Types of Seals for Turbomachinery

In the Turbomachinery industry there are a variety of seals used to prevent a machine's process gas from leaking into the atmosphere or keep oil from going into the process gas. These types of scenarios can lead to efficiency loss or severe damage to a machine. As temperatures and pressures vary, the choice of a seal will change to meet the specific need and create a suitable seal. To meet these needs, there are a lot of different materials used such as: rubber or graphite. The first choice for a design engineer when sealing is necessary is the Oring. 

The different types of seals are listed below:

-Orings: elastomers that are stretched and compressed to create the seal. Can be used in static and dynamic applications, as well as axial or radial sealing configurations.

-Gaskets: fills the irregularities between two mating surfaces while compressed (e.g. Flanges on pipes) in order to prevent leakage.

-Spring Energized Teflon (PTFE) Seals: These seals consist of a spring energized "U-shaped" jacket with different profiles and make use of different spring types and materials. They are used when normal elastomer orings cannot meet the temperatures (e.g. cryogenic temps in the Expander side of a Turboexpander), friction requirements, or chemical resistance of an application. These seals can meet temperature ranges from -328°F to 500°F or -200°C to 260°C.

Labyrinth Seal Example

-Labyrinth Seals: used for low pressure gas applications and where low leakage could be tolerated. They have the main advantage of being low cost. These seals are non-contact and use multiple "teeth" in order to create a difficult path for the process gas to overcome. Typically these laby seals use little to no friction with very small clearances and will sometimes wear into a mating babbitted bore after a machine is first started up. The main disadvantage with this type of seal is that it can be destabilizing at high pressures.

-Floating Carbon Ring Seals: One of the most commonly used shaft seals for the rotor assembly of an integrally geared compressor. Makes use of floating carbon rings with a very tight clearance. These type of seal has a reasonable cost and also lower leakage rates than labyrinth seals. The main disadvantage is the wear that occurs on the carbon rings over time and the risk of these rings locking up due to high pressure and leading to destabilization. 

Floating Carbon Ring Seal Exploded View

-Dry Gas Seals: The most expensive and complex seal available. Can have various configurations in order to have redundancy to aid in the event of a failure. This however, will require the shaft to grow axially to account for the larger seal configuration. The main disadvantage with this seal is cost and the rotordynamics challenge due to the larger shaft.

Tilting Pad Bearing Preload Calculator

 Here is a quick reference for calculating bearing pad preload. If you want to learn more about the characteristics of bearing assembly clearance, pad clearance and preload then visit my tilting pad bearing design parameters page. A good bearing engineer will look into rotordynamics and start with industry standards, and then fine tune the bearing preload in order to meet a good balance of all factors.

Diametral Clearance and Interference Calculator

Below is a calculator that I created in order to calculate the diametral clearance encountered when trying to assemble a shaft into a part with a bore (hole). 

Tips on Diametral Clearances

This is one of the most common and simple calculations that a designer must do in order to ensure ease of assembly and control critical areas of a machine that will need precise alignment. Clearances will also control the radial tolerance stackup of parts and needs to be attended to so that there will be no assembly issues. A designer should also consider the thermal growth and shrinkage of parts based on material properties during a machine's operation. A good engineer will understand as much as he/she can about the process conditions in order to proper material selection for their parts. A close and tight fit could come apart under operating conditions and become a loose fit due to thermal effects, which may lead to complete failure of a machine.

For example, the Rotor Assemblies and Bullgear Assemblies of an Integrally Geared Centrifugal Compressor (IGC) will need to use tight radial clearances with the fluid film bearings supporting these assemblies. The Impeller/Wheel and Shroud radial clearance of a Centrifugal Compressor or a Turboexpander is one the most critical clearances that is also held to a close clearance and takes into account the axial end play of a machine. Any change to this clearance may reduce the overall machine efficiency drastically, and its effect is relative to size. Therefore, the smaller the machine is, the tighter the clearance, and thus the more effect on efficiency that a clearance has. 

Thread Standard Types and Drawing Specification

 Threads are used both in internal and external applications for fastening two parts and creating a secure mechanical joint with the appropriate hardware combination which will vary with application and process conditions. For holes you use internal threads and for shafts you use external threads. When making an engineering drawing, you have to give the thread size, thread pitch, drill size, thread and drill depths, and thread class.

There are a variety of thread standards used in the world, but the most common include: American and Metric (ISO) thread standards. The American standard is better known as the Unified National Thread which includes UNC (coarse) and UNF (fine) that distinguishes the pitch type. Metric thread which is simply known as "M" also has fine and coarse types.

There are even special threads used for special purposes such as pipe thread standards which are used to join pipes and fittings. The most commonly used pipe thread standards are NPT (National Pipe Threaded), BSPP (British Standard Pipe Parallel) and BSPT (British Standard Pipe Tapered). These are used to define both straight and tapered threads. The British pipe standard BSPP and BSPT have become obsolete recently and have changed to be known as G and R/Rp/Rc designations, respectively. 

Another common thread in industry is the SAE thread type, which uses a straight thread sealing boss. It makes use of a BUNA-N rubber O-ring to ensure sealing of fittings.

When specifying a threaded hole callout there is a common method followed in engineering drawings. Typically you first specify a counterbore depth and size first (if needed), then the drill size and depth, and finally the tap size, pitch, and depth. For example, a threaded hole that is specified in the following way: 1/4"-20 UNC  1"  DP; Indicates that a hole has a thread for a 1/4" bolt that has a Coarse thread pitch that is 20 threads per 1 inch and will be tapped to a depth of 1".

Tilting Pad Design Parameters

Journal Tilting Pad Design:

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When it comes to journal bearings in the Turbomachinery industry, the tilt pad bearing is used for the high speed rotor assemblies of centrifugal compressors because of their design flexibility. Tilting pad bearings usually come configured with 3 to 6 pads. The most commonly used configuration used in the turbomachinery industry is a 5 pad Tilt Pad Journal Bearing (TPJB). There are two main pad orientations that are used to describe how these pads are loaded by the shaft. These are load-on-pad (LOP) or load-between-pads (LBP).
 

You can easily verify which configuration is present for your rotor. Look at your TPJB and if one pad is centered with the very bottom centerline of your rotor, then you have a LOP orientation. If two pads straddle the bottom centerline of your rotor, then you have a LBP orientation.

When designing a tilt pad you have a few common design parameters such as: pad clearance (Cpad), bearing (assembly) clearance (Cbrg), journal (shaft) diameter, and preload.

Typical Tilt Pad Bearing Bores

Bearing (assembly) clearance (Cbrg)

This is equal to the diametral clearance from the assembly bore diameter minus the journal diameter. To get an approximate real life assembly clearance, a lift check can be performed using a dial indicator on the bearing and multiplying this lift by an appropriate factor based on number of pads and orientation.

Pad clearance (Cpad)

This is equal to the diametral clearance from the machined pad bore diameter minus the journal diameter.

Preload (m)

One of the most common design parameters is called preload. As the oil wedge clearance changes during operation, a bearing's damping and stiffness changes. Preload is introduced to describe these variations. Preload is the fraction of distance between the bearing clearance (when concentric) to the pad clearance. Preload can typically vary from 0 to 0.75.

*If you want to calculate your own preload, you can use my own Preload Calculator. 

Preload (m) = 1 - Cbrg/Cpad

Preload is considered positive if the pad clearance is greater than the bearing clearance. When these clearances are equal then your Preload is zero. The downside of using zero preload is that when the shaft is not centered with the bearing, only some pads will create a hydrodynamic pressure which loads the bearing against the housing. The unloaded pads or lightly loaded pads may become unstable and lead to pad "flutter". 

To find preload you must start from somewhere. You typically know the shaft journal diameter where the bearing pads will ride. From there, you will have to establish a reasonable bearing clearance (radial gap) when the shaft journal is concentric with the bearing, between the shaft journal and the pad at the pivot position. Pad clearance is chosen based on achieving either a positive or negative preload, where positive preload is typically desired. The pad clearance is the difference between radius of curvature of the pad profile and the shaft journal radius. Next, you choose a pad thickness with a desirable amount of babbitt thickness for the size of your bearing. Thereby, setting your cage bore where your bearing pad will have its pivot contact area.

Fluid Film Journal Bearings in Turbomachinery

Fluid Film Journal Bearings:

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When it comes to journal bearings in the Turbomachinery industry, the purpose of a bearing is to support a load and reduce friction between stationary and rotating parts. A common type of bearing used in this industry is a fluid film bearing. These bearings use a lubrication system to supply oil in order to create an oil film to prevent metal to metal contact. As opposed to ball bearings, which do have metal-to-metal contact. Fluid film journal bearings are in the sliding contact category of bearings, while ball bearings are in the rolling contact category. 

Bearings which support rotating shafts can be classified into four categories: 
-Rolling contact: the load is supported by balls or rollers.
-Sliding - Hydrostatic: Load is supported by a high pressure fluid.
-Sliding - Hydrodynamic: Load is supported by a lubricant film.
-Magnetic - Load is supported by magnetic fields.

Fluid-film bearings are used in high speed, high load or precision applications. In these applications, Ball bearings will have limited life and cause excessive noise and vibration leading to Rotordynamic issues.


Hydrostatic vs Hydrodynamic:

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There are two types of fluid film bearings: hydrostatic and hydrodynamic. They both support a load using a thin fluid film, but they are different in how they generate the fluid film pressure. Hydrostatic bearings are pressured externally using a fluid such as: oil, water, or air. A pump is used to pressurize the fluid. A Hydrodynamic bearing uses the high speed of the journal (i.e. area where the bearing rests on a shaft) to self-generate the fluid pressure in a wedge-shaped film that forms from the relative motion between the surfaces.

Hydrostatic bearings do not depend on the relative motion between surfaces to maintain the fluid film, which means it can handle heavy loads at low speeds. These types of bearings utilize a recessed portion in between equally spaced stationary pads. Pressure is maintained in this recessed portion by pumping fluid through a flow restrictor. This pressure helps lift the shaft until flow out of the recessed portions and over the pads equals flow in. This forms a film thickness that stays constant for a certain recess pressure and bearing load. A flow restrictor is needed to create a pressure drop between the inlet feed annulus and the pad recesses to ensure that the required pressure for any individual pad never exceeds supply pressure. This is important because as the load direction changes, pressure on the loaded pad will change and the unloaded pads will decrease in pressure. The typical load restrictor used in hydrostatic bearings are orifices, which requires careful design to determine the optimum sizing.

Hydrodynamic Type Bearing: Tilt Pad Journal Bearing

For high speed shafts in the Turbomachinery industry, the most common type of Hydrodynamic bearing used today is the Tilt Pad Journal Bearing. In Integrally Geared Centrifugal Compressors, tilt pad journal bearings are used for the high speed pinion rotor assemblies. 

There are two major categories of Hydrodynamic bearings: 
-Fixed geometry bearings: 360° plain, partial arc, pressure dam and lobed bearings.
-Tilting pad bearings: bearing pads that are free to move about a pivot, line or a point. They have a lot of variety in their parameters: number of pads, preload, pivot offset, and load direction.