Published in Rock Products magazine, September 2014
As a dredge moves farther away from the discharge point, the pump speed is increased to maintain a proper slurry velocity within the pipeline. Once maximum pump speed has been achieved, further increasing the discharge distance causes the cost-per-ton of material moved to rise, while production declines. Of course, there is always a cost involved in pumping the water/material slurry and the associated fuel or power required to do so. Because of this, the idea of adding a booster pump to the dredging operation in order to reduce costs may seem counterintuitive. It is true that adding booster pumps requires an initial outlay of money for equipment, as well as ongoing costs for fuel or power supply and replacement of wear parts. However, the added benefits of greater production and less wear and tear on equipment means that a booster pump will typically pay for itself within a short period of time.
Turbulence within a pipeline is the product of velocity and friction. And turbulence is the key—because that is what keeps material suspended so that it can travel in the pipeline. If a pipeline is too long, the increased friction causes the slurry’s velocity to decrease. Because of this decrease in velocity, turbulence also decreases, meaning the maximum percentage of solids that can be safely transported in the pipeline also decreases. Since the slurry now has a greater chance of becoming too dense, it increases the possibility of pipeline plugging and, at the same time, causes a higher wear-rate on the pump because of poor pump efficiency and increased recirculation of solids within the pump itself. At the same time, the dredge is producing less material to pay for wear parts.
As long as the primary dredge pump is adjusted and working correctly, and it is being operated with sufficient power, a booster pump is the best option to improve pumping efficiency. A booster pump is installed between the primary dredge pump and the discharge point. It is used to increase the velocity of the material in the pipeline when the dredge has moved far enough from the plant that the main pump can’t provide sufficient turbulence to keep the material suspended in the pipeline. Dredging operations often balk at the idea of additional capital expenditure and the associated costs of adding a booster pump to the pipeline. But with the addition of a booster pump, production rates rise, revenues increase and wear costs decrease because the primary pump is not working beyond its capacity. In fact, the operation can often actually increase profits based on the booster pump installation.
As a conservative example, let us suppose that adding a new booster pump involves an initial capital outlay of $500,000 and results in an additional 100 TPH of production through increased pumping efficiency. Based on an 8-hour day and a 5-day week with year-round production, this translates to 208,000 tons of additional production each year. Even if material is only selling for $6 per ton, it would still result in an additional $1.2 million per year in revenue, and the booster pump has paid for itself inside of six months.
The addition of a second pump does raise both the capital acquisition and operating costs. But based on both the wear and tear on a dredge over its lifespan and its added production, it is easy to see that adding a booster pump to the operation may be an efficient and economical option.
Size, Power And Drive: It All Matters
Ideally, the booster pump should be sized to match the primary dredge pump output to reach the best efficiency point. Every pump will have a horsepower demand curve that takes into account the impeller size and range of output. It is important to purchase a booster pump with sufficient horsepower available for both the impeller size and the desired output.
While both variable-speed and fixed-speed booster pump drives are available, a variable-speed drive is the best option to ensure that the pump speed is always appropriate for the material flow. With a fixed-speed drive, there are two issues that commonly arise. The first is that the booster pump is often initially installed in the wrong location, making the speed incorrect for the material flow. The other issue is that even if the fixed-speed booster pump is initially installed in the correct location, when the dredge is moved to a different location, the booster pump is no longer in the correct location or operating at the correct speed. A variable-speed drive pump can adjust not only to the correct speed for the location, but also to the startup and shutdown pressure requirements of the operation.
Taking the above point into account, it’s clear that the location of the booster pump is critical for it to do its job efficiently. And elevation is one major reason dredgers often put the booster pump in the wrong location—because they have failed to take it into account. If the dredge is pumping material across 1,500 ft. of horizontal distance, it would seem logical to place the booster pump at the halfway point, which is 750 ft. But the booster pump deals with elevation in the pipeline more often than the primary dredge pump, as it must not only transport the slurry, but also lift it.
Slurry suspension, friction and pressure in the pipeline all affect the location calculation. The rule-of-thumb is that every 5 ft. of pipeline elevation is equivalent to approximately 100 ft. of horizontal pipeline friction. So a 50-ft. elevation is equal to 1,000 ft. of horizontal pipeline. Using the above scenario, with 1,500 ft. of horizontal pumping distance, and a 50-ft. destination elevation, it is equivalent to the dredge pumping the material 2,500 ft. in distance. In our example the booster pump should theoretically be placed at 1,250 ft. (“theoretically” because the dredger must also take into account any elevation change between the dredge and the booster, so there is some experimentation involved with exact placement).
To Automate Or Not To Automate
The answer to this question is a resounding “automate.” Automation comprises such a small part of the total booster pump cost that it only makes sense to automate. Relative to booster pump drive speed, automation senses the incoming and outgoing pressures in the pipeline and automatically changes the speed of the pump to accommodate the flow of material. A typical automation package adds less than 5% additional cost to the booster pump cost. And out of all available dredge automation systems, the benefit-to-cost ratio of booster pump speed automation is the best, due to the higher densities that can be carried at lower flow rates with less chance of pipeline plugging. The use of automation both decreases the energy cost and increases the dredge throughput and uptime.
Calculating More Than Just The Curve
Pump manufacturers use a variety of calculations to determine booster pump size, impeller size, power requirements and booster pump location. Challenges, however, can lie within the number and types of calculations the manufacturer uses. For instance, all pump manufacturers will use what is called a “pump curve” calculation for pump size. Theoretical pump performance is calculated based on pumping water in laboratory conditions. And many manufacturers will base dredging booster pump calculations on the traditional “curve,” taking only water into account. But in real-world dredging operations, the calculations must be based both on the water and on the material. Otherwise the specified pump will not have the horsepower required to move the material.
How seriously can bad calculations affect the bottom line? Let us suppose that a sand and gravel producer is selling sand for $7 a ton, and because of poor booster placement is producing 50 TPH less than it is capable of producing in a 10-hour day. That producer is losing $3,500 per day. In a six-day week, the losses amount to $21,000. After a year, the losses total more than $1 million.
The Bottom Line
For any dredging operation that is considering adding a booster pump, the best bet is to work with an established manufacturer that has experience with entire dredge operations. A good manufacturer can assess whether a booster pump would benefit the operation, and calculate the correct size, power and location for the pump. The best manufacturers also can train the operators on how to run the entire plant optimally, and provide technical support after the sale.
When Is A Booster Pump Not The Answer?
One common situation that often leads dredgers to assume that a booster is required is improper adjustment of the impeller on the dredge pump. The impeller must be properly adjusted to reduce the gap between the impeller and the front liner. If the gap is too great, the lack of pressure generating capability of the pump causes the slurry to recirculate. Production falls and pump wear increases.
By adjusting the pump impeller to reduce the gap to 1/16-in. at a maximum, pressure is restored, recirculation decreases and production increases.