【2025 Flow Meters】Negative pressure resistance of various lining materials for electromagnetic flowmeters

When a negative pressure (vacuum) is created inside the pipe, the atmospheric pressure outside the lining is much higher than the internal pressure; this pressure difference attempts to \"peel\" the lining away from the pipe wall or cause it to \"collapse\". If the strength of the lining material is insufficient or the bonding is weak, the lining may wrinkle, deform, tear, or even come off, resulting in permanent damage to the flow meter. The following is an analysis of the resistance to negative pressure for various common lining materials, ranked from best to worst: 1. Ceramic linings (such as alumina ceramics) • Resistance to negative pressure: Excellent • Note: Ceramic linings are essentially very rigid and extremely hard materials. It is not attached to the wall of the tube; rather, it is itself a part of the measuring tube. Therefore, it is completely unaffected by negative pressure, and will not deform or peel off. • Applicable scenarios: Highly abrasive slurries, corrosive media. It is the preferred choice for applications requiring resistance to negative pressure and wear, but it has the highest cost.

2. Polytetrafluoroethylene lining: PTFE linings need to be categorized separately, as their manufacturing processes differ and their resistance to negative pressures varies greatly. a. Polytetrafluoroethylene (PTFE) – Molding and sintering type / Integral winding and sintering type. Resistance to negative pressure: Excellent → Good. Note: PTFE linings produced using this process are relatively thick and bond tightly to the metal tube; they have a \"backing\" structure for support (such as an outer metal mesh or a special design), which gives them a certain degree of resistance to negative pressure. High-quality PTFE linings can withstand a certain degree of negative pressure, but there is still a risk of delamination if the negative pressure is too high or if there are sudden temperature changes. Key point: The ability of PTFE linings with skeletal support to withstand negative pressure is significantly enhanced. Applicable scenarios: highly corrosive media such as concentrated acids, alkalis, and strong organic solvents. When selecting, it is necessary to clearly ask the manufacturer about its negative pressure resistance specifications.

b. Polytetrafluoroethylene (PTFE) – adhesive film type. Resistance to negative pressure: poor. Note: This type involves applying a thin PTFE film to the tube wall using an adhesive. Since PTFE is non-stick by nature and has limited bonding strength, it can easily be “sucked” off at the joint or across the entire surface under negative pressure. It should be strictly avoided in any situation where negative pressure may be generated.

3. Perfluoroether rubber lining • Resistance to negative pressure: Good • Description: Perfluoroether rubber combines the elasticity of rubber with chemical resistance similar to that of PTFE. As an elastomer, it deforms under negative pressure, but its excellent elasticity and strong adhesion to metals enable it to return to its original shape after deformation, making it less likely to suffer permanent damage. Its resistance to negative pressure is generally better than that of PTFE, but inferior to that of PFA and ceramics. • Applicable scenarios: Applications that require both excellent corrosion resistance and a certain degree of flexibility, such as corrosive media containing particles.

4. Perfluoroethylene propylene coating • Resistance to negative pressure: Good → Medium • Note: FEP is a modified version of PTFE; it has better processability and a stronger adhesion to metals than PTFE. Therefore, its resistance to negative pressure is generally better than that of ordinary adhesive PTFE. However, risks still exist at higher negative pressures and temperatures. • Applicable scenarios: Corrosive media, operating temperatures below PTFE.

5. Polyurethane rubber lining • Resistance to negative pressure: Medium • Note: Polyurethane rubber is highly wear-resistant and possesses good toughness. Due to its elasticity and high mechanical strength, it can withstand a certain degree of negative pressure; however, under negative pressure over an extended period, even thicker linings may experience slight deformation. • Applicable scenarios: Neutral abrasive slurries such as water, sewage, and pulp.

6. Neoprene, nitrile rubber, EPDM lining • Resistance to negative pressure: Medium → Poor • Note: These general-purpose rubber linings are usually soft and have good elasticity. Under slight and brief negative pressure, there may be no major problems, but due to its soft material, it is prone to permanent wrinkles, bulges, or separation from the tube wall under continuous or higher negative pressure. Among them, rubber varieties with higher hardness exhibit slightly better performance under negative pressure. • Applicable scenarios: water, sewage, dilute acids, dilute alkalis, and other media with low wear and low corrosion properties. It should be avoided as much as possible in negative pressure conditions.

Key influencing factors and selection. 1. The effect of temperature is crucial; the strength and adhesion strength of almost all lining materials decrease as the temperature rises. o The negative pressure resistance value provided by the manufacturer is usually measured at a temperature of 20°C. Its tolerance decreases significantly at higher operating temperatures. For example, a PFA lining that can withstand a negative pressure of -0.09 MPa at room temperature may have a negative pressure resistance of only -0.04 MPa or less at 120°C.

2. Process and structure: There is a huge difference depending on whether a “skeleton” is used or not. o PTFE/PFA lining: This is the biggest variable. Be sure to ask clearly whether it is a regular lining or a lining with a metal mesh framework. The latter bonds tightly to the tube wall through a metal mesh, providing rigid support for the flexible fluoroplastic and increasing its resistance to negative pressure by several orders of magnitude.