6th International Research/Expert Conference "Trends in the Development of Machinery and Associated Technology"

TMT 2002, Neum, B&H, 18-22 September, 2002

AN ASPECT OF THE STABILITY OF THE LARGE DIMENSION MECHANICAL CONSTRUCTIONS

Dušan Vukojević Nedeljko Vukojević Nermina Zaimović – Uzunović Fuad Hadžikadunić

Samir Lemeš

Faculty of Mechanical Engineering in Zenica University of Sarajevo

Fakultetska 1, 72000 Zenica, B&H SUMMARY This paper deals with a part of the steam boiler construction which presents a large dimension construction. Stress strain state of the whole steam boiler and all it's parts depends on several acting factors. First of all, these are temperature conditions, transient processes during the starting up the system, system construction parameters, etc. All influences are acting simultaneously and not at the same manner at the different parts of the constructions. Compensators are links of the steam boiler connecting it to the supporting construction. During the period of acting the mentioned influences the stress strain state in compensators is changed, detected and analysed. On the base of measured results the behaviour of the whole steam boiler is analysed. The analysis is a contribution in defining total behaviour of the system presented. Keywords: steam boiler, compensator, strain, stress, displacement. 1. INTRODUCTION

Figure 1. Schematic picture of steam boiler TE "Kakanj"

1

4

3

2

Steam boiler is a mechanical construction whose consisting parts are compensators. It is steam boiler in TE "Kakanj" – B&H, power 230 MW (Figure 1). Large dimension construction and it's part (position 1) presents first gas tract which has its own guides. The second gas tract (position 2) is connected using compensators (position 3). The connecting point (position 4) of the two gas tracts is a weak area and huge fractures are appeared. Measuring of the deformations and stress calculating, at the part of the boiler (position 2) have been done in several cases of the starting up and stopping the system. 2. STRAINS OF THE COMPENSATORS Strain measurements were performed at the left and right compensators during the several working circles of the steam boiler, but in this paper only the left compensator will be shown. Measuring points MM1.1 and MM1.2 are shown at Figure 2. Measurements were performed using strain gauges connected to the UPM-40A instrument.

Figure 2. Left compensator and measuring points MM1.1 and MM1.2

Due to the acting of the temperature and other undefined outside factors, the strain state of the compensator was changed. Piston of the compensator changes its own position and the distance between measuring points is shortened. Only 10 mm long path of the piston is a full construction determined piston path. According to the steam boiler acting, piston of the compensator is moved through it's all path and leans on the cylinder wall. In the moment of contact, piston and the wall, compensator has become a rigid body. That case results in increased strains of the compensator. Time – strain dependance is shown at Figure 3. It is obvious and may be seen at the diagram (Figure 3) that strains rise during the period of time. That is a reason why compensator lose its function in a system of steam boiler.

LEFT COMPENSATOR Measuring points MM1.1 and MM1.2

MM 1.1

MM 1.2

Figure 3. Strains of the left compensator piston 3. STRESS STATE OF THE COMPENSATORS Stress state of the compensators is a result of the thermal expansion of the whole construction of the steam boiler. Figure of the stresses is changed inside the compensator after the moment in which compensator became the rigid body. Besides of that in the material of the compensator residual stresses are existing, and figure of stresses may be changed. Total stress state of the compensator can be defined on the base of the measured residual stresses and working measured stresses. It can be written as:

TOTAL= (a+M) ... (1)

where: a – value of the residual stress, M – average value of the measured stress during the working period of the compensator. Residual stress in the material of the compensator were performed and the value is a = 76,67 N/mm2. Directions of the principal stresses are shown at the Figure 4. They are calculated according to the equation:

2cos2

1

2

12121 n ... (2)

Figure 4. Directions of the principal normal stresses On the basis of the measured values at the measuring points MM1.1 and MM1.2 stresses were calculated as M1.1 = -52,3 N/mm2 and M1.2 = -27,7 N/mm2. Average value used for total stress calculating is –40,3 N/mm2. Total stress state defined after transient period is assigned with TOTAL = 36,35 N/mm2.

(1)

(a)

( )sa

( )sb

( )sc

(b)

(c)

(2)

f=-4,486o

4. CONCLUSION Large dimension mechanical constructions are subjected to a variety of complex working conditions. Cause of that, the problem of stability in the transient period is existing and results in stress strain unexpected changes. In this paper steam boiler unstability problem is analysed. The stress strain state of the compensator, as a part of the steam boiler, was analysed in the transient period. The results are important for making decisions in working parameters, production conducting and maintenance. 5. REFERENCES 1 Standard MSK – 64 skala, UNESCO, Pariz, 1964., 2 Vukojević D. : Theory of vibrations, Mechanical faculty, Zenica, 1998., 3 Brčić V. : Dynamics of constructions, IRO Graphic book, Beograd, 1981., 4 Fertis D.G. : Mechanical and Structural Vibrations, John Wiley & Sons, NY, 1995., 5 Thomson W.T. : Theory of Vibration with Applications, Chapman & Hill, IV edition, 1993.