Abnormal Operating Conditions in Oxygen Control Valves

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Analysis and Countermeasures for Abnormal Operating Conditions in Oxygen Control Valves

High-Pressure Oxygen Valves in Blast Furnaces: Flow Surges at Small Openings and Reverse Fluctuations in Opening-Flow Characteristics

In the oxygen-enriched injection systems of blast furnaces, the precise control of oxygen flow valves is critical to furnace stability and energy safety. This article addresses two typical user-reported issues, excessive flow at small valve openings (2–5%) and a counterintuitive flow decrease at 20–30% openings. By examining working conditions such as a high pressure drop (from 16 MPa to 3 MPa), a DN150 valve body with a DN125 trim, and high-inlet/low-outlet flow direction, we analyze the failure mechanisms and propose targeted solutions from the perspectives of fluid dynamics, valve characteristics, and process compatibility.

The Cv (flow coefficient) represents a valve's flow capacity at full opening. It's governed by the formula:

Cv = Q / (ΔP / ρ)

where Q is the flow rate, ΔP is pressure differential, and ρ is fluid density.

In this case, oxygen density varies significantly with pressure (about 180 kg/m³ at 16 MPa vs. 35 kg/m³ at 3 MPa).

If Cv is sized based on inlet conditions, actual flow capacity becomes severely oversized:

Design scenario:

At 16 MPa inlet and 3000 m³/h flow, Cv ≈ 150

Actual scenario:

At 3 MPa outlet, gas expansion per Boyle's Law increases flow to 8500 m³/h.

The required flow at 5% opening is 850 m³/h, but the existing valve delivers over 3000 m³/h at that opening.

Valve flow characteristics (equal percentage or linear) often enter dead zones or overshoot regions below 10% opening.

With a DN150 body and DN125 trim, the reduced port design theoretically lowers Cv, but under high differential pressure, fluid velocities exceed 30 m/s, well above the 25 m/s oxygen safety limit, leading to large flows at minor openings.

Cv Based on Outlet Conditions:

Recalculate Cv using outlet pressure (3 MPa) and target flow (e.g., 3000 m³/h), yielding Cv ≈ 80. This corresponds to a DN100 trim (Cv ∝ diameter²), aligning 5% opening flow to 150 m³/h.

Dedicated Low-Opening Trim:

Use a multi-stage cage-type trim with progressive throttling holes. For example, a single row of small holes for 2% opening and a double row for 5%. This ensures tight flow control and reduces velocity below 20 m/s, mitigating erosion risk.

Under a 13 MPa pressure drop, fluid force pushes the plug downward.

At 20–30% openings, the plug is partially open, causing flow velocities above 50 m/s through narrow valve gaps.

This generates Kármán vortex streets, leading to plug vibrations (20–50 Hz) and unstable flow signals.

Test Comparison:

Reversing the flow direction (low-inlet/high-outlet) reduces plug vibrations by 60%, significantly stabilizing flow.

When ΔP exceeds the critical pressure drop:

ΔPcr = P1 / (1 + √(ρ2 / ρ1))

(where P1 is inlet pressure, and ρ1/ρ2 are fluid densities), localized pressure inside the valve may fall below oxygen's saturation pressure (0.1 MPa at –183°C).

Although oxygen doesn't undergo phase change, high-speed turbulence mimics cavitation, causing density fluctuations that mislead flow sensors (recorded errors of –15% to –20%).

Physical Evidence:

Valve seat surfaces show honeycomb-like pitting (0.1–0.3 mm deep), a classic sign of erosion due to turbulent micro-jets.

In many blast furnace setups, the oxygen line includes an orifice plate.

As valve opening increases from 20% to 30%, internal turbulence may sharply raise local resistance (ΔP jumps from 10 MPa to 12 MPa).

With total system pressure fixed (16 MPa → 3 MPa), less pressure remains to overcome pipe resistance.

This results in "reverse regulation", an increase in opening causes a decrease in flow.

Change Flow Direction: High-Inlet → Low-Inlet

Reversing the direction reduces plug vibrations from 0.5 mm to under 0.2 mm, and narrows flow fluctuation from ±20% to ±5% (as per data from a steel plant).

Two-Stage Pressure Reduction with Flow Stabilization:

Stage 1 (16 MPa → 8 MPa): Use cage-type trim with gradually expanding ports (e.g., φ10mm to φ20mm), keeping flow speed < 30 m/s.

Stage 2 (8 MPa → 3 MPa): Integrate a labyrinth flow path at the seat to convert turbulence to laminar flow, and limit ΔP to ≤5 MPa per stage to suppress cavitation triggers.

Sensor Optimization:

Move flow sensor from 1D (pipe diameter) downstream of the valve to beyond 5D to avoid signal noise from turbulent zones. Accuracy improves from ±3% to ±1%.

Comparison Before and After Oxygen Valve Retrofit in a Steel Plant:

Cv Calculation: Start with the End in Mind

Always calculate Cv based on outlet conditions (pressure, density) to avoid flow misjudgment due to pressure expansion.

Flow Direction Based on Pressure Drop:

ΔP ≤ 5 MPa: High-inlet/low-outlet (better sealing)

ΔP > 5 MPa: Low-inlet/high-outlet (better flow stability)

Velocity Control Red Line:

For oxygen media, the velocity must stay below 25 m/s to prevent static buildup and pipe erosion. Use trim downsizing or multi-stage throttling to reduce speed.

The root of abnormal oxygen valve behavior lies in the mismatch between high-pressure fluid properties and valve control characteristics. Excessive flow at small openings stems from Cv oversizing, while reverse flow fluctuations result from deteriorated flow regimes, force imbalance, and system impedance feedback.

Solving such problems requires moving beyond isolated component fixes to system-wide analysis, from fluid properties to valve mechanics to system dynamics. In the critical and highly sensitive context of blast furnace oxygen control, even a 1% deviation in valve opening can cascade into large furnace fluctuations.

We invite technical professionals in high-pressure gas control to contribute insights on "oxygen valve cavitation suppression" and "precise low-opening control" to jointly advance universal, practical solutions through real-world data and collaborative engineering.