The following blog post is a preview of a more in-depth feature article on wet seal to dry gas seal conversions scheduled to appear in the March issue of Flow Control magazine. To register for a free subscription to Flow Control, go here.

The benefits of dry gas seals are such that conversions from traditional oil seals to dry gas seals may be advantageous to compressor operators. However, end-users should ask themselves several questions before deciding to retrofit their compressors with dry gas seals.

The decision to retrofit a compressor with these upgraded seals may be dictated by economic factors, HSE constraints, or technical considerations. Users should consider all of these factors when deciding whether or not to upgrade a compressor with dry gas seals.

In addition, the following precautions should be taken during project execution to ensure successful conversion: perform a detailed physical integration analysis of the dry gas seal in the existing compressor; conduct a detailed rotor dynamic analysis; select the proper gas seal system design for the compressor; and plan for operator training.

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To expect a totally leak-free sealing system between two parts in relative movement is unrealistic (e.g., between a static and a rotating part; between a housing and a shaft; in pumps, thermal motors, etc.). There are, however, efficient devices that may limit leaks, friction, and wearing at the interface of the moving parts.

Dry Gas Seal Cross Section & Cutaway
Cutaway and cross-section of a simplified gas seal.

Gas seals are among the most efficient means to minimize process gas leakage to the atmosphere and to reduce wear and friction.

Why Convert Wet Seals to Dry Gas Seals?

1.   The number one reason for retrofitting conventional wet seals to dry gas seals is reliability. Dry gas seals are non-contacting mechanical seals, which eliminates the issue of seal wear. Theoretical lifetime is limited only by the secondary sealing elements (usually o-rings or polymer-based seals) whose lifespan can be as long as 15 years. It is not uncommon to see dry gas seals operating for more than 10 years before being refurbished, which is much longer than is expected for oil seals.

2.   Local (or company-wide) HSE Regulations: Elimination of oil contamination by process gas has a positive environmental impact, since sour oil needs to be treated, stored, and disposed of. Sour seal oil treatment and disposal also has a significant cost.

3.   Reduced Operating Costs: Energy costs drop significantly, since seal oil pumps and degassing tank heating systems are not required when using dry gas seals. Power losses due to shear forces in gas seals are much lower than losses experienced in oil seals, which results in energy savings as well.

4.   Reduced Maintenance Costs: As stated above, the simplicity of gas seal systems means routine maintenance is less frequent and less costly than it is with oil seal systems.

5.   Reduced Emissions: Wet seal gas leakages are reduced 10-fold with gas seals, credited to the very thin running gaps between the seal faces. This results in cost savings for the end-user and reduced penalties on taxable gas flaring.

6.   Process Quality: Contamination of process gas by seal oil is eliminated, enabling higher quality process gas. Costs related to oil removal from process gas are also eliminated. A good example is closed loop/refrigeration processes where process gas treatment is costly.

7.   Maintainability: Some operators now have more experience with dry gas seals than with oil seals. This may compel end-users to retrofit a fleet at a specific plant or site to achieve consistent sealing technology throughout.

Dry gas seals are supplied as cartridges by vendors, and the gas seal OEM usually performs their maintenance/refurbishment.

These seven benefits may not be applicable to all situations, and it should be noted that wet seals to dry gas seals conversions are not straightforward. The following recommendations are offered to help make the retrofit project a success. A careful review of the system and its operating conditions is required. Provided all precautions are taken.

Raphaël Bridon began his career with Dresser-Rand in 1999 as a Technical Support Engineer. He then moved to a Project Development Engineer position for reciprocating compressors before working as a Key Account Purchaser in the aeronautic business. He returned to Dresser-Rand in 2007 as the Manager for the Gas Seals and Bearings Business Unit. He holds a master’s degree from Ecole Centrale Nantes (France).

www.dresser-rand.com