INCREASING THE EFFICIENCY OF BOILER OPERATION BY CONTROLLING DRUM LEVEL WITH BFP SCOOP OPERATION

Transcription

1 INCREASING THE EFFICIENCY OF BOILER OPERATION BY CONTROLLING DRUM LEVEL WITH BFP SCOOP OPERATION IN THREE ELEMENT MODE 1 Aditya Sindhu, 2 Aashish Bhaskar and 3 Avdesh Bhardawaj 1 Department of EEE, MAIT, Rohini, Delhi, India 2 Senior consultant, Indiabulls Power Ltd, Amravati, Maharashtra, India 3 Department of Applied Sciences, ITM University, Sector 23 (A), Gurgaon, Haryana, India ABSTRACT In order to meet out the steam requirement, to safeguard the boiler water tubes and to safe guard the turbine blades the boiler drum water level control plays an important role in thermal power station. The main objective of the boiler turbine system control is to meet the load demand of electric power while maintaining the pressure and water level in the drum within tolerances This paper demonstrates the increase in efficiency and energy savings by using the three element mode method of drum level control and compares it to the the traditional DP mode.traditionally in most power stations, the DP mode of operation is used because it is believed it provides more robust control of the throttle valve, however throttling losses do occur.this is primarily because it is a linear system of control. The linear system of control causes more losses and hence greater power consumption. Three element mode reduces these throttling losses by measuring the steam level, steam rate flow as well as the water level simultaneously, thereby resulting in a lesser loss of pressure and hence lesser demand. Pressure drop across the throttle valve can be optimized more efficiently using this method of control and the observations and results regarding this have been discussed.the results found were very encouraging and the method can be applied widely to all sub critical boiler systems across the power industry. KEYWORDS- boiler, three element, FRS, control valve. I. INTRODUCTION The control system for a boiler turbine unit usually needs to meet the requirement of the amount of water in the steam drum must be maintained at a desired level to prevent overheating of the drum or flooding of steam lines. This is critical for the safe and economic operation of a power plant. Scoop controller PID maintains speed of Hydraulic coupled Pump operated by constant speed motor. For Drum Level Control, Operator gives Set point for Drum Level Value. To achieve this set point value of Drum Level, 3 or 1 - element controller will maintain the Feed Station Control Valve position. The main purpose of scoop control is that this will reduce the throttling loss occurring due to high fluctuating DP across control valve. The Valve operates in better controllability position. The drum level must be controlled to the limits specified by the boiler manufacturer. If the drum level does not stay within these limits, there may be water carryover. If the level exceeds the limits, boiler water carryover into the super-heater or the turbine may cause damage resulting in extensive maintenance costs or outages of either the turbine or the boiler. If the level is low, overheating of the water wall tubes may cause tube ruptures and serious accidents, resulting in expensive repairs, downtime, and injury or death to personnel. A rupture or a crack most commonly occurs where the tubes connect to the drum. Providing tight water level control in a drum is mainly accomplished by utilizing one of three types of drum level control: single-element, two-element, or three - element. The term 'single-element' is derived from single variable: drum level influence on the feed-water valve position. While single - element drum 1402 Vol. 7, Issue 4, pp

2 level control is acceptable for steady boiler load conditions; as load changes become more frequent, more unpredictable, or severe; this type of level control cannot respond quickly enough to compensate. The term 'three-element' is derived from three variables: steam flow, feed water flow and drum level influence on the feed-water valve position. Wide Fast Combination of batch and continuous type operations such that plant steam load characteristics often varies continuously and unpredictably. The drum level must be controlled to the limits specified by the boiler manufacturer. If the drum level does not stay within these limits, there may be water carryover. Poor level control also has an effect on drum pressure control. Feed water going into the drum is not as hot as the water in the drum. Adding feed water too fast will result in a cooling effect in the boiler drum reducing drum pressure and causing boiler level shrinkage. This paper has been categorized into 4 main parts first one (background) introduces the problem and existing method theoretically. Observations were duly noted here. The next part, Technical and Financial Analysis introduces the solution both theoretically as well as diagrammatically. The third part, Impact of implementation gives us the observations in tabulated form and shows us the final derived values. This is then followed by Results II. BACKGROUND 2.1 Drum Level Measurement Fig. 1. Boiler drum level management The FIG-1 is an example of the arrangement of a differential drum level measuring transmitter. The differential transmitter output signal increases as the differential pressure decreases. The differential pressure range will vary between 10 and30 inches, depending on the size of the boiler drum, with a zero suppression of several inches. On the high pressure side of the measuring device, the effective pressure equals boiler drum pressure plus the weight of a water column at ambient temperature having a length equal to the distance between the two drum pressure connections. On the low pressure side, the effective pressure equals boiler drum pressure, plus the weight of a column of saturated steam having a length from the upper drum pressure connection to the water level, and the weight of a column of water at saturation temperature having a length from the water level to the lower drum pressure connection. 2.2 Existing Control System Drum Level Control is implemented in the PID controller. Changeover between single element and three element causes the drum level variation. Feed-water is controlled through Feed water regulating control valve by two constant speed motor driven pumps. The Drum level compensation includes steam temperature and drum pressure. Boiler drum level control is done by BFP scoop & feed water regulating station (FRS) control valve in two ways, namely: 1. DP mode 2. Three element mode, 1403 Vol. 7, Issue 4, pp

3 In DP mode BFP scoop maintains the DP across the FRS as per set point & control valve regulate feed water flow as per three element errors to maintain drum level as per set point. Whereas in three element mode, scoop maintain the drum level as per three element error keeping FRS control valve wide open. 2.3 Observation In most of units having MD BFP only, drum level control is accomplished through DP mode because it is believed that has robust control. But it results in appreciable energy loss due to throttling of FRS valve. DP set point for scoop is observed between the range of 7-10 kg/cm2; hence same is the pressure drop across FRS valve. Reason of high DP is set point to increase differential pressure to improve the control. Disadvantage of DP mode is increased pressure loss & Energy loss. III. TECHNICAL & FINANCIAL ANALYSIS Energy Saving potential in throttled FRS valve is given below which clearly indicates pump power loss can be optimized by minimizing pressure drop across the valve. Energy Saving Potential = Pressure drop across the valve x Pump Power Total Pressure Rise across the Pump DP optimization in DP mode gives no substantial energy saving due controlling action of FRS valve. Further pressure drop reduction is only possible in three element mode scoop operation where FRS valve is kept fully open. Pump losses % = Pump power (100- efficiency) Generated Load In 275 MW capacity unit having overall efficiency of 40 % and for pump power= 7 MW, DP =7.5 kg/cm2, Total pr. Developed by pump = 180 kg/cm2, Energy saving potential & Calculated pump loss is given below which are unusually high figure. Energy saving potential =29 % Calculated pump loss = % of Boiler Heat I/P As flow control by throttling in case of high power pump like BFP is inefficient, flow control by speed regulation by means of BFP scoop is adopted. Effect on System Curve with Throttling HEAD Fig 2 System Curve In the above system, pump system curve get shifted to lower efficiency region as throttling increases. Point A is the best efficiency point (BEP) where valve is full open at the pump s best efficiency point (BEP). But, in actual operation, amount of flow in valve full open condition is not necessary hence we 1404 Vol. 7, Issue 4, pp

4 throttle the valve. To point B to get desired flow. The reduction in flow rate has to be effected by a throttle valve. In other words, we are introducing an artificial resistance in the system. Due to this additional resistance, the frictional part of the system curve increases and thus the new system curve will shift to the left -this is shown as the red curve. So the pump has to overcome additional pressure in order to deliver the reduced flow. Now, the new system curve will intersect the pump curve at point B. At point B, pump head is increased the red double arrow line shows the additional pressure drop due to throttling. You may note that the best efficiency point has shifted from 82% to 77% efficiency. So what we want is to actually operate at point C which is on the original system curve for the same required flow. The head required at this point is reduced. What we now need is a new pump which will operate with its best efficiency point at C. But there are other simpler options rather than replacing the pump. The speed of the pump can be reduced or the existing impeller can be trimmed. The blue pump curve represents either of these options. It is not feasible to replace pump or trimming of impeller so the best solution is to reduce the speed. Hence flow control by speed control method is more efficient. DP control mode & three element mode schematic is given below: 1.) Scoop operation in DP mode 2.) Scoop operation in three element mode Fig 3 DP mode scoop operation Fig 4 Three element mode scoop operation DP mode is suitable in case of emergency & fluctuating load condition whereas three element mode is suitable in steady load condition. A basic flow chart of the existing DP as well as 3 element have been explained: 1405 Vol. 7, Issue 4, pp

5 DP MODE : Fig 5 DP mode control flowchart THREE ELEMENT MODE : Fig 6 Three element mode control flowchart 1406 Vol. 7, Issue 4, pp

6 Fig 7 Digitalized control and monitoring of boiler drum as seen from main control room IV. IMPACT OF IMPLEMENTATION BFP scoop operation was successfully tested in three element mode for each BFP. Pressure drop across FRS valve is observed to be3.5 kg/cm2 in valve full open condition. Ampere saving achievement is given below: Table 1 Observations and savings per BFP unit S.NO. BFP UNIT AMP. Gain/Hr KW saving/hr GHG reduction KG/Hr 1 BFP - 1A BFP - 1B BFP - 2A BFP - 2B BFP hydraulic coupling s response time and subsequent change in drum level shows that drum level control directly through BFP scoop operation in three element mode variation is quite feasible especially under steady operating condition V. RESULTS AND DISCUSSIONS As evident from the observations above made, there is a considerable decrease in pressure drop which was measured in full valve open condition, in the three element mode of operation. This lesser pressure drop results in lesser demand for the BFP to work continuously at full load, thereby generating an enormous potential for saving energy and increasing efficiency of the whole boiler operation system. Thus, it was seen that a slight loss in control can correspond too much greater efficiency and overall a better system of operation. VI. FUTURE WORK Although the three element mode was successively tested and results were encouraging, there is always a slight disadvantage to this method as compared to DP mode that is more degree of robust control over the throttle valve. Thus, more future research is needed in the context of how to increase the 1407 Vol. 7, Issue 4, pp

7 robustness of controlling the throttle valve as effectively as the DP mode and yet retain the same benefits of the three element mode of operation. VII. CONCLUSION A marked improvement in energy conservation and increase in overall efficiency of the system was observed when the drum level was controlled by the three element mode, wherein the throttle valve was fully opened as compared to the DP mode. ACKNOWLEDGEMENTS We are highly grateful to INDIABULLS POWER Pvt Ltd, Amravati unit for letting us use the boiler control unit as well as the various boiler equipments in their Amravati Power Plant for our research purposes. REFERENCES [1] INDIABULLS POWER Pvt Ltd, Amravati unit. [2] Operation and maintenance manual, dong fang electric corporation ltd. Released on [3] Invensys I/A Series hardware product specifications for controller, dong fang electric corporation ltd. Released on [4] M. Iacob, G.-D. Andreescu, Drum-boiler control system employing shrink and swell effect remission in thermal power plants, Proc rd International Congress on Ultra Modern Telecommunications and Control Systems and workshops (ICUMT), Budapest, Hungary, ISSN: , ISBN: , pp. 1-8, Oct IEEE [5] M. Iacob, G.-D. Andreescu, N. Muntean, SCADA system for a central heating and power plant, (in book chapter 8: Process Control and Automation Applications), in Instrument Engineers' Handbook Vol. 3: Process Software anddigital Networks, 4th Edition, Eds.: B.G. Liptak, H. Eren, CRC Press, USA, ISBN: , pp , Aug 2011 [6] Drum level control systems in process industries (Application Guide ), ABB Power. Published 2014 [7] A guide to boiler drum level equipment control concepts by The Clark-Reliance Corporation,16633Foltz Industrial Parkway, Ohio,USA. [8] Xu Lian,University of Nebraska-Lincoln, Optimized control strategies for a typical water loop heat pump system, 2012 [9] Steam pressure reduction : opportunities and issues, US department of energy. AUTHOR BIOGRAPHIES Aditya Sindhu is a B.Tech Third year student, currently pursuing electrical and electronics engineering from Maharaja Agrasen Institute of Technology (MAIT), under GGSIP University, New Delhi. He has been an academic topper throughout and published a paper in an international journal in the field of instrumentation. His research and technical interest areas include power, transmission, instrumentation and control. Aashish Bhaskar is working as a senior technical consultant at Indiabulls Power Ltd, a pioneer in the name of power industry today. He is an electrical engineer and has done his B.Tech from IIT Delhi. He has an experience of more than 5 years in the power sector and has led many teams and project site divisions. He is singly handedly responsible for saving a lot of company s investment by ingenious cost cutting methods. His research areas include energy conservation and machines Avdesh Bhardawaj is an accomplished academician, author, researcher and consultant. He has done B.Sc, M.Sc and M.Phil from premiere universities in India. He has extensive teaching experience in reputed colleges and universities. He is also working towards completion of his PhD from IIT Delhi. He has published a large number of research papers in International and National Journals and conferences. His research interests include EIA, mathematical modeling, energy conservation and pollution remediation Vol. 7, Issue 4, pp