TECHNICAL REPORT ON THE REPAIR OF DISHING ON A DIAPHRAGM

TECHNICAL REPORT ON THE REPAIR OF DISHING ON A DIAPHRAGM

1.0     INTRODUCTION

Definitions of dishing in a diaphragm is the form of deformation that occurs across the axis of the diaphragm.

The diaphragm is a hollow disk that holds the stator blades (nozzles) in its annular area. (The rotor blades are fixed on the turbine rotor.) In each stage, a diaphragm consists of an upper- and a lower-half. Each half of the diaphragm is fixed to the respective halves of the casing. This design makes the assembly/disassembly of the turbine feasible.

Figure 1    Image of a Diaphragm

1.1     Causes of Dishing in Diaphragm

         Operational rubs or severe creep deflection from exposure to high stream pressure differentials across the blade ring and inner Web when operating at high temperature.

1.2     Problem Statement

The dishing occurs such that the diaphragm experiences distorting across it cross axis and therefore causing it lose fit in the turbine.

The steam turbines diaphragm is exposed to about 600°C operation temperature which is sufficient to cause dishing.

The dishing or distortion took place on the web, this cause a change in form and could be within the range of  3mm and so on, in worse case scenario at locations illustrated below on the diaphragm web.

Figure 2 Marked Area (Press locations, region of high distortion/dishing)

1.3     Aim and Objective

To eliminate dishing from a steam turbine diaphragm by pressing the dished using a high pressure Hydraulic Pressing Machine.

2.0     Surveys

According to Orbital Energy Services newsletter, Dish removal entails pressing the dished area on the diaphragm to eliminate the dishing condition and restoring flatness. Once the dishing has been removed, the diaphragm can be reinforced by adding bridges or reinforcing the partition admission edges with weld to help maintain structural rigidity. The bridges distribute the stress experienced from creep effectively increasing the structural strength and rigidity of the diaphragm. The improved stress distribution throughout the structure improves component life post-repair, reducing the frequency of costly repairs and lower operating expense.

It has also been proved in various practices that this dish removal process provides a lasting and effective solution to dishing issues in Steam turbine diaphragms than other know methods of dishing removal such as

1.     Steam Seal Face Insert – Reposition the diaphragm up-stream to the correct axial position by installing an insert in the steam seal face.

2.     Offset Packing / Spill Strips – Supply new “offset geometry” packing segments and tip spill strips to re-establish the correct axial clearances between the rotating and stationary components. (MD&A,2016)

The above methods only provide a minimal solution to the dishing problem in steam turbines. These repair methods are selected based on the deflection found on the diaphragm. In order to proffer a lasting solution to dishing in diaphragm, Dishing removal by Pressing has proved to be the most effective method and will be applied in solving this problem.

  

3.0     Methodology

3.1       Locating the dished region.

The diaphragm was mounted and balance on a turning machine, this is to determine the displacement on dished region on the web. The process of determining the dished displacement allows to find the worst area/location of displacement. This is done using a dial gauge to feel the web to ascertain these area and marked out.

3.2       Operation on the Hydraulic Press 

The diaphragm was move to press platform and the Press pistons are placed at the marked area as shown below.

Figure 3 The hydraulic Press in operation

The pressure piston on the marked area are then regulated accordingly to yield the required flatness.

The table top vernier caliper were then  placed within the areas of distortion to verify it is at the allowable displacement value of 0.5mm.

4.0       RESULTS

The results on the diaphragm are tabulated below.

Figure 4 Diagram Illustrating the Press result location.

Diaphragm 1

Table 1 Results on a half of the Diaphragm 1 (wing 1)

wing 1
A	B	E	F	C	D
71.1	67.6	72.9	73.3	67.6	70.9
71.4	68.1			68	71.2
71.4	68.1			68	71.2
72.6	71.9			71.9	72.8
73	72.5			72.2	72.9
				72.5	73.1

Table 2Results on a half of the Diaphragm 1 (wing 2)

wing 2
A	B	E	F	C	D
71.2	68.4	73	74	68.1	71
71.4	68.8			68.5	71.8
72	69.6			69.3	72.2
72.1	70.8			71	72.3
72.4	71.6			72.3	72.9
72.9	72.3

The table above shows the reading of the displacement at the marked area after every attempt in the repair process. The distortion result was reduced to an average of 0.5mm after repair for the two halves diaphragms.

Diaphragm 2

Table 3 Results on a half of the Diaphragm 2 (wing 1)

wing 1
A	B	E	F	C	D
78.2	76.4	75.9	-	76.2	77.8
78.9	78.2	78.5	-	77.8	78.5
79.3	79.1	79.1		78.7	79.1

Table 4 Results on a half of the Diaphragm 2 (wing 2)

wing 2
A	B	E	F	C	D
78.9	76.2	76.7	-	75.9	78.4
79.1	77.8	79.1	-	77.2	78.9
79.2	79			78.9	79.1

The table above shows the reading of the displacement at the marked area after every attempt in the repair process. The distortion result was reduced to an average of 0.5mm after repair for the two halves diaphragms.

5.0       CONCLUSION

The diaphragms were effectively pressed to an allowable value which will best suit smooth running operation for the steam turbine.