The Particle Size 40% Rule Can Dictate Minimum Production Rate

We often hear customers say that there are only a few oversized particles in a deposit, and they will not present a problem. The truth is they will be a problem. Large particles are screened out and left behind if the pump has a screen. They eventually build up around the screen and stop the slurry from entering the pump.  This situation is often referred to as “ paving stoning the pump.” The alternative is to pump the large particles through the system, and the 40% rule then comes into play!

So what is the 40% rule? Well, in theory, the inside diameter of your pipe is approximately equivalent to the maximum particle size that could be transported through that pipe.  In practice, however, most applications require us to reduce the maximum particle size to 40% of the pipe ID. As the sketch below illustrates, this greatly reduces the possibility of two particles forming the beginnings of a plug.

Two particles over 50% of pipe ID

Two particles less than 40% of pipe ID

As illustrated below,  the possibility of three or more particles forming a plug still exists, but the physics of this makes it far less likely.

When an application is identified to have large particles and the numbers indicate that screening the pump will result in “paving stoning the pump,” the only alternative is to repeatedly clear the suction around the pump or size the pump and system using the 40% rule.  In short, pipes and pumps will all have to be a minimum of 2.5 times the size of the max particle.

For example, a customer may desire to run 24 hours a day through a  100 mm steel piped system to obtain 900 metric tons of sand daily.  However, the deposit contains an abundance of 55mm cobbles that also must be moved. The 40 % rule necessitates a minimum pipe of 55 x 2.5  or 138 mm. Moving to the next standard steel pipe size, the customer would have to use a 150 mm  (6-inch) pipe. To maintain a slurry velocity high enough to avoid settling in the larger pipe, flow rates would also have to increase by 2.5 times.   The net production would, therefore, be closer to 2000 metric tons per day.

In this example, the existence of some cobbles would force the customer to double the size (and expense) of the entire system. This is usually unwelcome news, but hopefully, a shorter workday or increased daily yield would help offset the increase in initial capital.

If you need help sizing your system, give us a call. Toyo engineers are experts.

Cheers

RJ