PRODUCTION • PROCESSING • HANDLING Fig. 1. Pump as turbine - operating point
T e recovered amount of energy is
given at the intersection point between the system curve and the pump characteristic. In this case the recovered energy would be about 3.6MW. T e pipeline fl ow is about 5,000m³/h, the fl ow through the bypass PCV, PCV-D is 1,620m³/h and an additional pressure loss of 50m must be generated at PCV-C. For turbines, the given ratio between
fl ow and pressure head leads to a design with operating points on the left side of the turbine characteristic. T e required energy can be recovered in the range between 1.5MW and 3.5MW. However, the turbine is oversized regarding the initial approach. T e potential energy recovery of the turbine is up to about 8MW. Using that turbine it was possible to
recover the requested energy with all diff erent pipeline operations. T e request to recover energy up to 3.5MW even at smaller fl ow rates requires turbine operation with high diff erential head. T is enables the turbine to recover energy up to 8MW, too. To handle the higher amount of energy, the electrical system needs to be adapted accordingly.
OPERATIONAL CHALLENGES T e installation of the system must be done in a way that the main goal of pipeline operation is possible all the time. T erefore, the energy recovery system needs to be installed in parallel to the existing line. In addition the existing task of the control
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valves must be kept. T at is to control the backpressure – to avoid slackline operation. T e energy recovery systems can only be used if the pipeline is operation. Furthermore, the energy recovery system must be easily started and stopped. In case of a pipeline emergency
shutdown (ESD), the turbine generator will be stopped immediately; energy can’t be recovered any more. In this case the grid load must be considered. T e evaluation of the grid stability is one of the main tasks. If the recovery system is designed for island mode, a kind of priority list can be installed to stop electrical consumers in preferred order. In island mode it is also important to ensure the energy balance between energy recovery and consumers. What happens if the island consumers need less energy than the minimum recoverable energy? In that case the installation of a load bank can help.
COMMERCIAL ISSUES What is the benefi t of an energy recovery system? It can be estimated using the following rule of thumb. Pressure loss (in bar) of the PCV divided by 10 and multiplied with the fl ow rate [m³/s] results in the potential energy (in MW). Assuming that 70% of the potential energy could be recovered this leads to:
P [MW] = dP [bar] / 10 * Q [m³/s] * 0.7”
T e energy recovery is only an add-on.
It is not required for the normal operation and main task of the pipeline. One important question needs to be clarifi ed: is it possible to feed the grid all the time? Together with the pipeline availability this results in the benefi t of such systems. Assuming 10 €/MW, the earning per year (in total 70% of the year energy recovery operation: 6,132 h) can be calculated:
Earning [€] = P [MW] * operating hours [h] * 10 [€] Example 1 = 3.5 [MW] * 6132 [h] * 10 [€] ~ 214.000€ Example 2 = 2.1 [MW] * 6132 [h] * 10 [€] ~ 129.000€
Installing energy recovery systems in oil pipelines is not something that’s done as standard. T ere is no general, ready-to-use solution. Each system must be designed individually and the required machines must be carefully selected. T erefore, there must be close contact with the manufacturers. In contrast to many conventional tasks, such projects also have to take into account the eff orts of the manufacturers, as otherwise it may be diffi cult to obtain the necessary information in time.
Thomas Rother is with ILF.
www.ilf.com
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