The capacity requirements, in terms of GPM flow rate, can be broken down into three general categories – structures, drinking water for animals, and fire-fighting. If the farmer irrigates crops other than his small vegetable garden, a separate irrigation system is required.
The capacity requirement of the structures is the same as described in detail in our October 2010 article. Each outlet – a toilet, tub, sink or outside hose faucet – is assumed to consume 1 gallon per minute. A typical farmhouse with two bathrooms might have 14 outlets: three for each bathroom (tub, sink and toilet), two for the kitchen (sink and dishwasher), two for the laundry room (washer and wash tub), and four outside hose faucets. To meet the flow requirements of the house, then, you would need – you guessed it – a 14-gpm pump. The milk house or barn might have an additional three faucets.
To determine the pump capacity needed to support the drinking water requirements of farm animals, we use the information provided by the Water Systems Council as shown in Table 1. These values are average, and do not include the extremes.
To translate this information into GPM pump capacity, the Water Systems Council recommends assuming the daily drinking requirements for animals will be met during a two-hour period. In other words, divide the total daily drinking water requirement by 120 minutes to get GPM.
Table 2 is an example for a small farm with 50 milking cows, 20 dry cows, 5 hogs and 100 chickens. In this example, the required pump capacity would be 18 gpm to meet the animals’ drinking needs. This is added to the requirements for the farmhouse (14 gpm) and barn (3 gpm), giving us a total pump capacity requirement so far of 35 gpm.
The last requirement to be met is fire protection. According to the Water Systems Council, the flow requirements of a fire protection system can vary from 20 gpm to 60 gpm for a small farm. This is a wide range because local codes vary considerably. As a minimum, go with what is required by local codes; if there are none, go with the flow rate recommended by the fire hose nozzle manufacturer.
As an example, let’s say your fire protection requ-irements are 60 gpm. If you are pumping directly from a well, you will need a pump with at least 60 gpm of capacity and enough pressure capability to get the water from the pumping level in the well to the pressure tank on the surface, plus enough to shut off the pressure switch (for instance, 50 psi for a 30/50 pressure switch). The pressure switch selection will depend on pressure requirements of the farm and fire hose nozzle as you will see in a moment. And since a 60-gpm pump is overkill for the normal 35-gpm daily demand, you should consider running the pump on a variable frequency drive.
If you are pumping from a storage tank, I would recommend using two 35-gpm pumps on an alternating system to provide 35 gpm for normal usage, and 70 gpm for fire-fighting. In such a system, you would install two identical booster pumps side-by-side at the storage tank. Each pump has the capability to meet the non-fire capacity requirements, and they run simultaneously to meet the fire needs. In non-fire service, they alternate from one to the other, using an alternating relay. Each pump is on every other cycle of the pressure tank.
A system like this has several advantages. First, the pumps last longer because they get extra cooling between cycles. Second, they save electricity, because they are sized for normal everyday demand. If a single, larger pump were used, it would be overkill 99 percent of the time. Third, a two-pump system offers redundancy, which is particularly important in a farm application where lack of water is more than just an inconvenience.
An alternating relay is an inexpensive device, available at most pump distributors or electrical supply houses. A second pressure switch is needed, set at a few pounds below the primary switch, to turn on both pumps simultaneously when the fire hose is in operation.
Pressure RequirementsThe system should provide usable pressure at the highest outlet in the system. Most appliances require a minimum of 10 psi at their inlet. Lawn sprinklers work best from 20 psi to 40 psi. Fire nozzles like 60 psi to 75 psi. Plus, there must be enough pressure to overcome the friction losses in the piping, which we will discuss in detail in the coming months. Refer to last month’s article on how to determine the pressure requirements for a full explanation.
Be conservative in your sizing considerations. The farmer may want to add outlets and fixtures in the future, or increase the number of animals he is raising. Also, pipes may calcify over time, increasing their resistance to flow, and the pump will lose some of its performance from normal wear. The addition of excess capacity over the calculated requirements is a wise investment. As with a simple single-family residential water system, a little planning will go a long way toward adding another happy camper to your customer list.
Next month, we will continue our discussion on system sizing with a look at pressure loss in pipes due to friction. ’Til then…