Suppose, for a moment, that you are lounging on a beach somewhere in the vicinity of the equator. If you were to draw side by side two lines in the sand, both heading due north, they will appear parallel. Although our brains will see them as parallel, they are not and—if extended—will eventually intersect at the magnetic North Pole. Often pumps operating in parallel can fool us into thinking that the expected flow will be much greater than the actual flow.
The general rules that describe the head and flow of two identical, centrifugal pumps operating in series or parallel are simple. When operating in series, flow remains the same as a single pump, but head is doubled at each flow point. In parallel operation, head remains the same as a single pump, but flow is doubled at each head point. Figure 1 below illustrates these rules. The blue curve is the one produced by a single pump while the green and red curves result from series and parallel operation.
It would be nice if predicting parallel pump flow could always be this simple, but in reality, the system conditions dictate the maximum rate of flow. For example, a typical sewage lift station illustrates the importance of evaluating pump performance against the actual system curve.
Figure 2 above shows the performance of two identical wastewater pumps operating in a simplex and duplex (parallel) environment. The black system curve is composed of a static head of 47-ft and the friction head produced by 300-ft of 6-in steel pipe. Valves and fittings increase the “equivalent” pipe length to 381-ft. The black marker on the curve represents the simplex design flow of 600-gpm, and it intersects the single pump, H/Q curve at 65-ft. As flow increases so does the system head, and the system curve crosses the duplex H/Q curve at approximately 79-ft.
The result is a maximum duplex or parallel flow of about 800-gpm, not the doubling some of us may have expected. (It should also be noted that each pump is operating at 400-gpm, which is 25 percent below BEP. We will address this in detail next month.) An 800-gpm peak flow may be adequate in some cases, but if not, the conditions that influence the system curve will have to change. Figure 3 below shows the same application with a couple of system changes. The discharge pipeline size has been increased to 8-in, and the pump impellers have undergone a small trim that allows them to meet the new design point head. The result is a system that performs quite a bit differently than the previous one.
Although the static head remains the same, the friction head is reduced substantially due to the increased pipe diameter. The original simplex design flow requires just 52-ft while duplex operation requires 60-ft. The result is a duplex flow of 1000-gpm or an increase of 200-gpm above that of the previous example. Additionally, the decreased friction reduces the power required at design flow from approximately 14-hp (6-in pipe) to 11-hp. Under duplex operation, the 24-hp required to produce 800-gpm in Figure 2 is reduced to just 22-hp at 1000-gpm.
Some duplex lift stations are not designed to accommodate parallel operation. Instead, the pumps are sized to meet maximum in-flow with a single pump. In these designs, the purpose of a duplex system is simply to extend pump life through alternation and provide back up in the case of a pump failure. Others are designed for parallel operation, but maximum flow can vary substantially. In some cases, the “as built” system will differ from the original design. I have seen this many times in subdivision wastewater stations (and also in the domestic water and circulation piping in commercial buildings). Often, these field changes are not communicated to the design engineer and the original drawings are seldom updated. These changes can affect both parallel operation and the ability of a single pump to meet its design flow.
In addition to the influence of the system curve, there are several other important conditions that should be considered when operating pumps in parallel. These include use of non-identical pumps, stable versus non- stable H/Q curves and various methods of VFD control. Next month we will take a detailed look at these topics and show how they can affect both constant pressure and lift station applications.
The Excel spreadsheets that produced Figures 1 to 3 are available for download on my website. Look for “Series-Parallel Pumping Calculator” and “Simplex-Duplex System Curve Analyzer.” The first one allows you to compare identical or non-identical pumps, operating in series or parallel, to a simple system curve. The latter allows comparison of simplex and duplex lift station pumps to the more complex system curves produced by a combination of vertical lift, force main pressure and friction.
Pumps & Systems, March 2008