GB2168433A

GB2168433A - Controlling centrifugal compressors

Info

Publication number: GB2168433A
Application number: GB08431325A
Authority: GB
Inventors: Vladimir Viktorovich Arkhipov; Gennady Fedorovich Velikanov; Khanafi Ibragimovich Muratov; Gilya Aizikovich Raer; Kir Borisovich Sarantsev; Valentin Gennadievich Soloviev; Konstantin Alexandrovic Telnov; Simon Zalmanovich Shabashov
Original assignee: NEVSKY Z IM V I PROIZV OB; PROIZVODSTVENNOE OBIEDINENIE NEVSKY ZAVOD IMENI V I LENINA
Current assignee: NEVSKY Z IM V I PROIZV OB; PROIZVODSTVENNOE OBIEDINENIE NEVSKY ZAVOD IMENI V I LENINA
Priority date: 1984-12-12
Filing date: 1984-12-12
Publication date: 1986-06-18
Also published as: GB2168433B; GB8431325D0

Abstract

The compressor comprises a housing (1), an axial intake tube (2) communicating at one end with a suction chamber (3) and at the other end with an impeller (4), a flow guide (5), and a cover (8) secured to the housing and having a streamlined o separating wall (9). Vanes (14) of the flow guide (5) are disposed so that their longitudinal axes are in parallel with the longitudinal centerline of the compressor, these vanes being rigidly affixed on the end face of the axial intake tube to embrace an annular baffle (16) connected to a piston (22) of a servomotor (21) for axially displacing the baffle to direct move or less fluid such as natural gas through the swirl imparting vanes (14). The length of this annular baffle is sufficient to block an annular passage (19) thereby directing the entire flow through the vanes (14) at low flow rates. <IMAGE>

Description

SPECIFICATION Improvement in or relating to centrifugal compressor This invention relates generally to turbocompressor engineering, and more particularly to a centrifugal compressor construction.

The invention can be used most successfully for controlling the volume flow rate of a constant-speed centrifugal compressor employed for transporting natural gas in main gas lines.

The world practice of compressor engineering has been plagued lately with the problem of matching constant-speed centrifugal compressor characteristics with the characteristics of a gas pipeline system served by such compressors due to an ever growing demand for providing a more economical way of gas transportation.

The present invention is directed toward the provision of such a centrifugal compressor with controlable volume flow rate at invariable rotational speed of its impeller the construction of which would prevent the negative effect of guide vanes on the flow of gas moving at the rated compressor capacity in an axial intake tube to ensure improved efficiency and reliability.

The essence of the invention resides in that in a centrifugal compressor comprising a barrel-type housing having arranged in the interior thereof in line with the longitudinal axis of the compressor an axial intake tube connected at one end to a suction chamber and at the other end to an impeller secured on a shaft in a cantilever fashion and communicable with a pressure chamber, a flow guide means for forming a flow of fluid to run toward the impeller and disposed at the inlet to the axial intake tube, and a cover cup secured to the housing and having a streamlined separating wall, according to the invention, vanes of the flow guide means are disposed so that their longitudinal axes are in parallel with the longitudinal centerline of the compressor these vanes being rigidly affixed on the end face of the axial intake tube to embrace an annular baffle arranged coaxially with the flow guide means and provided with a means for displacement thereof along the centerline of the compressor, this annular baffle having a length equal to the length of the vanes of the flow guide means, this length of the annular baffle being sufficient for blocking an axisymmetric annular passage confined by the streamlined separating wall and an intake lip provided at the end of the annular baffle facing this streamlined separating wall.

The above construction of the controllable capacity constant-impeller-speed centrifugal compressor improves its efficiency at the optimum (rated) volume flow conditions and makes it possible to increase the reliability and efficiency of main gas lines through reducing the amount of energy required for compressor operation at off-design flow rates.

This increased efficiency of the compressor at the rated volume flow conditions is accounted for by the fact that the guide vanes and intervane passages of the flow guide means are blocked by the annular baffle to obviate the adverse effect of the vanes of the flow guide means on the flow of gas passing in the axial intake tube at the optimum (rated) volume flow conditions.

Such an arrangement eliminates, firstly, flow constriction in the axisymmetric intake tube which otherwise would tend to cause energy losses; secondly, drag impact losses and turbulence losses at the trailing edges of the vanes; and thirdly, friction losses in the guide vanes.

The lack of reasons which may induce unsteady flows causing dynamic overloads in the impeller and compressor elements disposed downflow of the impeller improves the reliability of the compressor when it is operated at the rated capacity.

The centrifugal compressor embodying the features of the present invention is also more efficient when operated at off-design capacity than the known centrifugal compressors due to the rigid attachment of the flow guide vanes on the wall of the axial intake tube with the leading edges of the vanes at a zero angle of attack to the incoming flow running thereagainst from the suction chamber.

Because the guide vanes position relative to the axial intake tube is invariable at all offdesign flow rates, the incoming flow moves smoothly through the vanes thereby minimizing stalls and turbulences, which also results in reduced specific consumption of energy for gas compression and improved compressor reliability.

Improved operational reliability of the proposed centrifugal compressor is also ensured by the employment of a structurally simple, low-weight and low labor-cost annular baffle.

Preferably, the streamlined separating wall has a central hole therein, whereas the means for displacing the annular baffle along the longitudinal centerline of the compressor is fashioned as any known suitable servomotor secured to the cover cup of the housing, the actuating element of the servomotor being pivotally connected to a slider arranged in the cover cup to extend through the central hole of the streamlined separating wall and rigidly linked with the annular baffle.

The above construction of the means for displacing the annular baffle provides for the transmission of motion to the latter by way of a single kinematic linkage, particularly a rod, which simplifies the compressor structurally, makes it low-weight, assures less costly fabrication, and improves its operational reliability.

The pivotal connection of the servomotor rod to the slider obviates flexural forces and torques to act on the rod and the servomotor piston connected to the rod. The overall simplicity has been attained through dispensing with the conversion of the linear motion to rotational.

The actuator of the servomotor at the location where it extends from the streamlined separating wall can be sealed by any known suitable means to prevent contamination and erosive wear of the means for displacing the annular baffle.

Advisably, radially extending ribs are provided for rigidly connecting the slider of the servomotor actuator to the annular baffle.

This arrangement of the flow guide means enables to prevent undesirable swirling of the flow in the axial intake tube at the rated volume flow conditions which would be otherwise caused by turning of the flow from the radial to the axial direction which reduces the difference between the actual characteristics of the compressor and the rated characteristics.

The centrifugal compressor embodying the present invention has a constant rotational speed of its impeller of 4.800 rpm. It has a rated capacity of 394 m3/min (at a pressure of natural gas entering the suction chamber of 6.2 MPa and a temperature of 31.6"C), a pressure of natural gas escaping the compressor pressure chamber of 7.6 MPa, a power input of 9,700 kW, and a polytropic efficiency of 87%.

Other objects and advantages of the invention will become more fully apparent from a more detailed description of various preferred embodiments thereof that follows with reference to the accompanying drawings, in which: Figure 1 is a longitudinal sectional view of a centrifugal compressor embodying the present invention illustrating an annular baffle blocking the passage of swirled flow from intervane passages of the flow guide means to an axial intake tube at the rated compressor capacity; and Figure 2 is a section taken along the line Il-Il in Fig. 1.

With reference to Fig. 1, a centrifugal compressor according to the invention comprises a barrel-type housing 1 (Fig. 1), wherein there are disposed in line with the longitudinal centerline of the housing an axial intake tube 2 connected at one end thereof to a suction chamber 3 and at the other end to a flowthrough portion of an impeller 4, and a flow guide 5 for forming a flow of fluid, such as a natural gas, to run toward the impeller, the flow guide 5 being disposed at the inlet to the axial suction tube 2.

The suction chamber 3 is confined by the wall of the suction tube 2, wall 6 of a pressure chamber 7, inner surface of the housing 1, and a streamlined separating wall 9 secured on a cover cup 8.

The cover cup 8 is connected to the housing 1 by bolts (not shown). Stiffening ribs 10 connecting the wall 6 to the cover cup 8 are provided between the pressure chamber 7 and suction chamber 3, these ribs 10 serving to take up forces resulting from pressure differentials occuring in these two chambers. The impeller 4 having blades 11 is journaled on a shaft 12 in a cantilever fashion coaxially with the housing 1. The diameter of inlet port of the impeller 4 is equal to the diameter of the axial suction tube 2.

Outlet of the impeller 4 communicates with the pressure chamber 7 through a diffuser 13.

The flow guide 5 is defined by a plurality of vanes 14 longitudinal axes of which are in line with the longitudinal centerline of the compressor and a carrying ring 15 rigidly affixed on the end face of the axial suction tube 2.

The vanes 14 are rigidly secured on the carrying ring 15 so that their leading edges are at the zero angle of attack to the incoming flow, these vanes 14 embracing an annular baffle 16. Free ends of the vanes 14 are enclosed by a bondage ring 17. The annular baffle 16 is arranged coaxially with the flow guide 5 and has a means 18 for displacement along the longitudinal centerline of the compressor; the annular baffle 16 has a length substantially equal to the length of the vanes 14 of the flow guide 5, which is sufficient for blocking an axisymmetric annular passage 19. This passage 19 is defined by the streamlined separating wall 9 and an intake lip 20 arranged on the end of the annular baffle 16 facing the separating wall 9.The means 18 for displacing the annular baffle 16 along the centerline of the compressor may have the form of a servomotor 21 secured to the cover cup 8 of the compressor. A piston 22 of the servomotor 21 functioning here as the actuating member thereof is pivotally connected to a rod 23, whereas the rod 23 is pivotally connected to a slider 24 arranged in a boss 25 of the cover cup 8. The separating wall 9 has a central hole 26 to receive the slider 24, this hole 26 being provided with a labyrinth seal 27 serving to prevent the penetration of mechanical impurities carried by the natural gas to the elements of the servomotor 21.

The slider 24 is rigidly connected to the annular baffle 16 through a plurality of radially extending ribs 28.

The centrifugal compressor embodying the present invention operates in the following manner.

When operated at the rated capacity, the flow of natural gas is conveyed from the suction chamber 3 to the axisymetric annular passage 19 and therefrom to the interior of the annular baffle 16 and the axial intake tube 2 to thereafter run toward the impeller 4. By virtue of the fact that the vanes 14 and intervane passages 14a (Fig. 2) of the flow guide 5 are blocked by the annular baffle 16, these vanes 14 fail to affect the flow of natural gas traveling through the annular baffle 16 and the axial intake tube 2. Therewith, the piston 22 (Fig. 1) of the servomotor 17 assumes the right-most position.Among the attending advantages of the aforedescribed arrangement are: absence of constriction of the flow in the axisymmetric passage 19, which otherwise may cause energy losses; absence of impact losses when the flow runs against the leading edges of the vanes 14; trailing edge losses are prevented when the flow leaves the surface of the vanes; friction losses are avoided which are normally caused by the flow passing through the vanes; and also avoided are non-steady processes which generally cause dynamic overloads in the impeller 4, diffuser 13, bearings and seals (not shown) of the compressor. The above factors improve the efficiency and reliability of the centrifugal compressor operated at the rated capacity. The lip 20 provides for a smooth and continuous suction of the flow to the annular baffle 16.

At the minimum capacity the piston 22 of the servomotor 21 assumes the leftmost position. When the piston moves to the left, its rod 23 displaces the slider 24 and the annular baffle 16 until the latter completely blocks the axisymmetric passage 19.

The flow of natural gas conveyed from the suction chamber 3 and passing through the vanes 14 (Fig. 2) attains a circumferential component coinciding with the direction of rotation of the impeller 4, whereby the thus swirled flow moves along the axial intake tube 2 to enter the impeller 4.

Due to the coincidence of the projection of the absolute velocity vector on the circumferential velocity vector with the circumferential velocity vector of the impeller 4, head produced by the impeller 4 is reduced according to the known Euler equation. Also, due to the fact that the vanes 14 are fixedly secured on the carrying ring 15 with the leading edges of the vanes at the zero angle of attack relative to the incoming flow, the flow passes through the vanes 14 at a speed V (Fig. 2) smoothly with minimum of stalls and swirls to result in reduced consumption of power for the compression of gas accompanied by improved reliability of the centrifugal compressor.

When the centrifugal compressor operates at a capacity below the rated one and above the minimum one, the piston 22 of the servomotor 21 takes an intermediate position wherein the rod 23 (Fig. 1) acts to move the slider 24 from the rightmost position to the left along the longitudinal centerline of the compressor. The slider 24 therefore displaces the annular baffle 16 to partially block the axisymmetric annular passage 19 and partially open the flow guide 5.

The flow of natural gas is conveyed from the suction chamber 3 to the axial intake tube 2 through the axisymmetric annular passage 19 partially blocked by the annular baffle 16 and through the intervane passages 14a (Fig.

2) of the partially open flow guide 5. The flow swirled by the vanes 14 joins the unswirled flow passing through the annular baffle 16 to result in that the overall flow attains a circumferential velocity component with a vector that is less than the vector of the circumferential velocity component of the flow corresponding to the minimum volume flow rate of the compressor. Further, the thus joined flow moves along the axial intake tube 2 (Fig. 1) toward the impeller 4. By virtue of coincidence between the projection of the absolute velocity vector of the joined flow on the circumferential velocity vector and the circumferential velcotiy vector of the impeller 4, a reduction in the head produced by the impeller 4 results in accordance with the Euler equation.

The centrifugal compressor embodying the features of the present invention and operating at a constant rotational speed of the impeller makes operation of gas pipelines more economical due to matching its characteristics with the characteristics of a compressor operating in series or in parallel therewith and with the characteristics of a system served thereby.

Also, the centrifugal compressor according to the invention has a higher pressure-producing efficiency both at the rated capacity due to elimination of flow choking caused by the guide vanes of the known compressors continuously hampering the flow of incoming gas, and at off-design operating conditions thanks to reduced vane drag and the positioning of the leading edges of the guide vanes in the proposed centrifugal compressor in the area of relatively low flow velocities.

Finally, other features of the proposed invention are advantageous in that the construction of the flow guide means is simplified, its overall weight and size are reduced, while its fabrication requires less labor to be consumed.

Claims

1. An improvement in or relating to a centrifugal compressor comprising a barrel-type housing having arranged in the interior thereof in line with the longitudinal centerline of the compressor an axial intake tube connected at one end to a suction chamber and at the other end to an impeller secured on a shaft in a cantilever fashion and communicable with a pressure chamber, a flow guide means for forming a flow of fluid to run toward the impeller and disposed at the inlet to the axial intake tube, and a cover cup secured to the housing and having a streamlined separating wall, vanes of the flow guide means arranged so that their longitudinal axes are in parallel with the longitudinal centerline of the compressor, these vanes being rigidly affixed on the end face of the axial intake tube to embrace an annular baffle arranged coaxially with the flow guide means and provided with a means for displacement thereof along the centerline of the compressor, this annular baffle being in length equal to the length of the vanes of the flow guide means, this length of the annular baffle being sufficient for blocking an axisymmetric annular passage confined by the streamlined separating wall and an intake lip provided at the end of the annular baffle facing this streamlined separating wall.

2. An improvement in or relating to a centrifugal compressor as claimed in ciaim 1, in which a central hole is provided in the streamlined separating wall, whereas the means for displacing the annular baffle along the longitu dinal centerline of the compressor is fashioned as a conventional servomotor secured to the cover cup of the housing and having an actuator pivotally connected to a slider accommodated in the cover cup and slidably extending through the central hole in the streamlined separating wall to be rigidly connected to the annular baffle.

3. An improvement in or relating to a centrifugal compressor as claimed in claim 2, characterized in that radially extending ribs are provided for rigidly connecting the slider of the servomotor actuator to the annular baffle.

4. An improvement in or relating to a centrifugal compressor as described in the description, claimed in any of the preceding claims 1 to 3, and represented in the accompanying drawing 1 and 2.