The Use of Chemicals for Water Treatment Systems
The use of chemicals in water treatment systems affects the type of pipes, valves, injection pumps, and all parts of the water treatment plant.
Construction Materials
The construction materials for chemical storage tanks, feed pumps, piping, and other chemical system components influence the performance of the water treatment plant. Also influences equipment lifespan, personnel safety, water quality, and the environment. This is especially true for construction materials used in chemical systems operating at normal pressures and temperatures in water treatment plants.
In addition to chemical resistance, material selection should consider cost, pressure, temperature, velocity, mechanical strength, safety, internal versus external installation, buried versus exposed installation, soil corrosion, moisture, and gases and vapors that may come into contact with chemical system components. Life cycle costs are essential in material selection, taking into account manufacturing and installation costs and the expected replacement frequency. Often, many alternative materials are suitable for a particular application, and the material with the highest chemical resistance and longest lifespan is not necessarily the best choice. For example, one material may last 30 years, while another may have a lower life cycle cost even if it is replaced every 10 years. Owner preferences are equally important, and designers are cautioned against selecting materials based solely on economics.
Temperature Impact on Decision-Making
Temperature is an important factor in choosing plastic and metal materials. The pressure rating, and to a lesser extent, chemical resistance of plastic materials decrease as the temperature rises, particularly when temperatures exceed 100°F (38°C). Designers should consult manufacturers regarding pressure and temperature ratings for specific plastics. The chemical resistance and pressure rating of metals also decrease as temperature increases, but the effect on pressure ratings is negligible at normal operating temperatures in most water treatment plants.
Practicality is another important consideration. For example, polytetrafluoroethylene (PTFE) is resistant to almost all chemicals, but using PTFE gaskets for plastic pipe flanges is impractical due to the high torque required to achieve a good seal, which can easily cause the flanges to crack.
Moreover, the importance of each material selection factor depends on the application, and designers must consider all possible operating conditions. For instance, users may choose to dilute a chemical in the future, and if the chemical generates heat during dilution (e.g., sodium hydroxide), the chemical storage tank materials must be able to withstand periodic high temperatures.
Chemical Storage Tanks
Recommendations are made for materials used in chemical storage tanks, including liquid and day tanks. Plastic tanks generally have better overall chemical resistance and lower initial costs compared to metal tanks. Disadvantages of plastic tanks include lower durability, mechanical strength, and difficulty in repairing on-site. Plastic tanks require protection from sunlight if installed outdoors. Metal tanks, when suitable for the chemicals, provide greater durability and a longer lifespan.
Steel tanks handle pressurized applications and higher temperatures better than plastic tanks and are easier to repair on-site. Steel tanks are often chosen for their thickness and resistance to some expected corrosion (e.g., storing sulfuric acid in bare steel tanks). Additionally, steel tanks are typically used to accommodate vibrations applied to liquefy many dry chemicals. Steel tanks can be coated or lined with rubber, PTFE, or other organic materials.
Chemical Pipes and Valves
Valve body materials are usually the same as those of the connected piping, but there are exceptions. Plastic materials generally offer better overall chemical resistance and lower cost compared to metals. The disadvantages of plastic materials include lower mechanical strength, limited temperature range, expansion/contraction with temperature changes, and the possibility of cracking or failure due to shocks. Many plastic pipe and valve materials, such as polyvinyl chloride (PVC) and chlorinated polyvinyl chloride (CPVC), are prone to damage from hydraulic surges.
Some manufacturers publish formulas for calculating maximum allowable velocities, typically around 5 feet (1.5 meters) per second. PVC and CPVC pipes are not suitable for compressed gas service, as the pipes will shatter upon failure. For solvent-welded joints, solvent compatibility with the chemical is essential for reliable service. Plastic pipes also require protection from sunlight if installed outdoors. Metal pipe materials, when suitable for the chemical, are more durable and generally have a longer lifespan than plastic. Steel pipes and valves can handle pressurized applications and higher temperatures much better than plastic. But metal pipes and valves typically have higher installation costs than plastic ones.
PTFE-Lined Steel
Specialty piping materials, such as PTFE-lined steel, offer the chemical resistance of plastic and the mechanical strength of steel. Lined pipes are usually used only in high-pressure applications and with highly corrosive chemicals due to their relatively high cost. Carpenter 20Cb3 is the industry standard for sulfuric acid, particularly for valves, and is suitable for all concentrations up to 100°F (38°C). Titanium is suitable for many chemicals but is typically used only in high-pressure applications for chemicals incompatible with other metals due to its relatively high cost. Various materials are available for rubber hoses used to transport chemicals due to their corrosion resistance. Flexible pipes are often used as carrier pipes inside containment pipes in buried applications. However, verifying chemical compatibility with manufacturers is necessary, as hoses and flexible pipes may not have the same chemical resistance as rigid pipes made from the same material.
Gaskets and Seals
Gaskets and seals are required in nearly all chemical piping systems. Selecting appropriate gasket and seal materials for chemical piping systems at expected operating pressures and temperatures is crucial. Since gasket and seal failure is the primary cause of chemical leaks, selecting gasket and seal materials requires consideration of both chemical resistance and sealing ability at the maximum expected pressure and temperature. Soft materials like ethylene propylene diene monomer (EPDM) typically provide the best seal but are not necessarily the most chemically resistant. In contrast, relatively hard materials like PTFE are the most chemically resistant but do not provide the best seal. The best option is often a compromise between chemical resistance and sealing ability. Another complexity is that different grades of some gasket and seal materials vary significantly in chemical resistance. Additionally, the same material often has different trade names, which can cause confusion during design, construction, and maintenance.
Gasket and seal material selection for valves and equipment is often limited by the offerings of valve and equipment manufacturers, and this limitation can affect project specifications. Therefore, designers should consult with valve and equipment manufacturers regarding available and most suitable materials. Alternative and specialized gasket and seal materials are usually available at additional cost.
Feeders and Chemical Pumps for Water Treatment Systems
Gas feeders, dry feeders, metering pumps, and transfer pumps used in water treatment plants include components that come into contact with chemicals. These components consist of various metallic, plastic, and elastomeric materials. As with pipes, valves, gaskets, and seals, the proposed pipe pressures and temperatures influence material selection.
At CareWater, we offer a comprehensive range of water treatment plant solutions tailored to the source water type, chemicals used, and temperature, while considering all factors mentioned in this article. You can read more about the benefits and uses of chemical injection pumps.
