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Irrigation pipe: What's best size?Irrigation pipe: What's best size?

April 20, 2001

5 Min Read

The flexible plastic pipe used by many Delta farmers to irrigate cotton, rice, corn and soybeans varies greatly in price depending on the thickness and diameter of the pipe. The questions, then, are just what quality level is necessary for your farming operation, and what can you make do with in order to reduce your input costs?

A phone call to one large Mid-South supplier found flexible pipe prices ranging from $119 for a one-quarter mile roll of 12-inch diameter, 6 to 7 millimeters thick, two-ply pipe to $219 for one-quarter mile of 15-inch diameter, 9 to 10 millimeters thick, two-ply pipe. Multiplied over a large acreage, this $100 difference per roll of pipe could play a role in the profitability of your farming operation.

“They sell plastic irrigation pipe strictly by the pound. So, the thicker tubing will weigh more and that means you are going to pay more for the heavier 9- to 10-millimeter tubing than you would for the thinner, 6- to 7-millimeter tubing,” says Phil Tacker, irrigation specialist with the Arkansas Cooperative Extension Service.

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How many pennies you can shave off your plastic pipe bill, however, depends on your irrigation practices, the slope of your fields, and sometimes, whom you ask.

According to Tacker, the diameter and thickness of the plastic tubing you need depends on what you need it to do. The 6- to 7-millimeter thickness is fine for furrow irrigation, if you are simply punching holes in the pipe and running it down the row middles,” he says. “For multiple inlet rice irrigation, or border irrigation for soybeans, our recommendation is to go with the higher quality, 9- to 10-millimeter plastic piping.”

Irrigation specialist Jim Thomas at Mississippi State University in Starkville, Miss., recommends the 6- to 7-millimeter size for transfer pipe running down a grade, such as down an open ditch or a flume pad. The thinner flexible plastic pipe can also be used for zero, or near zero, grade fields if the pipe will be punched with holes only once and then left as is.

“The 6- to 7-millimeter thick pipe will work fine as long as you've got big enough pipe to handle the flow, and you've got a very smooth pad, a short run, and either a zero or 0.02, 0.04, 0.05 slope,” Thomas says. “But, if you are going to plug the holes in the pipe, or put gates in it, or have any variation in slope, you will want the 9- to 10-millimeter thick pipe.”

An important factor in pipe thickness, he says, is the field elevation change between the well and the end of the row in furrow-irrigated fields. “The thinner pipe could potentially be blown out by a slope of 0.1 or greater. What gets you is the repeated swelling of the plastic tubing. The flexible plastic pipe weakens each time you stretch it a little bit. The ultra-violet rays from the sunlight are already weakening it a little naturally,” he says.

Tacker adds, “A lot of producers just feel more comfortable using the heavier (9- to 10-millimeter) tubing in most applications, even in cases where the thinner tubing may work. It just has less tendency to weaken and rupture.”

Some companies have also begun manufacturing a heavier, 15-millimeter flexible plastic tubing for crop irrigation. The uses for such a thick plastic pipe will be more limited, especially considering the premium price of such a product and that you only get one-eighth mile instead of one-quarter mile, Tacker says. The primary uses he sees for 15-millimeter plastic pipe are where growers are pushing water up a steep grade, or through a low area (like a ditch) and up again or using it for a short discharge pipe from a pump or riser to a high in a field.

When it comes to choosing between the various pipe diameters available, Tacker says, there is some variation of opinions on size requirements.

In general, he says, he recommends a water capacity limit of about 1,000 gallons per minute for 12-inch tubing and 2,000 gallons per minute for 15-inch tubing. Anything much over 2,000 gallons per minute, he says, and a producer may need to consider going to 18-inch tubing, which has a holding capacity of approximately 3,000 gallons of water per minute.

The 12-inch and 15-inch probably account for about 90 percent of the tubing used in the Delta since most irrigation wells fall in the 1,000- to 2,000-gallons-per-minute range, according to Tacker.

“Those are the general considerations. Sometimes you can get a little more or a little less water through the pipe, depending on how it is being used ,” Tacker says. “Some consideration should also be given to run distance and whether or not water is released from the beginning of the tubing. The farther you have to go to get across the field, generally the larger the diameter of tubing you need.”

No matter what tubing size you choose for your irrigation needs, Thomas stresses the need to book flexible plastic pipe with your supplier as soon as possible. “Book it now. If you haven't booked it, you need to get it booked, because we faced a shortage last year as the dry weather increased the demand for plastic pipe. Also, with the price of petroleum so high, I wouldn't make a bet that the manufacturers are going to produce extra pipe that hasn't already been booked by farmers.”

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