Dry Gas Seals

COMPRESSORS CAN BENEFIT FROM A CORRECT DRY GAS SEAL UPGRADE

Dennis Schailbly

Anyone who decides to upgrade a process compressor from OEM oil seals to a Dry Gas Seal (DGS) should be aware of many important factors. Depending upon the age and design of the compressor, the first challenge is the number of available ports in the seal gland. OEM oil seals only need ports for oil-in, oil-out, sour drain and possibly buffer gas. A simple tandem DGS will need to convert these ports to “clean” gas supply, primary leak off, secondary leak off and barrier gas supply. This makes

A simple tandem DGS will need to convert these ports to “clean” gas supply, primary leak off, secondary leak off and barrier gas supply. This makes design of the DGS difficult without modifying the compressor case/seal gland. A new DGS with internal separation will need all those ports plus a secondary gas supply. To get from a basic oil seal with three ports to a DGS needing five ports would require new heads on both ends of the compressor. This factor alone can challenge the cost justification of the entire project. Possible alternatives include a simpler DGS design, using a single-face seal design or deploying another type of seal altogether. This is a decision that the project engineer will need to make early on.

Critical speeds will change

Once the decision to use DGS is made, a rotor dynamic study is necessary to determine changes that will result from removing the damping of the oil seals compared to gas seals. Rotor critical speeds will change, and if this puts the critical point too close to the operating point, the bearings will need to be modified to add more damping and keep the rotor stable during all operational conditions. Most units require a bearing upgrade.

The gas control panel will undoubtedly be the single greatest expense and by far the most important part of the upgrade. Thirty years ago, gas panels were simple and cost about the same as the seals. Today, they are complex and the price tag is many times the cost of the seals and any spares.

Considerations — gas control panel:

• Will buffer gases be controlled on Flow or on Delta P?

• What parameters of the panel will be monitored?

• What is the controls methodology (regulators or controllers with valves)

• What is the location & electrical area classification of the installation?

• Should an open rack or enclosed panel design be used?

• What warning and shutdown logic should be followed?

• What data will be gathered and recorded?

• What type of instruments should be used?

Now that the seals, new bearings and the gas control panel have been specified, it is time to look at the work ahead to complete the change out. This entails removal of the old wet seals from the compressor, typically in conjunction with a compressor rebuild. This is followed by demolition and complete removal of the seal oil system, separating it from the bearing oil system, if necessary. Be sure to thoroughly clean everything. This includes all flow paths for buffers and the signal connection to the seal. Once done, install the gas control panel, connect to the control room for data and control, connect to buffer source(s) and power, and connect to the compressor (but only after cleaning the lines).

The next step is to install upgrades to the compressor, such as the seals and bearings. Be sure to also conduct user training on seal operation and maintenance prior to startup. Begin startup with manufacturer support to fine tune the operation. Consider what will be spent on this upgrade. A rotor dynamics study is likely to cost $30,000 to $50,000. New bearings and spares cost about $4,000, plus about $3,000 per inch per bearing. Seals and spares will probably amount to $25,000, as well as $10,000 to $15,000 per inch of shaft per seal. Now for the biggest expense: $450,000 to $900,000 or more on the gas control panel.

But there are other expenditures involved. There is the cost of a compressor rebuild, demolition of the old seal system, installation of the gas panel, training, cleaning, and various other services and support. It is vital, therefore, to lay all this out to understand the total cost for the upgrade and see if this expense is justified compared to the cost of operation of the OEM seals.

Dennis Schaibly is Sales and Service Engineer at Kaydon Ring & Seal, A provider of mechanical seals of all designs for oil & gas, chemical process, natural gas transmission & aerospace. For more information, contact dschaibly@kaydon.com, (985) 264-7401 or visit website kaydonringandseal.com