Electricity + Control June 2017

PRESSURE + LEVEL MEASUREMENT

stant lines represent the minimum and maximum allowable steam flow limit of the turbine. All steam flow above the maximum limit (30 ton/h) cannot be utilised by the turbine and for every instance that the steam flow drops below the minimum limit of 10 ton/h, the turbine will trip if in operation. It should be noted that when power co-generation takes place in such fluctuating steam flow conditions, protection measures are in place to protect a turbine from tripping in close proximity. Common engineering practice is to allow for a time of steam stability, i.e. a continuous time interval before start-up where steam flow must be sufficient to have kept the turbine opera- tional. Under such protectionmeasures Figure 3 does not necessarily imply that six turbine trips would have been experienced within the 100 hour time period. Note that when a turbine is not operational, all power generation capability of the steam goes to waste, since it cannot be stored and utilised at a later stage. Under fluctuating steam availability steam shortages are ex- pected, however, trips should be limited as far as possible, since a turbine is designed to operate continuously and each trip occurrence depletes the life expectancy of the machine. The scenario as depicted in Figures 2 and 3 displays a situation where plant steam usages cannot be altered to assist turbines dur- ing low steam flow availability. The generic layout given in Figure 1 shows that parallel to off-gas steam production in boiler houses, off-gas flaring takes place. Thus, if plant steam usages cannot adapt to assist power co-generation, an investigation into off-gas flaring might prove to be advantageous.

Engineering considerations For the engineering plant under consideration the main focus for steam production is to ensure that plant process steam demands are satisfied and excess steam available for power co-generation is an additional advantage. Sufficient steam productions are therefore not measured in terms of available steamfor power generation. In a fluctuatingoff-gas pro- duction environment that results in fluctuating steam flow productions, it is to be expected that steam availability for power generation will not be constant and steam shortages will occur that may lead to unforeseen turbine trips. It should be noted that a turbine is designed to operate withinmaximumandminimumallowable steamflow limits. Steamflow above the maximum capacity is not allowed and cannot be utilised by the turbine. However, when the steam flow drops below the minimum allowable limit, protectionmeasureswill immediately shut down the inlet steamvalves to the turbine tripping themachine instantly. Theminimum steam flow limit helps to protect a turbine against damaging vibrations. Figure 2 demonstrates hypothetical steam flow productions from fluctuating off-gases over an arbitrary 100 hours’ time interval, where the fluctuating curve represents steam flow production (m. total ) in the boiler houses and the constant 50 ton/h line denotes plant usage (m. usage ) demands that need to be satisfied. The area between the two curves denotes excess steam available for power co-generation. As mentioned, plant steam demands are not constant and only chosen as such for illustrative purposes. If all of the excess steam from Figure 2 are utilised for power co-generation, the maximum average rate of power co-generation can be calculated by integrating the product of the area between the two curves with a conversion factor (C f ) and given by (1). This conversion factor is a function of the isentropic efficiency of the turbine, which in turn is a function of the steam flow. Some investment questions need to be addressed regarding the quantity of turbines and at what capacities need to be procured for power generation purposes. As mentioned above, this article does not address any investment questions and will only demonstrate the potential power co-generation effect if off-gas flaring is regulated.

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Excess steam (ton/h)

0

∫ 100 0

0 2 0 4 0 6 0 8 0 1 0 0

W

= ▒〖

C ƒ

(m •

– m •

)dh〗

(1)

Time (h)

max

total

usage

Figure 3: Hypothetical steam flow available for power co-generation over time, with turbine operating limits.

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The following section displays simulation results for the engineering plant where co-generation exists under conditions described in this section. Measured steam productions and plant usages are used to display power co-generation capabilities. An investigation is then launched into the off-gas quantities that were flared, from obtained measurements, and how the regulation of this flaring could poten- tially have impacted power generation at this plant. As mentioned, power co-generation is only selected to stipulate the effect of off-gas flaring and therefore simulations where boiler house capabilities are hypothetically increased to observe an even more positive power co-generation effect will not be discussed. All simulations are per- formed with the optimisation model from [2].

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0

0 2 0 4 0 6 0 8 0 1 0 0

Total steam production (ton/h)

Time (h)

Figure 2: Hypothetical steam flow production over time.

The hypothetical residual steam from Figure 2 is taken and plotted in Figure 3 with the addition of a turbine’s operating limits. The two con-

June ‘17 Electricity+Control

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