If we want to discuss centrifugal pumps we must learn the definition of some pumping terms.
Take a look at the above drawing. We’ll be using this drawing to learn some of these terms.
In the drawing I have described a pump with two discharge nozzles. We will pretend for a moment that it has only one and that one is pointing straight up. We’ll also be ignoring pipe friction in this part of our discussion.
With a given diameter impeller, and rotating at some given rpm, this centrifugal pump would throw the liquid straight up to some maximum height. W’ll call this maximum height the SHUT OFF HEAD of the pump. As mentioned this head is determined by the impeller diameter and shaft rpm. If I were to increase either one the shut off head would be a larger number. Note: that at this shut off head there would be no fluid coming out of the pipe. In other words the fluid would have no VELOCITY When we measure this head we use the units of either feet or meters.
If I rotated the discharge nozzle to a horizontal position, and again ignoring pipe friction, the fluid would come out at its maximum velocity. At this point there would be no discharge head. When we talk about velocity we express it as so many Feet Per Second or Meters Per Second. Pump curves do not show the term velocity, they show capacity and rate it in Gallons per Minute (gpm.) or cubic meters per hour (m3/hr.)
It should also be obvious that as the head increases the velocity will decrease and likewise if you increased the velocity of the liquid the head would decrease.
This centrifugal pump can give the fluid either Velocity, Head, or a combination of both.
As I rotated the discharge nozzle from the vertical to the horizontal position, the fluid would describe a PUMP CURVE. The shape of the curve would be determined by the shape of the impeller. The term we use to describe impeller shape is SPECIFIC SPEED
This head can be converted to a more familiar term if you wish. We can convert head to PRESSURE if we remember that 2.31 feet of 68 degree Fahrenheit fresh water is equal to a pressure of one pound per square inch. In the metric system 10,2 meters of fresh water at 20 C. equals one bar.
If we are pumping a liquid other than 68 F. fresh water we must consider the weight of that liquid to determine the correct pressure. We use 68 F. as the standard because 20 C. is the international standard for measuring things and in the Fahrenheit system 20 C. converts to 68 F.
We use the term SPECIFIC GRAVITY to describe the weight of a liquid. Water is given a value of one (1) so if the liquid will float in fresh water it will have a Specific Gravity of less than one (1). If it will sink in fresh water the specific gravity will be greater than one (1).
The formula for converting head to pressure looks like this:
In summary then we have learned that a centrifugal pump is a constant velocity device. We convert some of this velocity to head ( pressure ) to satisfy our pumping needs.
In any given pump application there is an ideal combination of head and velocity . When we’re pumping at this ideal we say that the pump is pumping at its BEST EFFICIENCY POINT
A very important point to remember is that the pump is pumping the difference between the discharge and suction heads. To know exactly what the pump is doing you must know the head at both the suction and discharge side of the pump. These readings can easily be measured with pressure gauges or chart recorders and the results converted to pressure, using the formula I gave you in this paper.
The best efficiency point (B.E.P.) is described as the point where we have the least amount of shaft deflection, and the power input is closest to the power output. In other words, the shaft is not bending and we are using the least amount of power for the job the pump is doing.
This best efficiency point is usually some where between 80% and 85% of the pumps shut off head, but you’re going to have to look at the pump curve to get an exact number.
SPECIFIC SPEED is a method of describing the shape of the pump impeller. Most of the popular pumps used in the process industry are of the Francis Vane type.
Take a look at the diagrams shown below. Picture “A” describes two pumps hooked up in SERIES If these pumps are the same size it will double the head out put of a single pump, but keep the capacity the same. If they are different sizes we will be limited to the capacity of the smaller pump, and the heads will add together.
Picture “B” describes two pumps hooked up in PARALLEL. In this configuration we will double the capacity of one pump, but the head will remain the same.
When ever you want to talk about heat you have to use the term BTU (British Thermal Unit) or CALORIE A btu. is defined as the amount of heat that is needed to raise one pound ( about one pint) of fresh water one degree Fahrenheit. In other words, if I heated one pound of water from 60 F. To !00 F. I must have used 40 British Thermal Units. A calorie is defined as the amount of heat necessary to raise one gram( one milliliter) of water one degree Centigrade.
We use the term SPECIFIC HEAT to describe how many BTUs., or calories are needed to raise one pound or one gram of a liquid, other than water, one degree Fahrenheit or Centigrade. Oil would typically have a Specific Heat of 0.35 so it would only take 0.35 BTUs. or Calories to raise oil one degree. Another way to look at it is that oil will get almost three times as hot as fresh water, if the same amount of heat is added to both of them.
- On February 17, 2018