The Importance of Hydraulic Seal Integrity
Hydraulic seals prevent leakage and loss of fluids from systems. When seals shrink or harden, they can crack and may lose elasticity, leading to seal failure. This can be caused by:
- High operating fluid temperatures
- Fluid degradation
The use of incompatible hydraulic fluids can cause swelling, and/or shrinkage of the seal or chemical attack that can lead to the failure of lip seals as well as O rings.
Types of Hydraulic Seals
The most commonly used material is acrylonitrile or nitrile butadiene rubber (NBR). Relatively inexpensive, NBR exhibits excellent resistance to petroleum-based hydraulic fluids for the temperature range -50°C to 120°C (-60°F to 250°F), but is not resistant to weathering. NBR can be used at temperatures up to 150°C (300°F) but service life will be shortened.
- Compatible fluids: Petroleum-based hydrocarbons including mineral oils, diesel and fuel oils; vegetable oils and greases; HFA, HFB and HFC hydraulic fluids; dilute acids, alkali and salt solutions up to moderate temperatures.
- Incompatible fluids: Highly aromatic hydrocarbons; ketones, acetic acid, and other polar solvents; strong acids, and glycol-based brake fluids.
The second most common material is fluorocarbon rubber (FPM/FKM), commonly known under the DuPontTM trade name VITON® *. It can be used over a wider temperature range of -40°C to 200°C (-40°F to 400°F). There are two main classes of Viton materials, type A and type G. Type A is more common and lower cost than type G, but type G offers improved compatibility with most fluids and resistance to both weathering and ozone.
- Compatible fluids: Petroleum-based hydrocarbons, synthetic hydraulic fluids, fuels, including gasoline/alcohol fuels, aromatics; many organic solvents and chemicals.
- Incompatible fluids: Glycol-based brake fluids; strong alkalis, amines, ammonia, formic and acetic acids, and superheated steam.
Other materials are used for specialised applications such as thermoplastic polyurethanes (TPU) with bio-hydraulic fluids.
Hydraulic Fluid Base Oils
To evaluate the interaction of hydraulic fluids and the sealing materials, it is important to understand the differences between hydraulic fluid base oils.
- API Group I base oils are commonly used for hydraulic fluids. They are refined by a solvent extraction method.
- API Group II and III base oils are refined by hydroprocessing techniques that convert waxes into iso-paraffins without using solvents. These base oils are water white in color and have superior oxidation resistance.
- API Group IV base oils are polyalphaolefins (PAO) whilst API Group V fluids include naphthenic mineral oils, polyalkylene glycol (PAG), natural triglyercide and synthetic esters and other synthetic products.
The majority of base oils used for hydraulic fluids need additives to enhance specific properties. These additives also introduce an additional set of potential interactions with the seal material.
Predicting the Behavior of a Fluid and Seal Material
It is possible to predict the swelling or shrinkage behavior of a seal material with reasonably accurate results under normal operating temperature conditions.
- Comparing the aniline point of the fluid in question to the aniline point of standard ASTM International reference oils produces a useful indication of the oil/ seal compatibility.
- More accuracy can be obtained using the Elastomer Compatibility Index (ECI) method that compares the behavior of a standard NBR compound in a variety of petroleum-based oils to the behaviors of various other nitrile compounds. The results of the compatibility tests are plotted on a graph and used to determine the Swelling Behavior (SB) of the compound in question. Knowing the ECI of the fluid and the SB of the seal material, one can calculate the expected physical interaction of the pair.
How to Pick the Right Seal Material for Your Application
Given the wide range of variables, it is wise to seek guidance from the manufacturers when selecting seal materials for either a new build or maintenance replacement.
It is particularly important to consider the likely impact of any change in hydraulic fluid on the seals already in use before proceeding with a replacement.
For API Group I based fluids operating at temperatures below 100°C (212°F), NBR seals are the most cost-effective choice. Fluoroelastomers (FKM) increase the maximum operating temperature to 200°C (400°F), which is well above the recommended range for ordinary fluids, whilst only slightly reducing low temperature performance. All the other materials exist to fill application niches not covered by NBR or FKM materials.
For applications currently using NBR or FKM seals that are not exhibiting unusual failure modes or unexpectedly short service lives, it is unlikely that changing to a higher performance – and usually more expensive – material will reduce operating or maintenance costs enough to offset the cost of the premium seals.
For new designs that operate with the same fluids in the same temperature ranges as existing systems, the same seal materials will probably work in the new system too. For new systems that don’t duplicate existing designs, either NBR or FKM is a logical elastomer to start with, unless there is something very unusual about the system or known issues with the fluid that will be used.
For more information on hydraulic fluids, elastomer or seal performance, contact your Lubrizol representative.
* Viton is a registered trademark of the E.I. du Pont de Nemours and Company