What Is a Pump?
A pump is a mechanism for moving a fluid (gas or liquid state) from one area to another. Both a trash pump and your heart are liquid pumps, but they work by different methods and are examples of the two main types of pumps.
Centrifugal pumps work by using a spinning disk, usually one with vanes on its face, to throw the fluid against a casing that directs the flow in the desired direction. Normally the intake is in the center of the disk (called an impeller) and the discharge is rotated 90° from the intake. Here's a simple example :
If you have a well, you probably have a submersible centrifugal pump at the bottom. Your water-cooled vehicle has a centrifugal water pump that moves the coolant through the engine and radiator. The fuel pump in newer (fuel injected) cars is a centrifugal pump. Centrifugal pumps are very common in industry.
- Easy to work on: as long as you have the correct seals on the drive shaft, the pump will run for years and parts are not built with excruciatingly tight tolerances.
- Long life: because the impeller and volute are not actually touching each other, there is very little wear between the two.
- Variable rates: by changing the speed of the drive shaft, you can easily change the rate of flow (within reason). There is a minimum speed below which the pump won't move anything, and a maximum speed where the spinning parts will fail, but there is a range of flow rates from a single pump.
- Tolerant of variable feed: properly designed, they can handle a limited amount of solids mixed in with the fluid feed. Liquid pumps will handle a limited amount of gasses in the feed.
- Good for pushing fluids long distances and up high vertical distances (AKA "head").
- Dead-head resilient: If the discharge is blocked, the pump will not build up much pressure. This is handy if you can't, or don't, want to deal with high pressure lines.
- High flow rates at lower pressures: I used to work with pumps that had up to 18,000 gallons per minute flow rates, but they were only working against about 6 feet of vertical head -- about 3 PSI.
- No dry start: the volute must be full before the pump will work. So-called "self priming" pumps simply have a method of filling the volute before starting the pump, or they have a volute that is designed to hold fluid after power is shut down.
- Poor suction: while they are capable of generating moderate discharge pressures, they won't suck up anything below the inlet port.
- Lower efficiency: only about 60% of the energy put into the pump is transferred to the fluid. There is a method of figuring out peak efficiency, but I don't want to get into how to read a pump curve in this post.
- Works best with thin fluids: oils and greases are too viscous to be moved through a centrifugal pump.
Positive Displacement Pumps
Unlike centrifugal pumps, positive displacement (PD) pumps use a wide variety of methods of moving fluids. They all have two things in common: check valves, and a chamber that changes in volume. The check valves may not be separate pieces, or they may be an integral part of the mechanism, but there has to be some way to prevent back-flow for a PD pump to work.
This is a peristaltic pump, commonly used as a "metering" pump because the volume of fluid is set by the size of the tubing. By changing the speed of the drive, you can easily meter out exact amounts of fluid. The changing volume is in the tubing, and the check valves are formed by the rollers as they pinch the tubing against the housing. They're easy to work on since the great majority of any wear is in the tubing, which is easily replaced.
- Often self-priming: they are designed to move gasses as well as liquids, so they can prime themselves from a dry start. A pitcher pump may need a cup of water dumped down the casing to wet the leather piston to form a good seal, which is why you'll sometimes find a full water jar next to the hand pump.
- Good for thicker fluids: high viscosity liquids like oil and grease are no problem for a PD pump. I've seen specialty PD pumps that will move wet concrete through a hose to the building site.
- High pressure: the common pressure washer that you use to clean your sidewalk or power-wash your siding uses a piston pump. Pressures in the thousands of PSI are easy to achieve with a PD pump.
- High pressure: if the discharge gets blocked, most PD pumps will create dangerous levels of pressure in a matter of seconds. I've seen someone close the wrong valve and have to clean up after a 6" diameter pipe blew out.
- Touchy to work on: much tighter tolerances in the manufacture means that PD pumps tend to be harder to work on, especially in the field. Special "clean rooms" are normally set aside for working on PD pumps to keep dust and dirt out of the internal parts.
- Lower flow rates than centrifugal pumps.
- Shorter life-span than a centrifugal pump: the tighter tolerances and constant motion of parts against each other lead to increased wear.
So picking a pump is not as easy as it seems. Like most of life, there are compromises and trade-offs in deciding which pump to choose if you're trying to empty a UST. I'm still doing some research and will update last week's article soon. Unfortunately I haven't been able to find a gas station that will let me field test any of my possible solutions; for some reason they're not fond of anyone spreading information that could help thieves...