Methods to Improve Pump Operation

Author: Ray Hardee, P.E.

In my most recent two Pumps and Systems columns, 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.

Operating Profile for Example System

Operating Conditions

Flow rate in gpm

Percent of time at operating condition

Maximum flow

400

5%

Normal flow

200

40%

Low flow

150

25%

Minimum flow

80

30%

 Table 1. Operating Profile for Example System – Load profile showing flow rates and percentage of time at each operating condition. (Graphics courtesy of author)

This system was designed to meet the maximum flow conditions of 400 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.

Figure 1. Sample Fluid Piping System - This graphic shows the system with a single pump and varying operational flow rates. The system controls have the pump start at a high level in the collection tank, and shut off on a low tank level.

Figure 1. Sample Fluid Piping System – This graphic shows the system with a single pump and varying operational flow rates. The system controls have the pump start at a high level in the collection tank, and shut off on a low tank level.

Our system controls consist of the pump starting on high-tank level and stopping on low-tank level. In this article, 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 articles, 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 1 foot. The inflow in the tank is based on the operating conditions outlined in Table 1.

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 1 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 the maximum flow conditions.

Figure 2. Collection tank level with one main pump and inflow of 400 gpm.

Figure 2. Collection tank level with one main pump and inflow of 400 gpm.

Performing the simulation for all the defined inflow conditions outlined in Table 1, we can see the number of pump starts with the existing transfer pump.

Conditions

Inflow (gpm)

Hours / year

Cycle time (h)

Starts/yr

Pump power (hp)

Max flow

400

400

1.2 hrs

338

22.3

Normal flow

200

3200

.86 hrs

2752

22.3

Low flow

150

2000

.98 hrs

2352

22.3

Min flow

80

2400

1.54 hrs

1558

22.3

Total

 

8000

 

7050

 

Table 2. Pump characteristics for various inlet conditions with existing transfer pump.

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 1 foot liquid level is reached in the collection tank.

After performing the system simulation, the multiple pump scenario results are displayed . Notice how using the design conditions with the maximum inflow of 400 gpm, both the pumps are required to lower the tank level, resulting in a cycle time of 3.4 hours as seem in Figure 3. Looking at the normal flow condition of 200 gpm, only one pump is running with a tank cycle time of 1.1 hours. 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. 

Figure 3. Collection tank level with two pumps and inflow at 400 gpm

Figure 3. Collection tank level with two pumps and inflow at 400 gpm

 

Conditions

Inflow (gpm)

Hours / year

Pumps running

Cycle time (h)

Starts/yr per pump

Pump Power (hp)

Max flow

400

400

2

3.4

117

21

Normal flow

200

3200

1

1.1

1454

12.35

Low flow

150

2000

1

1.1

909

12.35

Min flow

80

2400

1

1.51

794

12.35

Total

 

8000

 

 

3270

 

Table 3. Pump characteristics for various inlet conditions while operating two pumps in parallel.

As we can see from this system with multiple pumps operates for a longer time period, but generally with just one smaller pump. 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.

Conclusion

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. Common in these types of systems, most operating hours are spent well below the maximum designed flow rate. When operating at lower flow rates, the system with smaller pumps consumes less power for the operation. Additionally, the single pump system goes through more pump starts than the pumps in the system with two smaller pumps.

The cost of the second pump and driver will most likely add to the initial cost. Knowing how the total system will operate we can see that the two smaller pumps will consume less power and have fewer pump starts. This will save energy and maintenance costs each year.

In the past, one would have to wait until the system was designed, built, and placed in operation to look for ways to improve the system. Then it is already too late changes are difficult and costly after the plant is already operating. Using the simulation techniques discussed in this series, users can analyze how a system will operate over a wide range of expected conditions during the design phase and, ultimately, discover ways to improve the system.