All three are differential pressure (DP) flow meters working on the Bernoulli equation.
A constriction increases velocity and reduces pressure. The measured DP is proportional to the square of the flow rate.
The difference is in geometry. That geometry determines pressure loss, accuracy, cost, installation length, and suitability for dirty or high-velocity fluids.
The flow nozzle sits between the orifice plate and the Venturi in every performance category.
Better accuracy than an orifice plate, lower cost than a Venturi, moderate pressure loss, and compact installation. Standard choice for high-velocity steam and hot fluid measurement.
How All Three DP Flow Meters Work
All three elements use the Bernoulli equation to infer flow rate from a measured pressure difference.
As flow passes through a constriction, velocity increases and static pressure drops.
The DP transmitter measures this pressure difference. The flow computer converts it to a flow rate using the ISO 5167 formula.
Cd: discharge coefficient (dimensionless) — this is where the three types differ
d: throat diameter (m)
ΔP: differential pressure across the element (Pa)
ρ: fluid density (kg/m³)
β: beta ratio = d/D (throat diameter / pipe diameter)
Orifice plate Cd: 0.595 to 0.644
Flow nozzle Cd: 0.96 to 0.995
Venturi tube Cd: 0.984 to 0.995
In an ideal (inviscid, lossless) fluid, Cd = 1.0 exactly. In reality, boundary layer effects, flow separation, and viscous losses reduce the effective flow area.
The orifice plate has the lowest Cd because its sharp edge causes an immediate flow separation — the vena contracta (minimum jet area) forms downstream of the plate, not at the orifice itself. The flow nozzle and Venturi have shaped inlets that guide the flow more smoothly, minimising separation and giving higher Cd values closer to 1.0.
Orifice Plate: Simple, Cheap, and High Pressure Loss
An orifice plate is a flat disc with a central circular hole, clamped between pipe flanges. It is the simplest and cheapest of the three elements.
The sharp inlet edge causes immediate flow separation downstream of the hole.
The jet contracts to a vena contracta about 0.5 to 1 pipe diameters downstream, then expands — creating turbulence and permanent pressure loss.
Key Orifice Plate Facts
Pressure loss: 40 to 90% of measured DP is permanently lost. For a beta ratio of 0.5, permanent loss is typically 73% of ΔP.
Accuracy: ±0.5 to ±2% of rate. Cd varies significantly with Reynolds number below Re = 10,000.
Beta ratio range: 0.2 to 0.75 per ISO 5167. Smaller beta gives higher ΔP signal but higher pressure loss.
Tap types: Corner taps, flange taps (25.4 mm from each face), or D and D/2 taps. Each gives a slightly different Cd — never mix tap types without recalculating.
Limitations: Sensitive to upstream flow profile. Requires 20 to 40 pipe diameters of straight pipe upstream, 5 downstream. The sharp edge erodes in abrasive or high-velocity service, shifting the Cd.
Above this threshold, compressibility becomes significant and the gas expansion factor (ε) must be applied to the ISO 5167 equation. Ignoring this can cause a 2 to 5% positive flow error.
Most modern flow computers apply ε automatically, but legacy systems and manual calculations often omit it.
A rounding of just 0.1 mm on a 50 mm bore orifice changes the Cd by approximately 0.5 to 1%. Over time, this causes a systematic positive flow indication error — the meter reads higher than the actual flow.
In custody transfer metering (gas pipelines, oil terminals), orifice plates are often specified with a maximum allowable edge radius of 0.1 mm, verified by a micrometer or calibrated radius gauge at every maintenance inspection.
Flow Nozzle: High Velocity, Low Erosion, Compact
A flow nozzle has a shaped bell-mouth convergent inlet (elliptical profile per ISO 5167) leading to a cylindrical throat.
There is no divergent outlet section. The jet leaves the throat and expands freely into the downstream pipe.
The shaped inlet eliminates the flow separation an orifice plate suffers, giving a higher Cd (0.96 to 0.995).
Pressure loss is lower than an orifice plate but higher than a Venturi, because there is no recovery section downstream.
Key Flow Nozzle Facts
Pressure loss: 30 to 80% of measured DP permanently lost — less than an orifice plate, more than a Venturi.
Accuracy: ±0.5 to ±1% of rate with a calibrated Cd. Better than an orifice plate because the flow nozzle Cd is less sensitive to Reynolds number changes.
Beta ratio range: 0.2 to 0.8 per ISO 5167. Operates at higher flow velocities than an orifice plate at the same ΔP.
High-velocity suitability: The robust throat profile resists erosion at high flow velocities. Standard for superheated steam, high-pressure gas, and boiler feedwater where an orifice plate edge would erode rapidly.
Installation: Shorter than a Venturi but requires the same upstream straight pipe run as an orifice plate (typically 20 to 40D upstream). See the flow nozzle working principle guide for installation details.
At very low flows, the Reynolds number falls below the ISO 5167 valid range (Re less than 10,000 for a flow nozzle) and the Cd becomes unreliable.
For applications with a wide turndown ratio (more than 5:1), pair the flow nozzle primary element with a multivariable DP transmitter and a flow computer that applies the correct Cd versus Re correction at all flow rates.
Steam erosion destroyed orifice plate edges within months of installation. The curved bell-mouth entry of the flow nozzle solved this by reducing the jet velocity gradient at the throat edge, dramatically increasing service life.
Today, the flow nozzle remains the preferred primary element for superheated steam measurement in power generation, where it can survive continuous service for several years without edge rounding or significant Cd shift.
Venturi Tube: Lowest Pressure Loss, Highest Cost
A Venturi tube has three sections: a convergent inlet cone, a cylindrical throat, and a divergent outlet (recovery) cone.
The divergent section converts kinetic energy back to pressure after the throat.
This pressure recovery is what distinguishes the Venturi from the other two elements.
The orifice plate and flow nozzle discard throat kinetic energy as turbulence. The Venturi recovers it, losing only 5 to 20% of ΔP permanently.
Key Venturi Tube Facts
Pressure loss: 5 to 20% of measured DP permanently lost — the lowest of the three elements. See the orifice plate vs Venturi guide for a detailed loss comparison.
Accuracy: ±0.5 to ±1% of rate. The Venturi Cd is the most stable of the three elements because the shaped inlet and recovery cone prevent flow separation entirely.
Beta ratio range: 0.3 to 0.75 per ISO 5167.
Dirty fluid suitability: The smooth convergent profile is self-cleaning. Solids pass through without settling or plugging — unlike an orifice plate where particles lodge at the plate face and block pressure taps.
Cost and length: The most expensive of the three.
Physical length is 6 to 12 pipe diameters, requiring more installation space than a flow nozzle or orifice plate. See the Venturi tube guide for sizing details.
Too shallow an angle makes the Venturi physically very long. Too steep an angle causes flow separation in the recovery section, losing the pressure recovery advantage.
The optimum angle for maximum recovery with acceptable length is typically 6° to 7°. At this angle, boundary layer separation is just avoided and recovery efficiency reaches 80 to 95% of the throat kinetic energy.
Flow Nozzle vs Orifice Plate vs Venturi: Side-by-Side
| Parameter | Orifice Plate | Flow Nozzle | Venturi Tube |
|---|---|---|---|
| Discharge coefficient (Cd) | 0.595 to 0.644 | 0.96 to 0.995 | 0.984 to 0.995 |
| Permanent pressure loss (% of ΔP) | 40 to 90% | 30 to 80% | 5 to 20% |
| Accuracy | ±0.5 to ±2% of rate | ±0.5 to ±1% of rate | ±0.5 to ±1% of rate |
| Beta ratio range (ISO 5167) | 0.2 to 0.75 | 0.2 to 0.8 | 0.3 to 0.75 |
| Reynolds number range | Re greater than 5,000 | Re greater than 10,000 | Re greater than 200,000 (classical Venturi) |
| Physical length | Very short: plate thickness only | Short: 1 to 2 pipe diameters | Long: 6 to 12 pipe diameters |
| Upstream straight pipe | 20 to 40D typical | 20 to 40D typical | 10 to 20D (shorter due to shaped inlet) |
| Cost (relative) | Lowest | Medium | Highest |
| Erosion resistance | Poor: sharp edge wears quickly | Good: shaped inlet resists erosion | Excellent: smooth convergent profile |
| Dirty fluid / solids | Poor: solids plug at plate face and taps | Moderate: no plate face, but solids can settle in throat region | Good: convergent profile is self-cleaning |
| ISO standard | ISO 5167-2 | ISO 5167-3 | ISO 5167-4 |
| Best for | Clean liquids and gases, low flow velocity, cost-sensitive applications | High-velocity steam, high-pressure gas, boiler service, erosive conditions | Large pipelines, energy-cost-sensitive processes, dirty or abrasive fluids, liquid-gas mixtures |
DP Flow Meter Primary Element Selector
Watch: Venturi vs Orifice Plate: Key Differences (2025 Updated)
Flow Nozzle vs Orifice Plate vs Venturi Questions
External References
- ISO 5167: Measurement of Fluid Flow Using Differential Pressure Devices | ISO (2022 edition)
- Primary Flow Elements: Orifice, Nozzle and Venturi Selection | Emerson Rosemount
What We Learn Today
- All three are DP flow meters using the Bernoulli equation. The key difference is geometry: sharp edge (orifice), bell-mouth (flow nozzle), or convergent-divergent cone (Venturi).
- Pressure loss order: Venturi (5 to 20%) is lowest, flow nozzle (30 to 80%) is middle, orifice plate (40 to 90%) is highest.
- Use a flow nozzle for steam and high-velocity gas. Use a Venturi for dirty fluids and low-loss large pipelines. Use an orifice plate where cost is paramount and pressure loss is acceptable.