MECHANICAL SEAL SELECTION TN005
In this section I will be using the word “pump” to describe the piece of equipment that you will be sealing. If your equipment is anything other than a single stage centrifugal pump with an over hung impeller, the information still applies with a couple of exceptions:
- Mixers, agitators and similar pieces of equipment sometimes have severe axial thrust and shaft deflection problems due to their high L3/D4 numbers (The ratio of the shaft length to its diameter).
- Sleeve or journal bearing equipment allows more axial movement of the shaft than those pieces of equipment provided with precision bearings. Axial movement is a problem for mechanical seals because of the changing face load; especially at start up when the axial thrust reverses in a centrifugal pump.
- Open impeller pumps require impeller adjustment that could cause excessive axial movement of the shaft that will affect the seal face loading. Depending upon the severity of the abrasives being pumped, this could be a frequent occurrence.
- Multi-stage pumps are seldom as sensitive to operating off the best efficiency point (BEP) as single stage centrifugal pumps. The opposing cutwaters in these pumps tend to cancel out the radial forces created when the pump is operating off of its best efficiency point (BEP).
- Centrifugal pumps equipped with double volutes are not too sensitive to operating off the best efficiency point (BEP), but do experience all of the other types of shaft deflection.
- Specialized equipment such as a refiner in a paper mill will experience a great deal of axial travel as the internal clearances are adjusted.
Whenever I use the word fluid, I’m talking about either a liquid or a gas. If I say either liquid or gas, I am limiting my discussion to that one phase of the fluid.
Any discussion of mechanical face seals requires that you have many different types of knowledge. The first is, “should you be converting packed pumps to a mechanical seal?” Seals cost a lot more money than conventional packing and unless you are using split seals, they can be a lot more difficult to install. There is a packing conversion down side.
Assuming you have made the decision that the mechanical seal is your best choice for sealing, you must know how to select the correct design for your application. There are many different kinds of seals to choose from:
- Rotating seals where the springs or bellows rotate with the shaft.
- Stationary seals where the springs or bellows do not rotate with the shaft.
- Metal bellows seals used to eliminate elastomers that can have trouble with temperature extremes or fluid compatibility.
- Elastomer type seals utilizing O-rings and other shape elastomers.
- Single seals for most applications.
- Dual seal designs for dangerous and expensive products or any time back up protection is needed.
- Inside mounted designs that take advantage of centrifugal force to throw solids away from the lapped seal faces.
- Outside seals. Usually the non-metallic variety for pumps manufactured from non-metallic materials.
- Cartridge seals to ease installation and allow you to make impeller adjustments without disturbing the seal face loading.
- Split seal designs that allow you to install and change seals without taking the pump apart and disturbing the alignment.
- Hydrodynamic or non-contacting seals used for the sealing of gases.
- Hydrostatic designs are another version of non-contacting vapor seals.
There are some very desirable design features that you should specify for your mechanical seals:
- The ability to seal fugitive emissions without the use of dual seals, other than having the dual seal installed as a “back-up” or spare seal.
- Will the seal dynamic elastomer damage or cause fretting corrosion of the pump shaft? Almost all-original equipment designs do. Spring-loaded Teflon® and graphite are notorious for shaft destruction. There are many seal designs available that will not cause fretting corrosion or damage shafts and sleeves, and that is the kind you should be using.
- The seal should have built in non-clogging features such as springs out of the fluid.
- The seal should be able to compensate for a reasonable amount of both radial and axial movement of the shaft. There are special mixer seal designs that can compensate for axial and radial travel in excess of 0.125 inches (3 mm) and you should know about them
- The seal should be designed to be positioned as close to the bearings as possible to lessen the affects of shaft deflection. Ideally the seal would be located between the stuffing box face and the bearing case with a large diameter seal gland allowing plenty of internal radial clearance for the seal.
- The seal should generate only a small amount of heat. Seal face heat generation can be a problem with many fluids and there is no advantage in letting the seal faces, or the fluid surrounding them get hot
- Any heat generation between the seal faces should be efficiently removed by conduction away from the lapped faces and dynamic elastomer. Check to see if your design does it efficiently.
- Any dynamic elastomer (an O-ring is typical) should have the ability to flex and then roll, or slide to a clean surface as the carbon face wears.
- The seal face load should be adjustable to compensate for open impeller adjustments and axial growth of the shaft. Cartridge seals do this very well.
- Can you use universal materials to lower your inventory costs and avoid mix-up problems? All of the seal materials should be clearly identified by type and grade. You will need this information if you have to analyze a premature seal failure. Some seal companies try to make everything a secret, do not tolerate it!
- Will the seals be hydraulically balanced to prevent the generation of unwanted heat between the lapped faces? What is the percentage of balance? If you are using dual seals will the inner seal be a double balanced seal that is hydraulically balanced in both directions? Pressures can reverse in dual seal applications.
- You will want to become familiar with the effects of heat on:
- The seal faces, especially the carbon and plated or coated hard faces
- The elastomers, especially the dynamic elastomer
- Excessive corrosion of the seal components.
- The product. It can change with heat. It can vaporize, solidify, crystallize, coke or build a film with an increase in the product’s temperature.
- Internal tolerances of the seal especially face flatness and elastomer squeeze. Heat causes thermal growth of these components that will alter their critical tolerances.
We would like to be able to install the seal without having to modify the pump. The seal should be the shortest, thinnest design that will satisfy all of the operating conditions. Once you have the shortest, thinnest design that will satisfy the operating conditions there is seldom a need to modify any seal design.
The specific sealing application will dictate which seal design you should choose. If your seal application falls within the following parameters any stationary or rotating, “off the shelf” balanced O-ring seal should be able to handle the application without any serious problems.
- Stuffing box pressures from a one Torr vacuum to 400 psi. (28 bar). Note that stuffing box pressure is normally closer to suction than discharge pressure
- Stuffing box temperature from -40°F to 400°F. (-40°C to 200°C)
- Shaft speed within electric motor speeds. If the surface speed at the seal faces exceeds 5000 fpm. (25 m/sec) you will have to select the stationary version of the seal.
- Shaft sizes from 1 inch to 4 inches. (25 mm to 100 mm)
You may have to go to a special seal design if your application falls into any of the following categories:
- Stuffing box pressures in excess of 400 psi. (28 bar) require heavy duty seals.
- Excessive shaft movement of the type you find in mixers, agitators, and some types of sleeve or journal bearing equipment.
- The seal must meet fugitive emission standards.
- No metal parts are allowed in the system. You need a non-metallic seal.
- Nothing black is allowed in the system because of a fear of color contamination. You cannot use any form of carbon face; you must use two hard faces.
- There is not enough room to install a standard seal.
- You are not allowed to use an environmental control or no environmental control is available.
- Odd shaft sizes often dictate special seals.
- If the seal components must be manufactured from an exotic metal.
If any of the following are part of the application, you may need a metal bellows design that eliminates all elastomers.
- You are sealing a non-petroleum fluid and the stuffing box temperature exceeds 400°F (200°C) Petroleum fluids have coking problems that require cooling in the seal area.
- Cryogenic temperatures.
You should go to a dual seal application if your product falls into any of the following categories:
- You need two seals to control the seal environment outside the stuffing box.
- To control the temperature at a seal face to stop a product from vaporizing, solidifying, crystallizing, or building a film.
- To prevent a pressure drop across a seal face that can cause a liquid to vaporize.
- To eliminate atmospheric conditions outboard of a mechanical seal when there is a possibility of freezing water vapor in the air.
- To break down the pressure in a high-pressure application, by inserting an intermediate pressure between the seals. Two lower pressure seals can then be used to seal a high-pressure fluid that would normally require a very expensive high-pressure mechanical seal.
- To provide a lubricant if one is needed to prevent slip stick between lapped seal faces. This is always a problem when you are sealing a gas or non-lubricating liquid.
You need dual seals as a protection for personnel in the area if your product is any of the following categories:
- A toxic liquid or gas.
- A fire hazard
- A pollutant
- A carcinogen
- A radioactive fluid
- An explosive fluid
The other places we use dual seals are:
- Expensive products that are too valuable to let leak.
- You cannot afford to be shut down in the middle of a batch operation.
- You do not have a standby pump and experience shows that the seal failure is your highest probability of an unexpected shut down.
In the Sealing Application section you will learn:
- How to choose the correct seal materials.
- How to classify the fluid into specific sealing categories
- The environmental controls you might need to insure the seal will not fail prematurely.