Why PP Diaphragm Pressure Gauges are Restricted to Low and Medium Pressure Systems

In the Petrochemical and water treatment sectors, instrument selection is not merely about accuracy; it is fundamentally about system Safety and Reliability. A Diaphragm Pressure Gauge utilizes an isolation membrane to protect the internal movement from corrosive media. While PP (Polypropylene) is a highly cost-effective non-metallic material for acid and alkali handling, it is strictly categorized for specific pressure tiers.

Engineering standards typically limit PP diaphragm gauges to low or medium pressure systems, generally below 1.6 MPa (16 bar). This restriction is dictated by the molecular structure of the polymer, its physical mechanical properties, and its specific failure modes under industrial stress.

Physical Limitations of Mechanical Strength

PP is a semi-crystalline thermoplastic. Compared to metallic materials like Stainless Steel or Hastelloy, its Tensile Strength and Yield Stress are significantly lower. In a High Pressure environment, the diaphragm must withstand immense force per unit area.

When system pressure exceeds the material's load-bearing limit, the PP membrane undergoes significant elastic deformation and eventually enters Plastic Deformation. This permanent stretching leads to severe Zero Drift, rendering the gauge readings inaccurate. Furthermore, under prolonged high-pressure exposure, the polymer chains suffer from fatigue, increasing the risk of a catastrophic rupture that would allow hazardous media to leak into the instrument Case.

Creep Effect and Long-term Dimensional Stability

In Petrochemical processes, pressure is often accompanied by thermal fluctuations. PP exhibits a pronounced Creep effect, where the material slowly deforms over time under a constant stress load.

In high-pressure systems, this creep is magnified. Even if the initial installation pressure is within rated limits, the continuous load causes the PP diaphragm to thin or the sealing joints to relax. Unlike PTFE or metal diaphragms, which maintain better dimensional stability, PP reliability curves drop sharply when the pressure crosses the medium-pressure threshold. This makes it unsuitable for critical monitoring where long-term calibration stability is required.

Temperature-Pressure Rating and Negative Feedback

The mechanical performance of PP is extremely sensitive to Temperature. As the operating temperature rises from ambient toward its upper limit (approximately 60°C to 80°C), the Elastic Modulus of the material decreases rapidly.

This means a PP Diaphragm Pressure Gauge rated for medium pressure at room temperature might see its Maximum Working Pressure (MWP) reduced by 50% or more when processing 60°C chemical media. This Temperature-Pressure De-rating phenomenon leaves very little margin for error during process upsets or over-temperature events. To prevent instrument bursting, industry standards mandate keeping PP within low-to-medium pressure envelopes.

Sealing Integrity and Joint Strength Constraints

The overall construction of a PP pressure gauge—including the connector, upper flange, and lower flange—is often also made of PP or reinforced plastics. Under high pressure, these threaded or bolted connections are subjected to massive mechanical stress.

Because the Rigidity of plastic is inferior to metal, high-pressure cycles can cause microscopic shifts at the connection points. This leads to Fill Fluid leakage or the ingress of external media. Maintaining a Leaktight Connection requires keeping the physical tension on these plastic components within moderate limits, which is why low-to-medium pressure operation is a primary Safety requirement.

Selection Strategies for High-Demand Scenarios

For Petrochemical operators, the drive to use PP stems from its excellent chemical resistance and low cost. However, when pressures exceed 1.0 MPa or involve Pulsating Pressure, alternative strategies are recommended:

Upgrade Material: Consider PVDF, which offers higher mechanical strength and better temperature resistance. Lined Technology: Utilize a metal diaphragm (such as 316L) with a PTFE coating or lining to combine high-pressure capacity with chemical inertness. Safety Mapping: Strictly follow the manufacturer’s temperature-pressure curves to ensure the operating point remains within the safe envelope of the material.