A lantern ring is a perforated hollow ring located near the center of the stuffing box of a pump. Despite its simplistic appearance, it plays a crucial role in successfully operating a pump system.
The Lantern Ring’s Role
Lantern rings simultaneously serve three primary purposes:
- Protection — The term lantern ring comes from the primary use of the word lantern. A lantern is a vessel or device covering or protecting a light source. Lantern rings help protect the stuffing box by allowing flush water or pressure to enter. They also keep contaminants out. If the material being pumped or any other foreign matter enters the stuffing box, it could destroy the packing or damage the shaft or stuffing box.
- Outlet for cooling — Lantern is also an architectural design term for a structure on top of a roof or in an attic that allows for ventilation. These structures allow buildings to breathe. Lantern rings provide an opening and assist in cooling the stuffing box. This keeps the system from being completely closed off.
- Lubrication — Another primary function of the lantern ring is to facilitate lubrication. The fluid entering the lantern ring lubricates the packing. This reduces friction against the shaft. In addition, lubrication helps reduce the wear of the machine parts and the amount of excess heat generated within the system.
Only Used with Packing
In these three applications, lantern rings are only used with packing. It is a crucial element because packing tends to generate heat. Heat increases wear which could shorten the life of the packing and the shaft. Placing a lantern ring between packing rings extends the life of the packing, the stuffing box, and the shaft. Ultimately, this increases the lifespan of the whole system long term.
End users often need to pay more attention to lantern rings within the system because they do not perform the sealing function. In a stuffing box, teams focus on the packing and which style to use. However, the lantern ring is just as important as the packing. The packing will not function correctly without the lantern ring to provide protection, ventilation, and lubrication.
The Ideal Material: PTFE vs PEEK for Lantern Rings
Using lantern rings made from the right kind of quality material is critical. Low-quality plastic material options will soften and melt quickly and easily over time. If this happens, the ability to protect, cool, and properly lubricate the packing is greatly diminished. This will lead to premature packing failure.
Manufacturers make most lantern rings from polytetrafluoroethylene (PTFE) or polyether ether ketone (PEEK). Before PTFE was widely used, lantern rings may have been made from many different materials, such as brass, bronze, low-quality plastics, or metal. Some plants use these other materials in some instances.
Choosing the suitable material includes several considerations:
- Will the system operate in a water-tight or air-tight environment?
- What kind of lubricant will be used?
- What are the system pressure demands?
- How much flexibility is needed?
- Are there components with abrasive or hard-wearing qualities?
- What is the reactive nature of the materials contacting the lantern ring?
- Are there any hygienic considerations?
- What are the operating temperatures?
- Will maintenance be easy or difficult?
- Are there any financial limitations?
PTFE has become the standard for lantern rings because the flexible material holds a wide temperature range. It will tolerate temperatures from -300℉ to 500℉, which provides roughly an 800℉ temperature range. Few other materials can withstand such a wide range. Other benefits of using PTFE for lantern rings include that it complies with FDA regulations and is chemically resistant. These properties make it a universal material for lantern rings.
PTFE strengths are:
- Broad chemical resistance
- Flexible at low temperatures
- Low coefficient of friction
- Stable at high temperature
- Relatively easy to process
- High-frequency resistance
PTFE weaknesses are:
- Creep and cold flow
- Relatively soft
- Low thermal dissipation
- Poor dimensional stability
- High material shrinkage
- Low rigidity
- Low electrical conductivity
While PTFE may be the common choice for lantern rings in many instances, there are high-temperature applications in which the temperature of the stuffing box could potentially exceed 500℉. In these situations, PEEK is a better material to use for lantern rings. PEEK is always a good choice and performs well, but it is more costly. It is the premier material. However, because of its higher price, plants typically use PTFE about 95% of the time.
- Excellent tensile strength, elongation, and wear properties
- Superior chemical resistance
- Withstands long exposure to high pressure, water, and steam
- Excellent chemical resistance
- Very low flammability
- Resistant to gamma radiation
- Continuous use up to 500℉
- Not suitable for applications involving nitric or sulfuric acid
- More costly
- Difficult to process
Make the Right Selection
Choosing the suitable material for a lantern ring is critical when taking a total systems approach. Lantern rings should be considered equally because they are similarly crucial to packing.