Steel or Plastic?
Willard Says……
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Good stuff to know about hydraulic dredging.
The Discharge Pipeline—Steel or Plastic?
“Plastic” pipe refers to HDPE (High Density PolyEthylene)—the black material.
There are several factors to consider before deciding which type of pipe to select:
- Slickness
- Wear life
- Cost
- Flotation
- Inside diameter
- Pressure rating
Which is slicker, steel or plastic?
Several authoritative sources claim that plastic pipe is slicker and offers less resistance to flow than steel. Some experienced dredgers report that plastic “pumps” easier than steel based on their observations when they switched from steel to plastic pipe. However, they often fail to note that the inside diameter of the pipes was different—plastic pipes were larger. Pipe diameter significantly affects friction loss more than the pipe material. If the inside diameter of the plastic pipe replacing the steel pipe was larger, then the plastic pipe did indeed “pump” easier.
Conversely, if the inside diameter of the plastic pipe is smaller than that of the steel pipe, despite having the same outside diameter, the dredgers often find plastic pipes harder to use.
“Slick” plastic is a misnomer. Here's why:
According to Cameron Hydraulic Data, the Hazen and Williams Formula is widely used to calculate the pressure (head) loss due to friction from water flowing through pipes made of various materials. This formula requires a Friction Factor “C”. The value varies to account for the roughness of a pipe's inner wall.
The “C” number for clean new plastic pipe is 150, 130 for clean new steel pipe, and 60 for corrugated pipe. Higher “C” numbers mean smoother pipe walls. Thus, plastic pipes appear to be the best choice since their “C” number is higher than that for steel. Note that these values are for clear water.
However, new, slick plastic soon loses its slickness because dredgers insist on running gritty sand and sharp rocks through the discharge pipe.
Run your hand along the inside of a new HDPE plastic pipe. It feels smooth. Now run your hand along the inside of a used HDPE plastic pipe. It feels rough. The once-shiny surface is now scraped and scarred. Protruding plastic fragments impede the flow, reducing the slickness “C” number from its original value.
Run your hand along the inside of a recently used steel pipe and you'll find it smooth. Smooth as a baby’s bottom. Plastic can't compete with steel's smoothness after transporting sand and gravel.
In my view, friction losses are practically the same in plastic and steel pipes if the inside diameters are equal. A “C” number of 120 is reasonable for both steel and plastic dredge pipes.
Plastic Pipe Wear
The best way to determine if using plastic pipe is economically advantageous is to try a section of it in the dredge system. The following conditions can shorten plastic pipe's wear life:
- A large percentage of oversize (plus 1-inch) rock
- Sharp-edged sand and rock particles
- Higher-than-necessary flow velocity
Steel or Plastic Pipe?
Plastic pipe advantages:
- Large selection of sizes
- May not require expensive floats to support it in the water
- Flexible, thus needing fewer dredge sleeves
- Cost?
Plastic pipe disadvantages:
- Requires a fusion machine to join sections
- May not resist wear as well as steel
- Limited pressure rating
- Cannot be repaired
- Disposal problem
- Cost?
Pipe Size Selection
Plastic pipe offers a wider selection of inside diameters compared to standard steel pipe. Table 1 in WillardSays…Pipeline…How Far Can a Pump Pump lists common steel and plastic pipe diameters used for dredge pipelines.
If you are looking for more details, kindly visit HDPE Dredging Pipe Floaters manufacturer.
Recommended article:5 Things to Know Before Buying pipe floats
Usually, increased production and efficiency can be obtained by using a dredge pipe that is about 20 percent larger in cross-sectional area than the dredge pump discharge nozzle.
Steel Pipe
If conditions prohibit the use of plastic pipe on a small dredge (10 inches and under), it's best to use steel pipe that has the same inside diameter as the dredge pump discharge nozzle. Here are some cross-sectional areas for steel pipes:
6”—29 square inches 8”—50 square inches
10”—79 square inches 12”—113 square inches
Jumping from a 6-inch to an 8-inch steel pipeline would require a 73% increase in flow to maintain the same velocity. Few 6-inch dredges have enough power to achieve that.
It is not recommended to jump from an 8-inch to a 10-inch steel pipe because it would require a 58% increase in flow. Likewise, jumping from a 10-inch to a 12-inch pipe requires a 43% increase in flow. However, using a 14-inch steel pipe on a 12-inch discharge (an area increase of 22%) pump is usually acceptable. Jumping from a 14-inch to a 16-inch steel pipe (an area increase of 33%) is usually unsuccessful.
Plastic Pipe
One common mistake dredgers make is buying the wrong-sized HDPE pipe, ending up with the wrong inside diameter due to purchasing the wrong outside diameter.
The mistake stems from not understanding how HDPE pipe is sized.
HDPE pipe is specified by its outside diameter followed by an SDR number. The nominal outside diameter of a given size of HDPE and steel pipe is the same.
The SDR (Standard Dimension Ratio) number specifies the wall thickness of HDPE pipe. The wall thickness is calculated by dividing the outside diameter by the SDR number.
For example, 8-inch HDPE SDR13.5 pipe has the same outside diameter as 8.625-inch steel pipe. The wall thickness is 8.625” divided by 13.5, which equals 0.639 inches. Subtracting twice the wall thickness from the outside diameter gives an inside diameter of 7.35 inches. If you buy this pipe to replace 8-inch steel pipe, you end up with a pipeline that's about 19 percent smaller in cross-sectional area.
Pay attention to the pressure rating when selecting plastic pipe. The pressure rating correlates with wall thickness. Dredge pipeline pressure can exceed 100 psi, and many thinner-walled plastic pipes may not be rated for such pressure. Note that wear will reduce the wall thickness of plastic pipe, thus lowering its pressure rating.
Selecting a plastic pipe is more complicated than choosing a steel pipe. Buying a plastic pipe of the same nominal diameter as the pump discharge nozzle is almost always a mistake. The correct choice is generally to buy a plastic pipe one size larger than the pump discharge nozzle and select a wall thickness (SDR) that provides an inside cross-sectional area about 20% greater than the pump discharge port.
The most important factor is the inside diameter.
Tailor-Made Steel Pipe
Naylor Spiralweld steel pipe comes in numerous combinations of diameter, wall thickness, and alloy to meet specific needs that standard steel or plastic pipes cannot satisfy. Naylor pipes are available with Wedgelock grooved pipe ends and connecting collars, simplifying pipe connection to require only a hammer for assembly or disassembly.
Contact willard@willardsays.com with questions, comments, or criticism.
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