WO2014184368A1

WO2014184368A1 - Impeller with backswept circular pipes

Info

Publication number: WO2014184368A1
Application number: PCT/EP2014/060138
Authority: WO
Inventors: Mirco Innocenti; Alberto Scotti Del Greco; Iacopo Giovannetti; Libero Tapinassi
Original assignee: Nuovo Pignone Srl
Priority date: 2013-05-17
Filing date: 2014-05-16
Publication date: 2014-11-20
Also published as: ITCO20130019A1

Abstract

Low flow coefficient centrifugal impeller (100) for a rotating machine having a plurality of blades (3) backswept relative to the direction of rotation of the impeller and with inter blade flow passages (1) of substantially circular cross section. The circular cross section minimizes the wetted surface for the same passage area of a traditional impeller having rectangular cross section, thus reducing friction losses and increasing efficiency.

Description

IMPELLER WITH BACKSWEPT CIRCULAR PIPES

FIELD OF THE INVENTION

The present invention relates to an impeller utilized for example in a centrifugal compressor. BACKGROUND ART

A centrifugal compressor is a machine in which gas is compressed by an impeller or impellers mounted on a shaft in the compressor housing and rotating at high speed during operation. The impeller imparts velocity and pressure to the gas that is sucked into the impeller and delivered to an outlet chamber. A state of the art impeller comprises a back plate supporting an array of blades around a central hub. In a 3D impeller the blades have traditionally a prismatic shape and extend axially from the back plate and radially from the hub with a relative long base at the hub and a shorter tip at the external periphery of the plate. In a 2D impeller the blades have almost constant height with respect to the plane of the back plate and their shape in a transvers direction does not change. Present day impellers have usually so-called backswept curved blades i.e. the blades are bowed against the rotation direction at impeller outlet, to minimize aerodynamic losses. The blades normally curve from the base, i.e. from the larger base, to the tip with a typical angle comprised between 30° and 40°. The amount of backswept and increase in rotational speed are chosen such that the overall performance of the compressor is increased. Centrifugal compressors are nowadays largely used in the Oil and Gas industry. With rising oil prices, the performance of centrifugal compressors, i.e. high efficiency and wide operating range in order to easily cope with changing demands, is vital to the operation of oil and gas production facilities. The impeller is a key component to contribute to the overall performances of a centrifugal compressor. The impellers does all the work added to the gas flow, so it is impossible to realize a high efficiency compressor without a well design impeller. In order to improve the compressor reliability and efficiency and reduction of life cycle costs the improvement of the impeller design is therefore of vital importance. Most of the compressors used in the Oil and Gas industry have multistage impellers and generally the gas flowing through the last stage will have smaller volume and higher density and thus normally 2D or 3D low flow coefficient impellers are employed. While the total efficiency is mainly function of the earlier stages, the overall operating range id dominate by that of the latter stages. Thus an improvement in the efficiency of a low flow coefficient centrifugal impeller would contribute to an improvement in the overall performances of a centrifugal compressor, improvement that is highly needed.

BRIEF SUMMARY OF THE INVENTION

Therefore, there is a general need to realize 2D and 3D impeller of superior aerodynamic design. The inter-blade passages of a typical low flow coefficient centrifugal impeller, normally a 2D or 3D impellers, have a rectangular cross-section with a small aspect ratio. The equivalent hydraulic diameter is consequently small and thus also the Reynolds number. It is known that friction losses, that depend also on the surface roughness and wetting, are mainly responsible for the efficiency in the regime of low flow coefficient, and that these losses increase as the Reynolds number decreases. The main scope of this invention is therefore to realize a low flow coefficient 2D and 3D impeller where friction losses are minimize. Another scope of the invention is an impeller that can be easily manufactured, in particular that will be particularly suitable for additive manufacturing i.e. using a digital model to make three dimensional object by a manufacturing process where the object is created by adding successive layers of material. Nowadays the aerodynamics design of an impeller is automatically optimized by iterative processes of blade generation through the implementation of programs using Artificial Intelligence and Computation Fluid Dynamics for the results evaluation. From the virtual design of the impeller obtained by the computational method it is than convenient to use additive manufacturing to obtain directly the physical model and eventually the full production of the impeller. The present invention therefore achieve the above objectives providing a low flow coefficient centrifugal impeller for a rotating machine composed by an annular disk shaped main body having an internal circular surface forming a central hole and an external circular surface, a hub to be fixed to the rotor shaft of the rotating machine arranged in correspondence of the central hole, a shroud arranged spaced from the main body, a plurality of blades joining the main body with the shroud, and extending radially from the hub to the external circular surface, flow passages for compressing air formed between the blades, wherein the flow passages have a substantially circular cross section. The circular cross section minimizes the wetted surface for the same passage area of a traditional impeller having rectangular cross section, thus reducing friction losses that are responsible for efficiency at low flow coefficient. A second aspect of the present invention is a method for manufacturing a low flow coefficient centrifugal impeller having inter blades passages of circular cross section wherein the method consist in producing said impeller by additive manufacturing.

BRIEF DESCRIPTION OF THE FIGURES

Further characteristics and advantages of the invention will become more apparent in light of a detailed description of a preferred embodiment, but not exclusive, of an impeller for rotating machines, illustrated by way of example and not limitative, with the aid of the accompanying drawing in which:

Fig. 1 represents a perspective view of a 2D impeller according to the invention,

Fig. 2 represents a cross section of the impeller of Fig. 1 according to a plane perpendicular to the rotational axis.

Fig. 3 represent a diagram showing the performance of a 2D impeller designed according to the invention.

The same reference numbers in the figures identify the same elements or components.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION With reference to Figure 1 and 2 it is shown a perspective representation of a 2D impeller, according to the invention, for a rotating machine generally designated with the reference numeral 100. The impeller 100 is formed by an annular disk shaped main body having an internal circular surface and an external circular surface. The impeller is fixed to the rotating shaft of the machine through its internal surface, forming a hub 2. A shroud roughly following the shape of the main body is separated from this by the blades. Passages 1 for the air to be compressed are formed between the blades and have a circular cross section. In Fig. 2 is represented a sectional view of this type of impeller. The blades 3 and thus the air passage 1 have generally a complex shape. As seen from Fig. 1 and 2 they follow a curved path in the radial direction being backswept relative to the direction of rotation of the impeller. These characteristics of the impeller makes very complex and laborious the work of milling and welding even when using special tools suitable to be inserted to operate in those passages. These difficulties are detrimental to good accuracy, surface roughening and time. A shrouded impeller in general may be manufactured from one single solid piece of material or it may instead be constructed from two or more components, which are attached together to form the finished product. In general the various components are fixed by welding. The impeller can be constructed from one single solid piece by milling, but this process is very difficult and time consuming due to the limited accessibility given the complex shape of the flow passages. For milling it can be used Electrical Discharge Machining (EDM) that is a controlled metal-removal process by means of electric spark erosion. In this process an electric spark is used as the cutting tool to erode the single monolithic disc to produce the finished part to the desired shape. The metal-removal process is performed by applying a pulsating (ON/OFF) electrical charge of high-frequency current through the electrode to the work piece. This technique involve the step of machining the single piece by premilling followed by EDM roughing and EDM finishing. For the reason set before special shaped electrodes and special shaped tools must be used to conform to the complex passages design. The process is very time consuming and many working days are necessary to produce a single impeller. The impeller can be produced also by fabricating first two components i.e. by obtaining the blades by means of milling the main body or the shroud and subsequently by welding the two parts. With welding filling the internal junction between blades and main body or shroud given the lack of good accessibility is very difficult and welding defects easily occur in the fixing and thus the risk of cracking and deformation, i.e. thermal distortion, due to locally high temperatures, can become very high. Besides the surfaces of the flow passage will not have in general the smoothness required. In definitive it is very hard by using state of the art techniques to obtain an impeller with the high quality and reliability required. Alternatively, the impeller can be produced also by casting. Besides these drawbacks the realization of circular passage is almost impossible with these technologies. A technology that can be used for manufacturing the type of impeller described is additive manufacturing. The advantage of additive manufacturing compared with the other technologies are: -Reduced supply of raw materials.

-Reduced machining time which have great impact in the solution of full milling.

-Elimination of the cost for the construction of special electrodes and tools for the realization of the EDM machining.

In additive manufacturing the time of supply of the raw material is contained in a construction time of the pieces; a piece constructed by additive manufacturing technology does not require a starting raw piece, because the filler material of the process (suitably atomized metal powders) replaces the starting raw piece.

Compared to other technologies, the technology of additive manufacturing as well as being faster in terms of total cycle time, is able to realize paths of circular channels otherwise not accessible. Furthermore additive manufacturing allows to realize a light impeller with an interior lattice structure, surrounded by a solid structure continuous outer surface. Lattice structures can be obtained by suitably controlling the energy delivered by the high energy source of additive machine, such that the powder material is melted spot- wise, and individual volumes of melted powder material will solidify and ad-here one to the other leaving volumes there between, where the powder material is not melted and will subsequently be removed leaving an empty space within the solidified material forming the lattice structure.

Fig. 3 shows the results of calculation of performance predictions for a 2D impeller according to the invention represented with the letter A compared with a similar traditional 2D impeller having the same cross sectional area represented with the letter B. The graph shows that both efficiency and operating range are increased.

Claims

CLAIMS:

1. Low flow coefficient centrifugal impeller for a rotating machine composed by an annular disk shaped main body having an internal circular surface forming a central hole and an external circular surface, a hub to be fixed to the rotor shaft of the rotating machine arranged in correspondence of the central hole, a shroud arranged spaced from the main body , a plurality of blades joining the main body with the shroud, and extending radially from the hub to the external circular surface, flow passages for compressing air formed between the blades, wherein the flow passages have a substantially circular cross section.

2. The impeller of claim 1 wherein the blades and the flow passages are backswept relative to the direction of rotation of the rotor shaft.

3. The impeller of claim 1 or claim 2, wherein the main body, the blades and the shroud form one single solid piece of material.

4. A rotating machine including the impeller of anyone of the preceding claims.

5. Rotating machine wherein at least one impeller has inter blade flow passages having a circular cross section.

6. Method of constructing an impeller for a rotating machine having inter blade passages of substantially circular cross section consisting in utilizing the process of additive manufacturing.