一 、 Main types
1, Straight through sight glass
Structure: Similar to a short pipe, with flanges at both ends for connecting pipes, and observation glass embedded in the middle.
2, Glass tube sight glass (flow sight glass)
Structure: The main body is a transparent glass tube, fixed at both ends with carbon steel flanges or threaded joints.
Purpose: Not only can it observe the fluid state, but it can also roughly estimate the flow rate or liquid level through internal floats (such as rotor flow meters) or graduated measuring tubes.
3,Floating ball type sight glass (magnetic flap level gauge sight glass)
Structure: Usually used in conjunction with a liquid level gauge, it uses a magnetic float to drive the flip of an external indicator, thereby clearly displaying the internal liquid level externally.
Purpose: Mainly used for liquid level indication of storage tanks, boilers, reaction vessels and other equipment, making observation more intuitive and visible from a longer distance.
4, Equipped with scraper/cleaning mirror
Structure: A scraper or cleaning nozzle that can be manually or automatically operated is installed outside or inside the mirror.
Purpose: Used in situations where the medium is prone to scaling, crystallization, or dirt on the surface of glass, it can clean the glass without disassembly and maintain a clear view.
二 . Key Performance Parameters and Selection Points
When choosing carbon steel mirrors, the following factors must be considered to ensure safety and service life:
1，Nominal pressure (PN): must be higher than the maximum working pressure of the pipeline or equipment. Common types include PN1.0, 1.6, 2.5, 4.0MPa, etc.
2，Working temperature: depends on the material of the mirror glass and sealing gasket. For example, ordinary tempered glass has a temperature resistance of about ≤ 200 ℃, while borosilicate glass can reach ≤ 400 ℃.
3，Media compatibility (core points):
Carbon steel section: only applicable to non corrosive media such as water, steam, air, oil, etc. If the medium is corrosive (such as acidic or alkaline chemicals), stainless steel sight glasses (such as 304, 316L) should be selected.
Glass/window section: It needs to withstand medium corrosion and not react with the medium. Hydrofluoric acid, hot concentrated alkaline solution, etc. can corrode glass. In this case, special materials (such as quartz glass, fluoroplastic lined mirrors) need to be selected.
4. Sealing performance: The sealing gasket material (such as nitrile rubber, polytetrafluoroethylene, graphite) must be both temperature resistant, pressure resistant, and compatible with the medium.
5. Connection method: The most common is flange connection (HG/T, GB, JB, ANSI standards, etc.), but there are also threaded connections and butt welded connections.
6. Glass type:
Tempered glass: High strength, safe, and granular when broken.
Borosilicate glass: Good resistance to high temperature and thermal impact.
Quartz glass: extremely high temperature resistance, good corrosion resistance, and expensive price.
三、 Advantages and disadvantages
Advantages:
Low cost: Compared to stainless steel, carbon steel materials are cheaper.
High strength: Strong pressure bearing capacity, suitable for medium and high pressure working conditions.
Widely applicable: It is an economical and practical choice in non corrosive environments.
Disadvantages:
Corrosion resistance: This is the biggest limitation that limits its application in fields such as chemical engineering.
Maintenance required: In humid environments, the surface of carbon steel may rust and requires surface treatment (such as painting) or regular maintenance.
Heavy weight: heavier than stainless steel or plastic mirrors of the same specifications.
