Seal performance is one of the most important issues in a high-pressure turbo-compressor. The seal type and details can influence its rotordynamics behavior and overall stability. Generally, traditional labyrinth seals provide much lower levels of stiffness, damping and forces, which usually do not influence the rotordynamics and the damping. The forces generated by this type of seal are much less sensitive to clearance changes, gaps and seal details.
In the case of modern seals (such as honeycomb or hole-pattern, which usually presents a non-linear behavior), there is more stiffness and damping. However, vibration modes (such as bending modes) and natural frequencies can considerably affected by seal operation. These modern seal options could result in rotor instability if the system is not modeled and designed properly.
Non-linear approach
For traditional labyrinth seals, radial and tangential forces depend linearly on frequency and stiffness. Damping coefficients can be identified with reasonable accuracy by a linear curve that can measure forces in radial and tangential directions. However, this old-fashioned description of forces cannot be applied to a honeycomb or a hole-pattern seal. A non-linear approach should be selected for a proper modeling.
Honeycomb and hole-pattern seals have been successfully used in various compressor applications. Some theoretical studies suggest rotors at high pressure (for example, turbo compressors using honeycomb seals) would become unstable. If proper modeling methods are employed and correct designs applied, a stable high-pressure turbo-compressor can be obtained.
In high-pressure compressor applications, good damping and positive seal stiffness bring about better rotor stability. During start-up or shutdown, seal clearance could reduce because of thermal effects. In steady state operation, pressure differences can also affect seal geometry.
The margin to a seal’s stability threshold should be sufficiently high. Compressor seal design should be optimized with respect to leakage, performance, reliability and stability, particularly without impacting rotordynamics. It is recommended, therefore, to use damper seals, typically of the hole-pattern or honeycomb type for compressor internals to ensure stability when compressing high-density gases at high pressure.
Hole-pattern seals
Hole-pattern seals present a superior damping ratio (usually twice the damping ratio of the labyrinth seal). They exhibit excellent damping performance in the middle of the compressor characteristic curve where it is generally needed. Damping ratio increases as discharge pressure rises.
If the seal gap in operation is not sufficiently converging, the seal can show a weak changing tendency in the rotor natural frequency with an increasing discharge pressure. This is not the recommended design, because turbo-compressor stability could be affected over time by fouling, degradation or similar effects. The clearance of the seal should be kept as low as possible and convergent at all operating conditions.
Honeycomb seal case study
A case study is presented for a high-pressure centrifugal compressor with a rotor mass: 335 kg, a bearing span: 1350 mm, a total shaft length: 1710 mm, and a rated speed: approximately 12,300 rpm (around 205 Hz).
For the unloaded compressor, the 1st bending horizontal frequency and the 1st bending vertical frequency are around 121 Hz and about 129 Hz, respectively. Because of seal influence, horizontal and vertical mode turns into more circular forward and backward whirling modes for loaded operation.
The behavior of the loaded compressor rotor with labyrinth seal and swirl brake is not different from its unloaded operation. In loaded operation for a compressor with a labyrinth seal on the balance piston, the 1st bending backward frequency and 1st bending forward frequency are around 119 Hz and approximately 126 Hz respectively (say around 3-5% difference with unloaded ones). In the case of a labyrinth seal, radial forces are moderate and cannot influence the natural frequencies. Also, circumferential forces cannot destabilize the bending mode.
Large seal forces
For a compressor with a honeycomb seal on the balance piston, in loaded operation, the 1st bending backward frequency and 1st bending forward frequency are around 249 Hz and approximately 241 Hz respectively. For the honeycomb seal, there are considerable influences on the rotordynamics because of the large seal forces, particularly in the radial direction.
In other words, in a high-speed high-pressure compressor using the modern honeycomb seal, the rotor is started in operation, in a supercritical situation. Upon increasing the compressor load and gas density, the natural frequency will increase, thus bringing the rotor into a damped resonance (with a high damping) and then the resonance is passed by increasing the rotor speed to the rated speed. The 1st bending forward frequencies are around 152 Hz, 209Hz and 241 Hz for 40% load, 80% load and 100% load respectively.
(The author is a Chartered Professional Engineer in Australia, Queensland and U.K. An M.Sc. and B.Sc. graduate in mechanical engineering, he is a senior consultant specializing in rotating equipment, condition monitoring and reliability).
