Seal Selection Guide: Choose an O-Ring, Back-up Ring, Lip Seal, PTFE Seal, Packing or Gasket from Your Operating Conditions

1. Can a rubber seal survive? The temperature range and fluid screen out elastomers. If none is left, the answer moves to PTFE or metal (ORD 5700 §1.6, §2.2).
2. How does it move? Static, linear, rotary, oscillating or seat. Each has its own branch, as in the Parker handbooks.
3. Where is it, and how hard is the duty? Rod or piston, pressure direction, and the light / medium / heavy duty class of EPS 5370 Table 1-1.
4. Speed and pressure limits. Rotary seals are checked against surface speed and PV.
5. Follow-on choices. Back-up rings and their material, the filled-PTFE grade, the rubber compound, and add-ons such as wipers.

Each branch lists the seal types that fit, in the order of the Parker decision tree. Your requirements then move them up or down: zero leakage, field service, life, one-piece assembly, friction and harsh fluids. A type that breaks a hard limit is listed under "Ruled out" with the reason.

ORD 5700 §1.6 lists three exceptions: rotary contact speeds above 1,500 ft/min (7.6 m/s), an environment no elastomer survives, and a structure too weak for anything but a flat gasket. Everything else, it says, merits an O-ring first.

Most static failures are extrusion. Check back-ups above 1,500 psi (103 bar), and below that if the pressure pulses and the gland breathes. With no extrusion gap, none are needed (§4.2, §4.6).

O-rings suit short strokes on small diameters (§5.2). Very slow strokes twist them: no O-ring below 1 ft/min (5 mm/s) when the pressure difference is under 400 psi (27.6 bar) (§5.15.1). A moving O-ring always passes a thin film, which keeps it lubricated (§5.16.3).

• Up to 1,500 ft/min with a rotary compound, 800 psi (55 bar) max.
• -40 to 250 °F (-40 to 121 °C).
• Groove in the housing, never in the shaft. No stretch: a stretched O-ring heats, contracts and grips the shaft (Gow-Joule effect).
• Shaft hardened, about 55 HRC, true within 0.0005 in (0.013 mm) TIR.
• Smaller cross-sections at higher speed: the tool applies the limits of Table 5-5.

Contamination and side load also set the class. Use the highest class any condition falls in. Pneumatic duty is light up to 200 psi (14 bar) and heavy above.

Lip seals (U-cups) give low friction and wear but seal poorly at low pressure. Squeeze seals do the opposite. As pressure rises the difference fades (EPS 5370 §2). Pneumatics and gravity-return rams want lips; most hydraulics take squeeze or loaded-lip seals.

A heavy-duty rod gets a buffer seal (takes the spikes), the rod seal, and a wiper (keeps the dirt out; contamination causes most hydraulic failures, EPS 5370 §4). Side loads get wear rings, which widen the extrusion gap and so lower the seal's pressure rating.

• Pressure above 1,500 psi (103 bar) with an extrusion gap (ORD 5700 §4.2).
• Pulsing pressure or vibration, even at lower pressure.
• Pressure spikes, high temperature, or wear rings that open the gap (EPS 5370 §10).
• Moving seals rate about half of static ones (EPS 5370 §2).

Pressure from both sides needs one on each side. With one-sided pressure, Parker still recommends two: they are cheap, and a single ring is easy to fit on the wrong side. A single ring goes on the side away from the pressure (ORD 5700 §6.1, §6.2).

From ORD 5700 §6.3–6.4 and EPS 5370 §3, §10. Ratings are general guides; wear rings lower them.

Virgin PTFE is inert, slippery and food safe, but it creeps (cold-flows) and extrudes. Fillers trade a little chemical resistance and friction for strength and wear life (EPS 5370 §3).

• Virgin: cryogenic, gases, food. Weakest in creep.
• Modified virgin: less creep and gas permeation; still food grade.
• Mineral: food grade with more strength; gentle on soft surfaces.
• Glass: the common choice for back-ups and caps: best creep and extrusion resistance.
• Glass + MoS₂: high pressure and temperature on hardened shafts; rotary grade.

• Graphite: lowest friction, corrosive service, dry running, soft shafts.
• Carbon-graphite: wear and creep resistance, unlubricated service.
• Bronze: best for extrusion and heavy load; more friction, less inert.
• Carbon fiber: water, strong alkalis, hydrofluoric acid.
• Aromatic polyester: high temperature, soft dynamic surfaces; not steam.

Compare grades in the plastics compatibility chart.

• Made for vented, zero-pressure cavities; fine to 3 psi (0.2 bar). Pressure profiles reach about 300 psi (20 bar); PTFE beyond that.
• Nitrile to about 3,000 ft/min (15 m/s); FKM and PTFE toward 6,000 ft/min (30 m/s). Grease retention to about 2,000 ft/min.
• Size the lip compound for the underlip temperature: sump + √(ft/min) °F, which is 55 °F above the sump at 3,000 ft/min.
• Shaft carbon steel, at least 30 HRC (45 HRC with abrasives or pressure); bronze, aluminium and plastic shafts groove.
• The lip runs on an oil film. Dry, it hardens and cracks.

Non-contact labyrinths for ANSI pumps, IEEE 841 motors, pillow blocks, turbines and gearboxes where time between failures matters. They hold back oil splash or grease and keep out dust and wash-down spray, with virtually no torque and no shaft wear. Not for flooded seals or pressure.

• Under 150 psi (10 bar): spring-energized or PTFE lip below 1,000 ft/min; PTFE lip or metal-cased lip to 2,500; metal-cased lip to 5,000 ft/min.
• 150 to 750 psi: spring-energized or cased lip below 500 ft/min; cased lip to 1,000.
• 750 to 10,000 psi: spring-energized rotary seal, below about 1,000 ft/min. The preferred design for rotating unions.
• PTFE lips: typically to 6,000 ft/min, 150 psi and 450 °F (232 °C).
• Unlubricated: keep PV under about 150,000 psi·ft/min.

When the answer is a spring-energized PTFE seal, the tool follows Parker's profile trees and names the profile: rod & piston (static or dynamic, O-ring groove or Parker gland, sealing on the ID or OD), face (internal or external pressure), rotary and FlexiLip. The last letter is the spring: V cantilever (moderate load, scraper lips for abrasives), C canted coil (flat load, lowest friction), H helical (highest load, static and slow service). Standard radial FlexiSeals are rated to 3,000 psi; the extended-heel option raises that to 10,000 psi.

Packing is cheaper and needs less specialised maintenance. It is used where leakage is not costly or a problem, and it needs some leakage to stay cool and lubricated. Mechanical seals seal far better and are fitted to most pumps sold today, but they cost more, need clean conditions and precise alignment, and are harder to repair.

• Set rotating packing to leak about 8–10 drops per minute per inch of shaft diameter. Never stop the leak entirely: the packing burns and the shaft is damaged.
• Typical pump envelopes: general service 2,000 ft/min, 300 psi, 500 °F; high speed (graphite, ePTFE) to 4,500 ft/min; high temperature and pressure (graphite with metal-foil end rings) to 1,000 psi and 1,000 °F; carbon/graphite to 1,200 °F at 300 psi.
• Materials: acrylic or polyimide for general service; PTFE, ePTFE or graphite for chemicals (graphite not with strong oxidizers); aramid for slurry; FDA PTFE for food.
• Flush water through the lantern ring at least 1 bar (14.5 psi) above stuffing-box pressure.
• Mixers and agitators: slow but large runout, which packing tolerates. Valves: die-formed graphite with braided end rings for heat; live-loaded sets for low emissions. Reciprocating pumps to about 2,000 psi.

• Balance: unbalanced seals only to about 50 psi (3.5 bar), the FSA recommendation; balanced seals above.
• Double seal required for dangerous products (toxic, carcinogenic, explosive). Garlock cites EPA rules requiring a double seal for VOCs with specific gravity 0.4 or less.
• Double seal with barrier fluid at least 15 psi (1 bar) above the seal chamber for gases, vacuum and dry running, slurries, crystallizing or solidifying products.
• Hot water, refrigerants and light hydrocarbons can flash between the faces: cool-running faces, cool flush or a double seal.
• Carbon against silicon carbide is the coolest-running face pair when the fluid allows carbon.

• Soft: compressed fiber, graphite, PTFE, mica, rubber, cork. General and corrosive service at low to medium pressure; cheapest. Ratings drop with temperature and thickness.
• Semi-metallic: spiral wound, kammprofile, corrugated, jacketed. Low to high pressure and temperature; pressure set by the flange rating.
• Metallic: ring joints (ASME B16.20, API 6A; BX for 5,000–15,000 psi), lens, Bridgman and delta rings, welded gaskets. High pressure and temperature, high seating load.
• Temperature: PTFE -346 to 500 °F; flexible graphite to about 750–850 °F in air; mica and vermiculite to 850–1,800 °F; rubber only while pressure × temperature stays under 20,000 psig·°F.

The 300 psi switch from soft to semi-metallic gaskets in the tool is a KasperCalc screening value; check the gasket maker's pressure–temperature curve.

• Anaerobic: rigid machined metal flanges. Metal-to-metal contact, gap within 0.1–0.25 mm, Ra 0.8–3.2 µm, flatness 0.1 mm over 400 mm. Comes apart by prying. Will not cure on plastics.
• RTV: flexible flanges (stamped covers, oil pans, thin castings), metal-to-plastic joints, T-joints, gaps to about 0.3 mm. Cures over hours; old sealant must be scraped off for service.
• Soft gaskets and non-curing liquids need flanges stiff enough to load them evenly.

Metal O-rings, C-rings and E-rings take over where no elastomer or PTFE survives: Parker gives cryogenic to about 1,450 °F (790 °C) and vacuum to very high pressure (CSS 5129). Confirm the ring style, material and plating with the maker. For reliability estimates, see the NSWC-11 static seal calculator.

• Parker ORD 5700, Parker O-Ring Handbook, 2021 (50th Anniversary Edition).
• Parker EPS 5370, Fluid Power Seal Design Guide, 06/2014 with pages revised to 2018. Its rod, piston, wiper and back-up decision trees set the order of the reciprocating branch.
• Parker EPS 5340, PTFE Lip Seal Design Guide, 2018 printing. Its master decision tree sets the PTFE rotary bands.
• Parker EPS 5350, Rotary Seal Design Guide, 2018 printing. Rubber lip seal pressure, speed, underlip temperature and shaft limits; bearing isolators; the rotary seal decision tree.
• Parker Prädifa PTD3350, Hydraulic Seals. Polyurethane O-ring without back-ups.
• FSA/ESA Compression Packing Technical Manual, 4th ed. (2018). Packing service envelopes and leakage setting.
• FSA Gasket Handbook, 1st ed. (2017, rev. 2025). Gasket categories and temperature limits.
• US DOE and Hydraulic Institute, Improving Pumping System Performance, 2nd ed. (2006). Packing vs mechanical seals on pumps.
• Garlock KLOZURE Mechanical Seal Technical Manual (2010). Seal balance, double seals by application.
• Henkel Loctite Liquid Gasketing Design Guide. Anaerobic vs RTV formed-in-place gaskets.

The tool restates rules in its own words. It does not reproduce the sources' charts or tables.

• Raising the duty class for severity 3 or a no-replacement life.
• Recommending a back-up for moving O-rings above about 750 psi (half the static 1,500 psi rule).
• The 300 psi switch from soft to semi-metallic pipe-flange gaskets (the FSA says only "low to medium pressure").
• How each equipment type ranks packing, mechanical seals and lip seals against each other.
• Metal O-rings, C-rings and E-rings: temperature and pressure range only, from Parker CSS 5129.

The rules run in automated tests (tests/seal-select-logic.test.js): the ORD 5700 rotary example, every speed and pressure boundary above, the duty classes, the back-up and filler rules, lip seal underlip temperature, the packing envelopes, mechanical seal balance and arrangement, the gasket categories, and over 5,000 input combinations.