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Hedland Variable Area Flow Meters

Flow Meter Selection Guide A refresher on PPS

Given the popularity of PPS as a primary material of construction in Oval Gear Meters, we thought it would be useful to offer a quick refresher on PPS, otherwise known as polyphenylene sulfide. PPS is a polymer made up of alternating sulfur atoms and phenylene rings in a para substitution pattern. The highly stable chemical bonds of its molecular structure impart a remarkable degree of molecular stability toward both thermal degradation and chemical reactivity.

Or in plain English... When blended with glass fibers and other fillers, PPS is a versatile material which is recognized for its dimensional stability, high resistance to heat and it's chemical compatibility to Acids, Alkalies and other aggressive fluids.

PPS has been used as a manufacturing material in flowmeters for around 30 years, particularly by manufacturers of positive displacement flowmeters. In addition to the excellent mechanical properties, PPS can be molded into complex parts with very tight tolerances. This makes a flow meter with PPS components an accurate, but more importantly, a cost effective product to purchase and maintain.

Chemical Compatibility

PPS (Polyphenylene-Sulfide / Ryton) is a glass fiber reinforced thermoplastic polymer with a maximum service temperature of approximately 392°F. PPS is resistant to chemical attack from all non-aromatic (containing benzene), non-halogenated (containing halogens such as chlorine, fluorine, bromine) organic solvents at any concentration, even at elevated temperatures. PPS is suitable for service with some aromatic, halogenated, or amine based solvents at temperatures near ambient, however lifetime will be reduced, and suitability with these types of solvents should be confirmed prior to use.

PPS is also resistant to most water-based solutions of acids, bases, or neutral salts, with the exception of strong oxidizing acids such as nitric acid, hydrofluoric acid, and hydrochloric acid. Due to the susceptibility of the filler materials in reinforced PPS materials, it is not suitable for use with strong acids of any kind (pH<2) at elevated temperatures. Strong bases do not degrade PPS, however use of strong bases at elevated temperatures may cause a long term reduction in the mechanical strength of the rotors due to an acceleration of the hot-water effect described below.

PPS is generally not suitable for use with oxidizing chemicals in high concentrations, or at elevated temperatures, however service with these chemicals under mild conditions may be suitable depending on the particular application. For example; Hydrogen Peroxide, chlorine and Sodium Hypochlorite will cause swelling and plasticizing of PPS with extensive exposure above 5% concentration, or lower concentrations at elevated temperatures. However, PPS can withstand mild solutions of these chemicals used in common cleaning or disinfectant products at temperatures close to ambient.

One unusually dangerous environment for glass reinforced PPS is superheated water; this may seem surprising considering the good chemical resistance of PPS, however this effect is not due to polymer degradation, but to separation of the polymer from the glass reinforcement at its interface. The effect of hot water causes material swelling and a significant reduction in the strength of the polymer structure. Applications for hot water below 176°F are acceptable, even heated sea-water and water containing very low concentrations of chlorine are acceptable, however the reduction in meter life should be considered for applications involving water above 176°F. For all applications involving heated water the meter shouldn't be operated above 80% of maximum capacity due to the poor lubricating properties of hot water; if running above 80% of maximum capacity is unavoidable in a heated water application a reduction in service life should be expected.

Flow Specialist - Bill Michie Written By: Bill Michie  
Flow Applications Specialist
Cross Company
Instrumentation Group
Phone: (866) 905-9790 (M-F, 8am-5pm Eastern)
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Special thanks to the Technical Team at GPI Austrialia for gathering, publishing and giving us permission to use the valuable information and/or images provided in this post.
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