In a whitepaper, PPS Outshines Stainless Steel in Chemically Hostile Valve Applications, Emerson’s Steve Attri describes how this PPS material is highly resistant to corrosion in highly-corrosive applications.
He opens defining and describing PPS:
Polyphenylene Sulfide (PPS), is a high performance, fiber reinforced, advanced composite, linear thermoplastic polymer.
For over a decade, PPS has replaced conventional materials, such as stainless steel, for industrial applications that require superior corrosion resistance, non-sticking and high temperature performance.
PPS offers high strength-to-weight ratio, ease of machining, and cost savings through molded production. Some of the most recognized applications for PPS include blower and pump housings, impellers, flow meters, sensors, conveyor components, downhole oil production parts, and use as a protective coating.
Chemically-hostile environments cause deterioration of metals due to chemical and electrochemical reactions. Steve notes that PPS has higher resistance to corrosion than stainless steel, increasing operational life and sealing characteristics.
When used in valves, this enables them to:
…operate reliably and maintain tight sealing characteristics that meet or exceed the highest industry standards of 1 scfh @ 90% set pressure per valve.
As shown in the molecule depiction above, benzene rings are connected to sulfur atoms in polymer chains. These carbon-sulfur bonds between repeating molecules in the polymer are:
…extremely stable because the sulfur is a small compound; therefore, it is extremely difficult for other compounds to squeeze in between the benzene rings to replace the sulfur.
This arrangement in PPS provides excellent chemical resistance, even at elevated temperatures. By contrast, stainless steel:
…is more susceptible to corrosion because it deals with corrosion on a macroscopic level (porous granular steel alloy solution). PPS deals with corrosion on a microscopic level (tight repeating molecular crystalline structure).
Steve describes various types of stainless steel corrosion including intergranular, transgranular, stress cracking, imbrittlement, pitting and localized corrosion.
One excellent application for the use of PPS is for pressure vacuum relief valves that may be exposed to:
…many types of corrosive chemicals, such as weak and strong acids, oxidizing acids, alcohols, aldehydes, ketones, amines, bases, chlorinated organics, esters, ethers, hydrocarbons, inorganics, nitro compounds, phenols, salt spray, organic solvents, oxygenated fuels, auto-oxidizing fuels and other aggressive fuels.
Other materials may pit and crack when exposed to these chemicals causing valve seal degradation. This leads to excessive leakage, hindrance of relieving capacity and possible mechanical failure.
Enardo pressure vacuum vent valves for corrosive environments such as those containing H2S:
…provide protection against excess positive or vacuum pressure; prevent air intake, evaporative or blanketing product losses; and help contain odorous, hazardous and potentially explosive vapors.
…for the seat ring and pallet, which are the critical operational components of the safety valve. The pallet and seat are responsible for maintaining a vapor tight seal during normal tank storage or other process system operations. When tank breathing or process system interruptions cause positive or negative pressure buildup, the pallet and seat function together to relieve the pressure.
Beyond corrosive environments, Steve highlights the advantages of PPS in sticky and severe-cold processes, again due to the stable molecular structure.
Read the whitepaper for other physical properties such as a -50 to 428degF (-45.6 to 220degC) continuous usage temperature range and more.
Material selection for the operational components of the valve is crucial to maintain performance and reliability. Many applications subject the valve to stored or processed vapors, which can be chemically hostile, corrosive and sticky, as well as to harsh environmental conditions like extreme cold and heat. These conditions are the prime causes of deterioration, pitting, sticking and freezing, which cause leakage and possible valve failure. When used for valve seat ring and trim, PPS material provides greater resistance to these destructive factors compared to conventional metal materials such as aluminum, ductile iron, carbon steel and stainless steel.