction is clogged?
I think that it has something to do with the cavitation increasing the discharge pressure of the pump in "spurts". As the pump regains suction, the impeller is flooded and the pump has to start the column of liquid moving again. This inertia and the related pressure spikes exposes the motor to varying loads and amperage draw. Some of which cause the amperage to spike and the heaters in the starter to heat up and trip the overloads.
This is just my opinion.
Put a amp meter on the pump and monitor it.
My knowledge about pumps is learned from personal experience and study.
I may not have all of the numbers right, you be the judge.
I almost didn't post this after I saw how long it is, but here it is.
Sorry for the long post.
I hope that someone can use this information.
Feel free to critique, I'm here to learn.
steve
