Sodium Hypochlorite Storage Tank

Materials of Construction

There are many different materials of construction used as sodium hypochlorite storage tanks to eliminate damage to structural tanks and eliminate contamination of products. Four main types of materials used are linear high-density polyethylene (HDLPE), cross-linked polyethylene (XLPE) and fiberglass-reinforced plastic (FRP), chlorobutyl rubber-lined steel, and titanium. The choice of materials depends on available capital, tank location, and required service life. Some tanks may only last 3-5 years, others if properly specified and maintained could last 20-30 years. The only material known for over 30 years of service life is titanium.

Polyethylene

polyethylene tanks are used to store sodium hypochlorite. Historically, many tanks were made of linear high-density (HDLPE) or cross-linked polyethylene (XLPE). In the late 1990s, many crosslinked tanks failed prematurely. The major source of failure is at fittings on the sides of the tanks. One of the major problems with polyethylene tanks have to do with the outlet fittings on the bottom of the tanks (i.e., below the liquid level). This fitting allows attachment to the lowest point on the tank without metals or other materials contacting the sodium hypochlorite and the exact same material that the tank is manufactured from. Flanged fittings with titanium bolting should be used if the tank does not have an integrally molded-in, full drain fitting for larger tanks. These tanks typically have a lifespan of 4-7 years if exposed to direct sunlight, but may extend to 6-9 years with frequent painting. A tank may last up to 6-9 years indoors, but it should only be placed indoors if it can be replaced in the event of failure. These tanks should not be used in a construction application that allows for no easy replacement of the tank upon failure.

Fiberglass Reinforced Plastic

The use of fiberglass tanks to store sodium hypochlorite is common, and corrosion barrier inspections are usually performed every two years, with minor repairs as needed. Improper design and construction will result in corrosion barrier failure and structural damage within 2-3 years, requiring complete tank replacement. Typical specifications for FRP tanks would include hand-laid or "cross-wound" construction. Vinyl is used for the corrosion barrier and a structural layer of the tank, the interior of the tank corrosion barrier starts with 2 connecting veils. The final corrosion barrier is catalyzed by the BPO/DMA cure system and 4 hours post-cure. Double-layer FRP tanks using PVC and other materials as corrosion barriers. Detecting lining failure should be considered before possible damage to the outer FRP container.

Rubber Lined Steel

Rubber-lined steel tanks for sodium hypochlorite storage use a chlorinated butyl lining, typically 1/4 inch thick. These liners require a skilled applicator and heat curing. The service life is typically 3-6 years, at which point a complete lining replacement may be required. If the failure of the liner is not detected in time, the steel tank will be chemically attacked by sodium hypochlorite, resulting in iron contamination of the product.

Titanium

Titanium storage tanks are the best material choice for sodium hypochlorite. The grade commonly used is Commercial Pure Grade 2. Although the cost of titanium storage tanks is high, due to their long service life, post-maintenance costs are low. is the ultimate solution for sodium hypochlorite storage tanks

If the wrong materials of construction are used in any part of the processing system, product contamination will occur, resulting in accelerated decomposition of sodium hypochlorite and additional oxygen formation. All metals except titanium, tantalum, silver, gold, and platinum should be avoided. Metals such as stainless steel, Hastelloy, Monel, brass, or copper should be avoided at all costs. Hastelloy-C has been used for springs in some parts, such as ball check valves, and typically springs last about 12 months before needing replacement. These incompatible metals can be found in pumps, pump seals and water flush lines, electrodes in magnetic flow tubes, diaphragm seals for gauges and switches, temperature wells, and common piping elements such as hose connections, support clamps, and valves.

For metals in contact with sodium hypochlorite, the majority of construction for all process equipment is titanium. Tantalum is used for electrodes in magnetic flow meters and diaphragm seals. Silver and platinum are used for electrodes used to measure oxidation-reduction potential. There should be no other metal in contact with sodium hypochlorite.

Case: Stonecutter Island Sewage Treatment Works upgrading

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