Many processes with a pressure lower than 16 bar and a temperature lower than 120 °C are however equipped with expensive metal valves whereas the plastic valves would be more indicated.
The replacement of coated valves
It is common to find valves coated with PTFE or elastomer. These types of valves have the advantage of resisting high pressures and temperatures, because the body of the valve is in metal, while resisting the chemical attack and internal corrosion thanks to its internal coating.
1st problem
Coated valves are two to three times more expensive than thermoplastic valves
2nd problem
Coated valves do not resist external corrosion
Abrasion by salt water
3rd problem
PTFE coated valves suffer 100 times more than PVDF thermoplastic valves.
4th problem
The PTFE is coating is not flexible, due to forced demand to create tightness, it cracks
5th problem
Elastomer coatings erode
6th problem
When the PTFE or elastomer coating is damaged, the corrosive fluid attacks the metal and quickly pierces de valve
The solution
Very often, SAFI valves replace coated valves because in the majority of applications, the temperature is lower than 120°C and the pressure 16 bar.
On the picture, the white SAFI valve in PVDF replaced a PTFE coated valve on an installation where chloroparaffin emanations chemically attacked the outside of the coated valves.
Replacing valves made of alloys resistant to corrosion
Many installations, under the excuse that corrosive fluids are used in their piping, use valves in exotic metals and alloys such as stainless steel, Uranus B6, super duplex, etc.….if the pressure and temperature are high, these choices may be wise, however very often we find installations with low pressures and ambient temperatures using such valves, this represents a number of problems :
- 1st problem : Alloy valves are always more expensive than thermoplastic valves.
- 2nd problem : many alloy valves suffer from abrasion.
For example, stainless steel valves are made of an alloy of iron and nickel. This alloy allows a thin protection layer to create in contact with the chemical.
Valves having movable elements (like a ball in a ball valve) create friction between the elements. Moreover, if the fluid has solid particles, the abrasion increases, which erodes de protective film of the stainless steel and hollows the material.
By force of repetition, on the same area, here is what eventually happens.
The thermoplastic valves resist better than stainless steel to dynamic corrosion, that is, when corrosion combines with moving dynamic elements, like the ball of the valve. That is why there are installations with stainless steel pipes equipped with thermoplastic valves, because the pipe undergoes non-dynamic corrosion (no movable elements) while valves undergo dynamic corrosion.
Therefore the SAFI valves regularly replace exotic alloy valves.