As stated in the last post «optimizing pumping equipment» traditionally pumping equipment has been sized from the flow and pressure requirements for only a specific situation or scenario. On one hand, the network peak flow demand (when each irrigator freely manages its watering time hydrant) is usually calculated by the methodology of Clement from water needs of the alternative or crop rotation, among other considerations.
On the other hand, the engineer designer deems the pressure to be supplied by the system when all irrigation hydrants or a large percentage of them have the set pressure or higher.
However, the emergence of applications to optimize allows calculate the sizing of the pumping stations and irrigation networks much more accurately. Whether through Excel sheet (only for simple irrigation networks with few hydrants) or specific software such as GESTAR will determine the minimum total annual cost of the system (as the sum of the cost of depreciation of the investment and operating costs, simplifying the latter usually as the energy cost).
In this process, to increase the head pressure Hd, to the designed flow Qd, an inverse relationship exists between the energy cost (increasing) and the cost of the piping network (decreasing) as shown in figure 1: Total annual cost curve of the system depending on the available head pressure, Hd, sum of annual depreciation costs of the installed pipes (CAT) and energy cost (EC) required per year. In the overall energy cost, the energy term (product of energy consumed (kWh) by the average price (€ / kWh) is usually the most significant in annual electric bills.
The depreciation curve of materials leads to decreasing values as the diameters of the pipes are smaller. The opposite happens with the energy to be supplied by the system to reach the reference pressure through a network of pipes with diameters smaller (and thus cheaper).
So it comes to finding the minimum point on the curve of amortization of materials and energy costs.
As suggested by the figure, for the same cost curve pipes amortization (CAT) based on nominal head pressure (Hd), different assessments can be made in annual energy costs (CE1, CE2). They are based on different methodologies or charges applied, lead to different «optimal designs» (Hd1, Hd2). So, realistic approaches of energy costs must be made since the early stages of the project.
According to the latest data provided by the Irrigation Office of Aragon (North of Spain) price changes in electricity rates will mean, since August 2013, an increase between 10 and 30% for most Water User Associations. Therefore caution should be exercised when estimating the annual costs of the system and its impact on the decision of the minimum annual cost point.
Optimization of irrigation pumping equipment should be take into account that the need for header pressure is not constant in time for a network of irrigation demand. We must change the header flow calculated by Clement network by a cloud of points representing different flow and pressure relationships in header (network setpoint curve)
I collected the following figure 2 from the publication «improvements in the prediction of energy costs» presented at the National Irrigation Congress (Spain) by the research team GESTAR. It is illustrated, for a real irrigation network, the head pressure heights required, highlighting the maximum and minimum height. For each flow we have a wide range of pressures required that the system must be able to supply. We should be able to discriminate the values will not be representative. Through specific tools can generate thousands of random scenarios to help us discriminate situations «exceptional» and we can adjust the power of the station.
The good news is that we have tools to model direct pumping station with any regulation setpoint following a curve by several variable or fixed speed pumps. To do this, simply interpret the entire pumping station as a “Virtual Pump”. “Virtual Pump´s curves, absorbed Power (or Performance) vs. Net Flow curves, are the sum of the individual pumps taken into account the variable speed pumps can work in different points according the network setpoint curve).
It is desirable that the simulation model allows to configure and compare agile and flexibly any pumping station design. It must be offer the choice of compositions with arbitrary numbers of pumps constant RPM (BVF) and RPM variables (BVV) of equal or different size. It may be considered, if there are multiple BVV, action of variable sequentially (one pressure regulating. BVV time to time) or simultaneously (two regulating BVV simultaneously with the same speed), The results in terms of performance, power consumption, etc may differ notoriously from one option to another as illustrated in next Figure (Performance vs Flow)
Note: the images are taken from the presentation of the Technical Conference GESTAR issued by the group in June of 2,013 EUPSH, Huesca (Spain)