By Bill McNally, McNally Institute
Bill McNally shares his expertise on selecting the correct type of mechanical seal, installing proper environmental controls, choosing the right materials for your application, and installing seals correctly.
Let me mention here at the beginning that mechanical seals fail for only two reasons:
- One of the seal materials becomes damaged
- The lapped seal faces open and allow the product to leak.
Please keep that information in mind as you read this article.
We will begin with the type of seal you should be using. Seals fall into many categories:
Hydraulically balanced seals are the primary type to use because they generate less heat and they can handle a wide range of pressure, as well as a reasonable vacuum.
If the pump suction pressure (not discharge) exceeds 400psi (28bar) you might have to go to a special version of a balanced seal.
Likewise, if a vacuum on the suction side of the pump measures less than one “torr”, you will have to go to a seal with no elastomers, because elastomers “out gas” and shrink as the air is pulled out of the compound. This shrinking will cause the elastomers to leak and the vacuum can be destroyed.
The stationary version of the seal is the best design, because it will compensate for misalignment between the pump and its driver, as well as the stuffing box not being square or perpendicular to the rotating shaft.
This type of seal also has some real advantages if you are converting a packed pump to a mechanical seal or running at greater than electric motor speeds.
Metal bellows seals should be selected if the product temperature is in excess of 400°F (200°C) and no cooling of the stuffing box is allowed. Metal bellows seals are also a good choice for cryogenic (cold) service.
Remember that petroleum products have to be cooled because they will “coke or carbonise”, open the seal faces, and clog up seal components. Metal bellows seals are also a logical choice if no elastomers are allowed in the system.
Dual seals are becoming very important because of fugitive emissions laws. They can also act as a spare seal if you cannot afford an unexpected shutdown. Dual seals should always be designed with a “two way” hydraulic balance.
Non-metallic seals should be selected if the pump wetted parts are manufactured from non-metallic material such as carbon, Teflon, plastic, etc. These seals are normally mounted as outside seals and have no metal parts exposed to the fluid you are sealing.
Do not depend upon a flush into the stuffing box to protect the seal materials against corrosion, and do not try to second guess the application engineer that selected a non-metallic pump for this application. Stick with a non-metallic seal design in all non-metallic pumps unless you are a corrosion expert and willing to select the correct metal for the application.
Cartridge seals are necessary for ease of installation and to make pump impeller adjustments without changing the seal face loading, but some of them have problems. Cartridge seals also allow you to compensate for thermal growth of the shaft.
Be aware that there are special precautions you must understand if you are going to use cartridge mounted stationary seals.
Split seals can be used in most applications today, especially the larger shaft sizes. In most cases you can solve the packing conversion or seal leakage problem without taking the pump apart and getting involved in a lot of downtime.
Split seals are the only real alternative to cartridge seal designs. Today, they are available in both rotating and stationary versions.
Most products contain solids so the seal should have built in slurry features. With the exception of most finished products, accept the fact that you are pumping a slurry.
Be sure that the brand of seal you select will not frett or damage the pump shaft. This will allow you to use solid shafts instead of the inferior sleeved version with its high L3/D4.
After choosing the seal type, we must select the seal materials. Most of the time you will be choosing three different materials.
The metal components
You are concerned only about the parts that get wet in the fluid, unless the pump is in a very corrosive atmosphere. The wetted parts include the pump volute, impeller, shaft or sleeve, mechanical seal, stuffing box, etc.
The different grades of carbon/graphite
Carbon/graphite manufacturers supply a carbon/graphite chart that will give you the details of any particular grade they manufacture. With few exceptions an unfilled carbon such as Pure grade 658 RC is always a safe choice if any grade of carbon is acceptable in your fluid.
The hard face
There are many to choose from, but self sintered silicon carbide will probably be your first choice. Reaction bonded silicon carbide is also popular, but can be attacked by many high pH fluids. The common cleaner “caustic” is a fluid that will attack reaction bonded silicon carbide.
The rubber parts (elastomers) that are exposed to the fluid
The dynamic O-ring (the one that flexes and rolls) is the most critical.
Most chemicals and chemical combinations can be sealed with either a good grade of Viton of ethylene propylene. Most solvents and a few other chemical combinations require something a little better. The super elastomers have just about replaced Teflon® in those applications, but they cannot be used everywhere.
Remember also that all elastomers have an upper and lower temperature limit that must not be exceeded.
You must choose seal materials that are chemically compatible with all the fluids that might be passing through the equipment, and that includes any cleaners or solvents that might flush through the lines.
This is especially critical with the choice of elastomer (O-ring). The metal parts and face combination are usually compatible with most cleaners and solvents.
The next step is to place your chemical into the proper sealing categories. Keep in mind that most chemicals fall into several sealing categories at the same time.
After you have placed your fluid in the proper sealing categories, selecting the correct environmental control comes next. If the equipment is going to operate 24/7, without shutdown, you have a simple sealing problem.
But if you are on batch or intermittent service, or if a standby pump is run occasionally, then an environmental control will become critical. You need to know about the three most important environmental controls:
Control the temperature in the stuffing box area
You can raise the temperature, lower it or keep it within certain limits that will be dictated by the seal design and product characteristics. Temperature control is most important when the pump is shut down and the pumping fluid either cools down or heats up because of ambient temperature.
Control the pressure in the stuffing box area
There are many occasions where you will want to control stuffing box pressure to stop a fluid from vaporising, flashing or evaporating. We seldom have to lower stuffing box pressure, but if it is too high for a conventional balanced seal you will be better off going to a special high pressure design.
Cleaning up the product in the stuffing box
Clean products are easier to seal. Flushing is only one of your options, there are more.
Any of these environmental controls will work better if you have installed an oversized stuffing box on your pump. The seal needs lots of radial room to allow centrifugal force to throw solids away from the lapped seal faces and to lessen the generation of heat in the stuffing box.
Heat is a major cause of problems with both centrifugal pumps and mechanical seals. Anything you do that will help to remove heat from the stuffing box will add to the life of your seal and pump bearings.
If you have done all of the above, all that remains is to install the seal correctly, adjust the pump impeller (if it is an open, or semi-open design) and hook up the environmental control you chose to keep the seal functioning.
With a little luck, you should now get good seal life in fact, with a few modifications, your seals might never run out.