Mechanical Seal Guides, Applications, Materials and Sizing
Everything we know about choosing, sizing and troubleshooting mechanical seals and O-rings, written for engineers who have to make the decision.
Mechanical Seal Selection and Applications
Start here if you are choosing a seal for a pump, mixer, blower or bead mill.
Mechanical Seal Selection Guide
Seal types, face materials, elastomers and applications, with a five step selection method.
Seal Face Material Guide
Carbon, silicon carbide and tungsten carbide face pairs, and when each one belongs.
Seal Troubleshooting Guide
Diagnose leaks, weeping, squeal, blistering and premature failure by symptom.
Chemical Compatibility and Elastomers
Material choice is where most seal and O-ring failures are decided.
FFKM Chemical Compatibility
Which chemicals perfluoroelastomer resists, and the short list that beats it.
FFKM vs FKM Comparison
Temperature limits, chemistry, cost and every common question answered.
Kalrez Alternative Cross Reference
Cross referencing FFKM brands on properties, approvals and price.
EPDM or FFKM for SIP and Steam Duty
Choosing seal material on sterile lines, compression set, FDA and USP Class VI.
FFKM Resource Hub
What FFKM is, where it earns its cost, and where it is overkill.
Engineering Questions Answered
Short answers to the seal questions engineers and QA teams ask most.
Why does a mechanical seal fail again after a like for like replacement?
How can I tell whether a mechanical seal leak is a seal problem or a pump problem?
Why does a pump seal leak after CIP or SIP?
What does USP Class VI mean for an O ring or mechanical seal?
Does FDA 21 CFR 177.2600 mean a seal is approved for food contact?
Can I replace an EPDM seal with FFKM to stop repeat failures?
Why does an FFKM O ring fail when the chemical compatibility chart says it should work?
What documents should QA ask for before approving a product contact seal?
Why can a hygienic mechanical seal still create a product contamination risk?
What information do I need to specify a replacement mechanical seal properly?
Seal Failure and Identification Questions
What to check, keep and send when a seal fails or cannot be identified.
Why does the same mechanical seal keep failing on my pump?
If a second seal fails in the same pump, I would not start by ordering a third.
Why is my mechanical seal leaking just after installation?
The first question is when the leak starts.
Why is my O ring extruding or being cut at the edge?
An O ring with a ragged edge or a strip missing from one side may have been forced into a clearance gap.
Why is my O ring flattened and leaking after heat or cleaning cycles?
A used O ring will show some flattening where it was held in the groove.
How do I tell whether the seal has failed from chemical attack or heat?
A swollen or soft seal can suggest fluid incompatibility.
How do I measure an unknown mechanical seal for replacement?
If the part number has gone, do not order by the outside appearance of the spring.
What should I send to identify an unmarked or obsolete seal?
Send the complete old assembly if you can.
Is your seal flush wasting water?
Before anyone sells you a new seal, find out how much water the present arrangement uses.
Seal and O-Ring Sizing
Getting the size and groove right matters as much as getting the material right.
Large Diameter O-Rings
Rings from 200mm internal diameter upwards, 54 cross sections from 2.62mm to 60mm.
FFKM O-Rings
Supplier neutral FFKM selection guidance, chemical service, temperature and certification checks.
Seal Selector Tool
Answer five questions and get a material, face pair and arrangement to size against.
Large Diameter O-Ring Questions
Straight answers on sizing, supply, joints and tolerance for large rings.
Can I get a large diameter O ring when no standard size fits?
A large ring can be made to a non standard diameter by joined cord or moulding, so send the groove dimensions and duty and confirm the method against the enquiry.
How quickly can I get a large diameter O ring for a shutdown?
For a shutdown, ask for stock and a step moulded route, give the latest useful delivery date and duty, and decide whether a fast ring is temporary or permanent.
Should a large O ring be moulded or made from joined cord?
Joined cord can suit a single unusual static seal, while a compression step moulded ring gives better consistency where joints or movement are a risk.
Will a vulcanised O ring work in a pressure or vacuum seal?
Joined rings are not recommended for vacuum or high pressure dynamic duty, and for static duty a test and acceptance path should be agreed before manufacture.
What tolerance should I specify for a large diameter O ring?
Specify the tolerance on internal diameter and cross section, check squeeze at the gland limits, and ask for the tolerance class and inspection grade on the quote.
What should I send to get a quote for a non standard large O ring?
Send the groove dimensions or drawing, the duty, quantity and delivery date, and ask the quote to state construction, compound, approvals, tolerance and inspection grade.
Mechanical Seal Applications, Duty by Duty
The application decides the seal far more than the pump badge does. These are the duties we are asked about most, and what usually works.
| Application | Typical approach |
|---|---|
| Water and general process pumps | Carbon vs silicon carbide faces, EPDM or FKM elastomer, single seal in most cases. |
| Chemical transfer and dosing | Silicon carbide vs silicon carbide, elastomer chosen on the chemistry, not the temperature alone. |
| Slurries, pigments and bead mills | Hard vs hard faces, flushed or double arrangement to keep solids out of the faces. |
| Food, dairy and pharmaceutical | Hygienic design, cleanable surfaces, elastomers with the relevant food or pharma approvals. |
| Hot oil and thermal fluid | Temperature governs the elastomer; FFKM or high temperature FKM depending on the fluid. |
| Solvents, amines and steam | Classic FKM failure territory. Check compatibility before assuming the standard seal will hold. |
| Vacuum and gas duty | Low permeation elastomers, and rapid gas decompression grades where pressure cycles. |
| Dry running or intermittent flow | Face pair and flush plan matter more than material grade. Dry running kills carbon quickly. |
These are starting points, not specifications. Send the duty and we will confirm it, including when a cheaper material will last just as long.
Chemical Compatibility at a Glance
Elastomer compatibility is decided by the fluid, its concentration and the service temperature together. Use this as a first filter, then check the detail.
| Fluid | Usual elastomer | Watch out for |
|---|---|---|
| Water, glycol, mild alkalis | EPDM or NBR | NBR is not for hot water or ozone exposure. |
| Mineral oil, diesel, hydraulic fluid | NBR or FKM | EPDM swells badly in mineral oil. |
| Steam and hot water | EPDM or FFKM | Standard FKM degrades in steam. |
| Acids and oxidisers | FKM or FFKM by concentration | Concentration and temperature change the answer. |
| Ketones, esters, amines | FFKM or EPDM by fluid | FKM is often the wrong choice here. |
| Mixed solvent streams | FFKM | Mixtures behave differently from any single component. |
For aggressive or mixed chemistry, read the FFKM chemical compatibility guide and the FFKM vs FKM comparison.
Seal and O-Ring Sizing
O-ring standards
Imperial sizes follow AS568. Metric sizes follow BS ISO 3601. Quote the standard and the code and there is no ambiguity.
Measure the groove, not the ring
A used O-ring has swollen, compressed or shrunk. Groove diameter and groove width give the true size.
Cross section first
Cross section sets the squeeze and the groove depth. Get it wrong and the ring either extrudes or leaks.
Mechanical seal sizing
Shaft or sleeve diameter, seal chamber bore and depth, and the available axial length set what will physically fit.
Non standard and obsolete sizes
Large diameter rings can be made to your groove dimensions when no catalogue size exists.
What to send us
Shaft size, chamber dimensions, fluid, temperature, pressure and speed. A photo of the old part helps.
Need a size no catalogue carries? See large diameter O-rings.
Common Questions
How do I choose the right mechanical seal for my pump?
Work in this order: the fluid and its chemistry, the temperature, the pressure and speed, whether solids or dry running are possible, and finally how critical the pump is. Chemistry sets the elastomer, solids and dry running set the face pair, and criticality decides between a single seal and a double or flushed arrangement. Our mechanical seal selection guide walks through the same five steps.
How do I know which elastomer suits my chemical?
Match the elastomer to the fluid at its actual service temperature, not at room temperature. EPDM suits water, steam and many alkalis but fails in mineral oil. FKM suits oils and many acids but struggles with steam, ketones and amines. FFKM covers almost everything but costs far more, so it is worth confirming a cheaper material will not do the job first.
What size O-ring do I need?
Measure the groove rather than the old ring, because a used ring has swollen or compressed. You need the groove inner diameter, groove width and groove depth, which give the cross section and internal diameter. Imperial rings follow AS568 and metric rings follow BS ISO 3601.
Can you supply O-rings in sizes that are not in a catalogue?
Yes. Large diameter rings are made from 200mm internal diameter upwards across 54 cross sections from 2.62mm to 60mm, with no dimension limits, no mould costs and no minimum purchase. Send your groove dimensions and material requirement.
How do I measure a mechanical seal for replacement?
Record the shaft or sleeve diameter where the seal sits, the seal chamber bore, the chamber depth and the axial space available, plus the pump make and model if you have it. Those five figures identify almost any replacement, cartridge or component.
Which seal faces should I use with abrasive slurry?
Hard against hard, normally silicon carbide against silicon carbide, with a flush or quench plan that keeps solids away from the faces. Carbon wears fast in abrasive duty, so it belongs on clean fluids.
Is FFKM always the safest choice?
No. It is the broadest chemical resistance available in an elastomer, but it is expensive and often more material than the duty needs. A large share of the FFKM enquiries we receive are solved with a different FKM grade or a hardware fix.
What information do you need to quote a seal or O-ring?
Shaft or groove dimensions, the fluid and concentration, maximum temperature, pressure, speed, and any approvals you must meet. If you have the old part, a photo of the label or the part itself speeds it up considerably.
Free PDF: 40 Reasons Mechanical Seals Fail
Field diagnostic reference covering installation, material, operating and maintenance causes.