Because your process is a "mostly friction" one, with only a small elevation ("static head") component, your system curve is basically a parabola (blue in Part D). If you select an operating point on Part D by controlling the discharge pressure (or flow) at a particular value, you have, in effect, also set the other. For example, in the sketch, I have picked a flow controller (FIC) setpoint which equals 120% of the full capacity of one pump. This automatically sets the discharge pressure also P3 = 50% of one pump) and the speed at 63%.
Obviously, in your actual configuration the four pumps are not the same, (the main pumps are nearly three times as powerful as the boosters) and, consequently, the combined pump curves will be different from the ones I have shown in Part D, but the concept is the same.
Now, let me turn to the control of the the pumping station. If you want only stable operation, you can modulate the speed of the booster stage to maintain the suction pressure for the main stage, and can throttle the speed of the main stage to keep the discharge (flow or pressure) constant. This, with safety limits and start-up/shutdown logic is all you need.
What I will describe below is the system you should consider if you want to maximize the efficiency (minimize the energy consumption) of the pumping station. To do that, we have to understand that this process has four degrees of freedom (total load, boost/main load ratio, and load distribution in both pairs of pumps) and, therefore, we can place four controllers on it without causing excessive interaction ("fighting") among them. Also, because the oil is incompressible, the loops are fast, so I usually tune these loops for mostly integral behavior (very little proportional gain).
The first loop controls the discharge pressure of the main stage. I usually use a "neutral gap" PIC, so that the controller output remains unchanged as long as the pressure is within ±1% of setpoint. This stabilizes the loop, which has fast measurements, but relatively slow speed controller response, and is, therefore, noisy or interacts with other loops.
The second loop controls the load distribution between the boost and main stages (booster % = (P2-P1)/(P3-P1)) to guaranty maximum station efficiency. This interstage (booster discharge) pressure controller compares the above optimum desired setpoint with the actual value of (P2-P1)/(P3-P1), and adjusts the load on the booster stage until it is optimum. Naturally, this PIC setpoint is limited (high selector), so that it can not drop below the minimum suction pressure requirement of the main stage.
Similarly, we can optimize the other two loops by optimizing the distribution of the flows among the two pumps within each stage. This is accomplished by measuring the individual flows, and because we know the optimum flow distribution (optimum setpoint = F1/(F1 + F2), we can modulate the speeds to achieve that. In this configuration, the output of the stage pressure controller becomes the cascade master of the more efficient pump's flow controller, while the less efficient meets the remaining load.
To implement his type of optimized control system, the pump supplier has to provide reliable efficiency data for the pumps. If they can't do that, or if the unique piping configuration of the particular pumping station is to be also considered, efficiency data can be accumulated on the basis of past operating performance. This is done by comparing the kWs that were needed to deliver the same flows at the same ΔP at different pump loading combinations in the past, and selecting the least energy-demanding one. This strategy can also be used to signal the need for "pigging" (cleaning) the pipeline if the pumping power required to deliver the same load rises.
Such an optimizated variable-speed pumping station can reduce the energy cost of operation to one half of a constant- speed pumping station using control valves.
A: This is fundamental cascade control. Control of the booster pump is the inner loop, compensating for such things as tank level, suction pressure, etc. The goal of the booster pump is to provide a constant input to the main pump.
A: The issue of pump control has several aspects that require analysis.
- Is the booster pump receiving from an extremely low head pressure and raising the outlet pressure so that the main pump can supply the outlet pressure within the correct pump curve?
- Will the booster pump be in line with one or several main pumps, such that the booster pump(s) can deliver the required flow and pressure to the main pump(s)?
- If the booster is not dedicated to one single main pump, but rather to several, it may need the analysis of how many booster pumps you need to have.
- Since the premise is that the pumps are reciprocating, which means that the speed control and response time of the pumps are not instantaneous, the time delay of operating the booster and main pumps has to be linked based on time delays, as well as production needs.
Based on the basics defined before, we would be able to define the control requires as follows:
Assuming that the booster is dedicated to one main pump, the most effective method is that the speed of the motors be regulated and have dedicated bypass valves to avoid cavitation surges and overpressuring the system.
The operation would consist of starting up the booster and having it recirculate the flow (to avoid overheating the pump) through the bypass valve. Once the pressure is at the minimum operating head pressure of the main pump, then start this pump and recirculate the flow while the outlet pressure builds close to the line operating pressure.