Mechanical seals come in many configurations, each designed to handle a specific range of operating conditions. Understanding the fundamental categories — and the tradeoffs between them — is essential for selecting the right seal for any application. This guide covers every major type, from the most common to the most specialized.
The Fundamental Split: Pusher vs Non-Pusher
Every mechanical seal falls into one of two categories based on how it compensates for face wear.
Pusher Seals
Pusher seals use springs and dynamic O-rings (or other secondary seals) to push the rotating seal face against the stationary face. As the faces wear, the springs maintain contact. The dynamic O-ring slides along the shaft or sleeve to accommodate this movement.
Advantage: Simple, cost-effective, wide range of sizes and materials. Handles most general-purpose applications.
Limitation: The dynamic O-ring can hang up on the shaft due to coking, crystallization, or corrosion — a condition called "hang-up" that prevents the seal from tracking face wear and causes leakage.
Non-Pusher (Bellow) Seals
Non-pusher seals use a flexible bellow (metal, PTFE, or elastomer) instead of a dynamic O-ring. The bellow acts as both the spring element and the secondary seal. There is no sliding contact on the shaft, so hang-up is eliminated.
Advantage: No dynamic O-ring means no hang-up, better performance in high-temperature and coking services.
Limitation: Bellows are more sensitive to pressure and misalignment, and metal bellows can fatigue over time. Generally higher cost.
Pusher Seal Types: Spring Configurations
Pusher seals are further categorized by their spring design. The spring provides the closing force that keeps the seal faces in contact.
Single Spring Seals
A single large coil spring wraps around the shaft, providing closing force and driving the rotating face. The spring also transmits torque from the shaft to the seal face.
- Best for: General-purpose pump sealing, water, mild chemicals, and HVAC applications.
- Advantages: Robust, tolerant of moderate solids in the fluid, not easily clogged. The large spring is easy to clean and resistant to debris buildup.
- Limitations: The spring force is not perfectly uniform around the circumference, which can cause slight face distortion at larger shaft sizes.
- Typical sizes: 20 mm to 100 mm shaft diameter.
Multi Spring Seals
Multiple small springs are arranged around the circumference of the seal, distributing the closing force evenly across the face. This is the most common seal type in chemical processing.
- Best for: Chemical processing, oil refineries, petrochemical, pharmaceutical — any application requiring precise, even face loading.
- Advantages: Uniform face loading for better sealing performance. Compact design allows use in tight seal chambers. Available in balanced and unbalanced configurations.
- Limitations: Small springs can clog in dirty or slurry services. Not recommended where particulates may pack around the springs.
- Typical sizes: 20 mm to 175 mm shaft diameter.
Wave Spring Seals
A flat wave spring (resembling a corrugated washer) provides the closing force. These seals are extremely compact axially, making them suitable for equipment with very limited seal chamber depth.
- Best for: Space-constrained applications, OEM pump designs with short stuffing boxes, and light-duty services.
- Advantages: Minimal axial space required. Uniform spring force. Low cost.
- Limitations: Limited closing force, so not suitable for high-pressure applications. Typically used in smaller sizes.
Non-Pusher Seal Types: Bellow Configurations
Non-pusher seals use a flexible bellow element that replaces both the spring and the dynamic O-ring. The type of bellow material determines the seal's characteristics and application range.
Metal Bellow Seals
Welded thin metal discs (typically Inconel, Hastelloy, or stainless steel) form a flexible bellow that acts as the spring and eliminates the need for a dynamic O-ring. This is the premium seal type for demanding applications.
- Best for: High-temperature services (up to 400 degrees C), media that coke or crystallize, boiler feed pumps, hot oil, and heat transfer fluid systems.
- Advantages: No dynamic O-ring means no hang-up. Excellent thermal cycling tolerance. Handles high temperatures where elastomers fail. Very long service life in the right application.
- Limitations: Higher cost than spring seals. The welded bellow can fatigue from excessive vibration or misalignment. Not suitable for high-pressure applications as the bellow has limited pressure capability.
- Typical sizes: 25 mm to 100 mm shaft diameter.
PTFE Bellow Seals
A PTFE (or glass-filled PTFE) bellow provides complete chemical inertness. Since PTFE is resistant to virtually all chemicals, these seals are the go-to choice for aggressive chemical services.
- Best for: Strong acids (HCl, H2SO4, HF), strong alkalis (NaOH, KOH), oxidizing agents, solvents, and pharmaceutical applications where contamination must be avoided.
- Advantages: Near-universal chemical resistance. No elastomer in contact with the process fluid. FDA-compliant material for food and pharmaceutical use.
- Limitations: PTFE has limited memory — it can cold-flow under pressure and may not recover its original shape. Temperature range is narrower than metal bellows. The bellow is more fragile than metal and can crack if over-compressed.
- Typical sizes: 20 mm to 120 mm shaft diameter.
Elastomer Bellow Seals
A rubber bellow (typically NBR, EPDM, or Viton) provides the secondary seal and a degree of spring action. These are the simplest and most economical non-pusher seals.
- Best for: Clean water, HVAC systems, light-duty pumps, and OEM applications where cost is the primary driver.
- Advantages: Very low cost. Simple design with few parts. Good for applications with moderate temperature and pressure.
- Limitations: Elastomer degrades at high temperatures. Limited chemical resistance depending on the elastomer grade. Shorter service life than metal or PTFE bellows.
Cartridge Seals: The Installation-Proof Option
A cartridge seal is not a separate seal type — it is a packaging format. Any seal (single spring, multi spring, metal bellow, double seal) can be supplied as a cartridge assembly. The seal, gland plate, sleeve, and drive mechanism are pre-assembled on a cartridge unit and shipped as a ready-to-install package.
Why this matters: studies consistently show that 30-40% of premature seal failures are caused by installation errors — wrong compression, damaged O-rings, incorrect setting lengths. Cartridge seals eliminate these errors because they arrive pre-set. The technician bolts on the cartridge, removes the setting clips, and the seal is correctly installed.
- Best for: Critical equipment, hazardous services, plants with limited seal installation expertise, and any application where the cost of unplanned downtime justifies the premium.
- Advantages: Pre-set compression eliminates measurement errors. Faster installation and removal. Can be replaced without disassembling the pump. Available in single and double configurations.
- Limitations: Higher initial cost than component seals. Requires more axial space than a component seal.
- Typical sizes: 12 mm to 200 mm shaft diameter.
Quick Reference: Which Type for Which Application
| Application | Recommended Seal Type |
|---|---|
| General water / HVAC pumps | Single spring or elastomer bellow |
| Chemical processing (mild to moderate) | Multi spring, balanced |
| Aggressive acids / alkalis | PTFE bellow |
| High temperature (above 200 degrees C) | Metal bellow |
| Coking or crystallizing media | Metal bellow (no dynamic O-ring to hang up) |
| Toxic / hazardous media | Double seal (any type) with barrier system |
| Critical equipment, minimize downtime | Cartridge seal |
| Agitators / mixers / reactors | Heavy-duty cartridge or agitator-specific seal |
| Dry / gas applications | Dry running seal with hard face combination |
Choosing the Right Type
Understanding seal types is the foundation, but the final selection always depends on the specifics of your application — the media, temperature, pressure, shaft speed, and equipment constraints. For a step-by-step selection process, read our guide to choosing the right mechanical seal.
If you are not sure which type fits your application, that is exactly the kind of question our engineering team handles daily. Share your operating conditions and we will point you in the right direction.