Flygt submersible pumps, manufactured by Xylem, are among the most widely installed submersible pump brands in the global water and wastewater industry. From municipal sewage lift stations to industrial dewatering applications, Flygt pumps operate in demanding conditions where mechanical seal reliability directly determines pump uptime and maintenance costs.
This comprehensive guide covers everything distributors and service technicians need to know about Flygt pump seal replacement: understanding the dual seal system, selecting the correct replacement seal, installation best practices, and how aftermarket seals compare to OEM originals in terms of quality, performance, and cost.
1. Understanding the Flygt Dual Seal System
Unlike conventional pumps that use a single mechanical seal, Flygt submersible pumps employ a dual mechanical seal system with two independent seals separated by an oil-filled buffer chamber. This dual seal arrangement is fundamental to the pump’s reliability and motor protection strategy.
Lower Seal (Product Side)
The lower seal is the first line of defense between the pumped media and the pump’s internal components. It faces the harshest operating environment, directly exposed to sewage, sludge, abrasive particles, fibrous debris, and chemically aggressive wastewater. For this reason, the lower seal universally uses tungsten carbide versus tungsten carbide (TC/TC) face combinations — the hardest commercially available seal face pairing — to resist the constant abrasion and impact from solids in the pumped media.
The lower seal must also handle the full discharge pressure of the pump, which can reach several bar in deep wet-well installations. Its robust construction features heavy-duty springs and corrosion-resistant metal components designed to withstand years of continuous submersed operation.
Upper Seal (Motor Side)
The upper seal operates in the controlled environment of the oil-filled buffer chamber, separating the lubricating oil from the motor cavity. Because the buffer oil is clean and filtered, the upper seal does not face the same abrasive challenges as the lower seal. This allows the use of more economical face combinations such as SiC versus carbon graphite or ceramic versus carbon.
However, the upper seal serves an equally critical function: protecting the electric motor from moisture ingress. If the lower seal develops a leak, the buffer oil chamber provides a secondary containment zone. The oil level sensor (or moisture sensor in newer models) detects contamination and triggers an alarm before the leaked media reaches the motor windings. This dual-barrier design is a key reason why Flygt pumps achieve long service lives even in aggressive wastewater environments.
Buffer Oil Chamber
The buffer chamber between the two seals is filled with a specific grade of oil that serves three purposes: lubricating both seal faces, dissipating heat generated by the seal’s friction, and providing a visible leak detection medium. During routine maintenance, inspecting the buffer oil condition — checking for discoloration, milky appearance (water contamination), or reduced level — provides an immediate indication of seal health without disassembling the pump.
2. When to Replace Flygt Pump Seals: Warning Signs and Diagnostics
Flygt pump seals are wear components with a finite service life that depends on the application severity, pump operating pattern, and media characteristics. Knowing when to replace seals before they fail catastrophically can prevent expensive motor damage and unplanned downtime.
Elevated Motor Temperature: A failing lower seal allows process media into the buffer chamber. The contaminated oil loses its thermal conductivity, causing the motor to run hotter. If the motor temperature sensor triggers frequent high-temperature alarms, seal inspection is warranted.
Buffer Oil Contamination: Milky or discolored buffer oil is the most reliable early indicator of lower seal wear. During scheduled maintenance, always sample the buffer oil. Clean oil means the lower seal is intact; any signs of water or solids contamination indicate the seal is leaking.
Moisture Sensor Alarms: Modern Flygt pumps equipped with moisture detection in the motor cavity will alarm when water bypasses both seals. At this stage, both seals have likely failed and immediate pump pull and seal replacement is required to prevent motor winding damage.
Increased Vibration: Worn seal faces cause uneven loading and shaft deflection that manifests as increased pump vibration. Vibration monitoring can detect seal degradation before a catastrophic failure occurs.
Visible Leakage at Cable Entry: In severe cases, process media can travel along the motor shaft and exit through the cable gland, indicating complete dual seal failure.
3. Flygt Seal Model Cross-Reference: Matching Seals to Pump Models
Flygt’s product range spans dozens of pump models across the 2000, 3000, 4000, 5000, and 7000 series. Each pump model uses a specific seal size and configuration determined by the shaft diameter, seal chamber dimensions, and the pump’s intended application.
The following cross-reference covers the most commonly serviced Flygt pump families:
20mm shaft seals: Flygt 2610, 2620, 2630, 2640, 4610, 4620 — Compact plug-in seal design for small dewatering and drainage pumps.
35mm shaft seals: Flygt 2870, 2670, 3153, 5100, 2100 — Medium-duty municipal and industrial wastewater pumps. One of the highest-volume replacement seal sizes.
45mm shaft seals: Flygt 3171, 4650, 4660 — Larger capacity wastewater and stormwater pumps.
60mm+ shaft seals: Flygt 3301, 5150-300/310/350/360 — Heavy-duty pumps for large municipal treatment plants.
When ordering replacement seals, the critical information required is the pump model number (found on the pump nameplate), the shaft diameter, and whether the seal is for the upper or lower position. Aftermarket manufacturers designate their Flygt-compatible seals using proprietary naming systems, but a reliable supplier will maintain a complete cross-reference database that maps every Flygt pump model to the correct upper and lower seal part numbers.
4. OEM vs. Aftermarket Flygt Seals: Quality, Cost, and Performance Comparison
Original Xylem/Flygt seals are engineered and tested specifically for Flygt pumps, but they carry a premium price — often 3 to 5 times higher than quality aftermarket alternatives. For distributors and municipal maintenance departments managing large installed bases of Flygt pumps, the aftermarket seal market offers significant cost savings without necessarily compromising on quality.
What to Look for in Quality Aftermarket Flygt Seals
Not all aftermarket seals are created equal. The key quality differentiators to evaluate are:
Face Materials: The lower seal must use TC/TC (tungsten carbide vs. tungsten carbide) faces. Some low-cost aftermarket seals substitute ceramic or SiC for the stationary face, which dramatically reduces abrasion resistance in sewage applications. Always verify the face combination before purchasing.
Elastomer Grade: Premium aftermarket seals use FKM (Viton) elastomers as standard, which provide superior chemical and temperature resistance compared to the NBR (Buna-N) used in many economy-grade aftermarket and some OEM seals.
Metal Components: Hastelloy C-276 drive bands and compression springs provide the best corrosion resistance in wastewater. Lower-quality seals use 304 or inferior grades of stainless steel that corrode within months in aggressive sewage.
Dimensional Accuracy: The seal must be manufactured to OEM dimensional specifications to ensure proper fit in the seal housing. Seal face flatness should be verified to within 2 light bands (0.00058mm) under monochromatic light.
Spring Design: The spring must provide consistent closing force throughout the seal’s wear life. Clogging-resistant spring designs that prevent fiber buildup are essential for sewage pump applications.
5. Flygt Seal Installation Best Practices
Proper installation is critical for achieving maximum seal life. Even the highest-quality seal will fail prematurely if installed incorrectly. Follow these best practices for Flygt pump seal replacement:
Step 1 — Clean the Seal Chamber: Remove all old seal debris, hardened deposits, and corrosion from the seal housing and shaft. Any contamination left in the seal chamber can damage the new seal faces during the first hours of operation.
Step 2 — Inspect the Shaft and Sleeve: Check the shaft surface for scoring, wear grooves, and corrosion. The shaft surface finish at the seal mounting area should be between Ra 0.4 and 1.0μm. A worn or scored shaft will damage the new seal’s O-ring and cause premature leakage.
Step 3 — Lubricate Before Assembly: Apply a thin film of clean barrier oil or silicone grease to all O-rings and the shaft surface before sliding the seal into position. Never install dry O-rings — the friction during installation can twist or nick the O-ring, creating an immediate leak path.
Step 4 — Verify Seal Orientation: Flygt seals are designed for a specific rotational direction in some models. Verify the spring rotation direction matches the pump’s shaft rotation before final assembly.
Step 5 — Fill with Correct Buffer Oil: After installing both seals, fill the buffer chamber with the specified oil grade to the correct level. Underfilling reduces cooling capacity; overfilling creates back-pressure on the upper seal.
Step 6 — Run-In Period: Avoid subjecting the pump to maximum load immediately after seal replacement. A gradual run-in period of 2 to 4 hours at reduced load allows the seal faces to bed in and establish a stable fluid film.
6. Extending Flygt Seal Service Life: Operational Best Practices
Beyond proper installation, several operational practices can significantly extend Flygt pump seal life and reduce total cost of ownership:
Monitor Buffer Oil Regularly: Implement a quarterly buffer oil inspection schedule. Early detection of oil contamination allows planned seal replacement before the motor is damaged.
Avoid Dry Running: Ensure the pump operates only when fully submerged or when a minimum submersion level is maintained. Even brief dry-running episodes can destroy the lower seal faces in minutes.
Manage Solids Content: In applications with exceptionally high solids content or fibrous material, consider installing a seal flushing system that directs clean water to the lower seal area, reducing the abrasive load on the seal faces.
Maintain Cooling: Submersible pumps depend on the surrounding liquid for motor and seal cooling. If the pump operates in a poorly ventilated sump or handles hot media, ensure adequate cooling arrangements are in place.
Stock Critical Spares: For installations with multiple Flygt pumps, maintaining a stock of upper and lower seal sets for the most common pump models eliminates emergency procurement delays and reduces downtime from hours to the time required for physical seal replacement.
Conclusion
Flygt submersible pump seals are precision-engineered components that protect the pump’s most expensive element — the electric motor. Understanding the dual seal system, recognizing early failure indicators, selecting quality replacement seals with verified materials, and following proper installation procedures will maximize seal life and minimize the total cost of maintaining your Flygt pump fleet.
Yongbang Seals manufactures a complete range of Flygt-compatible replacement seals covering the full 2600 through 7100 pump series. Our ASI series seals feature FKM elastomers, TC/TC face combinations, and Hastelloy-grade metal components as standard. Contact us for a cross-reference lookup on your specific Flygt pump model.
