GB2026099A - Steam turbines - Google Patents

Abstract

A steam turbine high pressure casing comprises outer and inner shells 4, 2, respectively, between which is disposed an intermediate shell 3 welded to inner shell 2 to increase casing stiffness. The shells 2, 3 each support some of the diaphragms of the H.P. stages and provide a gap 22 through which H.P. steam is bled to flow via ha chamber 23 to glands 18, 19. Intermediate shell 3 also supports the first stage stator blades 21 of the intermediate pressure turbine. <IMAGE>

Description

SPECIFICATION Turbines This invention relates to steam turbines employing high pressure and intermediate pressure sections within the respective cylinders.

Some of the objects of the invention are as follows: (a) to effect reduction in the thickness of the flanges at the horizontal joints in the high pressure section; (b) to effect greater symmetry in the cylinder construction to reduce thermal stresses; (c) to effect cooling of high temperature glands and cooling of first few stages in the intermediate pressure section and the rotor; According to this invention, the following features are proposed to be incorporated and these will be more clearly described subsequently with the help of the accompanying drawings: (a) between the outer cylinder and the inner cylinder of the high pressure stage there is provided an outer shell for the said inner cylinder which shell is spaced both from the said outer cylinder and said inner cylinder.

The outer shell extends upto the glad between the high pressure and intermediate pressure cylinders; (b) a sealing ring is fitted in the passage between the outer cylinder and the outer shell of the inner cylinder; (c) the outer cylinder is connected to the outer shell of the inner cylinder.

Thus the proposed high pressure and intermediate pressure reheat turbine cylinder design includes an outer shell for the high pressure inner cylinder which will result in overall stiffness against torsion and opening forces at the horizontal joint. The said outer shell extends upto the first stage diaphragm of the intermediate pressure section of the turbine.

The provision of a flexible seal or sealing ring between the said outer shell and the outer cylinder, results in forming the boundary of steam path for the reheat steam.

The said outer shell supports a gland carrier and provides passage for relatively cooler steam to reach the intermediate pressure first wheel, unlike the existing system where the steam from the high pressure section reaches the high temperature gland supported by the high pressure inner cylinder.

The salient features of the improved turbine construction will be described with the help of the accompanying drawings in which: Figure 1 shows longitudinal section of a part of the high pressure and intermediate pressure cylinders of a turbine; Figure 2 shows an isometric cut out section of the high pressure stages of the turbine; Referring to the drawings, A and B respectively show the high pressure (HP) and intermediate pressure (IP) stages of the turbine.

Referring first to the HP stage, 1 is the 360" admission type of nozzle box, 11 is the inlet pipe for admission of steam into the nozzle.

5 is the rotor which is common both for the HP and IP stages. 2 is the inner cylinder and 4 the outer cylinder of the HP section. 9 is the casing for IP section.

According to this invention there is now provided an outer shell 3 spacedly disposed from both the outer cylinder 4 and the inner cylinder 2. The inner cylinder 2 supports (in the embodiment shown) the diaphragms of seven high pressure stages, while the outer shell 3 supports the diaphragm of the next three stages. The next three stages are supported by an independent liner 1 2 (Fig. 2).

1 3 are the keys between the outer shell 3 and the outer cylinder 4. A sealing ring 7 is fitted between the shell 3 and cylinder 4 to prevent the entry of reheated steam in the annular chamber to the right of said sealing ring 7.

The provision of the outer shell 3 is responsible for the following functions: (a) it increases the torsional stiffness of the HP inner cylinder 2 thus obviating the necessity of having the inner cylinder 2 of the thicker section which is generally necessary to account for the torsional stiffness required of the inner HP cylinder; (b) all HP cylinders have to be bolted at the joints and the flanges at the joints have to be exceedingly thick so that the part of the flange between any two bolts does not deflect and cause leakage of steam. This large thickness of the flanges compared to the thickness of the outer cylinder 4 has always resulted in dissymmetry causing differential heating and consequent thermal stresses. The provision of the outer shell 3 will enable the flange to be of reduced thickness thereby making the cylinder shell more symmetrical.

8 is the high pressure outlet which is reheated and fed to the first stage of the IP cylinder through chamber 6.

1 8 and 1 9 are the high temperature and low temperature glands respectively. The first diaphragm 21 of the IP stage is held at its outer end in the outer cylinder 4 and at its inner end by the outer shell 3, with the provision of a flexible annular ring 20 provided at the outer end of diaphragm 21 to account for differential expansion.

A small quantity of steam from the HP stage escaping through an opening 22 flows through space 23 and enters a clearance 24 in the inner cylinder 2 and escapes through glands 1 8 and 1 9 to seal the gland 1 9.

The steam from space 23 escaping through clearance 24 passes through the low temperature gland 1 9 and cools that part of the rotor 5 which is sealed by the gland 1 9 and also cools the region close to the bore of the first wheel of the IP stage. Because of this cooling it becomes permissable to use rotor of larger diameter which means higher velocity of the turbine blades resulting in the reduction of total number of stages in the IP section and consequently reduction in the overall length of the rotor.

The improved construction thus proposed a combined high pressure and intermediate pressure reheat turbine cylinder design, which provides for an outer shell for the HP inner cylinder to increase the overall stiffness against torsion as well as opening forces at the horizontal joint, the said outer shell extending upto the first stage diaphragm of the IP section of the turbine with provision of flexible seals on either side to take up differential expansion while also forming the boundary of steam path for the reheat steam, the said outer shell supporting a gland carrier and providing relatively cooler sealing steam to reach the IP first wheel compared to the sealing steam reaching the high temperature gland supported by the HP inner cylinder.

Claims (6)

1. A steam turbine in which the high pressure casing comprises an outer cylinder and an inner cylinder, the inner cylinder comprising an inner shell and an outer shell welded together to form an integral unit, the inner shell supporting diaphragms of the first few high pressure stages, the outer shell supporting the diaphragms of the subsequent high pressure stages, a gap being provided between the said two shells to allow steam after its expansion in the inner shell to occupy the annular space between the said two shells for effecting equal distribution of the pressure drop between the said two shells and the said outer cylinder.

2. A steam turbine as claimed in claim 1 in which the outer cylinder is extended to support the first few stages of the intermediate pressure section.

3. A steam turbine as claimed in claim 2 in which an annular space is provided for admission of intermediate pressure inlet steam between the high pressure steam inlet pipe and inlet branch of the high pressure outer cylinder, flexible seals being provided between the inner and the outer cylinder of the high pressure stages for accommodating differential expansion on opposite side of the high pressure steam inlet branch and for preventing intermixing of the intermediate pressure inlet steam and exhaust steam from high pressure stages.

4. A steam turbine as claimed in claim 2 or claim 3 in which means are provided for passing steam at relatively low temperature from the space between the said two shells to sealing glands of the inlet of the intermediate pressure stages.

5. A steam turbine as claimed in any of the preceding claims in which the outer shell of the inner cylinder is provided with openings with removable covers to provide access for the insertion and tightening of horizontal joint bolts.

6. A steam turbine substantially as herein described and illustrated in the accompanying drawings.