Conveying height

Conveying height describes how far a fluid can be transported against gravity and flow resistance. In demolition and cutting technology, it influences whether power units reliably supply tools – from the use of concrete demolition shears on higher floors to rock and concrete splitters in shafts, tunnels, or on slopes. Those who understand conveying height plan lines, power units, and tools so that performance, response time, and safety on the construction site are correct. Accurate assessment of conveying height also reduces commissioning effort and prevents costly oversizing or heat-related inefficiencies.

Definition: What is meant by conveying height?

Conveying height is the specific energy supplied to the medium (e.g., hydraulic oil), expressed as an equivalent height in meters. It consists of the geodetic elevation difference (static component), pressure losses due to friction and fittings, and, where applicable, a velocity component. In practice, one often speaks of the manometric conveying height, i.e., the sum of the elevation difference to be overcome and the loss heights in the system. The greater the conveying height, the higher the pressure and/or flow rate of the power unit must be designed so that tools such as concrete demolition shears, rock splitting cylinders, or combination shears operate reliably. In formal terms, the hydraulic head H relates to pressure via H ≈ Δp/(ρ·g), where ρ is the fluid density and g is the gravitational acceleration.

Physical fundamentals and units

Conveying height H, pressure difference Δp, and density ρ of a medium are closely related: the higher the density, the lower the height effect of a given pressure. Guide values help with classification: 1 bar corresponds approximately to a 10 m water column; for hydraulic oil (typically lower density than water) rather about 12 m. Conversely, a height difference of 25 m with hydraulic oil means an additional pressure requirement of around 2 bar for the static component. Added to this are friction losses in supply and return, fittings, couplings, and valves, which increase strongly with flow velocity (flow rate and hose inner diameter). The velocity component v²/(2g) is usually small in hydraulic circuits but can become relevant with very high flow velocities in undersized hoses.

• Quick check: Δp ≈ 0.8-0.9 bar per 10 m with hydraulic oil, ≈ 1 bar per 10 m with water.
• Density effect: lower density yields greater height for the same pressure difference; use actual data sheets for precise sizing.
• Friction scaling: losses rise roughly with the square of velocity – moderate flow speed via larger inner diameters over distance.

Significance of conveying height in hydraulics and demolition technology

In closed hydraulic systems of demolition tools, conveying height determines whether the differential pressure required at the tool is present. For vertical lines to higher work areas (e.g., strip-out of a high-rise), the power unit must overcome the geodetic height in addition to the tool load. At lower-lying points of use (e.g., rock excavation in a tunnel), the static shares are partially supportive, yet line lengths and friction losses often dominate. For tools with high operating pressure requirements such as concrete demolition shears or rock and concrete splitters, a precise assessment of conveying height is crucial to avoid performance losses, heating, or delayed response times. If the available pump pressure is largely consumed by static head and line losses, actuators may stall, control becomes sluggish, and thermal load in the oil rises.

Power units and conveying height

Hydraulic power units supply flow at a set pressure. Conveying height determines the operating point on the pump curve: if the required height (including losses) increases, the available flow at the tool decreases for the same drive power. Pressure relief valves must be set so that they cover both the conveying height and the tool demand without unnecessarily stressing the system. Smart placement – as close as possible to the tool’s elevation – reduces the static component and improves dynamics. Variable-displacement and load-sensing concepts can help keep pressure reserves available without excessive bypassing through the relief valve, thereby reducing heat generation at partial load.

Line routing on the construction site

Long hose bundles, tight bend radii, and many couplings increase the loss height. Larger nominal diameters reduce flow velocity and thus friction. On construction sites with changing levels, such as in special demolition, a routing with few changes of direction, short free-hanging sections, and a hose size selected for the flow rate is recommended. This is particularly relevant for tools with pulsating demand such as multi cutters, steel shears, or tank cutters. Mechanical protection, proper support, and kink protection prevent cross-sectional constriction and additional, avoidable pressure losses.

Influencing factors: medium, temperature, hose system

• Medium and density: Hydraulic oil has a lower density than water. This changes the conversion between pressure and height and directly affects the geodetic component.
• Viscosity and temperature: Warmed oil is less viscous; friction losses can decrease, while leakage increases, reducing effective performance. Cold, viscous oil increases friction losses and starting pressures.
• Hose length and nominal diameter: Long, small-diameter lines cause high pressure losses. Larger nominal diameters lower velocity and thus loss height.
• Fittings and couplings: Every transition, every valve, and every quick coupler adds individual losses. High-quality, flow-optimized fittings reduce total loss height.
• Elevation difference: Rising or falling lines create static components. When operating concrete demolition shears on high floors, the additional pressure requirement must be considered.
• Return line and filter: The return line also causes losses – clogged or overly fine filters increase total conveying height.
• Pulsation and dynamics: Shears and crushers generate alternating demand peaks. Accumulators and adequately sized lines stabilize the operating point.
• Installation layout: Vertical risers and high points can trap air; careful bleeding and avoiding siphon-like routing improve repeatability.
• Surface condition and contamination: Internal roughness and deposits in hoses or valves increase friction and reduce available pressure at the tool.

Conveying height in interaction with power units and lines

The sizing rule is: first determine the elevation difference, then define routing and nominal diameters, and finally select the power unit in terms of pressure and flow rate. For tools with high peak loads, e.g., rock and concrete splitters, a sufficient pressure reserve is advisable to safely overcome the sum of geodetic height and friction losses. For concrete demolition shears, conveying height noticeably affects closing speed and holding force, especially with long hose runs on higher floors. During commissioning, comparative measurements with pressure gauges at the unit and at the tool connection clarify whether the calculated reserve is actually available under load.

Calculation and conversion of pressure, conveying height, and flow rate

Practical approximations simplify planning: per 10 m elevation difference, about 1 bar must be added for water; for hydraulic oil approximately 0.8-0.9 bar per 10 m (depending on actual density). Friction losses depend strongly on flow rate and hose inner diameter: doubling the velocity multiplies the losses, which is why larger nominal diameters are often more efficient over long distances. For dimensioning, add the components: geodetic height plus line losses in supply and return plus fitting losses. The result gives the required pressure reserve so that the necessary operating pressure is available at the tool. Concrete designs should always be based on solid data on density, viscosity, hose characteristics, and tool demand. In short: H_total ≈ H_static + H_friction,supply + H_friction,return + H_fittings + H_velocity.

• Rule of thumb: if line length doubles at constant diameter and flow, expect friction losses to rise significantly – consider upsizing by one nominal diameter for long runs.
• Verification: compare pressure at the power unit with pressure measured near the tool during operation; the difference approximates total loss height.
• Thermal check: high oil temperature at modest tool load indicates excessive throttling losses due to undersized lines or unnecessary restrictions.

Practical examples from concrete demolition, strip-out, and rock

Example 1: Concrete demolition shear in a high-rise

The power unit is at ground level; the concrete demolition shear works on the 8th floor (approx. 24 m difference). The static component alone requires about 2 bar extra (with oil). With a 40 m hose bundle and several couplings, additional losses occur. Measures: position the unit one level higher, choose a larger hose nominal diameter, reduce couplings, set the pressure relief so that the required operating pressure at the tool plus reserves is available. Result: consistent cutting force and better cycle times. A small hydraulic accumulator close to the tool can buffer demand peaks and improve response at long distances.

Example 2: Rock and concrete splitter in the tunnel

The tool is topographically lower than the unit; the static component acts positively. However, the line routes are 70-100 m long. Low friction losses are therefore decisive: large-dimensioned lines, flow-optimized fittings, and clean routing without tight bends. This ensures the required pressure peak for the splitting process is reliably provided. Additionally, monitor the differential pressure across return filters to prevent a creeping increase in total loss height.

Example 3: Deconstruction in a tank facility with a tank cutter

In special operations in vessels or on platforms, elevation levels vary. Temperature conditions influence oil viscosity. A well-matched combination of unit performance, hose dimension, and filter condition ensures consistent cutting quality and prevents excessive heating due to unnecessary loss heights. For cold environments, suitable oil viscosity classes and preheating reduce start-up losses and shorten ramp-up times.

Planning and design: step by step

1. Record the elevation difference between unit and tool (consider ascents and descents).
2. Determine the tool’s flow rate and pressure requirements (including reserves for peak loads).
3. Plan line routing: as short and direct as possible with large nominal diameters and few couplings.
4. Estimate pressure losses: include supply, return, fittings, filters, and valves.
5. Select and set the power unit: consider pressure relief, cooling, filtration, and output.
6. Monitor during operation: use oil temperature, response times, noises, and pressure indications to detect conveying height-related losses.
7. Document setup: record hose lengths, diameters, valve settings, and measurement points to enable reproducible performance and later troubleshooting.

Common mistakes and how to avoid them

• Underestimating friction losses in long or small-diameter lines – avoid with larger nominal diameters and fewer couplings.
• Unfavorable placement of the unit below or far from the point of use – position at the tool’s level whenever possible.
• System pressure set too low without reserve – adjust the pressure relief valve to suit practice.
• Neglected return-line losses – check return line and filter sizing.
• Poor temperature control – overheating increases losses and reduces performance; pay attention to cooling and oil quality.
• Insufficient measurement – missing pressure readings at the tool mask actual loss heights and lead to misadjustment.

Safety, environment, and legal notes

Working under hydraulic pressure requires prudent action. Settings on the unit and changes to lines and couplings may only be carried out by qualified personnel. The operating manuals of Darda GmbH tools and the hydraulic power units are authoritative and must be observed. Requirements from occupational safety, environmental protection, and construction site regulations can vary depending on the place of use; careful, project-specific planning is essential. Before work begins, depressurize circuits in a controlled manner, check emergency stop functions, and provide spill containment to prevent environmental damage in the event of a line failure.