Material Selection for Magnetic Level Indicator in Hydrofluoric Acid Level Measurement

Generally, materials for chamber and display panel of a magnetic level indicator are selected according to the properties of the measured medium.

Common materials used for chamber are stainless steel 304, stainless steel 316L, stainless steel 304/316L lined with F4 or F46 fluorine tube and plastic PP. Among them, stainless steel 304 and 316L are more popular. If the measured medium could have chemical reaction with the compositions in stainless steel to cause corrosion, fluorine lined stainless steel or plastic PP should be selected as a proper material for the chamber. If the measured medium is at normal temperature and under normal pressure, plastic PP will be a suitable choice. If the measured medium is at abnormal temperature and under abnormal pressure, fluorine lined 304/316 will be a better choice because it can withstand process temperature up to 120℃, process pressure up to 1.6MPa.

Generally, display panel of a magnetic level indicator is made of aluminum alloy or plastic panel, and plastic panel is used in the working environment that is corrosive to aluminum alloy.

No matter if it is aluminum alloy display panel or plastic display panel, its transparent front cover can be either ordinary glass or acrylic:

Patent - Transparent Panel Connect Structure of Jiwei Magnetic Level Indicator

Ordinary glass: Ordinary glass is an inorganic non-metallic material which is made from a variety of inorganic minerals. The main composition is silicon dioxide and other oxides. Because of good transparency for easy observation and its stable chemical properties (no chemical reaction with most of liquids), front cover on display panel is often made of ordinary glass.

Acrylic: Acrylic is also called plexiglass, or polymethyl methacrylate. Acrylic is a polymer material with good plasticity. It is difficult to make and manufacturing cost is high. However, because of its high transmittance (above 92%), stable chemical properties and good durability, it is often used for applications with complicated working conditions or just to meet customers´ special requirements.

As we can see from the table above, the corrosion resistance of acrylic and PTFE is better overall. They are ideal materials for measuring highly corrosive liquids.

Hydrofluoric acid is an aqueous solution of hydrogen fluoride gas. It is clear, colorless, fuming, volatile and quite corrosive. It has a strong pungent smell. When the concentration of hydrofluoric acid reaches more than 5mol/L, it will be auto-ionized.

According to the properties of hydrofluoric acid, plastic PP is a good choice for the magnetic liquid indicator used in level measurement of hydrofluoric acid. If the hydrofluoric acid is under good ventilation and certain process pressure, fluorine lined 316L is a better choice.

Although aluminum alloy display panel with glass front cover can be used for most of liquids, glass cannot be used as the front cover if the instrument is used for hydrofluoric acid. Hydrofluoric acid is more corrosive to some metals, glass and material containing silicon, because hydrofluoric acid dissolves glass (main composition: silica) to generate gaseous silicon tetrafluoride. The reaction equation is as follows:

SiO ₂+4HF=SiF ₄+2H ₂O

SiF ₄+2HF=H ₂ [SiF6]

Therefore, hydrofluoric acid should be stored in plastic (for example, PTFE), waxy or lead containers.

Hydrofluoric acid is often used in chemical reduction process of rare metals in industrial production, for example, in production of potassium fluorotantalate, niobium oxide, and tantalum oxide.

In level measurement of hydrofluoric acid, a material with better corrosion resistance should be selected for the display panel of the magnetic level indicator, especially for the front cover of display panel. Otherwise the front cover will be blurry with the time and liquid level reading will be affected. It may even cause safety accident if the situation is severe. Therefore, material for front cover of the magnetic level indicator for measuring the level of hydrofluoric acid must be acrylic sheets. In applications for level measurement of hydrofluoric acid, glass front cover is often corroded and blurry because of improper selection of materials, as shown in pictures onsite below:

 

Fig.1 Corroded blurry front cover onsite

Manufacturers should fully communicate with users to select proper magnetic level indicator made of the most suitable material for real application working conditions.

Jiwei Automations has always paid great attention on this. Our staff communicates with each user patiently to realize the properties of the measured medium and understand working conditions and help the customer select the best suitable magnetic level indicator. Furthermore, we have overcome several technical problems in real applications of magnetic level indicator and hold multiple invention and utility patents on this product. Jiwei Flap-11 magnetic level indicator is designed with wider display panel for easy observation onsite. Its front cover can be made of glass plate or acrylic sheet which is selectable for measuring level of different media. Unlike some manufacturers who use 2 mm acrylic sheets in order to reduce cost, Jiwei Flap-11 is made of 3 mm acrylic sheets (if selected) to ensure quality of the product. In addition, the magnetic coupling system in our magnetic level indicator is scientifically designed so the magnetic flap can work reliable to avoid wrong flipping. The ingress protection rating of the instrument reaches to IP66 /IP 67. The high reliability of Jiwie magnetic level indicator has been widely praised by our users.