An Experimental Investigation of The Dynamic Effects in Upper Drum Boiler Under Steam Mass Flow Rate Variation

The dynamic effects in the “upper“ drum of the boiler (mechanical’s effects) are very complex problems and critical due to the pressure disturbance in the upper drum of the boiler. This work deals with an experimental investigation of the dynamic behavior of the boiler drum level system is reported from plant data (Al-Quds power plant) in Baghdad / Iraq under a particular condition of operating, and the mathematical equations of the boiler model variables explained and defined. The dynamic effects are investigated with increasing steam mass flow rate (10% and 20%) at the outlet of the boiler. The results showed that the rapid rise in the rate of steam mass flow causes the dynamic effects to increase (shrink and swell) by 15%, rates of evaporation and thus causes an increase in the volume of water inside the upper drum boiler that causes overheating in the tubes.


H I G H L I G H T S A B S T R A C T
 Rapid rise in the steam mass flow rate causes the increase in dynamic effects.  The pressure disturbance in the boiler's upper drum cause mechanical effects.  Drum level fluctuations cause interactions with the controls of boiler combustion.  Interactions with the controls, resulting inefficient and dangerous operations.
The dynamic effects in the "upper" drum of the boiler (mechanical's effects) are very complex problems and critical due to the pressure disturbance in the upper drum of the boiler. This work deals with an experimental investigation of the dynamic behavior of the boiler drum level system is reported from plant data (Al-Quds power plant) in Baghdad / Iraq under a particular condition of operating, and the mathematical equations of the boiler model variables explained and defined. The dynamic effects are investigated with increasing steam mass flow rate (10% and 20%) at the outlet of the boiler. The results showed that the rapid rise in the rate of steam mass flow causes the dynamic effects to increase (shrink and swell) by 15%, rates of evaporation and thus causes an increase in the volume of water inside the upper drum boiler that causes overheating in the tubes.

Introduction
Boilers are closed containers in which water is transferred to steam under pressure and through the application of heat. The furnace of a boiler can convert the chemical energy in the fuel to heat, and it is the function of the boiler to transfer this heat to the contained water in the most efficient technique. This heat is transferred by radiation, conduction, and convection to the water in the boiler [1]. The relative percentage of each depends on the type of boiler, the heat transfer surface built for, and the type of fuel. In the boiler drum, the water-steam interface occurs where the boiler drum level in the safe operation of a boiler is a critical variable [2]. The Low-level water in the drum makes increasing in the steam, and when the water level high that causes carryover water into the steam header and decreasing the steam in the drum [3]. If the amount of water in the upper drum reaches the limits, the boiler water brings into the turbine, super heater, and this can cause harm resulting in comprehensive repair costs, either the turbine or the boiler failures [4]. The decline in the pressure reasons to a small portion of the saturated water in the boiler immediately vaporizes; this produces a large magnitude of boil-up from most of the boiler tubes. So that, the level controller sensing an elevation in the level of the pressure cycles of the drum will be caused variation in drum level [5].
Steam generation water distribution management is a critical issue that must be considered for boiler safety and reliability. Boiler shutdowns are common due to poor management. One of the most serious issues of boiler technology is water distribution in natural circulation drum boilers [6,2]. Poor water circulation may lead to tube rupture, which can result in an unplanned boiler shutdown, disrupting plant operations. Weak circulation can be caused by operational issues such as rapid increases in boiler load, which causes rapid increases in heat flux as a result of rapid increases in fuel flow speeds. Due to the non-uniformity of heat flux transmission in the furnace, each boiler tube faces different heating conditions. Poor circulation water has been blamed for several boiler explosions in recent years. So, to cool the tubes that constitute the boiler walls, proper water circulation is needed [7]. Figure 1 shows the schematic diagram of the upper drum.

Water level variation
Under any load inside the boilers, the drum water, mud drums, and boiler tubes cohabit with the generated steam bleb. The resulting development of steam flow from the drum causes a decrease in pressure if the request steam is unexpectedly increased [8]. Because the drum pressure function is the rate of steam production, the pressure drop would immediately cause additional steam to be produced. This implies that the water "inventory" of the steam boiler is accompanied by additional steam bubbles as the steam rises under the water surface within the boiler to water ratio [9].
The drum level increases swell until the rate of the supplementary steam generation stabilizes due to the steam already having a greater specific volume than water. Once the stabilizing rate has been achieved, unless the feed water flows rise immediately, the lack of equilibrium of mass movement from both the feed water and steam flows would lead to the drum level dropping. And this makes managing the drum level difficult [4,10].
The decline in the pressure reasons to a small portion of the saturated water in the boiler immediately vaporizes; this produces a large magnitude of boil-up from most of the boiler tubes. So that, the level controller sensing an elevation in the level of the pressure cycles of the drum will be caused variation in drum level. Can describe the linearized behavior of a wet surface ( ) at the operation level and the perversion in the drum level (L) we measure from its normal operating level is [11]: (1) Figure 2 shows the variation in Drum level, if the rush order for steam decreases instantly the same effect happens, but in the opposite direction. Drum pressure rises rapidly in this situation, reducing the steam production and steam-to-water ratio [12,7]. This shows the initial declines in the drum level before the new steam-generating rate supplies stabilize. Whenever the stem rate is stabilized, the drum level will start rising, except if the feed water stream is decreased to settle with the latest steam flow rate immediately [13].

Swelling and shrinking phenomena
The downstream mechanism regulates the flow of steam from the boiler. The strain in the steam drum and boiler circuit is affected by a sudden rise or decrease in steam flow. The boiling point and density of water and steam will both change as the pressure changes. The amount in the steam drum will quickly increase or decrease as a result of these simultaneous reactions [14,15]. The swell and shrink reactions are the rise and fall of water level caused by pressure changes.The swell and shrink reactions are in the opposite direction of the boiler's natural activity. As a result, swell and shrink reactions wreak havoc on steam drum level power. The pressure in the boiler circuit will rise if the steam output from the boiler is abruptly decreased [16]. Due to a higher inflow than outflow, the drum level will initially shrink and then raise as the pressure rises. As the steam output from the boiler is abruptly raised, the opposite is real. Pressure fluctuations caused by unstable energy supply to the boiler will also induce the same swell and shrink reactions, making steam drum level control incredibly difficult [17].

Mathematical formulation
It is very difficult to achieve the solution of mathematical equations as a synthesis of the mathematical description. In several phenomena, the number of variables, nonlinearities, and uncertainties contribute to the complexity of the problem [18]. The governing equations consist of the preservation of the total system's mass and energy, the phase shift in the drum boiler, including the volumes of water and steam within the drum boiler, and the rate of steam condensation and flow circulation in the riser-down comer loop, which controls the mass, energy and momentum transport.
The equation simultaneously solved using the explicit method by time step 0.3 second for a total time of 400 seconds [11,19].
Equations (8 and 9) are representing the equilibrium conditions. All parameters are measured and calculated such as, drum pressure, steam mass flow rate, drum volume, riser, and downcomer volumes, water surface area, feed water temperature, and thermal conductivity of the pipe material. There are many constants according to the boundary conditions are taken from power plant characteristics.

Experimental procedure
The experimental process of this work was applied to the steam generator of the boiler (water tube) in Baghdad / Iraq (Al-Quds power plant) model (szs20-1.0-210y). Table I shows the technical characteristics of the steam plant generator. All the parameters were determined when the plant was in a state of thermal equilibrium, and other data was reported from the technical characteristics sheet of the steam generator. Figure 3 shows the water tube boiler in (Al-Quds power plant).
The experimental operation work requires the analysis of the fundamental parameters as follows: Figure 4 shows the steam generation unit's drum pressure gauge. It varies depending on the firing rate and steam demand (turbine load), so to calculate the drum strain, we start the plant at a steady firing rate and let it run until it stabilizes. While the steam pressure and temperature in different points of the circuit, as well as the temperature of the cooling water at the condenser's outlet remain constant. When the drum pressure has stabilized, the following adjustments are used to determine the drum pressure: -(i) raising the steam load by 10%, (ii) increasing the steam load by 20%. The drum pressure is measured with time at each transition.

Drum Level Measurement
The drum level of the steam generation unit has a complex engineering design (90 mm gauge "length glass") [ Figure 5]. Reading the drum level directly from the glass gauge to record the drum level's reaction to the firing rate, synchronization, measuring the drum level to increase the steam mass flow rate ("10 percent and 20 percent") at the outlet of the boiler. This represents the swelling phenomenon. The same process is repeated in the opposite direction, drum level reaction with the following modifications: (i) reducing the steam load by 10%, and (ii) reducing the steam load by 20%. This represents of shrinkage phenomenon. Liberalized behavior can be represented by the wet surface ( ) at the functional level may be viewed. The water tube boiler in Figure 3: (Al-Quds power plant) Baghdad / Iraq.

Fuel level measurement
The amount of fuel (LDO oil) in the tank was determined at normal operating and the reaction of using the fuel with the following modifications: (i) increasing the heat input by 10%, (ii) increasing the heat input by 20%. Figure 6 shows demonstrates the experimental behavior of firing rate fluctuations over time at normal operation and at changing the heat input by increasing 10% and 20%. Figure 7 Presents an experimental drum pressure behavior with time during normal operation while Figure 8 explains how the pressure in the drum behaves when a sudden increase in heat input . When the firing rate is at its highest, the drum pressure increases, allowing the firing rate to reach the mean by (2.1) minute due to an increase in heat input. As a result of a 10% step increase in the heat supply, the pressure increases at a steady rate of ⁄ = 0.0314 bar/s. And the rate of pressure increases by 0.065 bar/s in the case of a 20% step increase in the heat supply. To explain the increase in drum pressure, when the firing rate rises, the amount of water volume inside the upper drum boiler rises as well, causing a disturbance (oscillation) in the drum pressure, indicating that the disturbance value in the minimum firing rate is less than the maximum firing rate.

Results and discussion
The experimental measurements for distribution firing rate In the upper drum, there is a large amount of water and steam bubbles. During a quick increase in the steam load, a substantial rise in the water/steam level may occur due to an increase in the volume of these bubbles. The increase in volume was influenced by a decrease in steam pressure due to the increased load, as well as a rise in steam production due to the increased rate of fuel firing to match the increased steam load. Figure 9 Shows steam-water level at regular operation with fuel firing rate, where the change in fuel firing rate has an influence on drum level, but rapid changes in drum pressure that expand or shrink the steam bubbles due to load changes are the most important cause of shrinkage and swelling. The drum pressure rises and the firing rate changes when there is a decrease in steam demand, if the pressure of the drum decreases, the boiling point of the water in the drum and the evaporator also decreases. This reduces the volume of the bubbles (i.e., bubbles get smaller). The water in the drum is actually forced up by the rising bubbles to create more bubbles. Figure 10 shows the steam quality in the boiler, as the rate of evaporation rises in tandem with the increase in heat supply, the steam quality at the riser's exit improves. During the first 10 seconds, the output improves rapidly. Figure 11 shows that the amount of water in the system has increased ( increases). The rise in is due to the rise in as well as the condensation of steam under the water level in the drum. The growth in water content is in line with the increase in total water volume, . Figure 12 shows the amount of steam according to the relationship [ , the overall steam capacity, , decreases. Where V denotes the system's total volume. Figure 13 demonstrates the influence on swelling phenomena of the rapid change of steam load. It is clear that when the steam load suddenly changes, the effects of swelling phenomena are increased and that the complex disruption in the watersteam level is increased. Figure 14 demonstrates the influence on shrinking phenomena of the rapid change of steam load. It is clear that when the steam load suddenly changes, the effects of shrinking phenomena are increased and that the complex disruption in the watersteam level is increased.
The Drum water level (Al-Quds Figure 9: power plant) in Baghdad / Iraq.When the fire rate (heat input) changes by 10 percent & 20 percent at the normal operator Variation of steam quality for Figure 10: step rise in heat input Variation of steam volume for Figure 11: step rise in heat input.
Variation of water volume Figure 12: for step rise in heat input.
Effect of suddenly change of Figure 13: the load on swelling phenomena Effect of suddenly change Figure 14: of the load on shrinking phenomena.

Conclusions
The current study looks at the dynamic consequences of rapid changes in steam mass flow rate (10% and 20%) at the water tube boiler's outlet, the results indicate that: A sudden rise in the loaded steam causes an increase in the upper drum's shrinking and swelling 1) phenomena, when the temperature has increased, then the bubbling increases the volume of the drum and the level will first go up, and then the level begins to go down as the steam flow increases. Due to the inverse response of the level to changes in steam demand, the drum level is hard to control.

2)
To prevent overheating of the tubes, the drum level system must ensure that the water levels are always 3) mainlined above the top of the risers/down comers.
Drum level fluctuations cause interactions with the controls of boiler combustion and can generate cyclical 4) or even unstable boiler control, resulting in inefficient and dangerous operations. If the level of the drum drops too low, the boiler may be damaged by thermal stress.

5)
If the level gets too high, some water particles may be carried by steam leaving the drum, which can cause 6) harm to turbines or other steam users. The future perspective is to use a control system to reduce the dynamic effects and improve the 7) performance of the upper drum boiler.