Flow control valves for corrosive and abrasive chemicals
Control valves are often short lived in corrosive chemicals, in particular in the presence of solid particles.
This is an area of chemical engineering where applying traditional knowledge is simply not sufficient.
SAFI Control valves are designed to offer the best resistance to corrosive and erosive chemicals and a low pressure recovery at a very attractive price.
The purpose of a control valve is to create a pressure drop following the requirements of the process. SAFI V-orifice flow control ball valves are designed and built according to the process data.
The V-orifice is shaped by CNC machining to deliver the required flow characteristic curve. All new patterns are tested on our dynamic test bench to confirm the flow curve.
The ball and the V-orifice plate are usually made of PVDF to provide the maximum resistance to erosion.
TYPICAL FLUIDS :
- Chlor-Alkali Plants
- Water treatment plants
- Paper pulp mill
- Battery materials plan
- Wet chlorine
- Hydrochloric acid
- Caustic soda
- Sodium carbonate
- Sodium hypochlorite
- Ferric chloride
- Zinc sulfate
- Copper chloride
- Sodium chlorate
- Sulfuric acid
- Chlorine dioxide
- Sodium sulfite
- Lime water
- Aqua regia
- Copper sulfate
- And more
The "Vena contracta"
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 pressure drop (ΔP).
Where the flow velocity is high, corrosive chemicals cause all sorts of damage to the materials.
Contact our technical team to understand better the Vena Contracta phenomenon, its consequences and our solutions.
PVDF: the best material against abrasion
The corrosion resistance of metals is generally determined in still fluids. However, 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.
In such situation, solid polymers offer an economic solution that will outperform even “special metals” and other exotic materials.
Polymers are not equal when it comes to resisting high flow velocities, turbulence and erosion, as well as chemical attack. The table here shows the abrasion resistance of several polymers, as determined by the “TABER” ASTM D4060 test (wheel CS-10). Materials which resist best to erosion have the lowest figure on this table. Note that PVDF has a better resistance than stainless steel.
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.
V-Orifice seats and Needle valves
Depending on the type of valve (globe, diaphragm, butterfly, V-port ball, etc.) the pressure drop in the Vena Contracta will be more or less important.
A big pressure drop induces high flow velocity. High flow velocity in the Vena Contracta is what causes damage in corrosive and abrasive media.
Globe valves have a shallower pressure drop in the Vena Contracta than diaphragm , butterfly and full port ball valves.
But with proper design, V-orifice ball valves have a pressure drop as shallow as a globe valve. The pressure drop 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.
The velocity in the Vena Contracta varies in the opposite way to the pressure drop. Globe and V-Orifice ball valves therefore have a lower maximum velocity in the Vena Contracta. Diaphragm and butterfly valves are the worst.
Thanks to their unique design, our V-orifice ball and needle V-notch valves have a FL similar to Globe valves.
The characteristic curve must be adapted to the process. There are various “depth” of Equal% depending on the process data.
Our valves offer adapted orifice and needle profiles that are designed according to customer-specified process conditions for the best results.
If you are not sure which design you need, do not hesitate to contact us.
Positioner and actuator
Non-metallic components are preferred for corrosive environments.
SAFI valves are equipped with corrosion resistant accessories. Our engineers have experienced the best results with the positioners Samson with polyphthalamide casing, and with the actuators Max-Air with polyarylamide cylinder body.
All our designs can be adjusted to customer's specifications.
Example of application
Plant: Aditya Birla Chemicals, Thailand.
Service: Sodium Hypochlorite (NaOCl)
In service since 2013.
SAFI ball valve (2029 serie) in PVDF with FPM (VITON) seals, Samson Positioner and Max-Air Actuator.
You may observe that the support structure for the previous heavy metallic valve is no-longer necessary for this light-weight valve!