Patent Grant
9476427
Conventional turbo compressors known in the art are designed to compress a gas. They are normally composed of many stages (rotating impellers and static diffusers) stacked on a flexible shaft rotating at relative high speed. Critical mechanical elements such as bearings and thrust balancing devices are often exposed to the process fluid.
Any impurities in the process fluid such as solids or liquid are detrimental to both the thermodynamic and mechanical performance. When impurities or liquid are expected to be present in the process stream different types of auxiliary equipment are utilized to clean or dry the process gas upstream the compressor. Typically a gas scrubber and/or heat exchangers may be used to remove liquid from the process fluid.
Known attempts to modify conventional turbo compressors to be so called “liquid tolerant” have had very limited success and only very low liquid fractions can be accepted in some rear cases. However, even in these cases the presence of liquid will cause deterioration in the thermodynamic and mechanical performance.
The challenges are even greater when designing a gas compressor for use in a subsea environment. In particular, the robustness of the design and physical dimensions of the compressor should be considered when the compressor is to be deployed in subsea environments with challenging weather conditions. For example in the arctic, where there are significant oil- and gas resources, subsea deployment techniques such as via a ship's moon pool are of great benefit due to the presence of moving ice.
This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter.
According to some embodiments, a subsea deployable counter-rotating compressor for compressing a fluid is described. The compressor includes: a first elongated member rotatable about a longitudinal axis; a first plurality of impellers fixedly mounted to the first member and being shaped and arranged so as to exert force on the fluid in a direction primarily parallel to the longitudinal axis when the first member is rotated in a first rotational direction about the longitudinal axis; a second elongated member rotatable about the longitudinal axis; a second plurality of impellers fixedly mounted the second member such that the first plurality of impellers is interleaved with the second plurality of impellers, the second plurality of impellers being shaped and arranged so as to exert force on the fluid in the same direction as the first impellers when the second member is rotated in a second rotational direction about the longitudinal axis, the second rotational direction being an opposite rotational direction to the first rotational direction; and a motor system mechanically engaged to the first member so as to rotate the first member in the first rotational direction, and mechanically engaged to the second member so as to rotate the second member in the second rotational direction.
According to some embodiments the first elongated member is a hub and second elongated member is a sleeve that surrounds at least a portion of the hub. The first and second pluralities of impellers can be arranged in a plurality of rows of first impellers and a plurality of rows of second impellers, respectively, with the first and second rows of impellers being mounted on the hub and sleeve in an alternating pattern of rows with each row of impellers making up a stage of the compressor that counter rotates with respect to each adjacent stage. Fluid passing through the compressor during operation can include gas and liquid phases and are at substantially mixed by the counter rotation of the stages.
According to some embodiments, no static diffuser elements are positioned between the alternating rows of impellers, and the impellers are mounted directly to the hub and sleeve without any intermediate structural members. According to some embodiments, the motor system includes a first motor for rotating the first member in the first rotational direction and a second motor for rotating the second member in the second rotational direction. According to some embodiments, the compressor is dimensioned such that it can be deployed from a moon pool of a ship.
According to some embodiments, a method for compressing a fluid including a gas and liquid phases is described using a counter-rotating compressor on a sea floor. The method includes: rotating a first elongated member about a longitudinal axis in first rotational direction, the first elongated member having a plurality of first rows of impellers mounted thereon; rotating a second elongated member about a longitudinal axis in a second rotational direction, the second rotational direction being an opposite rotational direction to the first rotational direction, the second elongated member having a plurality of second rows of impellers mounted thereon; and sucking the fluid successively and alternatingly through the first and second rows impellers, with each row of impellers exerting force on the fluid in a direction primarily parallel to the longitudinal axis.
According so some embodiments a method for positioning a fluid compressor on a sea floor is described. The method includes: deploying a ship having a moon pool opening for installing subsea equipment to the sea floor; and lowering a compact turbo fluid compressor from the moon pool opening to the sea floor, the turbo fluid compressor being dimensioned so as to be deployable through the moon pool opening and being mechanically robust so as to reliably compress subsea fluids containing a mixture of gas and liquid phases.
The subject disclosure is further described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of embodiments of the subject disclosure, in which like reference numerals represent similar parts throughout the several views of the drawings, and wherein:
The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the subject disclosure only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the subject disclosure. In this regard, no attempt is made to show structural details of the subject disclosure in more detail than is necessary for the fundamental understanding of the subject disclosure, the description taken with the drawings making apparent to those skilled in the art how the several forms of the subject disclosure may be embodied in practice. Further, like reference numbers and designations in the various drawings indicate like elements.
It has been found that in conventional dry gas compressors the presence of liquid in the process stream will cause significant deterioration in the thermodynamic and mechanical performance of the compressor. A main reason for this is that the gas and liquid tends to separate, and that compressors with static diffusers tend to have poor performance with separated gas and liquid phases. Furthermore, it has been found that many conventional dry gas compressors designs will actually induce separation in gas-liquid flows. When even small amounts of liquid separates from the main gas, the gas streamlines are disturbed and performance deteriorates. Boundary layer separation can occur and form local re-circulation zones and liquid concentration areas with large thermal gradients. Phase separation and boundary layer separation are two different phenomena which have been found to influence each other and both have negative effect on both the thermodynamic and mechanical performance of the compressor. Liquid concentration in particular can cause large dynamic and transient imbalances which can be detrimental for the mechanical performance of the compressor.
The operating envelope of a conventional turbo compressor is bounded by a “surge line” at low flow rates and by a “stone wall” at high flow rates. For flow rates lower than the “surge line” boundary layer separation occurs and causes performance degradation to such a degree that the compressor cannot operate. For flow rates higher than the “stone wall” choking occurs as the local velocity reach the sonic velocity and the flow rate cannot be increased further. It has been found that the presence of liquid and the effects thereof as described above will move the “surge line” to a higher flow rate and further limit the operating envelope. Additionally, the sonic velocity in a gas-liquid stream may be significantly lower than the sonic velocity in the single-phase gas and single-phase liquid stream. The presence of liquid will therefore move the “stone wall” to a lower flow rate and further limit the operating envelope. It has been found that the phenomena described above affecting the “surge line” and the “stone wall” are very complex and influenced by a large number of variable and transient parameters. These limits can therefore in general not be considered fixed for a compressor operating on a gas-liquid stream.
In
As can be seen, the interleaved rows of impellers mounted to the inner hub and outer sleeve are stacked successively to each other and rotate in opposite directions. In this way, each row of impellers forms a separate stage of the compressor. Note that in this design there are no guide vanes or diffusers between the successive adjacent stages. Rather, the fluid discharged from a stage rotating in one direction immediately enters into the stage rotating in the opposite direction and so on through a number of successive contra rotating stages.
By introducing successive contra rotating impeller blade stages in this way, guide vanes and diffusers can be omitted. An interesting aspect of this design is that the lengths of the inner hub and the outer sleeve that transfer the rotational energy to the impellers from lower motor 122 and upper motor 124, respectively, can be significantly reduced. By reducing these lengths over other designs, the number of stages provided can be reduced for the same energy input, allowing the required energy input or number of stages to be fit on a short stiff shaft. Furthermore, by shaping and arranging the impeller blades so as to directly impart axial motion (parallel to the rotating central axis) to the process fluid, the impeller blades can be mounted directly to the inner hub and outer sleeve, thereby eliminating the need for additional cross member structures such as disks or arms that extend from the hub and/or sleeve. By eliminating additional structures and mounting the impeller blades directly on the hub and/or sleeve, the design is even more mechanically robust when compared to other designs. The compressor shown with relatively short dimensions of the inner hub and outer sleeve members, as well as mounting the impeller blades directly on those members, has been found to be significantly less prone to any unbalance due to uneven load distribution which can result from the presence of separated liquid and gas phases in the process stream.
In an example wherein the gas and liquid phases enters the compressor as a homogeneous mixture, the contra rotating impeller blade stages has been found to provide effective inter-stage mixing. And since diffusers or guide vanes are not present, the gas and liquid fluids remains in a well mixed homogeneous state throughout all the compressor stages.
When the process gas and liquid are in a well-mixed homogeneous state, the process fluid can be considered as “single-phase” with equivalent fluid properties.
Ro=GVF*RoG+(1−GVF)*RoL
Cp=GMF*CpG+(1−GMF)*CpL
In particular, the equivalent mixture density will increase significantly with increasing liquid content. The increased density will translate a given head into an increased pressure ratio.
P2/P1=(Ro1*g*H/(f′n/(n−1)*P1)+1)^(n/(n−1))
Also, the equivalent mixture heat capacity will increase significantly with increasing liquid content. The increased heat capacity will reduce the temperature rise for a given pressure ratio and efficiency.
T2−T1=T1*((P2/P1)^((Z*R)/(Cp*Eff))−1)
This inter-cooling effect will also contribute to an increased density that again results in a further increase in the pressure ratio.
Thus, the contra rotating impeller blade arrangement shown is: (1) structurally more robust by allowing for shorter effective shafts lengths (i.e. the inner hub and outer sleeve lengths) for applying energy to the impeller blades; (2) enhances for inter-stage mixing of gas and liquid phases; and (3) omits guide vanes and diffusers which allows for the gas liquid stream to remain well mixed and homogenised throughout all the compressor stages. It has been found that the arrangement shown provides for phase mixing so well, that the process fluid can be considered as “single-phase” with equivalent fluid properties. Accordingly, the presence of liquid in the process fluid will have an enhanced density effect that will increase the pressure ratio and an enhanced heat capacity effect that will reduce the temperature rise and further increase the pressure ratio.
Another significant advantage of the contra rotating design shown is that it is more compact, in width and length than many other designs. In particular, by mounting the impeller blades directly on the inner hub and outer sleeve structures, intermediate structures between the impeller blades and the hub and sleeve can be eliminated, thus leading to a reduced overall width of the compressor. Further, the contra rotating design allows for the use of two smaller motors instead of one larger motor, which has been found to further reduce the unit's dimensions. An important aspect of the physical compactness of the design is its ability to be deployed using certain types of deployment techniques. In particular, for subsea deployment and retrieval, the use of an open moon-pool vessel is a significant advantage, since larger compressor designs may have to be deployed using floating cranes and/or barges.
While the subject disclosure is described through the above embodiments, it will be understood by those of ordinary skill in the art that modification to and variation of the illustrated embodiments may be made without departing from the inventive concepts herein disclosed. Moreover, while the preferred embodiments are described in connection with various illustrative structures, one skilled in the art will recognize that the system may be embodied using a variety of specific structures. Accordingly, the subject disclosure should not be viewed as limited except by the scope and spirit of the appended claims.
| Number | Name | Date | Kind |
|---|---|---|---|
| 2234733 | Jendrassik | Mar 1941 | A |
| 2406959 | Millard | Sep 1946 | A |
| 4830584 | Mohn | May 1989 | A |
| 20080267716 | D'Souza et al. | Oct 2008 | A1 |
| Number | Date | Country | |
|---|---|---|---|
| 20140147243 A1 | May 2014 | US |
