Double Eccentric Butterfly Valve Flow Coefficient Testing: Standards, Methods, and Applications

Flow coefficient testing of double eccentric butterfly valves measures the true flow capacity (Kv or Cv) of the valve under controlled laboratory conditions, verifying manufacturer ratings and providing essential data for piping system design. This article explains the test standards, sample requirements, test items, procedures, and calculation methods used by accredited third-party laboratories for double eccentric butterfly valve flow coefficient evaluation.

Technical Overview

A double eccentric (double offset) butterfly valve is a quarter-turn valve in which the stem axis is offset both from the disc sealing plane and from the centerline of the pipe bore. This geometry produces a camming action during opening and closing, minimizing seat rubbing, reducing operating torque, and enabling tight bidirectional shutoff with extended seat life. Because the disc clears the seat almost immediately after opening, these valves offer relatively high flow capacity, which must be quantified precisely for correct system design.

The flow coefficient expresses the amount of fluid a valve can pass at a given travel position and pressure drop. Two units are commonly used: Kv, defined as the flow rate in cubic meters per hour of water (5 °C to 40 °C) passing through the valve with a pressure drop of 1 bar, and Cv, defined as the flow rate in US gallons per minute of water at 60 °F producing a pressure drop of 1 psi. Flow coefficient testing determines the rated Kv/Cv at full open, the flow coefficients at intermediate openings, and the inherent flow characteristic of the valve.

Testing is typically performed in accordance with IEC 60534-2-1 (sizing equations), IEC 60534-2-3 (test methods), and IEC 60534-2-4 (inherent flow characteristics and rangeability), together with the GB/T 17213 series, which is the corresponding Chinese national standard family. Valve design requirements are covered by standards such as API 609, EN 593, and GB/T 12238, while flow measurement methods reference ISO 5167 and ISO 4185.

Testing Samples

Wafer-Type Double Eccentric Butterfly Valves

Short face-to-face valves designed to be clamped between two pipeline flanges with through bolts. These are the most frequently submitted samples and cover the majority of general-purpose industrial and utility applications.

Lug-Type Double Eccentric Butterfly Valves

Valves with threaded or tapped lug bosses around the body periphery, allowing dead-end service on one side while permitting downstream piping removal. They are tested in the same manner as wafer-type samples.

Flanged Double Eccentric Butterfly Valves

Valves with integral raised-face or flat-face end flanges, commonly supplied in larger nominal sizes and higher pressure classes. Test sections must match the nominal diameter and flange drilling of the sample.

Sample Preparation and Quantity

• Samples should be production-representative units, complete with stem, disc, seat, and packing as delivered.
• Typical test scope covers DN40 through DN600 on standard laboratory benches; larger sizes may require special rig arrangements.
• Common pressure classes include PN10 to PN63 and ASME Class 150 or Class 300 equivalent ratings.
• Usually one to three samples per size and model family are submitted to establish representative flow data.
• The bore and disc surfaces must be clean, free of coating overspray, burrs, and shipping debris that could distort the results.

Testing Items

Flow coefficient test programs for double eccentric butterfly valves generally include the following items:

• Rated flow coefficient (Kv100 or Cv) at 100% disc opening.
• Flow coefficient at intermediate travel positions, typically tested in 10% travel increments or at defined opening angles such as 30°, 45°, 60°, and 75°.
• Inherent flow characteristic curve, plotting relative flow coefficient against relative travel.
• Deviation of the measured characteristic from the specified characteristic (linear, equal percentage, or quick opening where declared).
• Pressure loss coefficient (zeta factor) at specified flow velocities.
• Pressure recovery factor FL, where liquid service cavitation assessment is required.
• Rangeability, defined as the ratio of maximum to minimum controllable flow, when the valve is intended for throttling duty.

Testing Methods

Test Apparatus and Instrumentation

Testing is conducted on a closed-loop or open-loop hydraulic test bench using clean water as the test medium. The rig comprises a reservoir, a pump with variable-speed or throttling control to establish steady flow, a calibrated flow measuring section, and precision instrumentation. Flow rate may be measured with an electromagnetic flowmeter, a turbine meter, a differential-pressure device built per ISO 5167, or by the gravimetric method per ISO 4185. Differential pressure across the valve is measured with calibrated transducers, upstream pressure and water temperature are recorded, and disc position is set with an angle indicator or actuator with position feedback.

Test Section Configuration

• The test pipe internal diameter must match the valve nominal size, with concentric alignment to avoid artificial pressure losses.
• Pressure taps are located 2 pipe diameters upstream and 6 pipe diameters downstream of the valve end connections, in accordance with IEC 60534-2-3.
• Adequate straight pipe lengths are maintained upstream and downstream to ensure fully developed, swirl-free flow.
• The test differential pressure is selected to avoid choked flow and cavitation, keeping downstream pressure safely above the vapor pressure of water at the test temperature.

Test Procedure

• Install the sample in the test line and verify full-range disc movement and accurate position indication.
• Set the disc to the first test travel position, typically starting from fully open.
• Establish steady flow and allow readings to stabilize before recording.
• Record flow rate, upstream pressure, downstream pressure, and water temperature at a minimum of three distinct flow rates for each travel position.
• Repeat the sequence at each successive travel setting down to the minimum specified opening.
• Verify that the Reynolds number remains in the turbulent regime and that all measurements stay within the calibrated ranges of the instruments.

Calculation of Results

For water in the non-choked, non-cavitating turbulent regime, the flow coefficient is calculated from the measured values. In its general form the flow coefficient is expressed as Kv = Q × (ΔPn/ΔP)^(1/2) × (ρ/ρ0)^(1/2), where Q is the volumetric flow rate in m³/h, ΔP is the measured pressure drop in bar, ΔPn is the reference pressure drop of 1 bar, and ρ/ρ0 is the relative density of the test water. For practical water testing this simplifies to Kv = Q / (ΔP)^(1/2). The imperial coefficient is derived from the relation Cv = 1.156 × Kv, or equivalently Kv = 0.865 × Cv. The pressure loss coefficient is calculated from the measured pressure drop and the mean pipe velocity, and the inherent characteristic deviation is evaluated by comparing measured relative flow coefficients with the declared characteristic in accordance with IEC 60534-2-4.

Repeatability and Acceptance

At each travel position, the individual flow coefficients obtained from the three flow settings should agree within a tight repeatability band, typically a few percent, confirming stable test conditions. The measured rated flow coefficient is then compared with the manufacturer's published value, and the uncertainty of the determined coefficient is stated in the test report. Results are reported with full traceability to calibrated instruments in test reports issued by accredited third-party laboratories.

Applications

Flow coefficient test data for double eccentric butterfly valves directly support engineering decisions across many industries:

• Water supply, distribution, and pumping stations, for valve sizing and pump head calculations.
• HVAC chilled and hot water systems, for pressure balancing and energy efficiency optimization.
• Petrochemical and chemical process plants, for line sizing and hydraulic network analysis.
• Power generation circulating water and cooling water systems.
• Oil and gas facilities, tank farms, and metering stations.
• Fire protection, marine, and shipbuilding piping systems.
• Product development, CFD model validation, and tender compliance verification where certified flow capacity is a contractual requirement.

Accurate Kv and Cv data allow engineers to predict pressure losses, avoid oversizing that wastes pumping energy, and confirm that selected valves will deliver the required flow under installed conditions.

FAQ

What is the difference between Cv and Kv?

Cv is the imperial flow coefficient in US gallons per minute at a 1 psi pressure drop, while Kv is the metric coefficient in m³/h at a 1 bar pressure drop. They describe the same physical capacity and are converted using Cv = 1.156 × Kv.

Which standards govern the test?

The test methods are governed by IEC 60534-2-3, with calculations per IEC 60534-2-1 and characteristic evaluation per IEC 60534-2-4. The corresponding Chinese national standards are the GB/T 17213 series, and flow measurement references ISO 5167 and ISO 4185.

What test medium and conditions are used?

Clean water, generally between 5 °C and 40 °C, is used as the standard test medium. Flow is regulated to keep the flow regime fully turbulent and to prevent cavitation or choking during measurement.

Is the flow coefficient test destructive?

No. The test is non-destructive and does not alter the valve seating surfaces or mechanical function. It is distinct from shell and seat leakage tests, which are performed separately under standards such as ISO 5208 or GB/T 13927.

Do all disc positions need to be tested?

For verification of rated capacity, testing at 100% open is sufficient. When the valve is intended for throttling or control duty, testing at incremental openings across the travel range is performed to establish the complete inherent flow characteristic.

What does the final test report include?

The report documents valve identification, test setup and instrumentation calibration status, measured data at each travel point, calculated Kv and Cv values, the flow characteristic curve, deviation analysis, and the measurement uncertainty, all issued by accredited third-party laboratories with full traceability.