In my two most recent Pumps & Systems columns (read them here), we discussed methods for sizing pumps according to the design scenario.
In many applications, the normal flow rate is less than the design flow rate for pump selection. Previously, we discovered that the flow rate through the sample system operates as outlined in Table 1, below.
This system was designed to meet maximum flow conditions of 400 gallons per minute (gpm), but as we can see from Table 1, this occurs in 5 percent of the operation. The remainder of the time, the flow rate through the system varies from 200 gpm to 80 gpm.
Our system controls consist of the pump starting on high-tank level and stopping on low-tank level. In this column, we will simulate the operation of the entire system. This will allow us to optimize the system under the full conditions outlined in Table 1. Further, we will assume the system will be in operation for 8,000 hours per year.
Normally, one must wait for the system to be placed in operation to see how the total system operates. Using the methods outlined in previous columns, we can simulate how the system will operate during the design process.
Since the maximum inflow of the system is 400 gpm, the transfer pump was sized for 500 gpm at approximately 22 hp. This allows the collection tank to be pumped down.
The collection tank is a vertical cylinder with a 10-foot diameter and 12-foot height. The pump starts with a high tank level of 11 feet, and shuts off when the tank level reaches one foot. The inflow in the tank is based on the operating conditions outlined in Table 2.
A simulation was performed on the design condition, and the level in the collection tank was calculated. Figure 2 shows how the level changes with a 400 gpm influx.
We can see it takes 15 minutes to fill the collection tank before the high-level control is activated. The pump starts and runs for 58 minutes until the level in the collection tank returns to the low level. This takes one hour and 13 minutes (or 1.2 hours) to cycle completely through a tank operation. At 1.2 hours per tank cycle, the pump goes through 338 pump starts per year under maximum flow conditions.
Evaluating Two-Pump Operation
In the next example, we will evaluate the same system with changes to the pumping characteristics.
Rather than a single pump, there are two identical pumps sized so each provides a design flow of 210 gpm with 100 feet of head.
The system controls are set up so the first transfer pump starts on a level in the collection tank of 10.5 feet, with the second transfer pump starting slightly higher at 11 feet. The pumps are both stopped when the one foot liquid level is reached in the collection tank.
After performing the system simulation, the multiple pump scenario results are displayed in Table 3.
Notice how using the design conditions with the maximum inflow of 400 gpm, both pumps are required to lower the tank level, resulting in a cycle time of 3.4 hours as seen in Table 3. Looking at the normal flow condition of 200 gpm, only one pump is running with a tank cycle time of 1.1 hours.
This system with multiple pumps operates for a longer time, but generally with just one smaller pump. When using just one pump there are additional power savings. Each of the smaller transfer pumps consumes 12.35 hp when operating compared to the larger pump at 22.3 hp. Further, since the controls alternate pump starts between the two pumps, the total number of starts for each pump is less than the operation of the larger single pump.
When comparing the operation of the system with one large transfer pump and the system with two small transfer pumps, their performance is similar in the design case of maximum flow. However, this only occurs during 5 percent of the operating hours. Commonly in these types of systems, most operating hours are spent well below the maximum designed flow rate.