Replacing Wet Seals with Dry Gas Seals in Centrifugal Compressors

The dry seal design was first patented in 1951. The latest spiral groove was patented in 1968. However, centrifugal compressors installed in the late 1980s/early 1990s still have wet seals. It makes them an excellent case for retrofitting dry seals to reduce operational costs.

This article aims to provide solid reasons to convert from earlier wet seals to Dry Gas Seals (DGS), explain their working mechanism, and outline their key benefits for centrifugal compressor operators. If you have a compressor with wet seals, read on to learn why and how you should convert it to a modern dry-gas-sealed compressor!

1. Why Convert Wet Seals to Dry Gas Seals?

Converting from legacy wet seals to dry gas seals offers economic, operational, and maintenance benefits. There is a trend from environmental protection agencies to promote the use of dry gas seals as they reduce emissions and operate cleanly. Converting wet seals to dry gas seals is a strategic business decision.

1.1 Economic Drivers for Conversion

According to lessons learned from Natural Gas STAR Partners (EPA, 2006), a dry gas seal can save up to $135,000 per year compared to a wet seal compressor. Which means that in some scenarios, the switch allows payback in 14months. The savings from preventing gas leaks alone justify the retrofit. For most cases, the payback period is less than one year. Owing to the nature of the dry gas seal, it helps avoid parasitic power losses. Wet seals, or oil seals, require seal oil pumps, fan coolers, and degassing systems to operate. Therefore, the power consumption of dry seal is 5kW/hr, whereas wet seals require a substantial 50 to 100 kW per hour. Switching to DGS results in reduced power consumption, annual cost savings, and efficient operation.

1.2 Environmental and HSE Imperatives

As of now >85% new compressors use dry gas seals. This highlights the rapid adoption and maturity of the technology. There will be enough support for the operation and maintenance of these seals in the near future. Moreover, the environmental impact of wet seal is methane emissions ranging from 40 to 200 standard cubic feet per minute (scfm). The gas is absorbed in seal oil and then vented. In contrast, a dry gas seal emits only 6 scfm.

For health and safety, the removal of oil use reduces the chances of fire hazard or explosion. Moreover, there is no change in oil leaking into the process gas, which can cause system degradation or contamination.

1.3 Technical Obsolescence of Wet Seals

Many of the problems associated with the older wet-seal design are due to the complex seal-oil system. It's simple: increasing the number of pieces of equipment increases the chances of failure. Periodic maintenance and surveillance to ensure equipment performance is also complex in wet seals.

In a dry gas seal, there is no wear during operation. It uses only a small amount of gas to create the non-contacting gap, which is why DGS systems often operate for 5 to 10 years without refurbishment, drastically improving compressor uptime.

2. Working Principles of Dry Gas Seals in Centrifugal Compressors

Understanding the working principles will help our readers determine whether they can retrofit their centrifugal compressors with modern dry seals. Let's begin with the core mechanics first:

2.1 Core Mechanical Design

The dry gas seal is typically installed at the location where it separates the high-pressure process gas from the bearing (atmospheric) side.

To simplify the core mechanical design, there are two sets of rings that are pressed against each other when the compressor is not running.

• Rotating Ring: The rotating ring is fixed on the compressor shaft and rotates with it. It is a ring with precision-engineered grooves laser-cut.

• Stationary Ring: The ring is held static with the dry gas seal housing. A set of springs pushes this ring against the rotating ring.

These rings are made from low-friction materials such as Silicon Carbide (SiC), Carbon, and Tungsten Carbide (TC). These seals can be either in tandem or single configuration, depending on the pressure and emission requirements. The grooves on the rotating ring can be API Plan G (APG) or T-groove design. The most recent innovation is the bidirectional groove, which is highly efficient even at low rpms to create a reliable high-pressure zone during slow roll or rotation in either direction.

2.2 Hydrodynamic Lift-Off Mechanism

The heart of the dry gas seal is its hydrodynamic lift-off mechanism. It is a precision-engineered product that allows the seal faces to operate without contacting each other, drastically reducing wear and tear.

The Lift Mechanism

The grooves on the rotating part are the key to the lifting mechanism. These grooves are microscopic in dimensions and typically have a spiral or T-groove pattern. Typically, these grooves are 2.5µm to 10µm deep. As the shaft starts to rotate, the grooves act like a gas compressor, drawing in sealing gas and increasing its pressure toward the center. As a result, a high-pressure zone is created, pushing the stationary ring away from the rotating ring. The result is a precise 3µm to 10µm non-contact gas film.

Stability and Reliability

The key to a stable performance and reliability of the dry seal is maintaining the gap between the two ring faces. The hydrodynamic pressure is balanced by the spring force. The dry gas seal is highly reliable and can operate at very high peripheral speeds, up to 250 m/s. Modern, patented bidirectional grooves generate hydrodynamic pressure even at low speeds in the opposite direction, significantly reducing wear and extending the life of the dry seal in centrifugal compressors. The outward flow of gas removes contaminants, further enhancing reliability.

2.3 Gas Supply and Conditioning System

The filtered, dried, and heated gas from the compressor discharge is returned to the primary port of the dry gas seal. The gas is then routed via a balance line, ensuring seals are exposed to lower suction. Differential pressure control is key to ensuring the correct gap. In tandem configuration, the N2 (Nitrogen) gas is injected into the intermediate labyrinth seal to buffer the secondary seal. It helps prevent the process gas from escaping into the air. 

3. Benefits of Dry Gas Seals for Centrifugal Compressors

3.1 Operational Reliability and Uptime

• Fewer parts → higher mean time before failure

• >2x service life vs wet seals

• Non-contact → zero wear during operation.

• No oil pump failures → less downtime.

• Excellent in H₂S, benzene, and toxic media

• No seal face damage from transients.

• Predictable performance over time.

• Lower vibration → protects bearings.

3.2 Energy Efficiency Gains

• 5 kW/hr vs 50–100 kW/hr for oil pumps

• No parasitic load from the circulation system.

• Improves pipeline efficiency

• 5–10% of wet seal power

• Lower carbon footprint per unit of gas

• No heat from oil friction.

• Reduced cooling water needs.

• Higher overall compressor efficiency.

3.3 Emission Reduction and Compliance

• ≤6 scfm vs 40–200 scfm methane

• 97% reduction in case study (75 → 2 Mcf/day).

• Zero leakage in chemical pumps (Lepu LPDGS21).

• Meets API 692 (N₂ labyrinth required).

• No degassing emissions from oil.

• Supports global methane pledges.

• Cleaner gas → no downstream scrubbers.

• Quantified ESG impact.

4. Advantages of Replacing Wet Seals with Dry Gas Seals

4.1 Quantified Cost-Benefit Analysis

• $240,000/year gas saved

• $135,360 implementation → 14 months payback

• $73,000 O&M savings

• Lower spare parts inventory.

• Reduced insurance from fire risk.

• Higher resale value of the retrofitted unit.

• Tax incentives for emissions reduction.

• Long-term ROI >10 years.

4.2 Elimination of Oil Contamination

• No oil in pipeline → prevents catalyst poisoning

• No oil disposal → lower hazwaste cost.

• Cleaner gas → higher market value.

• No oil mist in bearing cavity.

• No oil leaks during maintenance.

• No oil filter changes.

• No oil sampling required.

• No oil-related corrosion.

4.3 Mechanical and Maintenance Simplicity

• No oil pumps, degassers, or filters (EPA).

• Lower labor costs → fewer technicians.

• Easier troubleshooting → fewer alarms.

• No oil level checks.

• No oil heating/cooling.

• No relief valve testing.

• No seal oil trap cleaning.

• Digital monitoring simpler

4.4 Safety and Risk Mitigation

• No high-pressure oil → no fire/explosion

• Safer maintenance → no oil spills.

• Lower personnel exposure to hazards.

• No hot oil burns.

• Reduced confined space entry.

• No oil vapor inhalation.

• Compliant with OSHA process safety.

• Lower risk in H₂S environments.

5. Retrofit Process: Replacing Wet Seals with Dry Gas Seals

We believe you are now convinced to replace the wet seals on your centrifugal compressors with the latest dry gas seals. The process requires careful evaluation of your current equipment and ensuring that the new seal will be compatible with your system:

Step 1: Physical Integration Analysis

The first and most critical step is focusing on the mechanical compatibility of the new seal cartilage with the existing centrifugal compressor.

• Detailed Analysis: Determining the extent of rework will confirm whether the new dry gas seal fits within the original head/cavity.

• Dimensional Check: Review all the inboard and outboard diameters, the seal cartridge length, and the locking system to the compressor shaft.

• Casing Modification: Check if rework on the compressor shaft and compressor head is required. Early identification can help reduce future rework costs and project delays.

• Port Requirement: Buffer gas and sealing gas ports will be required. Ensure the compressor casing can accommodate at least 4 ports.

Step 2: Rotor Dynamics Re-Analysis

When the compressor is running, dynamic forces come into play. In the case of oil- or wet-seals, the damping characteristics are superior to those of dry seals. Therefore, the absence of the film can alter the vibrational response. A detailed rotor dynamic analysis is key to ensuring that the amplification factor and logarithmic decrement remain within acceptable limits. For a large centrifugal compressor with long shafts, consider additional upgrades, such as damper bearings or hole pattern seals, to maintain stability.

Step 3: Gas Seal System Design — Filters, Dryers, Heaters

The DGS (dry gas seal) requires a robust gas seal system that supplies reliable, dry, filtered gas to the seal. It should provide a seal under all conditions, including startup, shutdown, and pressurized standby. The system is referred to as Gas Seal Panel (GSP). The system should include a pre-filter, a booster, and a heater. There should be a sufficient 20 °C margin as per API 614 standards to maintain above the dew point. The presence of any moisture can damage the dry seal system.

Step 4: Operator Training — Alarms, SOPs

Operator training is key, as a dry gas seal is viewed as a “Black Box”. Training is necessary to ensure that the operators can interpret the indicators to assess the seal's health.

Recommendations for Retro-Fitting Dry Gas Seals

To ensure the retrofit goes smoothly, consider brands such as Lepu Seal, which provide a comprehensive retrofit guide with each of their dry gas seals. Moreover, their on-site supervision as OEM ensures that all tasks are performed correctly. Here are some things to consider for the process:

• Comprehensive Pre-Shutdown Planning

• Consider Specialized Vendor Solution

• Perform Post-Install Tests

• Check Rotor Dynamics

• Ensure Leakages are Within Limits

• Update P&IDs and Other Documentation

6. What's The Difference Between LEPU's Dry Gas Seal And Others?

6.1 Direct OEM Replacement Capability

LEPU Seals are designed for direct OEM replacement. They have a comprehensive drop-in dry gas seal solution for major global brands. Models like the LP-28 are 100% equivalent to John Crane's popular 28-series seals (including 28AT, 28XP, and 28ST), while the LPGAS is a direct equivalent to the Flowserve GASPAC, featuring identical T-groove or APG patterns. The universal LPDGS 801 is specifically designed for retrofitting units from John Crane, Flowserve, and EagleBurgmann.

Key Takeaway: LEPUs' drop-in dry gas seal features eliminate the need for any housing modifications.

6.2 Cost and Delivery Advantage

Leveraging direct contact with the OEM reduces cost. There is no middleman man which reduces the lead times to 2-4weeks from 12-16 weeks often quoted by OEMs. The pricing of LEPU seals is 30-50% lower than that of major OEMs such as John Crane and Flowserve. To facilitate retrofit trials, LEPU offers free samples (one set per client) and maintains a large in-stock inventory of over 500 units for common equivalents, backed by a 12-month price lock guarantee.

6.3 Advanced Engineering and Testing

LEPU utilizes advanced design and rigorous in-house testing that meets or exceeds global standards. A key proprietary feature is the Seal Protector™, which dramatically eliminates the need for a seal gas booster and reduces contamination risk by over 90%. The seals feature a patented bidirectional groove design to ensure film stiffness and reliable lift-off even at low speeds (below 1,000 RPM). The APG (Advanced Pattern Groove) design provides a 30% higher lift-off speed than standard grooves. Seals are validated in the test lab up to 50,000+ RPM, confirming that final leakage rates are held below 6 scfm at full operational pressure. Additionally, cryogenic models undergo thermal-shock testing (e.g., 40 °C to 200C °C in 30 seconds), and all designs are pre-validated using digital twin simulations to predict a 10-year wear life.

6.4 Global Field-Proven Performance

LEPU dry gas seals are field-proven globally, operating reliably in some of the most challenging industrial applications. Case studies include SINOPETRO, where over 10 units have achieved more than five years of continuous operation in CO2 reinjection service, and YANGZI PETROCHEMICAL, which reported zero failures using the LPDGS21 in a hydrogen recycle compressor. The LP-28 dry gas seal model is proven in extreme climates, running in Gazprom's Siberian pipeline at 50 °C, and the LPGAS model is used in demanding sour gas applications for NIOC in Iran. Across over $1,000 global clients, the company maintains an internal KPI of over 98% on-time reliability, with client data showing a 97% reduction in emissions post-retrofit. The company holds ISO 9001, API Q1, and CE certifications.

7. FAQs

Q1. How do dry gas seals work, and what are dry gas seals used for?

Dry gas seals use spiral grooves on a rotating ring to pump clean gas, creating a 3–10 µm non-contact film that prevents leakage. They are used in centrifugal compressors to isolate high-pressure process gas from bearings, eliminating oil and reducing emissions.

Q2. Why is a dry gas seal better than an oil seal? Can all compressors be retrofitted?

Dry gas seals emit ≤6 scfm vs 40–200 scfm, use ~5 kW vs 100 kW, and require less maintenance. There is no oil contamination or fire risk. Not all compressors can be retrofitted due to housing design or operational constraints. Brands like LEPU seals offer drop-in dry gas seals for quick replacement.

Q3. When should we replace the dry gas seals?

Replace dry gas seals when signs appear: abnormal vibration, pressure drop, increased leakage (>6 scfm), overheating, or power spike. A scheduled overhaul every 5–7 years, or after 50,000+ operating hours, ensures reliability. Monitor via seal gas flow and temperature.