Chemical Equipment Update

Fig. 3. Modern Loos International steam boiler system with fully automated water analysis and monitoring.

to the boiler safeguard pressure, this problem can be avoided. Modern burner managers are able to variably specify

the burner actuating time, ie the running time between the burner low load and high load position. At the same time, the reaction speed of the burner to setpoint value deviations can be influenced via the regulation parameters in the performance regulator. A shell boiler, with its high proportion of material and high water content, is a comparatively slow reacting system. Performance regulators that are set too ‘quickly’, possibly in combination with very short burner actuating times, lead to a rapidly rising heat entry in the flame tube. On the water side, this heat entry is removed primarily by steam bubbles which form and rise in the steam area (Fig. 4). However, these steam bubbles form after a certain time delay. This causes brief local overheating and additional temperature change stresses, which in the long term speed up the process of material fatigue in the area of the heated boiler walls. It is strongly recommended that commissioning is carried out

by specialist staff, who will then set the burners and regulating systems. If multiboiler systems are not fitted with an automatic sequence control, the operating team has a major role to play. The members of the team must switch off boilers if the power take-off no longer justifies the operation of several boilers. . A sequence control concept is advisable even for boiler

systems with two steam generators and is absolutely essential if there are three or more boilers in one boiler house. The type of sequence control (addition and disconnection

of boilers as a function of volume or pressure) that is used will depend firstly on the number of boilers and secondly on what pressure fluctuations can be accepted on the consumer side. With sequence controls that are dependent on the volume of steam, the viable pressure fluctuation range can be kept much lower. The following should also be noted: steam generators in

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multiboiler systems must be hydraulically separated from each other to prevent them influencing each other (eg using non- return valves); and even at the planning stage, it should be borne in mind that secondary boilers must be fitted with a floor heating coil in order to avoid temperature layering of the boiler water during the heat maintenance phase. Starting up the system from the cold state represents the

greatest mechanical load on the boiler body. The reason is the greater temperature difference between the flame tube and boiler casing at the cold start compared with standard operation at operating temperature. The flame tube thrust (difference between change in length of the boiler casing and the flame tube) is higher during the start-up process, which leads to considerable additional strains which the boiler body has to deal with. This problem is even more acute if, during the start- up procedure, steam bubbles cannot form or can only form to a limited extent, which is the case, for example, if the steam removal valve is closed. The natural circulation that normally exists in the steam boiler (Fig. 5) does not occur, resulting in temperature layering in the boiler (cold at the bottom, hot at the top) with additional thermal stresses. In the case of very frequent cold starts, these extreme changeover stresses can lead to cracking of the material, or, in the worse case, to complete failure of the system. The following should be noted if the start- up load is to be reduced:

❒ Start-up from cold state to operating temperature with the smallest possible burner load.

❒ During the start-up procedure, a small quantity of steam should always be able to flow off in order to start the natural circulation through pushing up the steam bubbles.

❒ Ideally, the system should be fitted with an automatic start-up switch, which regulates burner operation and load removal as Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44