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D5390-21 – Standard Test Method Technical Guide
🧐 Scope and Application of Palmer-Bowlus Flumes
ASTM D5390−21, formally designated the Standard Test Method for Open-Channel Flow Measurement of Water with Palmer-Bowlus Flumes, provides a detailed procedure for measuring the volumetric flow rate of water and wastewater in sewers and open channels. Under the jurisdiction of Committee D19 on Water, the standard ensures consistent application by requiring that all measurements be recorded in inch-pound units. The Palmer-Bowlus flume is specifically designed for installation in existing circular conduits and channels to establish a definitive head-discharge relationship.
📏 Key Terminology and Hydraulic Principles
A precise understanding of the standard’s terminology is essential for accurate flow measurement. The method operates on the principle of critical flow, where the specific energy in the channel is minimized for a given discharge. The standard defines critical flow as occurring when the Froude number equals unity. The head, measured upstream relative to the throat floor, is the primary variable used to calculate flow rate via the rating equation. The boundary layer displacement thickness describes viscous effects in the throat that influence the calibration equations.
| 🟦 Term | 🎯 Symbol | 📐 Definition |
|---|---|---|
| Critical Flow | N/A | Channel flow where specific energy is minimum; Froude number equals 1. |
| Froude Number | Fr | Dimensionless ratio of inertial to gravity forces (V / √(g × d)). |
| Head | h | Depth of flow referenced to the flume throat floor, measured at an upstream location. |
| Boundary Layer Displacement Thickness | δ* | Normal distance accounting for velocity deficit from viscous friction in throat. |
⚠️ Critical Flow Constraint: The Palmer-Bowlus flume requires critical flow to maintain a unique stage-discharge relationship. If the tailwater submerges the flume throat beyond the allowable limit, the rating ceases to apply. The Froude number must be determined to confirm operational validity per the standard.
⚙️ Referenced Documents and Operational Standards
D5390−21 does not exist in isolation. It is cross-referenced with several essential standards for water testing. General terminology is defined in D1129. The calculation of precision and bias must follow the guidelines of D2777. Alternative flow measurement methods can be found in D1941 (Parshall Flume), D3858 (Velocity-Area), and D5242 (Thin-Plate Weirs). The standard also aligns with international practices outlined in ISO 4359 and ISO 555.
| 🟦 Designation | 📌 Application / Description |
|---|---|
| ASTM D1129 | Standard Terminology Relating to Water |
| ASTM D2777 | Practice for Determination of Precision and Bias |
| ASTM D3858 | Open-Channel Flow Measurement by Velocity-Area Method |
| ISO 4359 | Liquid Flow Measurement in Open Channels (Flumes) |
| ASME Fluid Meters | Hydraulic theory and application for fluid meters |
💡 Upstream Condition Best Practice: The accuracy of the head measurement depends on the approach flow. A straight, unobstructed channel upstream of the flume is required for the minimum distance specified in the standard to ensure a stable water surface and accurate flow rate calculation.
❓ Frequently Asked Questions
🔍 What is the primary purpose of ASTM D5390−21?
It specifies a test method for measuring the volumetric flow rate of water and wastewater in open channels and sewers using Palmer-Bowlus flumes.
⚡ Why is the Froude number significant in this test method?
The Froude number indicates whether critical flow has been established. When Fr equals 1, the specific energy is minimized, providing a unique, mathematically stable relationship between the measured head and the flow rate, which is fundamental to the method’s accuracy.
📐 What measurement units are considered standard by this method?
Values expressed in inch-pound units are regarded as the standard for this test method. SI unit conversions are provided in parentheses but are classified as informational and non-standard.
💡 How is the precision of this test method evaluated?
The precision and bias of D5390−21 are determined in strict accordance with ASTM Practice D2777, which provides the necessary statistical framework for analyzing interlaboratory test data.