Seals dry gas

To this day, balanced mechanical seals using o-rings are the standard in industry. Meehanical seals eontinue to evolve in sealing face technology, computer design, finite element analysis, cartridge designs, split seals, double or dual seals, and dry gas seals. 

Dry gas seals, when eeonomie eonsiderations allow, are another alternative for redueing gas leakage. Dry faee-seal systems have been used sueeessfully and field experienee has proven their effeetiveness. 

Two or four lobe bearings are used in these expanders, determined by the funetion of the rotor-dynamie requirements. A double-aeting thrust bearing may be mounted, if required. Labyrinth seals with eonneetions for seal gas are standard equipment, however, dry gas seals are available. 

CASE STUDIES ON ACTIVE MAGNETIC BEARINGS (AMB) AND DRY GAS SEALS (DGS) ... 

Active Magnetic Bearings (AMB) and Dry Gas Seals (DGS) in Hydrocarbon Processing... 

In this case, details of turboexpander design with dry gas seals and active magnetic bearings are presented in enough detail that the application to this specific process is clarified. 

Turboexpanders with Dry Gas Seals. As mentioned above, the application of dry gas seals in centrifugal compressors dates back to the early 1980s. They are useful in process compressors because seal gas leakage is considerably reduced. Also, the contact of process gas and other constituents that may dilute lubricating oil is minimized. 

The first documented use of dry gas seals in turboexpanders was in 1989. At that time, an ethylene plant in Scotland worked with a dry gas seal manufacturer to retrofit a turboexpander-integral gear-generator package. The partial success of that project was sufficiently encouraging for both user plant and turboexpander manufacturer to undertake a redesign of all three expander stages. Dry face or gas seals were installed at that time. 

This successful experience led to the design, manufacture, and use of turboexpanders with dry gas seals. Table 6-2 lists the installations and major design parameters of dry gas seals. 

Turboexpander manufacturers and dry gas seal vendors recognizing the above limitation, worked together to develop a dry gas seal design that is an excellent compromise given the space limitation and customer requirement. Figure 6-16 shows a cross-section of the dry gas seal developed for turboexpanders. 

The turboexpander dry gas seal consists of the conventional dry gas seal mating ring and primary ring, an outboard labyrinth, an inboard labyrinth, and tlie cavity to be vented, if desired. Tlie outboard labyrinth reduces warm seal gas leakage to the process side efficiency deterioration is thus minimized. The inboard labyrinth, on one hand, provides an additional seal between the process and lubricating fluids. On the other hand, it allows injection of an inert gas, if desired. In the latter case, inert gas leaks to the bearing side and to the cavity between the... 

Clair, M., Application of Magnetic Bearings and Dry Gas Seals to an Overhung Compressor, Revolve Proceedings, April 1992. 

The use of dry gas seals in proeess gas eentrifugal eompressors has inereased over the last 30 years, replaeing traditional oil film seals in most applieations. Over 85% of eentrifugal gas eompressors manufaetured today are equipped with dry gas seals. 

The power loss can also be increased with seal contamination. The seal surfaces being noncontacting under dynamic conditions the power loss associated with dry gas seals is very small. The power loss for a 10-in. (254 mm) seal operating at 1000 psi (69 Bar) and 10,000 rpm is about 12-14 kW. With damage seal surfaces, these losses can be increased by 20-30%. 

Contamination from Bearing Lubrication Oii. A barrier seal is required on the outboard side of the dry gas seal, between the gas seal and the compressor bearing. The primary function of the barrier seal, typically buffered with air or nitrogen, is to prohibit the flow of bearing lubrication oil into the gas seal. Contamination of the dry gas seal from lube oil can occur when the barrier seal fails to function as intended. 

Contamination from Seai Gas Suppiy. Contamination from the seal gas supply oeeurs when the sealing gas is not properly treated upstream of the dry gas seal. Gas seal manufaeturers have stringent requirements for seal gas quality. Typieally, the sealing gas must be dry and filtered of partieles 3 mieron and larger. Filters are normally provided in the gas seal system to meet this requirement. 

Dry gas seals operate under extremely tight toleranees, whieh demand that speeial eare be taken in the design of the gas seal environment, and in the operation of the eompressor and gas seal system. While the threat of seal degradation and redueed seal life due to outside influenees is real, the detrimental effeets of these faetors ean be minimized. 

The replaeement of meehanieal seals by dry gas seals must be elosely examined. There have been many eases where the replaeement has eaused the eompressor to operate in an unstable manner. This is due to the faet that removal of the meehanieal seal eauses a ehange in the damping of the rotor and ean eause the rotor to operate eloser to its eritieal speed. 

Evenson, R.S., Mason, B., Frederick, D.V., St. Onge, and Alain, G. Development and Field Application of a Single Rotor Design Dry Gas Seal, Proceedings of the 24th Turbomachinery Symposium, Texas A M University p. 107 1995. 

Mayeux, T., Paul, Feltman Jr., and Paul, F., Design Improvements Enhance Dry Gas Seal s Ability to Handle Reverse Pressurization, Proceedings of the 25th Turbomachinery Symposium, Texas A M University, p. 149, 1996. 

Takeuchi, Takao, Kataoka, Tadashi, Nagasaka, Hiroshi, Kakutani, Momoko, Ito, Masanobu, Muraki, and Ryoji Advanced Dry Gas Seal by the Dynamic Ion Beam Mixing Technique, Proceedings of the 27th Turbomachinery Symposium, Texas A M University, p. 39, 1998. 

In dry compressors, shaft end seals are generally one of five type.s. These are labyrinth, restrictive ring, mechanical contact, liquid film, and dry gas seal. The labyrinth type is the most simple but has the highest leakage. The labyrinth seal is generally ported at an axial point between the seals in order to use an eductor or ejector to control leakage and direct it to the suction or a suitable disposal area. Alternatively, a buffer gas is used to prevent the loss of process gas. Appendix D presents a calculation method for use with labyrinth seals. 

The dry gas seal is a variation of the mechanical contact seal. It differs in that it uses a microscopically thin layer of gas to separate and lubricate the faces. The seal is configured in a tandem or double-opposed seal arrangement. More complete details are covered in Chapter 5 under Dry Gas Seals. 

Dry gas seals are in the positive seal class and have the same basic design features as mechanical face seals with one significant difference. The dry gas seal has shallow grooves cut in the rotating seal face located part way across the face. The grooves may be in a spiral pattern the exact location and pattern vary from one manufacturer to another. Lubrication and separation is effected by a microscopically thin film of gas. This implies some finite amount of leakage, which is quite small but must be accounted for in the design. 

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