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ASME PTC 19.5.2-1971 scan — Application, Part II of Fluid Meters
📏 ASME PTC 19.5.2-1971 is the Interim Supplement 19.5 on Instruments and Apparatus containing the “Application” portion — Part II — of the classic report Fluid Meters: Their Theory and Application by the ASME Research Committee on Fluid Meters (Sixth Edition, 1971). Aimed squarely at engineering use, this Part is built around differential-pressure (head-type) flow measurement: it presents installation requirements, equations for computing rates of flow, worked examples, and tolerances for a family of primary elements. It has long been a technical bible for flow measurement in the Performance Test Codes and a historical source for later flow-measurement standards. The U.S. Department of Defense adopted the document in 1992 for military use.
💡 Historical significance: the Research Committee on Fluid Meters was organized in 1916, and its report has been the authoritative reference for flow measurement since the first edition of 1924. Part II “Application” works with Part I “Theory and Mode of Operation”: Part I develops the theory, while Part II provides equations, charts, and tolerances ready for direct engineering calculations.
1. Document Structure and Scope 📌
The Application part is organized as follows:
- Chapter II-1: conversion factors, constants, and data on fluids and materials;
- Chapter II-2: general requirements for fluid metering — installation;
- Chapter II-3: primary elements and equations for computing rates of flow;
- Chapter II-4: examples;
- Chapter II-5: tolerances.
| Primary Element | Type | Typical Line Sizes | Key Characteristics |
|---|---|---|---|
| Thin-plate square-edged orifice | Concentric orifice | 2–36 in. and beyond | Simple, low cost, larger pressure loss |
| Eccentric and segmental orifices | Eccentric/segmental opening | Dirty or wet-gas service | Allows solids and liquids to pass |
| Small precision-bore orifice meters | Small-bore precision orifices | Small flows, laboratory | High accuracy, precision machining |
| Flow nozzles | Converging nozzle | Medium and large lines | Lower loss than orifice, wear-resistant |
| Venturi tubes | Converging-throat-diverging | High flow, loss-sensitive duty | Minimum pressure loss, high accuracy |
| Nozzle-venturi | Combination | Special duty | Balances accuracy and loss |
| Sonic-flow primary elements | Critical-flow nozzles | High pressure drop, gas metering | Flow independent of downstream; ideal for calibration |
| Elbows | Differential across an elbow | Existing elbows | No added element; lower accuracy |
| Electromagnetic flowmeters | Faraday induction | Conducting liquids | No pressure loss, no moving parts |
⚠️ Although the document centers on differential-pressure elements, it also includes chapters on elbows and electromagnetic flowmeters. Select the primary element by flow range, allowable pressure loss, media properties (dirt, corrosion, conductivity), and accuracy — do not default to orifice plates out of habit.
2. Installation Requirements 📐
Chapter II-2 specifies installation requirements that govern measurement accuracy:
- Straight run: adequate straight pipe upstream and downstream of the element to remove swirl and asymmetric velocity profiles;
- Flow conditioning: when straight run is insufficient or strong disturbance sources exist (valves, elbows, pump discharge), install straighteners or flow conditioners;
- Pressure-tap arrangement: flange taps, corner taps, or D and D/2 taps each suit different situations — configure strictly to the selected standard;
- Element alignment: the orifice bore must be concentric with the pipe, the element face perpendicular to flow, and gaskets must not protrude into the flow path;
- Distance to disturbances: establish minimum straight-run requirements by valve type and opening and by the number of upstream elbows.
🚨 Poor installation is the most common source of flow-measurement accidents: insufficient straight run or a protruding gasket introduces a systematic differential bias that cannot be corrected. Before using any head-type meter, check upstream/downstream straight runs, tap type, and element installation item by item, and verify with calibration data where possible.
3. Flow Equations 📊
Chapter II-3 presents the flow equations for each primary element. The basic equation for differential-pressure meters is:
Q = K · A · √(2g·Δp/ρ)
where Q is the volumetric flow rate, K is the flow coefficient (determined by the element geometry and Reynolds number), A is the throat or orifice area, Δp the differential pressure, and ρ the fluid density. For gases, an expansion factor accounts for compressibility. Discharge coefficients, Reynolds-number correction curves, and gas expansion factors are given as charts; Chapter II-4 provides step-by-step examples and Chapter II-5 gives tolerances for the elements and measurement systems.
💡 The accuracy of head-type meters depends on the reliability of the flow coefficient: as long as the element geometry complies with the specification, installation meets the straight-run requirements, and differential and density measurements are accurate, the discharge coefficient can be taken directly from the published charts without wet calibration — a key reason these meters are the first choice for performance tests.
4. Engineering Practice Essentials 🛠️
- 🔍 Turndown and transmitter selection: differential signal varies with the square of flow; at low flow the differential is small and relative error grows — size the DP range sensibly and handle small signals carefully at low load;
- 🔧 Gas measurement corrections: gas flow requires simultaneous static-pressure and temperature measurement to correct density; for wet gas determine humidity — at high water-vapor content the correction is substantial;
- 📊 Impulse lines: avoid liquid accumulation in impulse lines (tap above the line for gases, below for liquids); use seal pots when needed and keep both legs at equal head;
- 📋 Element maintenance: sharp-edge wear, fouling, and erosion of nozzles change the flow coefficient — inspect critical elements periodically and re-verify geometric dimensions per the specification.
5. Frequently Asked Questions ❓
❓ Q: How do I choose among orifice plates, flow nozzles, and venturi tubes?
A: The trade-off is accuracy, pressure loss, and cost: orifice plates are cheapest with the highest loss; flow nozzles give moderate loss and are wear-resistant; venturi tubes have the lowest permanent loss but are large and costly. For long-term loss-sensitive large lines, prefer the venturi.
A: The trade-off is accuracy, pressure loss, and cost: orifice plates are cheapest with the highest loss; flow nozzles give moderate loss and are wear-resistant; venturi tubes have the lowest permanent loss but are large and costly. For long-term loss-sensitive large lines, prefer the venturi.
❓ Q: Why do head-type meters require long upstream and downstream straight runs?
A: The elements are calibrated for fully developed turbulent flow with an axisymmetric velocity profile; upstream swirl or asymmetry changes the static pressures at the taps, producing a bias that no flow-coefficient correction can remove. Install flow conditioners when straight run is inadequate.
A: The elements are calibrated for fully developed turbulent flow with an axisymmetric velocity profile; upstream swirl or asymmetry changes the static pressures at the taps, producing a bias that no flow-coefficient correction can remove. Install flow conditioners when straight run is inadequate.
❓ Q: Why must pressure and temperature be measured with gas flow?
A: The differential signal reflects a combination of volumetric flow and density; gas density varies strongly with pressure and temperature. Measure static pressure and temperature to compute the operating density and correct to standard conditions or mass flow — otherwise errors reach several to tens of percent.
A: The differential signal reflects a combination of volumetric flow and density; gas density varies strongly with pressure and temperature. Measure static pressure and temperature to compute the operating density and correct to standard conditions or mass flow — otherwise errors reach several to tens of percent.
🔍 Q: What is special about sonic (critical-flow) nozzles?
A: When upstream pressure is sufficient to choke the throat, the flow depends only on upstream stagnation pressure, temperature, and the nozzle area — independent of downstream changes. This gives exceptional repeatability and stability, making critical-flow nozzles the standard for gas-flow calibration and for precision flow measurement in compressor tests (PTC 10).
A: When upstream pressure is sufficient to choke the throat, the flow depends only on upstream stagnation pressure, temperature, and the nozzle area — independent of downstream changes. This gives exceptional repeatability and stability, making critical-flow nozzles the standard for gas-flow calibration and for precision flow measurement in compressor tests (PTC 10).
