Every trenchless project needs a method to remove excavated soil from the bore. Some methods use augers, while others could even be hand excavated. But, many trenchless construction projects use slurry systems for this purpose.
A slurry mixture – or mud containing water, bentonite and other additives – is pumped down the drill string. The mud pushes out of the drilling head itself, helping to lubricate the side walls of the bore. This makes it easier for the pipe to slide through the soil. Slurry also maintains a pressure inside the bore to withstand the effects of groundwater pushing in. A positive pressure inside the bore is important to prevent tunnel collapse. Microtunneling and HDD are typical trenchless methods which use a slurry system. (For more on mud, check out Bentonite and the Use of Drilling Mud in Trenchless Projects.)
Slurry pumps supply enough pressure to push the mud through the bore, carrying spoil from the excavation back to the surface with it. From there it enters a separation unit to remove the solids before being recycled in the system. These pumps operate in harsh conditions and often require both high pressures and high flow rates.
Sizing a slurry pump correctly is a critical part of the pump selection process. This ensures that it will operate within its design parameters, avoiding premature pump failures and costly downtime.
How do you size a slurry pump to match your needs?
Understanding Pump Capacity
There are a wide variety of types of pump, including reciprocating, diaphragm and centrifugal pumps. Many slurry systems use centrifugal pumps because of their robustness and performance characteristics. A centrifugal pump works by rotating an impeller which pushes the slurry through the system.
Pump capacity is primarily defined by two parameters: head pressure and flow rate. Manufacturers supply each pump with a pump curve where pressure and flow are plotted against each other. It is this curve that we use to determine whether a pump is suitable for our application.
Chemical Engineering describes a detailed process for engineers to follow when sizing a pump. The more complex the application, the more important it is to have expert help in establishing the pump design characteristics needed. Nevertheless, there are some fundamental steps that can be applied in every situation.
1. Calculating the Dynamic Head
Dynamic head is effectively the pressure at the discharge of the pump in order to supply the energy needed to push the slurry through the system. A major factor that affects the dynamic head is changes in level from the slurry supply tank to the lowest point in the bore and back up again. The pump must overcome the forces of gravity to push the mud back out of the ground.
Another significant factor is the friction of the system. As the slurry passes through the drill pipe and back up to the surface, it encounters friction against the surface walls of the pipe. Slurry pumps must provide enough energy to overcome these forces of friction too.
2. Calculating Flow Rate
Besides supplying sufficient dynamic head based on the calculations above, a slurry pump must also push the required volume of slurry through the system. This is defined in terms of flow rate. The flow rate required for a trenchless project is heavily dependent on the soil conditions and the size of the bore. Sandy soils need the lowest volume of slurry to carry the spoil to the surface in an HDD project. But, shale conditions may require up to 20 times as much. (To learn more about ground conditions, see When Ground Improvement is Needed During Trenchless Rehabilitation.)
3. The Effects of Pump Efficiency
No pump operates at 100 percent efficiency. In fact, efficiencies can range from as low as 50 percent up to over 90 percent. This means that from 10 to 50 percent of the energy you supply to the pump is not actually used to move the slurry through the system – it is lost. The better the efficiency of the pump, the lower your operating costs will be. However, efficiency is based on where on the pressure/flow curve the pump is operating.
How the Pump Curve Works
A pump curve is essentially a graphic representation of the performance of a pump. If the head and flow rate of your application sits on or below the pump curve, then the pump is capable of performing that task. A steep curve means that it can generate a lot of head but limited flow rate. Flat pump curves represent pumps that can generate high flows but have a limited head pressure.
Pump manufacturers actually supply a series of curves for each pump that represent the pump’s performance for different diameter impellers. Efficiency curves can be overlaid on the pump performance curve to indicate where the pump is operating in its best efficiency range.
Selecting an efficient pump is important, as it saves on operating costs like electricity or generator fuel. One pump may be cheaper to buy but cost much more to run in the long term. It may be better to buy a more expensive pump up front, but one that has a good efficiency and lowers your operating costs.
The Risks of Getting It Wrong
Operating on the left side of the pump curve will lead to symptoms of temperature rise, noisy operation due to cavitation and vibration, which will ultimately result in low bearing and seal life and regular maintenance outages.
The same effects can occur if a pump is operated on the right-hand side of the pump curve. The best operating point for the pump is in the middle of the pump curve, where it is most efficient and least likely to have component failure.
When analyzing pump failures, it is important to look at the whole system and not just the pump itself. Monitor and record pressure readings in the mud circulation system and check friction calculations. An error in these calculations could mean that you have the wrong pump for your service and therefore have a solution that is prone to failure.
As in most process applications using pumps, it is often not the cost of a pump repair that is the primary problem. The problem is the interruption to the project while the repair is being executed.
Man hours lost and overheads are still incurred while there is no progress on the trenchless construction activity itself.