Patent Application
20170022994
The present disclosure relates to subsea fluid processing machines. More particularly, the present disclosure relates to surge free rotating fluid processing machines such as subsea compressors.
Conventional turbo compressors are typically designed to compress dry gas. They normally consist of several stages, each including rotating impellers and static diffusers. The impellers are typically stacked on a shaft rotating at relatively high speed. In order to achieve good performance, i.e. large capacity, high pressure increase and good efficiency, the operating envelope becomes narrow. Also, a relatively complex control system is relied upon to ensure that the compressor always operates within acceptable boundaries and limits. In particular, conventional turbo compressors often rely on anti-surge control systems to maintain stable performance and mechanical integrity.
An anti-surge system is typically complex and costly. It typically uses fast acting valves and flow rate measurements, and therefore it is difficult to remotely control over long distances. Anti-surge systems are more difficult to implement for subsea applications. Anti-surge systems are further complicated in multiphase applications. Reliable fast action valves and flow rate measurements as used by compressor anti-surge control systems are currently inadequate for subsea multiphase applications.
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 fluid pressure increasing machine is described. The machine includes: an elongated member rotatable about a longitudinal axis;
a motor system mechanically engaged to the member so as to rotate the elongated member about a central longitudinal axis in the rotation direction; and a plurality of impellers each having a leading edge, a trailing edge and a chord line defined by a line between the leading and trailing edge. Each impeller is fixedly mounted to the first member such that a chord angle, defined by an angle between the chord line and the rotation direction, is less than or equal to a stall angle at which a maximum force is exerted on a fluid in a direction primarily parallel to the longitudinal axis when the member is rotated in the rotation direction.
According to some embodiments, the machine is a contra rotating design and includes a second elongated member rotatable about the longitudinal axis in a second rotation direction being opposite to the rotation direction; and a second plurality of impellers fixedly mounted to the second member such that the plurality of impellers are interleaved with the second plurality of impellers. Each of the second plurality of impellers also have chord angles that are less than or equal to the respective stall angles.
According to some embodiments, the fluid processing machine is of one of the following types: gas compressor, wet gas compressor, multiphase compressor, gas pump, liquid pump, multiphase pump, and electric submersible pump (e.g. either on the seafloor or in a wellbore.) According to some embodiments, the machine is free from an anti-surge control system.
According to some embodiments, a method of imparting force on a fluid is described. The method includes rotating an elongated member about a longitudinal axis in a rotation direction. The elongated member has a plurality of impellers mounted thereto each having a leading edge, a trailing edge and a chord line defined by a line between the leading and trailing edges. Each impeller is mounted such that a chord angle, defined by an angle between the chord line and the rotation direction, is less than or equal to a stall angle at which a maximum force is exerted on a fluid in a direction primarily parallel to the longitudinal axis.
According to some embodiments, a subsea fluid pressure increasing machine is described. The machine includes: an elongated member rotatable about a longitudinal axis; a motor system mechanically engaged to the member so as to rotate the elongated member about a central longitudinal axis in a rotation direction; and a plurality of impellers each having one or more gaps or openings that effectively increase a stall angle at which maximum force is exerted on a fluid in a direction primarily parallel to the longitudinal axis when the member is rotated in the rotation direction.
According to some embodiments, the gaps/openings allow fluid from a higher pressure side of the impellers to pass through to a lower pressure side of the impellers. This delays boundary layer separation from the lower pressure side of the impellers. According to some embodiments, each impeller includes a main blade portion and leading slat portion positioned in front of a leading edge of the main blade portion. A gap is formed by the space between the main blade portion and the leading slat portion. According to some embodiments, openings include a combination of holes and a slot(s) passing through each of the impellers. According to some embodiments, the machine is a wet gas compressor with contra rotating impeller stages.
According to some embodiments a method of imparting force on a fluid is described. The method includes rotating an elongated member about a longitudinal axis in a rotation direction. The elongated member has a plurality of impellers mounted thereto, each having one or more gaps or openings that effectively increase its stall angle.
According to some embodiments, one or more of the described systems and/or methods can be used in topside or subsea fluid processing equipment in an analogous fashion.
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.
According to some embodiments, techniques for achieving a surge free compressor operation are described that do not rely on an anti-surge control system. Compressor surge occurs when the flow approaches the impeller blades with an incident angle that is so large that the flow is no longer able to stay attached to the low-pressure side of the impeller blade (i.e. the “suction” side of the impeller blade).
According to some embodiments, the impeller blades are positioned such that their chord angles are less than their respective stall angles. If all the impeller blades in the compressor meet this condition, then compressor surge does not occur for any positive flow rate. However, the low chord angles to meet the desired surge free operation can unduly constrain the nominal flow rate for the compressor. The resulting undesirable constraint on flow rate is so great that such designs are often not practical.
According to some embodiments, impellers having chord angles less than the stall angles are used in a contra-rotating impeller arrangement without static diffusers. The contra-rotating impeller arrangement provides for much larger nominal flow rates than conventional single rotating impeller arrangements with the same chord angles. Accordingly, a surge free design is provided without excessively compromising the nominal flow rate. According to some embodiments, a surge free compressor includes impellers such that the chord angles of all blade airfoils are less than the corresponding airfoils stall angles. By positioning successive impeller stages without static diffusers in a contra rotating arrangement, the nominal flow rate is sufficiently large to justify the low, surge-free chord angle design of the impellers. Thus, a compressor is provided that has reasonable nominal flow rates, is inherently surge-free for all positive flow rates and does not rely on separate surge control systems. Such a compressor is particularly suitable for remote, subsea and multiphase applications.
Note that as used herein the term “airfoils” refers to any impeller blade design, regardless of whether the processed fluid is air, another gas, a mixture of gas and liquid, or a liquid.
In
From
Cz=(U−Cx)·tan(β1),
where Cx is negative for contra rotating impellers. The nominal flow rate can be defined at a zero incident angle, i.e. when V1 is tangential to the airfoil leading edge camber line, which for a cambered airfoil normally results in a small AOA with β1 close to the airfoil chord angle. As the nominal flow rate is proportional to Cz it can be observed from the above equation that the nominal flow rate increase with increasing magnitude of Cx for contra rotating impellers since Cx then is negative. In comparison, for a conventional single rotating impeller arrangement with static diffusers, Cx will ideally be zero but normally has a small positive value.
The relative increase in flow rate for a contra rotating impeller arrangement compared to an ideal single rotating impeller arrangement with static diffusers with the same impeller chord angles becomes:
ΔQnom/Qnom=−Cx/U>0; for negative Cx.
Thus, according to some embodiments, the use of contra rotating impeller stages allows for higher nominal flow rates which makes the surge free condition (each of impellers has its chord angle less than or equal to its stall angle) practical, especially for applications such as subsea deployments and/or wet gas compressors. Note that impellers 510 and 520 are shown to be arranged such that they force fluid downwards so as to be more understandable to those familiar with the concept of aerodynamic lift. According to some embodiments, however, such as shown in
According to some embodiments, the impeller blades are cylindrical (i.e. its shape does not changing along the radial direction). In such cases the chord line can simply be drawn between the leading and trailing edges of the impeller. In some embodiments, however, the impeller blade is non-cylindrical in that its shape changes in the radial direction. In such cases a mean cord line is defined and can be used for calculating the chord angle. Examples of non-cylindrical shapes include slight changes in chord angle to accommodate the fact that locations of the impeller further from the central axis “see” a slightly higher fluid velocity. Other examples include impellers having elements to enhance stall characteristics such as slots which may not run the whole width of the impeller. Examples are shown in
According to some embodiments, impeller blades having enhanced stall characteristics are provided. In particular, by increasing the stall angle of the impellers blades, a surge free design is practical without excessively compromising the nominal flow rate. Increasing the stall angle of impeller blades can be accomplished in a number of ways, some illustrative examples of which are described herein.
In general, impeller blades and airfoils that are designed for high maximum lift will also have high stall angles. A number of different impeller blade/airfoil designs and design features are available for this purpose. According to some embodiments, further increase in the impeller blade/airfoil stall angle is achieved by introducing a slot arrangement near the leading edge of the impeller blade/airfoil. According to some other embodiments, an increase in the impeller blade/airfoil stall angle is accomplished by using multiple elements for each impeller blade/airfoil. By applying impeller blade/airfoils with increased stall angles, the nominal flow rate of the compressor can be made sufficient large so as to justify surge-free chord angle positioning of the impellers.
According to some embodiments, the orifices 812 are circular holes with diameters of about 2% of the airfoil chord length are distributed along a straight line from hub 318 to tip on the high pressure side 702 of the impeller blade 700 at the approximate location of the stagnation point for incipient boundary layer separation at a high angle of attack. According to some embodiments, the holes 812 penetrate about 75% of the impeller blade thickness before they are manifolded in a slot 814 pointing out and backwards on the suction side 714 of the impeller blade 700 with an angle of approximately 20 degrees to the impeller blade surface and located upstream of location of incipient boundary layer separation at a high angle of attack.
The pressure difference between the high pressure 702 and suction (or low pressure) side 704 of the impeller blade will cause a positive flow from the pressure side 702 through the holes 812 and slot 814 to the suction side 704 of the impeller blade, thereby helping to delay boundary layer separation.
By using one or more stall angle enhancement techniques such as orifices, slots, slats, and gaps, the stall angle of the compressor impellers can be increased. Increasing the stall angles of the impellers allows for larger impeller chord angles and higher nominal flow rates while still maintaining surge free performance without reliance on anti-surge systems. According to some embodiments, the stall angle enhancements described increase nominal flow rates enough that simple rotation (i.e. non-contra rotating) compressor designs can be used. According to some other embodiments, the stall angle enhancements described are used in combination with a contra rotating arrangement to even further boost surge-free nominal flow rates over what would achievable without such enhancements.
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 some 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.
