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Selecting Valves for Corrosive Media Like Strong Acids and Alkalis
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Selecting valves for corrosive media like strong acids and alkalis is a critical and highly specialized issue. Improper selection can lead to rapid valve failure and media leakage, resulting in safety incidents, environmental pollution, and production interruptions.
The following key points, steps, and common valve types for selecting valves for highly corrosive media provide a systematic selection guide.
1. Core Selection Principles
For corrosive media, valve selection must adhere to the following four core principles:
Corrosion Resistance First: The valve material must withstand long-term chemical and electrochemical corrosion from the media. This is the most basic and most important principle.
Sealing Reliability: The valve's sealing structure (including the seals of the seat, valve disc/ball, and valve stem) must effectively prevent leakage of hazardous media, both internally and externally.
Operational Safety and Convenience: The valve should be easy to open and close and adapt to operating conditions (such as temperature, pressure, and frequency). For high-risk media, remote operation or automatic control is often considered. Economical: While meeting the above requirements, select the valve with the best value for money, taking into account procurement cost, maintenance cost, and service life.
2. Selection Steps and Considerations
Step 1: Determine Process Parameters (Selection Basis)
This is the starting point for selection and must be as detailed as possible:
Media Properties: What is the specific acid/base? What is the concentration? (For example: 98% concentrated sulfuric acid, 37% hydrochloric acid, 50% liquid caustic soda). Note: Different concentrations can significantly affect corrosiveness.
Temperature: The media's operating temperature, maximum/minimum temperature.
Pressure: The media's operating pressure, differential pressure, and maximum shutoff differential pressure.
Flow Rate and Diameter: The pipe size (DN) and the required flow characteristics (regulating or switching).
Operation Frequency: Will the valve require frequent opening and closing, or be normally open/closed?
Step 2: Select Valve Body Material (Critical Decision)
The choice of material directly determines the valve's lifespan.
Metal Material:
Stainless Steel: 304/316L stainless steel is suitable for oxidizing acids such as dilute sulfuric acid and nitric acid, but is not resistant to hydrochloric acid and chloride ions. 316L has slightly better resistance to chloride pitting corrosion than 304.
Duplex steels, such as 2205 and 2507, offer excellent resistance to chloride stress corrosion cracking and are suitable for chloride-containing media, such as salt water and hydrochloric acid (medium and low concentrations).
Hastelloy C-276/C22: A leading corrosion-resistant alloy, resistant to most strong acids (including hot concentrated hydrochloric acid), strong oxidizing media, and wet chlorine gas, but extremely expensive.
Titanium (Ti) and titanium alloys: Offer excellent corrosion resistance to chloride ions, hypochlorite, wet chlorine gas, and nitric acid, but are not resistant to fuming sulfuric acid and hydrofluoric acid.
Nickel (Ni) and nickel alloys, such as Monel, offer excellent corrosion resistance to hydrofluoric acid and fluorine media.
Non-metallic/Lining Materials:
Polytetrafluoroethylene (PTFE): The "King of Plastics," resistant to nearly all strong acids, bases, and organic solvents (except molten alkali metals, fluorides, and chlorine trifluoride). It has a wide operating temperature range (-196°C to 200°C). It is commonly used as a lining or sealing material for ball and butterfly valves.
Perfluoroelastomer (FFKM): Such materials as Kalrez® and Chemraz® are the highest-grade elastic sealing materials, offering corrosion resistance comparable to PTFE while retaining the elasticity of rubber. They are used for demanding O-rings and valve seats.
Polyvinylidene fluoride (PVDF): Resistant to most acids, bases, and halogens, it offers superior mechanical strength to PTFE, but has a lower temperature limit (~90°C).
Reinforced polypropylene (RPP): Excellent acid and alkali resistance and low cost, but with poor temperature and pressure tolerances. It is often used for smaller diameters and mild operating conditions. Plastic/Rubber Lining: The valve body is lined with PTFE, PO, F46, etc., offering a cost-effective balance between the strength of the metal body and the corrosion resistance of non-metallic materials.
How to choose? Consult the "Corrosion Data Sheet" or consult the material supplier to determine the most suitable material based on the specific media, concentration, and temperature. Avoid relying on experience.
Step 3: Select Valve Type
Different valve types are suitable for different corrosive conditions:
Ball Valve
Advantages: Fast opening and closing, low flow resistance, excellent sealing (can achieve bidirectional zero leakage), and compact structure.
Applications: Widely used for switching corrosive media. Full-bore ball valves are particularly suitable for applications requiring pipeline cleaning or slurry media.
Selection Notes:
Fluorine-lined ball valve (F46 Lined Ball Valve): The valve body is lined with F46 (FEP), the ball is coated with F46 or PFA, and the stem seal is PTFE or FFKM. This is one of the most common and cost-effective options for handling strong acids and alkalis, and is suitable for most medium- and low-temperature (≤150°C) corrosive conditions. Metal-Seal Ball Valves: Made of special metals such as Hastelloy and titanium, they are suitable for demanding applications involving high temperatures, high pressures, or both corrosive and abrasive conditions.
Butterfly Valve
Advantages: Simple structure, compact size, light weight, low cost, and fast opening and closing.
Applications: Cutting and regulating corrosive media with high flow rates, low pressure differentials, and large diameters (DN>100).
Selection Notes:
PTFE-Lined Butterfly Valve: The valve body is lined with PTFE, and the disc is coated with PTFE or a special metal (such as 316L coated with NiP). Cost-effective compared to PTFE-lined ball valves, they are an economical choice for large-volume, large-diameter corrosive gas/liquid pipelines.
Duplex Steel Butterfly Valve: Suitable for corrosive applications requiring some metal strength.
Diaphragm Valve
Advantages: No stuffing box, absolutely zero leakage. Smooth flow path, not prone to clogging. The media only comes into contact with the valve body lining and diaphragm. Applications: Highly hazardous, highly toxic, and high-purity media (such as semiconductors and pharmaceuticals), slurries, and viscous fluids.
Selection Notes: The diaphragm is a key component, and the diaphragm material (e.g., PTFE-lined rubber diaphragm, all-PTFE diaphragm) must be selected based on the media. Pressure and temperature resistance are relatively low.
Plug Valve
Advantages: Simple structure, fast opening and closing, unobstructed flow path, and excellent sealing.
Applications: Suitable for corrosive media containing solid particles or slurries, such as electroplating solutions and wastewater.
Selection Notes: Fluorine-lined (F46) or all-plastic (e.g., PVDF) construction is commonly used.
Bellows Seal Globe Valve
Advantages: Utilizing a metal bellows structure, it completely eliminates leak points at the valve stem (zero external leakage) and offers high reliability.
Applications: Corrosive media that are highly toxic, expensive, radioactive, or highly volatile (e.g., phosgene and hydrofluoric acid). Selection Considerations: High cost and high flow resistance make them typically only used in critical applications with small orifices where absolute leakage must be guaranteed.
Step 4: Select Sealing Materials
The materials of the stem seal (packing) and seat seal must be compatible with the media.
PTFE: The most commonly used material, offering excellent corrosion resistance, but cold flow and thermal expansion properties require consideration.
Reinforced graphite: High-temperature and corrosion-resistant, suitable for stem packing in high-temperature applications.
FFKM: For the most demanding elastic seals, but expensive.
PTFE composite packing: PTFE packing containing graphite and stainless steel wire offers superior performance.
Step 5: Consider the Operation Method
Manual: For infrequent operation and non-hazardous applications.
Pneumatic/Electric: For applications requiring frequent on/off switching, remote control, or integration into automated systems. For corrosive media, actuator selection should also consider environmental corrosion (e.g., choosing a cylinder or motor with an anti-corrosion coating).