Fluid does not move through a manifold without encountering resistance. Passages, turns, valve openings, fittings, filters, meters, hoses, and tubing all contribute some restriction to the flow path.
The engineering objective is not to eliminate all pressure drop, which is impossible, but to control it so the system delivers the required performance without unnecessary energy loss.
System Resistance
What Causes Pressure Drop?
Pressure drop is the difference in fluid pressure between two points caused by resistance to flow.
As fluid moves through smaller passages, valves, bends, restrictions, filters, fittings, and other components, energy is required to overcome those restrictions.
Increasing flow through the same restriction generally increases the amount of pressure loss.
Internal Geometry
Passage Diameter Matters
Smaller passages force a given volume of fluid through less cross-sectional area.
This increases fluid velocity and can increase pressure loss through the manifold.
Larger passages can reduce restriction, but manifold size, material between passages, valve cavities, machining access, and structural requirements limit how large those channels can become.
Flow Velocity
Flow Rate and Fluid Velocity
Flow rate describes how much fluid moves through the circuit over time, while velocity describes how quickly that fluid moves through a particular passage.
Two passages can carry the same total flow while producing different velocities if their internal diameters differ.
Passage velocity influences friction, pressure loss, noise, heat, and response within the system.
Valve Restrictions
Valves Can Become the Limiting Flow Area
Increasing manifold passage size does not remove a restriction caused by an undersized valve.
Valve openings, internal metering edges, cartridge geometry, directional spool passages, check-valve seats, and control orifices can all influence flow capacity.
The entire fluid path should therefore be reviewed from source to actuator or process connection.
Valve Manifold Selection Guide →Hydraulic Systems
Pressure Loss Becomes Heat
In hydraulic systems, energy lost across restrictions is converted largely into heat.
A circuit that continuously forces high flow through a large pressure drop can therefore contribute significant thermal load.
Heat can influence fluid viscosity, seal life, component efficiency, and overall system reliability.
Pneumatic Systems
Compressed-Air Pressure Loss
Pneumatic systems can lose usable pressure through regulators, filters, tubing, fittings, manifolds, valves, and exhaust restrictions.
A compressor may provide adequate pressure at the source while an actuator receives significantly less pressure during peak airflow demand.
Evaluating pressure under actual operating flow is therefore more useful than checking only static supply pressure.
Pneumatic Manifold Sizing →Fittings & Lines
External Connections Matter Too
A properly sized manifold can still perform poorly if hoses, tubing, fittings, filters, or connectors create greater restrictions elsewhere in the circuit.
Line length, internal diameter, bends, fitting geometry, and flow rate all contribute to total system pressure loss.
Hoses, Tubing & Fittings →Diagnostics
Measuring Pressure Drop and Flow
Pressure can be measured at two locations to determine how much pressure is lost across a component or section of the circuit.
Industrial flow meters can measure liquid movement, while specialized air and mass-flow technologies can be used for gas measurement.
Combining pressure and flow data helps determine whether a system problem is caused by insufficient source capacity or excessive downstream restriction.
Engineering Review
Pressure Drop Design Factors
Complete-System Analysis
Evaluate the Whole Flow Path
Pressure drop should be evaluated from the fluid source to the point where useful work is performed.
A manifold is only one portion of that path, so pumps, compressors, filters, valves, hoses, tubing, fittings, meters, actuators, and return or exhaust circuits should all be considered.