The pressure drops in the slurry circuit of a microtunneling machine They are calculated by estimating the pressure drop that occurs when pumping the slurry through pipes, hoses, elbows, valves, fittings, the excavation chamber, the discharge and return lines, and the separation plant. The calculation must take into account flow rate, the internal diameter of the pipes, circuit length, elevation difference, roughness, density, viscosity, solids content, particle size distribution, flow regime, and localized losses.
In a microtunneling project with hydro shield, the slurry circuit serves three critical functions: stabilizing the face, transporting the excavated material, and maintaining a flow rate compatible with production. Therefore, the calculation of pressure drops must be coordinated with the sludge separation plant, face pressure, excavation rate, pumping distance, and the rheological properties of the fluid.
What's included in the mud course?
A mud course typically includes:
| Element | How It Affects Pressure Drop |
|---|---|
| Drive Line | It transports clean or conditioned slurry to the tunnel boring machine |
| Excavation Camera | Exchanges pressure and load with the cutting face |
| Return Line | Transports slurry containing excavated solids to the surface |
| Pipes and Hoses | They generate friction losses |
| Elbows, valves, and fittings | They cause localized losses |
| Pumps | They must compensate for losses, elevation differences, and operating pressure |
| Attack Pit | Add elevation changes, vertical sections, and connections |
| Separation Plant | It introduces internal losses and affects the recirculated flow rate |
| Measuring Equipment | They allow for the measurement of flow rate, pressure, density, and viscosity |
In works of pipe ramming y microtunneling, the return line is usually more demanding because it carries solids, its density may increase, and it is more susceptible to wear, sedimentation, or blockages.
Key variables in the calculation
Pressure drops depend on several groups of variables:
1. Circuit Geometry
The following are considered:
- Total drive length.
- Total return length.
- Inner diameter of pipes and hoses.
- Number of elbows.
- Valves and fittings.
- Section breaks.
- Height difference between the platform, the shaft, and the tunnel boring machine.
- Depth of the well.
- Drive length.
As the tunnel boring machine advances, the pipeline length increases, and consequently, pressure losses may increase.
2. Circulation Flow Rate
The flow rate must be sufficient to transport the excavated material and maintain a stable cutting face. If the flow rate is too low, sedimentation and insufficient scouring may occur; if it is too high, head losses, energy consumption, and wear increase.
The flow rate is coordinated with:
- Excavation diameter.
- Feed rate.
- Volume of excavated solids.
- Pumping capacity.
- Capacity of the separation plant.
- Required front pressure.
3. Properties of Sludge
The rheological properties of sludge They directly influence the hydraulic calculation.
The following are reviewed:
- Density.
- Viscosity.
- Elastic limit.
- Gel strength.
- Solids content.
- Sand content.
- Particle size distribution in suspension.
- Temperature.
- pH and chemical stability.
- Tendency to settle.
A denser or more viscous slurry results in greater pressure drops and requires more pumping power. A slurry with too many solids can also increase abrasion, sedimentation, and the risk of blockages.
4. Type and quantity of excavated solids
Return slurry does not behave the same way as feed slurry. When excavated soil is added, its density, viscosity, abrasiveness, and transport capacity change.
The following is analyzed:
- Percentage of solids.
- Maximum particle size.
- Sand, silt, clay, or gravel.
- Abrasiveness.
- Risk of sedimentation.
- Plant separation capacity.
- Minimum conveyor speed.
How it is calculated conceptually
The calculation combines three components:
1. Friction Losses
These are the losses caused by the friction between the slurry and the pipe walls. They depend on the length, diameter, flow rate, roughness, and properties of the fluid.
Conceptually:
Greater length + smaller diameter + higher flow rate + higher viscosity = greater pressure drop.
During the design phase, they are calculated by section, separating the supply and return lines, because their flow conditions may differ.
2. Localized Losses
These are the losses associated with accessories and changes in direction:
- Elbows.
- Valves.
- Reductions.
- Widenings.
- Incoming and outgoing.
- Connections in a well.
- Measuring elements.
- Separation equipment.
Each component introduces an additional loss that must be added to the friction in the pipes.
3. Elevation Difference and Working Pressure
The pumping system must also overcome the elevation difference between the surface, the shaft, and the tunnel boring machine, in addition to maintaining the working pressure required for the water shield.
Therefore, the total pressure required by the pumps is obtained by adding:
- Friction losses.
- Localized losses.
- Geometric difference in elevation.
- Pressure needed on the front lines.
- Operating margin for variations in terrain, solids, and wear.
Difference between the supply line and the return line
| Line | Features | Main Risk |
|---|---|---|
| Propulsion | Cleaner sludge, better-controlled properties | Losses due to flow rate, viscosity, and length |
| Return | Slurry containing excavated solids, with higher density and abrasiveness | Sedimentation, wear, blockages, and increased losses |
| Vertical sections | The Effect of Elevation Difference and Accumulations | Increased pumping demand |
| Long sections | Friction and cumulative losses are increasing | The Need for Appropriate Pumps and Speed Control |
The return line is usually designed with special care because it must transport the excavated material without allowing solids to settle in the pipe.
How Is the Calculation Verified During Construction?
During execution, the calculation is validated by comparing predicted values with actual measurements:
- Supply pressure.
- Return pressure.
- Blood flow.
- Slurry density.
- Viscosity.
- Solids content.
- Sand content.
- Pressure drops per section.
- Pump power consumption.
- Temperature.
- Performance of the separation plant.
- Excavated volume versus transported volume.
- Sedimentation or blockage issues.
- Wear and tear on pipes, valves, and pumps.
If the actual pressures increase beyond what was expected, this may be due to a higher solids content, excessive viscosity, partial blockage, equipment wear, a reduction in effective diameter, sedimentation, dosing errors, or changes in the soil conditions.
Risks of Underestimating Pressure Drops
An insufficient estimate can lead to:
- Undersized pumps.
- Insufficient flow.
- Loss of transport capacity.
- Sediment buildup in pipes.
- Circuit blockages.
- Pressure drop across the orifice.
- Lower excavation performance.
- Increased wear and tear.
- High energy consumption.
- Unplanned stops.
- Saturation or imbalance in the separation plant.
- Security and control issues on the front lines.
At river crossings, submarine emissaries o sea water catchments, these risks may also affect environmental monitoring, spills, turbidity, stockpiles, and operational continuity.
Information Required for Design or RFQ
To calculate or review the pressure losses in the slurry circuit of a microtunneling machine, it is advisable to provide the following information: excavation diameter, driving length, pit depth, circuit layout, discharge length, return length, pipe inner diameters, number of elbows, valves, fittings, elevation differences, target flow rate, advance speed, face pressure, slurry type, density, viscosity, yield point, solids content, particle size distribution, sand content, temperature, pump capacity, available power, slurry separation plant capacity, distance between the plant and the shaft, allowable pressure drops, safety margins, environmental requirements, and control criteria during construction.
Request a Technical review of the slurry circuit, pressure drops, and pumping for the hydro-shield before shutting down pumps, pipes, the separation plant, or RFQ documentation.
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