[2025 Practical Guide] Key Selection Criteria for Double-Flange Level Gauges in Special Operating Conditions Such as Negative Pressure, Corrosion, Crystallization, and High Viscosity
Thread Content
This post was last edited by zhoudingshengs on 2025-6-15 at 15:29. Flange level gauges (differential pressure level gauges) calculate the liquid level by measuring the differential pressure between the diaphragms at both ends of the container. When selecting such gauges, it is necessary to take into account specific operating conditions (such as negative pressure, crystallization, viscosity, corrosion, etc.), and decide whether special treatments are required for key components such as the diaphragm material, the filling fluid in the capillaries, and the diaphragm structure.
I. Negative pressure conditions: In an environment with high negative pressure, the pressure inside the container is lower than atmospheric pressure, which can cause the liquid filling the capillaries to vaporize (especially under high-temperature conditions), lead to deformation of the diaphragm or failure of the seals, affect measurement accuracy, and even damage the instrument.
Key selection criteria: Diaphragm material: Prioritize materials with resistance to negative pressure and strong deformation resistance, such as 316L stainless steel (suitable for normal negative pressures) and tantalum (for conditions involving both corrosion and negative pressure). Capillary filling fluid: Avoid easily vaporizable silicone oils (such as ordinary DC200 silicone oil), and use filling fluids with low vapor pressure instead (such as fluorinated oils, suitable for negative-pressure environments from -50 to 200°C). Structural design: Choose reinforced diaphragms or those with greater rigidity, such as tantalum diaphragms, to reduce the risk of diaphragm deformation. Verify the negative pressure range: Ensure that the transmitter’s range covers negative pressure values (e.g., -0.1~1.6 MPa) to avoid damage due to operating outside the specified range. Alternative (deep vacuum environment): Use radar/ultrasonic level gauges: completely contactless and unaffected by pressure.
Selection advice: In high-negative-pressure conditions, if a double-flange level gauge must be used, it is necessary to clarify the following parameters with the manufacturer: 1. Minimum absolute pressure； 2. Medium temperature (affects the choice of filling fluid) ； 3. Process connection material (high sealing performance required for deep vacuum) ； 4. Is explosion-proof certification required (in the chemical/pharmaceutical industry)?
Double-flange level gauges can be used under medium to low negative pressure conditions thanks to optimized design, but they present significant risks in deep vacuum environments; it is recommended to prefer pressure-independent level measuring instruments.
II. Crystallization conditions: When the medium crystallizes on the surface of the flange diaphragm (such as in salt or sugar solutions), it can block the gaps in the diaphragm, cover it, or cause corrosion, thereby leading to inaccurate measurements.
Key selection criteria: Diaphragm material: Choose a material that is resistant to crystalline corrosion and easy to clean, such as 316L stainless steel (resistant to corrosion in most crystalline media) or Hastelloy C276 (for conditions involving both strong corrosion and crystallization). If the crystallization temperature is high, Monel alloy can be considered (resistant to high-temperature chloride crystallization). Optimization of diaphragm and flange design: 1. Prefer flat diaphragms to prevent crystal accumulation (avoiding crystal buildup in the grooves) ； 2. Diaphragm surface treatment: polishing or spraying a Teflon coating (resistant to high temperatures, corrosion, and material adhesion) to reduce crystal attachment. 3. For high-viscosity crystals, an insert-type diaphragm design is used, allowing the diaphragm to extend deep into the equipment and avoid the crystal zone. Auxiliary measures: Install a purging device at the flange (such as nitrogen purging) to regularly remove crystals from the surface of the diaphragm ； Or choose a flange design with a flushing interface for easy online cleaning. Change the installation method: As indicated by the red circle in the figure, this prevents crystal accumulation and allows the crystals to be removed by their own gravity.
Selection recommendations (key requirements): 1. Flat diaphragm with no dead corners + PTFE coating (Teflon)； 2. The material has been upgraded to Hastelloy/tantalum due to its crystalline corrosion resistance ； 3. Mandatory heat tracing to maintain temperature > freezing point ； 4. Install the flushing ring and dual-valve assembly (for easy maintenance) ； 5. Go deep into the container, avoiding the crystallization zone ； 6. Change the conventional installation method. 7. Be sure to provide the specific crystal composition, concentration, and temperature profile to the manufacturer for customized verification!
III. Viscous operating conditions: High-viscosity media (such as asphalt, heavy oil, and polymer solutions) may adhere to the diaphragm surface, resulting in delayed diaphragm response or measurement errors.
Key selection criteria: Diaphragm material: Choose a smooth material with low adhesion, such as 316L stainless steel (which has low adhesion to most viscous media) or diaphragms coated with PTFE (the PTFE coating significantly reduces adhesion to viscous media; however, note that the maximum temperature tolerance of PTFE is around 260°C). Diaphragm structure: Prefer convex diaphragms (with a raised surface design, allowing viscous media to slide down more easily due to gravity, thus reducing adhesion) ； Avoid flat diaphragms (viscous media tend to accumulate).
Installation: If necessary, install a heating device (such as electric heating or a steam jacket) to reduce the viscosity of the medium.