Pumps – Basic Principles of Operation
This post on the basic principles of pump operation will be quick. Before getting too deep into the different types of pumps, and things like where we might see the pumps in buildings, let’s discuss the basic engineering principles behind the operation of a pump.
As we learned about in the earlier pump post on like what they are, they are a device that uses an external power source to apply force to a fluid in order to move the fluid.
A shaft, or long metal cylindrical pole, with a motor on one end that can turn the shaft and a head or device (impeller) that move the fluid.
Pump Operation Principles – the start & some engineering terms
Also written about earlier – a pump develops none if its own energy. It converts the energy from the external power source (usually in our case a motor) into mechanical kinetic energy (the energy of a body or a system with respect to the motion of the body.) The kinetic energy is then consumed to perform work. The work we want. To generate power, to turn a fan belt, to move fluid in a pump.
Every pump has a power end and a fluid end. The power end is yep, you guessed it, where the external power source is connected and does its thing. The fluid end is where the fluid comes and goes, the pump suction and discharge.
No, not on a cold mug of beer. Oh, not what you were thinking about?
Pump head. When a pump applies energy to the fluid, it almost always cause an increase in pressure. This is what is called pump head, or head.
According to the Navy, there are 4 types of head:
- Net positive suction head
- Suction head
- Discharge head
- Total discharge head
Since in building engineering we don’t often actually have to calculate pump sizes, we don’t need to get into the technical definitions of all 4 of these. In simple terms so I can understand them lol, we have 2 suction heads, 1 discharge head, and then 1 total head.
For the suction side, take a boiler feed water pump. Usually these pumps are underneath the deaerating feed tank. This tank is a big cylinder, half filled with water. Having all of this water ABOVE the feed water pumps adds to the energy available at the pump suction.
Potential energy and height and all that. If the pumps are 5 ft below the tank, that is 5 feet of potential energy we can convert to kinetic energy. Ya dig?
Discharge head just means the pressure of the fluid leaving the pump.
And total head is the net difference between the suction head and discharge head. This basically indicates what kind of work this pump can do. Like if you have a huge pump in the basement of a 45 story building and you need to get that water to the roof…
Think about the weight of 45 stories of water filled piping acting against the pump. For that pump to get that water up to the roof, it has to be about to work hard enough to overcome 45 floors of that weight of water. Crazy right?
How can a pump accomplish this? Well one easy way is to have the water return to the pump suction side from that height as well. Which is usually how buildings are designed. So that we can use those 45 floors of gravity as potential energy, then convert it to kinetic, and then work. Nice.
When a pump operates below the level of a liquid – it gets delivered due to gravity.
When a pump operates above the level of a liquid – the pump must be able to create a vacuum great enough to receive the liquid.
Ha ha, this helped me out, going over this again. Thanks everyone!
Certain liquids will vaporize a little (like gasoline, oil) on the suction side of the pump because of the vacuum there. If too much vaporizes – vapor bound pump.
I guess that is it for the day. We went over some basic engineering principles of operation for pumps. We briefly covered the 4 types of pump head – net positive suction head (npsh,) suction head, discharge head, and total discharge head.
We defined what a pump is again, how it converts energy, and how vapor binding of a pump can occur. Let me know what you think. What is the biggest pump you have ever seen? I think, not sure though, the largest one I ever saw was one of the condenser water pumps in Seattle. This is one of the pumps that had to go up 45 floors. It was rated for 3000 gallons per minute and was a multistage centrifugal pump. We had two of those and then 2 more 1500 gpm pumps.