Pump Choices and Settings

Here are the basics of selecting a pump, and for determining float settings in demand-dose situations.
Pump Choices and Settings

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We get a lot of questions about pumps and how they are used in onsite systems. From an installer standpoint, if there is a pump in the system, either you or the designer should have worked out its size and characteristics for the specific site and application.

We will highlight some of the basics of selecting a pump and of determining float settings in pumpout situations.

A single-stage pump has one impeller, is easy to maintain, and handles small pieces. Lift and horsepower are directly related. A multi-stage pump has more impellers and smaller tolerances and has a higher head with lower horsepower. Multi-stage pumps have been used for a long time in delivering water from wells.

Key specifications

We like to think a pump has two key specs: the first is the delivery capacity in gallons per minute, and the second is the total dynamic head or pressure necessary to push the water to the delivery point.

In pressure distribution systems, the flow rate is set by the distribution network, the number of perforations, their size and the discharge pressure. The total dynamic head is set by location, which is a combination of the elevation difference, the piping length and size, and the amount of friction loss through the system.

The simplest application of a pump is to pump up to a higher elevation to a drop box or distribution box and have the effluent distributed by gravity. To select a pump for this application, we only have to know the amount the pump needs to deliver (say, 15 to 45 gpm), and the total dynamic head based on the elevation difference between the pump and the discharge point.

If that was determined from the design as being 16 feet of total dynamic head, we would be looking for a pump able to deliver more than 15 gpm, but less than 45 gpm, at 16 feet of head. This information can be taken to the pump supplier and, based on the pump curve for selected products, a suitable pump can be selected. One note of caution here: To ensure operating efficiency in any pump selection, a pump should be chosen that does not lie with 10 percent of either end of the pump curve.

Pressure systems

In a pressure system, the routine is similar. Here, the head is determined by the elevation difference, the flow rate and the friction loss, while the pump capacity is determined by the number of pipe perforations, their size, and the operating pressure. So if we received a design calling for a pump that could deliver 30 gpm at 11 feet of head, we would find a pump able to meet those requirements.

Another note here is that these are the minimum requirements, and the actual operating point on the pump curve is going to be higher. Since the pressure system is self-compensating, this is not a problem, as long as the design values are under the pump curve.

So now let's assume we want to determine how to set the floats in a demand-dose system to deliver the desired dose. In a demand-dose configuration, the pump is turned on whenever a prescribed volume of effluent flows into the pump tank and activates a switch by way of floats or sensors. Thus the dose to the next component is subject to the variations in water usage.

Getting control

This is a socially controlled system, regulated by the people using it. It is the simplest form of dosing, but it results in a variable delivery of effluent. The people using the system are in control. Now, how many of the people using their systems know how those systems work? Very few. So there is little or no true control in a demand system, except that only the set dosing volume is sent to the next component at any one time.

The simplest control for this type of system is a piggyback plug. A float switch and a pump are both plugged into an outlet, and the float turns the pump on and off. The problem with this design is that there is zero ability to manage it. There's no method to keep track of the flow or to see if the pump is delivering too much or too little.

So what we've done with the piggyback control is take out the one place where we can have a little bit of knowledge. It is better to use a panel, because then we have a few more tools for management, including cycle counters and timers. The advantage of the piggyback is that it is the simplest setup, and most service providers can legally do the maintenance.

How much per inch?

The first step in setting floats is to find the amount of effluent that must be delivered. Then we have to determine the gallons per inch contained in the pump tank. Typically, the dose volume in a demand system is set by delivering 25 percent of the daily flow, plus drainback. So a 600 gpd system should deliver 150 gallons, plus any drainback from the supply pipe.

So suppose we have a rectangular pump tank, 7 feet long by 4 feet wide, we want to deliver 150 gallons per dose, and the drainback is 10.2 gallons. How far apart do we set the floats to deliver the desired pump dose?

First, we determine the tank gallons per inch. To do this, we calculate the area of the tank in feet (length times width), assume one foot of depth, and multiply by 7.5 (the number of gallons in a cubic foot). Then we divide by 12 (the inches in a foot).

So for the tank in this example, 7 feet (length) times 4 feet (width) equals 28 square feet, or 28 cubic feet at one foot of depth. Now, 28 cubic feet times 7.5 gallons per cubic foot equals 210 gallons per foot. Dividing that by 12 yields 17.5 gallons per inch in the tank.

Now, if we need to pump 160.2 gallons, we divide that by the gallons per inch (17.5), yielding 9.38 inches. So we need to set the floats 9.38 inches apart. See how easy that was? In some of our upcoming articles, we will further explore the concepts of dosing.


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