Mechanical Seal Selection Guide
Seal types, face materials, elastomers and applications, in the order a specialist actually works through them.
In one line: Select a mechanical seal by working through the fluid, the temperature and pressure, the speed and shaft size, the equipment condition, and the cost of failure. Those five decide the seal type, the face pair, the elastomer and whether you need a double seal with a support system.
Most seals that fail early were never wrong on the drawing. They were specified against the product and not the cleaning cycle, or fitted into a pump that was already misaligned. Selection is as much about the duty around the seal as the seal itself.
The Five Step Method
1. Define the fluid
Product, concentration, solids, viscosity, and every cleaning or flushing fluid that passes through. Rate the material against the worst of them.
2. Fix temperature and pressure
Peak process temperature, seal chamber pressure, vacuum conditions and any pressure cycling. Above roughly 10 bar, specify a balanced seal.
3. Take the speed and dimensions
Shaft or sleeve diameter, speed in rpm, and the seal chamber bore and depth. Surface speed at the faces sets the heat you have to remove.
4. Assess the equipment
Pump condition, alignment, pipe strain, bearing wear and how far it runs from best efficiency point. A worn pump destroys correct seals.
5. Price the failure, not the seal
If an unplanned stop costs thousands an hour, the cheap seal is the expensive one. Run the numbers on the total cost of ownership calculator.
Compare options on lifetime cost with the total cost of ownership calculator.
Seal Types, and When Each One Wins
Component seals
Loose parts fitted and set on site. Lowest purchase price, highest dependence on fitting skill.
Best for: Simple duties, in-house fitters with a setting procedure, tight budgets.
More detailCartridge seals
Pre-assembled and pre-set on a sleeve and gland. Fit, tighten, remove the clips.
Best for: Anywhere downtime is expensive or fitting skill varies between shifts.
More detailSingle seals
One face pair, sealing against the product with atmosphere on the other side.
Best for: Clean, lubricating, non-hazardous fluids at moderate temperature and pressure.
More detailDouble seals
Two face pairs with a barrier or buffer fluid between them, back to back or tandem.
Best for: Toxic, flammable, crystallising or abrasive fluids, and any zero emission duty.
More detailMetal bellows seals
A welded metal bellows replaces the spring and the dynamic elastomer.
Best for: High and cryogenic temperature, hot oil, and duties where an elastomer will not survive.
More detailSplit seals
Split into halves so they fit without dismantling the equipment.
Best for: Large shafts, agitators and pumps where stripping the drive train is the real cost.
Gas seals
Non-contacting faces separated by a gas film, usually nitrogen.
Best for: Dry running risk, zero product contamination, high speed duty.
Lip and oil seals
Radial lip seals for grease, oil and low pressure rotary duty, not process pressure.
Best for: Gearboxes, bearing housings and shaft exits, not pump product sealing.
More detailFace Material Pairings
| Pairing | Role | Typical duty | Character |
|---|---|---|---|
| Carbon vs silicon carbide | The general purpose pairing | Clean fluids, water, oils, general process | Forgiving, low friction, tolerates brief dry running |
| Silicon carbide vs silicon carbide | The hard pairing | Abrasives, slurries, aggressive chemistry | Excellent wear life, unforgiving if it runs dry |
| Tungsten carbide vs carbon | The tough pairing | High pressure, shock loads, pressure cycling | Very strong, heavier and more thermally conductive |
| Carbon vs ceramic | The budget pairing | Clean water, light duty, low pressure | Cheap, limited chemical and thermal range |
| Diamond coated faces | The specialist pairing | Dry running risk, extreme abrasion, long overhaul intervals | Highest cost, longest life where duty justifies it |
Full breakdown of hardness, thermal behaviour and pairing rules in the seal face material guide.
Elastomers, the Usual Weak Link
| Elastomer | Good for | Limits |
|---|---|---|
| NBR (nitrile) | Oils, water, fuels to about 100C | No ketones, no strong acids, poor with ozone |
| EPDM | Hot water, steam, caustics, brake fluid to about 150C | Never with mineral oil or grease |
| FKM (Viton) | Oils, fuels, many chemicals to about 200C | Poor with ketones, esters, amines and hot water |
| PTFE encapsulated | Very broad chemical range | Low elasticity, needs the right groove design |
| FFKM | Almost all chemistry, to 325C in high grades | Cost, and weak low temperature performance |
Figures are typical for general purpose compounds. If the chemistry is the problem, check the chemical compatibility chart and FFKM vs FKM.
Selection by Application
Water and wastewater
Abrasive grit and intermittent dry running dominate. Silicon carbide faces, EPDM or nitrile, and a cartridge design to survive rough fitting conditions.
Chemical processing
The cleaning cycle usually decides the material, not the product. Silicon carbide faces with FKM or FFKM, and a double seal where the fluid must not escape.
Food, dairy and pharmaceutical
Hygienic design, FDA or USP Class VI elastomers, and materials that survive repeated CIP and SIP without hardening.
Oil, gas and refining
API 682 arrangements, balanced seals, explosive decompression resistant elastomers and a properly designed flush plan.
Paints, inks and dispersions
High solids and high viscosity. Hard faces, a flush that keeps solids out of the faces, and realistic overhaul intervals.
High temperature and hot oil
Metal bellows seals remove the dynamic elastomer, with cooling or quench arrangements to keep the faces stable.
Six Ways Selection Goes Wrong
- Specifying on the pump model alone, ignoring what actually goes through it
- Forgetting the cleaning cycle, which is often harsher than the product
- Using the average temperature instead of the peak
- Choosing a component seal where nobody has a setting procedure
- Upgrading the seal when the real fault is alignment, cavitation or bearings
- Ignoring the flush plan, so a correct seal still cooks itself
If a seal is already failing, diagnose it before you reselect, using the seal troubleshooting guide.
Seal Selection, Questions Answered
How do I select a mechanical seal?
Work through five things in order: the fluid, the temperature and pressure, the shaft speed and size, the equipment and its condition, and the consequence of failure. Those five decide the seal type, the face pair, the elastomer, the metal parts and whether you need a support system. Choosing on price alone is what produces the repeat failures we get called out to.
What is the difference between a cartridge seal and a component seal?
A component seal is a set of loose parts fitted and set on site, so it is cheaper but depends entirely on the fitter getting the working length right. A cartridge seal arrives pre-assembled and pre-set on its own sleeve and gland, so installation is faster and far harder to get wrong. Where fitting skill varies or downtime is expensive, cartridge wins on total cost even though it costs more to buy.
When do I need a double mechanical seal?
When the fluid is toxic, flammable, carcinogenic, crystallising, or simply must not reach atmosphere, and when a single seal has no reliable liquid film to run on. A double seal with a barrier or buffer fluid also protects the faces from dry running and from solids in the product. It costs more and it needs a support system, so do not fit one where a single seal in the right material would last.
What is a balanced mechanical seal?
Balancing reduces the net closing force on the seal faces by changing the hydraulic area. An unbalanced seal is simpler and fine at low pressure, typically under about 10 bar. A balanced seal runs cooler and survives higher pressure and higher speed, so it is the default choice on anything demanding.
Which seal face materials should I choose?
Carbon against silicon carbide is the general purpose workhorse: forgiving, low friction, good on clean fluids. Silicon carbide against silicon carbide handles abrasives and aggressive chemistry but is unforgiving of dry running. Tungsten carbide is tough under pressure and shock. There is a full breakdown on the seal face material guide.
Which elastomer should I use with my seal?
Nitrile for oils and water at moderate temperature, EPDM for hot water, steam and caustics but never with mineral oil, FKM for oils, fuels and many chemicals to around 200C, PTFE encapsulated where chemistry is broad, and FFKM where the chemistry or the temperature beats everything else. The elastomer is the most common cause of a chemically driven seal failure.
What information do you need to specify a seal?
Pump make and model, shaft or sleeve diameter, the fluid and its concentration, operating and maximum temperature, seal chamber pressure, shaft speed, whether solids are present, any cleaning cycles, and any standard you have to meet such as API 682, ATEX or FDA. A photo of the old seal and the failure surfaces helps more than most people expect.
What is API 682 and does it apply to me?
API 682 is the standard for shaft sealing on centrifugal and rotary pumps in refining, petrochemical and gas duty. It defines seal categories, types, arrangements and qualification testing. If you are in general industry it is not mandatory, but its logic, especially on arrangements and support systems, is a sound guide to specifying reliably.
Can I fit a seal to an old or obsolete pump?
Usually yes. Most obsolete seals can be matched by dimension and duty, or converted to a modern cartridge design that fits the existing chamber. Send the old part or its dimensions and we will identify a like for like or an upgrade, including for pumps the original maker no longer supports.
Why does the same seal keep failing?
Because the seal is rarely the root cause. Repeat failures usually trace back to misalignment, pipe strain, cavitation, running off the best efficiency point, bearing wear or dry running. Change the seal ten times and the pump will keep eating them. Diagnose the failure surfaces first, then choose the seal.
What does the wrong seal actually cost?
The seal set is the smallest number in the calculation. Lost production, callout labour, contamination and emergency shipping dominate it. Our total cost of ownership calculator lets you compare a cheap seal that lasts eight months against a properly specified one that lasts five years.
Do you help with selection free of charge?
Yes. Send the duty and the pump details and you get a straight recommendation, including when the cheaper option is the right one and when refurbishing what you have beats buying new.
Send the duty, get the specification
Pump details, fluid, temperature, pressure and speed. You will get a straight recommendation, including when refurbishing what you have beats buying new.