Serial Pumps
Process Performance Limited by Pump Max Pressure
As an example, let’s analyze the case of a filtration process, which requires high pressures to overcome the resistance posed by the filter (schematic in Figure 1).
Since the system is very restricted, the system load curve is a steep parabola. The required pressure is so large that is falls out of the operational limit of the pump (see Figure 2).
Serial Pumps Performance
Since the process performance is limited by the pressure made available by the pump, a possible solution is to install two pumps in series (Figure 3). In this way, each pump provides a pressure jump, which add up to obtain the target pressure. The flow rate is the same for both pumps.
To estimate the new flow pressure performance of a system of serial pumps, it is necessary to multiply the pressure of a single pump by the number of pumps in series, while the flow rate remains the same (each pump elaborates the same flow rate, and the pressure jumps provided by each pump are added up together, as shown in Figure 4).
Redundancy and Hydraulic Loop Modifications
Theoretically, it is possible to achieve partial or total redundancy in the system with a set of pumps in series. In practice, it poses some challenges in case of a failure, since it becomes difficult to isolate and remove a pump without interrupting the process.
If a set of pumps in series is intended to function as a redundant system, each pump should have a set of valves and bypasses to isolate it without interrupting the pumping process.
Control of Pumps in Series
Pumps in series can be operated in different ways:
- In speed control
- One pump in speed control, the second pump in process control
In the first case, the speed is defined by a higher-level PLC/SCADA system (which can run a closed loop control based on a process parameter such as pressure or flow rate). All pumps receive the same speed set-point, and run at the same speed.
In the second case, one pump operates at a fixed speed to provide (for example) the minimum pressure possibly needed by the plant, while the other pump operates in pressure control mode do adapt to the changes in the pressure requirements.
Boosting the Pressure at Specific Locations
Dispensing Fluid at Multiple Points-of-Use
A common installation setup (in particular in the semiconductors manufacturing) is composed of a big re-circulation loop, powered by a powerful pump, from which multiple tools in parallel draw small amounts of fluid. In these setups, the main pump provides the majority of the pressure and supplies flow to all the tools, but the tools might require different pressures because they run different processes.
For this reason, each tool has its own booster pump do draw and pressurize the fluid at the required process point. In this case, the re-circulation pump will typically operate at constant speed, while the booster pumps will adapt their speed to match the target process point.
Maximum Pressure Limited by Components
Let’s take as an example a hydraulic setup where the Point of Use requires high pressure, and on top of it there is a large static head to overcome due to height difference. In total, a pressure of 10 bar is needed. A single Levitronix pump can’t deliver it, so two pumps in series are needed. If the pumps are placed one immediately after the other (as in Figure 6), the outlet pressure of the second pump will be 10 bar, which is more than the elbow fitting can withstand, therefore it is not a safe installation. Therefore, the second pump must be placed at a higher altitude (as in Figure 7), so that at the outlet of each pump the pressure is 5 bar (which can be sustained by the fittings).
