Differential pressure (DP) flow meters use Bernoulli’s principle: inserting a restriction in the pipeline creates a pressure differential (ΔP) across it — ΔP is proportional to the square of flow velocity. Four variants share this principle: orifice plate (square-edged hole in a flat plate); Venturi tube (converging-diverging throat); flow nozzle (bell-shaped convergence without diverging section); and Pitot/Annubar (multi-port averaging probe sampling velocity pressure at multiple points across the pipe cross-section). All require a DP transmitter to convert the differential pressure into a 4–20 mA flow signal.
For an orifice plate: high-pressure tap upstream of the plate and low-pressure tap at the vena contracta (minimum pressure point just downstream of the plate) connect to a DP cell. The DP cell measures the differential and the flow computer applies Q = Cd × A × √(2ΔP/ρ) — Cd being the discharge coefficient calibrated for the specific β ratio (orifice-to-pipe diameter ratio). Venturi tubes follow the same equation with higher Cd (0.98 vs 0.61) and 90–95% pressure recovery vs 40–60% for orifice plate. Pitot/Annubar measures velocity pressure (dynamic pressure minus static pressure) at multiple annular positions and averages them to provide a representative pipe-traverse mean velocity.
Challenges
Rangeability Limited to 3:1–5:1 Before Accuracy Degrades (ΔP ∝ Q²)
At low flow (below 30% of design), ΔP falls to <10% of maximum — DP transmitter resolution limit reduces accuracy to ±5–10%.
Orifice Plate Wear and Edge Erosion Shifting Discharge Coefficient Cd
Orifice sharp edge erodes in abrasive/particulate service — Cd shifts by 1–3%; recalibration or replacement required.
Condensate Accumulation in Impulse Lines Causing Zero Offset
Wet gas or steam service fills impulse lines with condensate — zero shift up to 10% of span if lines not equalised.
High Permanent Pressure Loss (Orifice 40–60% of ΔP Permanent)
Orifice plate generates permanent pressure loss — increases pump/compressor energy consumption.
Straight Run Requirement (20D upstream, 5D downstream) Limiting Installation
Upstream disturbance (elbows, valves) introduces velocity profile distortion — errors up to ±3% without adequate straight run.
Solutions
- Dual or Triple DP Transmitter Ranging (Low/Mid/High Range Stacked): Three transmitters covering 3:1 overlapping ranges extend total rangeability to 10:1–15:1 — automatically switches at 30% span each.
- Wear-Resistant Orifice Material (SS 316L Hard-Faced Tungsten Carbide for Abrasive): WC or ceramic hard-facing on orifice bore and leading edge resists erosion — maintains Cd within 0.3% over life.
- Condensate Pots on Gas Service + Self-Draining Impulse Line Slope (1:10): Condensate pots with vent valves equalise impulse line fill — eliminate zero offset in steam and wet gas service.
- Venturi or Annubar for Low Pressure Loss Service (<10% of Orifice Plate ΔP): Venturi tube recovers 90–95% of differential — 10× lower permanent pressure loss vs equivalent orifice plate.
- Flow Conditioning Plate (Gallagher or NOVA-SWIRL) Reduces Straight Run to 5D–10D: Perforated conditioning plate homogenises velocity profile — reduces upstream straight run requirement from 20D to 5–10D.
Applications
- Utilities: Steam, water, compressed air, nitrogen flow measurement on main headers — orifice plate standard.
- Oil & Gas: Natural gas and crude oil custody transfer metering — Venturi and flow nozzle for high-accuracy fiscal metering.
- Chemical: Liquid and gas process stream flow measurement — robust, no moving parts, wide material options.
- Power Generation: Boiler feedwater, steam, and cooling water flow measurement — ASME PTC 19.5 compliant metering.
- HVAC: Chilled water, hot water, and air duct flow measurement — annubar preferred for large duct/pipe sizes.
Models & Capacities
DP Flow Meters — Technology Comparison & Sizing Reference | ||||||||||
Type | Pipe Size DN [mm] | Flow Range | Accuracy [% reading] | Permanent Pressure Loss | Rangeability | Reynolds No. Min | Particle Tolerance | Viscosity Limit [cP] | Calibration | Best Application |
Orifice Plate (square edge) | DN 15–DN 1,500 | Gas, liquid, steam | ±0.5–1.0 | 40–60% of ΔP | 3:1–5:1 | Re >5,000 | Poor (>50 µm erodes) | <5 | In-situ or lab | Utilities, general industrial — low cost, replaceable |
Orifice Plate (conical inlet) | DN 15–DN 600 | Gas, liquid, viscous | ±1.0–2.0 | 40–65% of ΔP | 3:1 | Re >100 (low Re type) | Moderate | 5–50 | In-situ | Low Reynolds number viscous liquid service |
Orifice Plate (eccentric/segmental) | DN 50–DN 600 | Two-phase, slurry, wet gas | ±1.5–2.5 | 40–60% | 3:1 | Re >5,000 | Good (eccentric cut) | <200 | In-situ | Slurry, two-phase, dirty liquid with solids |
Venturi tube | DN 50–DN 1,200 | Gas, liquid, steam | ±0.5–1.0 | 5–10% of ΔP | 3:1–5:1 | Re >75,000 | Good (no sharp edge) | <20 | Lab (factory) | High-pressure / high-value fluids — minimum pressure loss |
Flow nozzle | DN 25–DN 600 | High-velocity steam/gas | ±0.5–1.5 | 20–30% of ΔP | 3:1 | Re >50,000 | Good | <10 | Lab (factory) | High-velocity steam, superheated steam, boiler feedwater |
Pitot tube (single point) | DN 50–DN 5,000 | Gas, liquid, air duct | ±2–5 | <1% | 3:1 | Re >10,000 | Poor | <5 | In-situ | Large duct/chimney flow surveys — low cost, portable |
Annubar (multi-point pitot) | DN 25–DN 5,000 | Gas, liquid, steam | ±0.5–1.5 | <1% | 3:1 | Re >10,000 | Moderate | <10 | In-situ | Large pipe/duct permanent installation — low pressure loss |
Wedge (high-viscosity) | DN 15–DN 600 | Viscous, slurry | ±0.5–1.5 | 25–40% | 3:1 | Re >500 | Excellent | Up to 100,000 | Lab | Viscous liquids, slurry, heavy fuel oil |