Choosing the Correct Valve Material(PVDF : concern to high corrosive, high permeating, high temperature, steam and abrasive media)

PVDF needle valveDegassing valve Quick damp valve PVDFfor corrosive and ultra pure liquid

Process engineers are faced with the decision of choosing the correct valve materialfor corrosive applications. Decision criteria are sometimes based on cost, availability,reliability, durability, and performance history.Fluoropolymers are often times the preferred choice for valves in corrosive servicesdue to their relative cost position compared to high alloy valves and due to theiravailability. Many times the proof test to determine if a fluoropolymer lined valve willwork or not is if the process temperature if below 400°F (204°C). If the temperatureremains below 400°F it is assumed that the fluoropolymer lined valve will work asexpected. One pitfall of this approach is that fluoropolymer lined valves can bespecified in certain applications resulting in unexpected performance due to otherprocess conditions such as permeation, steam, and abrasive media.Polyvinylidenefluoride, or PVDF is a partially fluorinated fluoropolymer. Meaning thatnot all of the carbon atoms are surrounded by fluorine atoms resulting in a lowerchemical resistance then the fully fluorinated fluoropolymers. Hydrogen atoms havereplaced some of the fluorine atoms, which have a weaker bond and are moresuspect to chemical attack. Ketones, esters and organic amines can attack PVDF.While these PVDF may not offer the superior chemical resistance of the fullyfluorinated polymers they do, PVDF offer significant improvements in strength,impact toughness and wear resistance.

The benefit of PVDF to valve users is in its high mechanical strength and itspermeation resistance. PVDF has good cold flow resistance and high impactstrength resulting in great abrasion resistance. PVDF can be melt processed andtherefore is found more commonly as a valve liner. PVDF is also used to linelarge parts such as vessels due to its ability to resist stress cracking whencooling.

Abrasive and Corrosive Media

Solids or particulate have a negative effect on fluoropolymer lined valves.Especially, when a valve element such as a ball, plug or disk are being turnedinto the body/seat. The presence of abrasive material can drastically effect thelife of a fluoropolymer lined valve causing them to leak past the seatprematurely. To counter this effect one can specify a ceramic ball or metal plugto eliminate the wear point of this plastic component.However, the turning motion of the element is still present and will continueto have a negative effect on the life of the PTFE seat. Other alternatives wouldbe to use a stronger material such as PVDF. Or, one could use a valve that doesnot have a turning element such as a diaphragm valve.

Study :Tribological Behaviour of Polymeric Materials in Water Lubricated Contacts**author : A.Golchin, G.F.Simmons, S. Glavatskih, B. PrakashThe present study aims at investigating the tribological behaviour of several unfilled polymer materialssliding against 316L stainless steel in distilled water. The tests were carried out in a uni‐directional pin on discconfiguration with an initial apparent contact pressure of 5 Mpa at room temperature. The test were carriedout at constant sliding speed (0,13 m/s) considering the lowest practical rotational speed of the tribometer.The surface roughness of stainless steel discs were Ra=0,2 µm. The worn surfaces were examined usingscanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) techniques.

Result : in this study UHMWPE provided the lowest wear rate with PET and POM. Thewear resistance of PVDF and PA almost similar. PTFE showing highest wear ratesimilar to PC (polycarbonate).

Permeating mediaFluoropolymer lined valves provide for excellent chemical resistance to aggressivemedia. However, one of the downfalls of these materials is that certain chemicalscan permeate the fluoropolymer and reach the base metal components causingvalve failure. Media to be concerned with are those that contain chlorine,fluorine, or bromine. These could be hydrochloric acid, hydrofluoric acid orhydrobromic acid. Factors effective permeation are concentration of the media,temperature of the media, time of exposure and liner thickness.

Along with molecular size, molecular polarity plays a key role in permeationthrough polymer barriers. PVDF is somewhat polar (~ 1.0–1.7) and will thus bequite resistant to benzene, Cl2, Br2, O2, N2, H2, especially with compared withnon‐polar fluoropolymers (PTFE, PFA, FEP) and non‐polar polyolefins (PE, PB, PP).High crystallinity gives PVDF reasonably good resistance to even very polarchemicals like methanol and sulfuric acid.

source : www.Arkema‐Inc.com

SteamLastly, steam cleaning of process piping is a common procedure in some industries.Steam can also have a negative effect on the life of fluoropolymer lined valves. Thefailure mode for fluoropolymer lined valves that have failed from steam cleaning isdeformation of the fluoropolymer or delamination from the base metal. Steam is asmall molecule and can permeate the surface of the fluoropolymer similar tochlorine, fluorine, or bromine. When a valve is cooled the steam can then condensein the liner. When the valve is steamed again the condensed liquid will vaporizecausing the liner to deform or delaminate from the base metal.

PVDF Valve Seat High PressureHigh TemperaturePVDF (W) is perfect intermediate highpressure seat, with the performancebetween the Delrin (Y) (POM Polyoxymethylen) (relatively low operatingtemp. at maximum pressure +60°C)and the PEEK (K)/ virgin PEEK (L)(which withstands higher temperaturebut exerts very high operating torque).

Graphic : performance plastics comparationwith pressure and temperaturesource : www.habonim.com

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