Elastomer compatibility: why some compounds swell or harden seals

1. What elastomer compatibility means

An elastomer seal is a network of long polymer chains with a share of liquid components. A grease or sealant touching it is never chemically inert. Its base oil, thickener and additives diffuse slowly into the polymer.

Plasticisers and processing oils diffuse out in return. In plain terms, compatibility describes the size of this two-way exchange. A well matched compound leaves the seal close to its original size, hardness and strength.

A poor match changes at least one property, sometimes beyond recovery. Valve seals are thin and loaded, so small changes matter. Stiff movement or a new leak after a compound change suggests a compatibility problem.

The valve lubrication guide covers choosing the compounds themselves. This page follows what those compounds do to seals.

2. Why seals swell

Swelling is absorption of the base oil into the elastomer. The rubber takes in oil that is chemically similar to itself. The seal then grows larger, softer and weaker.

A swollen seal can fill its groove and extrude under pressure. It can also press against a ball or stem and make the valve stiff. Stiff operation soon after injecting a new compound suggests swelling, but only inspection confirms it.

Swelling is a common result of choosing a grease for convenience rather than for its chemistry. General-purpose grease is not automatically a substitute for a valve lubricant or sealant. The seal reacts to what is actually injected.

3. Why seals harden or shrink

Hardening is the opposite process: material leaves the seal instead. The compound or the service fluid draws plasticiser out of the elastomer. The seal loses flexibility, shrinks slightly and cracks where it bends.

Heat, oxidation and dry gas all accelerate this loss. Organic sealants can harden in dry gas and seize valves; this is recognised industry knowledge. Dry-gas leakage and seal behaviour covers that failure path in detail.

A seal that has hardened often leaks slowly and worsens with time. This indicates an aged or chemically attacked seal. When a stem or body leak resists treatment, seal replacement is the honest endpoint.

4. How common seal families behave

Different elastomer families respond differently to oils, solvents and chemicals. A qualitative map helps, but it is only a starting point.

• Nitrile rubber (NBR) works well with mineral oils but is attacked by some synthetic fluids and aromatics.
• Fluoroelastomer (FKM) resists a broad range of fluids, at higher cost.
• EPDM suits water and glycol services but swells badly in hydrocarbon oils.
• Silicone rubber copes with wide temperature swings but has lower mechanical strength.
• PTFE is nearly inert, though it is a plastic and seals differently.

Treat these notes as tendencies rather than fixed rules. Exact resistance depends on the fluid, the temperature and the formulation of the seal itself.

5. Cleaners and sealants carry the same risk

Grease takes the blame, but cleaners and sealants also touch seals. Cleaner solvents are chosen to dissolve hardened deposits. Stronger chemistry and longer contact both raise the risk to elastomers.

Dwell time matters as much as chemistry. A cleaner held in a valve absorbs into the soft parts it reaches. Keep dwell to the plan, and cycle the valve as scheduled.

Sealants add a further constraint. The sealant must suit the seal material and the lubricant already in the valve. A curing sealant can lock a swollen seal in place. Non-hardening sealant chemistry avoids that particular failure path.

6. How to verify compatibility before use

Verification before injection is a short and ordered process. Work through it before any compound meets a seal in service.

1. Identify the seal material from the valve data sheet or the OEM. Where OEM documentation exists, consult it for limits and warranty terms.
2. Check the compound against the seal family and the service fluid together. Gas composition matters as much as the grease, and sour and CO2-bearing gas changes the picture.
3. Ask the compound supplier for compatibility guidance. Where products were mixed in the past, read what mixing compounds from different suppliers can do.
4. For critical service, request an immersion check on a spare seal before full injection.

Three gates stand before any injection job on a live valve. The injection fittings must be in good condition. The valve design and the service must suit injection. Site rules and the permit to work must allow the job.

Treatment by injection in service is not possible on every valve. We formulate valve lubricants in-house and state the seal families each one suits.