February 21, 2022 | 5 minute read
Dry gas seals are an integral part of modern centrifugal compressors. They provide reliable shaft sealing on turbomachinery and reduce process gas leakage. The separation seal is located between the dry gas seal and bearing cavity and uses separation gas flow to protect the dry gas seal from oil ingress and prevent leakage from the dry gas seal into the lube oil cavity. It is integral to the dry gas seal and a key element for the safe and normal operation of the compressor.
Separation seals can be in the form of labyrinths, contacting bushing seals like the John Crane Type 83, and non-contacting bushing seals like the John Crane Type 93FR. Labyrinths require a large clearance to ensure contact-free operation. Both the thermal clearance and radial shaft movements from oil film bearings are drivers of this clearance gap. As a result, labyrinths suffer from the highest separation gas consumption. By comparison, contacting bushings have the minimum clearance and gas consumption, but incur wear.
API 692 recommends the separation seals should be non-contacting bushing seals . However, gas consumption of traditional non-contacting bushings is not constant because the clearance of non-contacting bushings is largest during cold/static conditions and is smaller during hot/dynamic conditions. This is due to the coefficient of thermal expansion for carbon being only half of the surrounding metal sleeve. Therefore, to mitigate the contact, the traditional non-contacting bushing design needs to account for the difference in thermal coefficient of expansion.
With nearly 30 years of separation seal design and application experience, John Crane developed a new separation seal that improves oil restriction capability and mitigates the disadvantages of non-contacting variants.
The Type 93LR is a lift-off, non-contacting, bushing seal which combines dry gas seal and separation seal technology. In the static unpressurized condition, the Type 93LR carbon ring closely fits with the shaft/sleeve, leading to a tight restriction preventing oil ingress. In the pressurized condition, the Type 93LR lifts and forms a stiff film for a wear-free operation. In high vibration conditions as described in ISO 7919-3 and API 617, the stiff gas film helps prevent wear.
The Type 93LR also features a patented joint design to further minimize the oil ingress and unwanted gas leakage through the joints. The separation gas velocity in static and dynamic conditions will be higher than the API 692 requirement of gas velocity to prevent dry gas seal oil ingress, while the gas consumption is ~50% lower than the Type 93FR non-contacting bushing and ~80% lower than a labyrinth in the flow control mode. For customers, this can dramatically improve energy efficiency and decrease expenditure. This solution has no restriction to gas types or dew point.
The Type 93LR has undergone intensive testing programs that validate the concept, showing no wear and no oil ingress while maintaining consistent low separation gas consumption. Particularly valuable to end-users with legacy compressor systems, the Type 93LR can be retrofitted to installed separation seals with minimum modifications to the gas seal support system to ensure the fastest payback. This makes the Type 93LR an ideal choice for every application that requires a separation seal with minimum consumption, while also providing constant protection against bearing oil.
 API Standard 692: Dry Gas Sealing Systems for Axial, Centrifugal, Rotary Screw Compressors and Expanders, 1st ed., American Petroleum Institute, Washington D.C, June 2018.
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