Advanced technologies to improve your process

Controlling the flow of corrosive chemicals




Any type of  flow control valve has a restricted passage area called « Vena Contracta ». In this area, the flow is accelerated to a high speed. Turbulence is created, dissipating energy and therefore creating the desired ΔP. Where the flow velocity is high, corrosive chemicals cause all sorts of damage to the materials.

Why non-metallic materials?

The corrosion resistance of metals is generally determined in still fluids. But at high flow speeds and in the presence of turbulence, a process known as « erosion induced corrosion » starts destroying even the so-called « corrosion resistant » metals. Erosion Induced Corrosion is a particular form of accelerated corrosion linked to fluid motion, high flow velocity and turgulence. The think film of naturally formed passivation layer

In such situation, solid polymers may offer an economic solution, that will outperform even « special metals » and other exotic materials. Unlike metals, properly selected polymers resist the chemical « in the bulk », and not by virtue of a thin autopassivation layer.

Which non-metallic materials?

Polymers are not equal when it comes to resisting high flow velocities, turbulence and erosion, as well as chemical attack.  The table below shows the abrasion resistance of several polymers, as determined by the « TABER »  ASTM D4060 test. The lower the figure, the better is the resistance.

Apart from polyamid, which cannot be used because of its low chemical resistance, the best abrasion resistant material is PVDF.  This material should be preferred for all parts of the valve making up the « Vena contracta ». When PVDF cannot be used for chemical reason, such as in concentrated hydroxides, then PPH and ECTFE are acceptable alternatives.

It is readily seen that PTFE, which has the lowest abrasion resistance of all polymers, is the worst choice for contact with the flow in the Vena Contracta. However, because of its almost universal chemical resistance, PTFE is often found as a liner in control valves, either globe valves or diaphragm valves. Such valves suffer a fast deterioration of the flow throttling parts, such as discs, seats, diaphragms etc…


Material

Weight loss (mg/1000 rev.)
ASTM D4060, wheel CS-10

PVDF homopolymer (SOLEF®)

5 - 10

PVDF, SOLEF® 21508/31508

5 - 8

PA 6 polyamid

5

ECTFE, Halar®

13

PP homopolymer

15 - 20

PTFE

500 - 1000

304 stainless steel

50

Source : SOLVAY S.A.

What type of control valves ?

The purpose of a control valve is to create a pressure drop, according to the requirements of the process. For the control valve, he Delta-P is a goal, not a consequence. Depending on the type of valve (globe, diaphragm, butterfly, V-port ball….) the pressure drop in the Vena Contracta will be more or less pronounced. Globe valves have a shallower pressure dip in the Vena Contracta than diaphragm valves, butterfly valves and full port ball valves. But with proper design, V-port ball valves have a pressure dip as shallow as that of a globe valve. The pressure dip in the “vena contracta” is characterized by the “Pressure Recovery Coefficient” FL of the valve. The value of FL  varies between 0 and 1.  A value close to 1 means a smaller pressure drop.

V-port ball valves

V-port ball valves are not to be confused for V-ball or C-ball valves. V-port ball valves have fixed mask in front of the ball orifice that acts as a «variable area orifice plate». V-port ball valves designed by TECHLINK exhibit a value of FL similar to that of globe valves, and a flow charateristic very close to a perfect theoretical Equal-% curve.  

V-port ball valves, (unlike standard ball valves) perform very well in abrasive and corrosive service, because of the high FL value, giving a moderate flow velocity in the Vena Contracta.


Designed and built according to the process

TECHLINK V-port flow control ball valves are designed and built according to the process data. The ball and the V-port mask are usually made of PVDF to provide the maximum resistance to erosion. The V-port is shaped by CNC machining to deliver the required flow characteristic curve. All new patterns are tested on a loop to confirm the flow curve.



Materials

A variety of high-technology, corrosion resistant engineering  materials are  available for the construction of TECHLINK non-metallic regulation valves. The choice must take into account not only the chemical resistance of the materials, but also erosion problems that may occur in the flow restriction area (vena contracta).


Solid polymers

When this is allowed, entirely non-metallic valves are preferred. TECHLINK’s flow control valves are then built around the integral flanged solid polymer valves from SAFI, France, worldwide reputed for their long lasting strength, stability and precision manufacturing.

The result is a valve that outlasts any metallic or lined metal valve for the control of corrosive chemicals in harsh environments, where external corrosion is just as much of a problem as the chemical itself.



PFA lined stainless steel


Flow characteristics of standard profiles (Cv)


Typical fluids



Applications

Chlor-Alkali plant

Brine from saturator to treatment tank

Brine flow to brine filter

Brine flow to electrolyzer

Weak brine tank level control

Weak brine tank acid injection valve

Dechlorinator brine tank level control

 Dechlorinator caustic and sodium bisulphite dozing valve

Catholyte flow to electrolyzer

Catholyte tank level control

DM water flow control to electrolyzer

Chlorine pressure control to hypo

Electrolyzer differential pressure control ( hydrogen pressure )

Primary hypo ORP control

Secondary hypo tank level control

Weak chlorine blower bypass pressure control valve

HCl unit chlorine flow control

HCl unit weak chlorine flow control

HCl unit DM water flow control


Water  treatment plant

Hydrochloric acid, caustic soda

Sulphuric acid

Poly-aluminium chloride

Ferric chloride

Copper, Aluminium or Ferric sulphate

Benzalkonium chloride

Chlorinated water

Sodium Hypochlorite

Ozone, Hydrogen peroxyde

Chlorine dioxide



Paper pulp mill

Chlorate cell hydrogen pressure control

Chlorate flow to ClO2 generator

HCl FLOW to ClO2 Generator

Air flow to ClO2 Generator

Weak chlorine recycle flow to ClO2 Generator

Chilled water flow to ClO2 Absorber

ClO2 pump tank level control valve

Weak chlorate evaporator level control

Weak chlorate evaporator pressure control

Chlorate flow temp control valve to ClO2 Generator

The velocity in the Vena Contracta varies in the opposite way to the pressure dip. Globe and V-port ball valves therefore have a lower maximum velocity in the Vena Contracta. Diaphragm valves and butterfly valves are the worst.

Features include :

 Lightweight,  best choice for FRP piping.

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