ISO 5167: Differential Pressure Flow Measurement Standard Explained

Differential pressure (DP) flow measurement is one of the oldest and most widely used methods in industrial practice. It's based on a simple principle: when fluid flows through a restriction (orifice, nozzle, or venturi), pressure drops, and this pressure difference is proportional to flow rate.

ISO 5167 is the international standard that governs how these devices must be designed, installed, and used to achieve repeatable, accurate measurements. If you're specifying a DP flowmetre—whether orifice plates, venturi tubes, or nozzles—this standard defines the engineering rules.

What ISO 5167 Covers

ISO 5167 is divided into five parts:

• ISO 5167-1: General principles and requirements (measurement uncertainties, installation conditions)
• ISO 5167-2: Orifice plates
• ISO 5167-3: Nozzles and venturi tubes
• ISO 5167-4: Wedge-shaped primary devices
• ISO 5167-5: Cone and quarter-circle orifices

Most industrial applications use orifice plates (Part 2) or venturi tubes (Part 3). Wedge and cone devices are specialised (viscous fluids, low flow).

The standard applies only to incompressible flow (liquids) and compressible flow (gases and steam) up to Mach 0.3. For transonic or supersonic flows, the standard does not apply.

The Fundamental Equation

ISO 5167 is built on the energy equation derived from Bernoulli's principle. Mass flow rate is calculated as:

Q_m = C × d² × √(2 × ρ × ΔP) / √(1 - β⁴)

Where:

• Q_m: Mass flow rate (kg/s)
• C: Discharge coefficient (dimensionless, 0.6–0.99 depending on device type and Reynolds number)
• d: Orifice or nozzle diameter (metres)
• ρ: Fluid density (kg/m³)
• ΔP: Differential pressure (Pa)
• β: Diameter ratio (orifice diameter / pipe diameter)

The discharge coefficient C is the critical variable. It varies with:

• Reynolds number (Re = ρ × v × D / μ)
• β ratio (diameter ratio)
• Tap locations (corner taps, flange taps, D-D/2 taps)
• Orifice geometry (sharp edge, bevelled edge, chamfered)

ISO 5167 provides tables and correlations for C based on these parameters. Manufacturers typically use computational fluid dynamics (CFD) or empirical testing to validate their orifice designs against these tables.

Device Types: Orifice, Venturi, Nozzle

Orifice Plates (ISO 5167-2)

A flat metal disc (typically stainless steel) with a hole in the centre, inserted between two flanges in the pipeline. The fluid is forced through the hole, creating a pressure drop.

• Cost: Lowest (GBP 200–1,000)
• Accuracy: ±2–3% under ideal conditions, depending on Reynolds number
• Pressure recovery: Lowest (70–75%); significant permanent pressure loss
• Maintenance: Susceptible to fouling; requires frequent cleaning if fluid is dirty or viscous
• Application: Clean fluids (natural gas, refined petroleum, potable water), large pipes (4" to 24"+)

Venturi Tubes (ISO 5167-3)

A tapered tube that gradually narrows to a throat (convergent section), then gradually expands (divergent section). Fluid flows smoothly through without sharp edges.

• Cost: GBP 2,000–10,000
• Accuracy: ±1–2% under ideal conditions
• Pressure recovery: High (95%+); minimal permanent pressure loss
• Maintenance: Robust; tolerates some fouling and viscous fluids better than orifice
• Application: Large-diameter pipes where pressure loss must be minimised, dirty or slightly viscous fluids

Flow Nozzles (ISO 5167-3)

A streamlined short tube that sits inside the pipeline. Similar accuracy to orifice plates but with slightly better pressure recovery.

• Cost: GBP 800–3,000
• Accuracy: ±1.5–2.5%
• Pressure recovery: 75–90%; moderate pressure loss
• Maintenance: Better fouling tolerance than orifice; still requires care in dirty service
• Application: Steam and high-velocity gas applications; moderate-cost alternative to venturi

Installation Requirements: Why They Matter

ISO 5167 is strict about upstream and downstream straight pipe requirements. Deviating from these requirements introduces measurement errors.

Upstream Straight Run

• Minimum: 10–20 pipe diameters upstream of the orifice (depending on β and upstream disturbances)
• Why: Allows turbulent flow to stabilise. Elbows, tees, or valves close to the orifice cause velocity profile distortion, increasing measurement error
• Cost impact: GBP 500–2,000 in additional pipe and support steel to achieve required straight run in retrofit installations

Downstream Straight Run

• Minimum: 5–10 pipe diameters downstream (less critical than upstream)
• Why: Allows pressure to stabilise in the outlet pressure tap

Pipe Diameter Constraints

ISO 5167 applies to pipe sizes D ≥ 0.05 m (50 mm). For smaller pipes, alternative methods are recommended. For very large pipes (D > 1 m), special considerations apply (high Reynolds number effects, large diameter ratio β, expensive orifice plates).

Pressure Tap Locations

The standard specifies three common tap configurations:

• Corner taps: Pressure taps immediately adjacent to the orifice on both sides. Most common in compact installations. Highest differential pressure reading.
• Flange taps: Taps located 1 inch (25 mm) upstream and downstream. Standardised location; compatible with manifold blocks.
• D and D/2 taps: Upstream tap at 1 pipe diameter upstream; downstream tap at 0.5 diameters downstream. Used in large pipes where flange taps are impractical.

Your tap location determines the effective discharge coefficient (C) used in the calculation. ISO 5167 provides C values for each configuration.

Pressure Drop and Energy Cost

The permanent pressure loss (not recoverable) across a DP device is a significant operational cost. For a large orifice plate in a 10 m³/h flow, permanent pressure loss might be 50 kPa, requiring constant pump power to overcome.

Annual energy cost example:

Assume: 50 kPa loss, 10 m³/h flow, 0.85 pump efficiency, GBP 0.12/kWh electricity.

Annual energy cost = 50 kPa × 10 m³/h × 8,760 h/year / (0.85 × 3.6e6) × 0.12 = GBP 1,700/year

Venturi tubes recover ~95% of this pressure, reducing annual energy cost to GBP 85/year. Over a 10-year instrument lifetime, this difference is GBP 16,150—far exceeding the GBP 5,000–8,000 extra cost of the venturi.

For custody transfer or high-value fluid applications, the lower ongoing energy cost of venturi tubes often justifies higher capital cost.

Accuracy and Uncertainty

ISO 5167 quantifies measurement uncertainty based on several factors:

• Device uncertainty: The uncertainty in the discharge coefficient (typically ±2% for orifice, ±1.5% for venturi)
• Installation uncertainty: Deviations from specified tap locations, straight run violations, surface roughness
• Differential pressure measurement uncertainty: The accuracy of your DP transmitter (±0.5%, ±1%, or ±2% depending on model)
• Fluid property uncertainty: Density must be known accurately; errors here propagate to flow calculation

Combined, realistic accuracy is typically ±2–3% for orifice and ±1–2% for venturi in well-installed systems.

Important: This is volumetric flow at the measurement conditions. If you need mass flow, temperature and pressure must be measured simultaneously, and uncertainty increases.

When to Use DP Flow Measurement

Advantages

• Extremely well-established; ISO 5167 is referenced globally
• Low capital cost for orifice plates
• Works with any fluid (liquid, gas, steam) within ISO 5167 bounds
• No moving parts; inherently safe and durable
• Suitable for custody transfer if calibrated and verified to MID standards

Disadvantages

• Permanent pressure loss (except venturi, which still has 5% loss)
• Requires accurate differential pressure transmitter; DP signal is often small and must be measured with high resolution
• Vulnerable to fouling (orifice plates); regular maintenance required
• Cannot handle multi-phase flow (gas-liquid mixtures)
• Accuracy degrades at very low Reynolds numbers (viscous fluids) or very high Reynolds numbers (compressible flow effects)
• Straight run requirements impose installation constraints

Specifying a DP Flowmetre Installation

When requesting quotation for a DP flow measurement system, provide:

• Fluid type: Liquid, gas, steam; fluid name (water, natural gas, nitrogen, saturated steam)
• Normal operating conditions: Pressure (bar), temperature (°C), expected density (kg/m³)
• Flow range: Minimum and maximum expected flow (m³/h or kg/h)
• Pipe diameter: Nominal diameter (mm or inches)
• Available pressure drop: Maximum acceptable permanent loss (bar or kPa)
• Required accuracy: ±1%, ±2%, or ±3%
• Fluid cleanliness: Expected fouling rate; any particulates or condensables
• Installation location: Straight run available upstream/downstream (metres)

An experienced flow measurement engineer will recommend orifice, nozzle, or venturi based on your constraints and calculate the exact diameter required to achieve your accuracy and pressure drop targets.

DP Flow Measurement vs. Modern Alternatives

When should you choose DP measurement over newer technologies?

Choose DP if:

• Budget is constrained and you need the lowest capital cost
• Your fluid is extremely challenging (slurries, highly viscous, cryogenic)
• Custody transfer with MID certification is required and your installation already has DP tradition
• You have extensive in-house DP expertise and calibration capability

Choose Coriolis or EM instead if:

• Accuracy >±1% is required (Coriolis achieves ±0.1–0.5%)
• Pressure loss must be minimised for energy efficiency
• Mass flow (not volume) is required without separate T/P measurement
• Multi-phase flow is present
• Your fluid properties vary significantly (Coriolis compensates automatically)

Real-World Example: Natural Gas Metering

Scenario

A gas distribution utility must measure flow through an 8-inch (200 mm) transmission line. Expected flow: 50,000 m³/h at 50 bar, 15 °C. Accuracy required: ±2%.

Solution: Orifice Plate with DP Transmitter

• Device: Orifice plate, corner taps, carbon steel construction
• Orifice diameter: ~80 mm (calculated to produce ~100 kPa DP at design flow)
• Permanent pressure loss: ~50 kPa
• DP transmitter: Rosemount 3051S or equivalent, 0–150 kPa range, ±0.5% accuracy (0–50 mV output to SCADA)
• Capital cost: Orifice plate GBP 500, transmitter GBP 2,000, installation/flanges GBP 1,500 = GBP 4,000
• Annual energy cost: GBP 2,500–3,000 (pump running cost to overcome pressure loss)

Alternative: Venturi Tube

• Cost: Venturi tube GBP 5,000, transmitter GBP 2,000, installation GBP 1,000 = GBP 8,000
• Annual energy cost: GBP 125–150 (95% pressure recovery)
• Payback: (8,000 - 4,000) / (3,000 - 150) ≈ 1.5 years

For a gas utility operating 24/7/365, the venturi tube pays for itself in 18 months through energy savings, and is preferred long-term despite higher initial cost.

Next Steps

1. Determine your fluid and operating conditions: Pressure, temperature, flow range, density.

2. Check available installation space: Measure straight run available upstream; confirm tap locations compatible with your pipework.

3. Define accuracy and pressure loss budgets: Decide acceptable DP device type (orifice, nozzle, venturi) and maximum acceptable permanent loss.

4. Request engineering design: Engage a flow measurement specialist to calculate orifice diameter and expected accuracy.

5. Pair with appropriate DP transmitter: Choose transmitter accuracy (±0.5%, ±1%, or ±2%) appropriate for your overall system accuracy target.