Mechanical Seal

Mechanical Seal designs and types

A mechanical seal is an essential part of modern industry. A seal is typically used as a method to contain fluid or gas within a pump, compressor or vessel. Shaft seals are Used in pumps, mixers, or anything where a rotating shaft passes through a stationary housing. Mechanical face seals are a specific type of seal used where leakage must be reduced to a vapor.
Seals have a wide spectrum of applications; therefore, their usage is not limited to one type of equipment. These applications range from automobiles, ships, rockets and industrial pumps, compressors, to residential pools and dishwashers, and many more.
Seals come in various forms which include single springs pump seals, compressor seals, multi-spring seal designs, metal bellows seals, mixer and agitator seals, and single and double cartridge seals. Considering that there is more than one type of seal, it’s important to be able to identify the temperature range, application, and environment the seal will have to endure to choose the correct seal that is right for your application. No one seal will work in all applications. The correct seal for the job will prevent environmental contamination, and result in cost savings, through improved machine operating efficiency, longer life, and machine safety. To learn more about which mechanical seal is best for your applications, visit mechanicalseals.net and sealsales.com for pricing of common styles, or contact American Seal & Packing at 714-361-1435. One of our helpful representatives will be more than happy to assist you.

Basic Set of Parts

Mechanical seals are manufactured using three basic sets of parts. Each seal will have a set of primary seal faces, a set of secondary static seals, typically O-rings, wedges and, or, V-rings, and a spring to maintain face contact. The most common methods of energizing the seal faces include a single spring, multiple springs or metal bellows. American seal & packing can help make the proper selection on what mechanical seal is best needed for your application. The proper selection is made when you best understand the full operating conditions. First, identify the exact liquid that is going to be handled. Metal parts must be corrosion resistant, usually stainless steel, Monel, or Hastelloy. Mating faces must also resist corrosion and wear, so carbon, ceramic, silicon carbide or tungsten carbide are the most common. Bronze, Aluminum Bronze, Glass Filled PTFE, and other exotics may be considered. For stationary sealing, Buna, EPR, Viton, and PTFE will be very common selections. When choosing the right mechanical seal, keep in mind the pressure, temperature, and characteristics of the liquid and the proper type of seal will be based on the pressure on the seal and the seal size (including whether it’s balanced or unbalanced). The temperature can determine the choice of secondary seals to handle the temperature of the liquid or gas. The characteristic of the liquid is also important because abrasive liquids create excessive wear and shorten seal life so it’s important you how abrasive a fluid is when selecting your materials.

Selecting a mechanical seal Design

Before selecting your mechanical seal design there are three things you want to remember. All of the seal materials must be chemically compatible with any fluids that will be pumped through the system and that includes solvents, cleaners or steam that might be introduced into the system to flush or clean the lines. It also includes any barrier fluids that are used to circulate between dual mechanical seals. The seal faces must stay together, so if the seal faces open, the seal will leak and allow solids to penetrate between the faces and the solids will eventually destroy the lapped surfaces. A good seal life is defined as running the mechanical seal until the carbon face is worn away. Any other condition is called a seal failure; therefore, it's important to identify the proper materials needed that will aid in preventing wear and tear, longevity. The goal is to successfully prevent leakage all while giving you the best longevity for your application.

When dealing with the temperature limit when selecting a seal, you must not exceed the temperature of the seal components and you must not exceed the temperature limit of the fluid you’re pumping. Always design for the weakest link. One important thing to keep in mind when doing so is that many fluids will change from a liquid to a gas, solid, or crystal at elevated temperature, so it is very important that you understand the characteristics of the liquid you’re trying to prevent from leaking. When selecting the correct mechanical seal for your application, be sure to determine the correct O-ring and gaskets that can properly be used. When choosing an O-ring, or any other elastomer, be sure to take into consideration any cleaners or solvents that might be flushed through the lines or that could come into contact with the seal. If you are having any issues selecting the correct O-ring for your mechanical seal at, contact one of our representatives info@enerzhy.com . We are more than happy to help you. Contact us today.

Understanding the materials needed is also essential. Selecting the faces and the metal parts of the seal based on what type of liquid, pressure, temperature, and characteristics of the liquid is ideal. When selecting the faces of the seal, ceramic versus carbon is a good choice for oxidizing chemicals for example. If you’re going to use Tungsten Carbide as a face material, we recommend a nickel base Tungsten Carbide as opposed to a cobalt base. Cobalt is too hard and can crack with normal seal face differential temperatures. These are just a few examples of how selecting the proper face material can vary. There are many more options you can take when selecting a face material, and American Seal and Packing can help decided which is best for you.

Multiple Seal Design

There are multiple seal designs available for the mechanical seal configuration. A mechanical seal will have a conventional, pusher, non-pusher, unbalanced, balanced or cartridge configuration. It is important to understand how each work, as it will allow you to select the correct one for the appropriate type of application.

Pusher Seals

Pusher seals use a secondary seal that moves axially along a shaft that will have the contact at the seal faces. This will accommodate wear and will help with any absorption of the misalignment of the shaft. These types of seals have the advantage of being inexpensive and are available in a wide range of sizes and configurations.

non-pusher HYPERLINK "https://www.mechanicalseals.net/Metal_Bellows.htm" seals do not have a secondary seal moving along the shaft that helps maintain seal face contact. These seals have the secondary seal in a static state at all times (generally an O-ring, PTFE wedge or Flexible Graphite wedge). Even when the pump is operating, the secondary seal will remain in this static position at all times. The primary seal wear is usually accommodated by welded metal that moves to help in the compression of the rotary to stationary seal faces. The advantage of non-pusher seals is that they have the ability to handle high and low-temperature applications due to the metal bellows, and they won’t require a rotating secondary seal. Not requiring a rotating secondary seal means that it won’t be prone to wearing of the shaft. Non-pusher seals also have some disadvantages that must be noted. The thin bellows cross-sections must be upgraded for use in corrosive environments (such as Hastelloy), and they have a higher cost of metal bellows seals.

Cartridge seals have the mechanical seal pre-mounted on a sleeve, and these fit directly over the shaft. The advantage with cartridge seals is that they eliminate the requirement for seal setting measurements upon installation. Cartridge seals also have lower maintenance costs and have lower seal setting errors. One of their primary disadvantages is that they do have a higher short-term cost. Over the life of the seal, with reduced failures due to installation, and better engineering design the long-term cost is usually less. Careful dimensional evaluation prior to installation will ensure they will fit into existing stuffing box or seal housings. ANSI and API designs are standard. There are multiple and custom seal designs available.
Single spring seals

Single Spring Mechanical Pump Shaft Seals are the most common type of pump seal. A single pump spring seal will have a single spring coiled in a right or left-hand design. This design in position will help accommodate left and hand turning pumps. Using single spring mechanical seals allows flexibility to accommodate misalignment, any deflection of the shaft, and any form of breakaway shock loading. A single spring seal also has the capability to resist clogging even in the most extreme vicious conditions by containing different types of fluids. Single seals don’t always meet the shaft sealing requirements of today’s refinery and chemical application pumps due to the small amount of required. The seal industry has configurations that incorporate two sets of sealing faces with a clean barrier fluid injected between these two sets of seal face. The choice to choose between a double or single seal comes down to the initial cost to purchase the seal versus the cost of operation, maintenance, downtime caused by the seal, length or preventive maintenance cycles and the environmental and user plant emission standards for leakage from the seal.

Double (dual pressurized) seal

One of the more common multiple seal configurations is called a double (dual pressurized) seal. A double dual pressurized seal has two seal face sets that are oriented in opposite directions. These two seals will be arranged in a series. The primary seal keeps the product contained within the pump housing and the secondary seal prevents the flush liquid from leaking into the atmosphere. The features of this seal arrangement include potentially five times the life of a single seal in severe environments, the metal inner seal parts are not exposed to the liquid being pumped which makes it good for viscous, abrasive, or thermosetting liquids, and the double seal life is virtually unaffected by process upset conditions during pump operation. Double mechanical seals are offered in two arrangements that include back-to-back or face-to-face. Some common applications of double mechanical seals include if the fluid and its vapors are hazardous to the operator or environment and must be contained, and at high pressures or temperatures. Double seals allow the use of mechanical seals on these difficult liquids. A double seal costs less to purchase and install than a tandem seal, but keep in mind that you usually get what you pay for.

Tandem (dual unpressurized)

The other multiple seal configuration is called a Tandem (dual unpressurized) arrangement. With this arrangement, the two individual seals are positioned in the same direction. A tandem seal will have two completely independent seals that don’t depend on the other one for sealing. Seals that are oriented in this configuration can withstand much higher pressures in the pump casing when compared to a double seal pump. This type of arrangement is very common in a submersible wastewater pump that is commonly used between the pump and motor with oil as the barrier liquid. With this type of arrangement, you can expect that the pressure between seals is lower than the seal chamber pressure. The external fluid only lubricates the most outside set of faces and pumped fluid lubricates most inside faces. The outside seal serves as a safety seal or containment device, and leakage to the atmosphere is external fluid, possibly mixed with small amounts of pumped fluid. to assist you with the many choices we have available and choosing what’s best for your application.

Conclusion

We use mechanical seals to stop any visible leakage. A mechanical seal will leak vapor as the fluid film on the faces reaches the atmospheric side of the seal faces, but any visible leakage will be nonexistent. Modern cartridge seal designs are essential in today’s industry because they do not damage the pump shaft, and maintenance is reduced as seals have inboard springs or a metal bellows. These energizers make the seal self-adjusting as the faces wear. Seals will have lightly loaded faces which will consume less power than any form of compression packing. Bearing contamination is reduced as the lubricant does not become affected by seal leakage and wash out. Corrosion of plant equipment is also at a minimum if the product is contained within a pump. Overall, a mechanical seal is the best way to go. They will provide prevent wasted product resulting in reduced costs.