A common question asked by users of air operated double diaphragm (AODD) pumps is, “What is the maximum viscosity of a process fluid that can be transferred by an AODD pump?” In truth, the answer has little to do with the selected pump and much to do with the piping system to which the pump is connected. Users often forget this, since most AODD applications are transfer applications with a relatively low viscosity fluid. While a complete discussion of more accurate methods for evaluating pump systems is beyond the scope of this article, pump users can employ the following techniques to estimate the factors that impact flow rates in AODD systems with high viscosity fluids. Consider the following simple fluid transfer system in which the user wants to transfer 20 gallons per minute (gpm) using a 1-inch AODD. To determine if an application is possible, three questions must be answered:
Changes to the piping system can help AODD pumps work beyond their typical applications.
All-Flo Pump Co.
01/31/2018
Figure 1. Example system (Images courtesy of All-Flo)
A typical 1-inch AODD may have dry lift capabilities of 15 ft-H20 or 6.5 psi. In practical terms, this means the pump cannot operate in systems where suction line loss exceeds 6.5 psi. Using a 1-inch suction line as depicted in Figure 1 results in a suction line loss that exceeds the pump’s capabilities. To meet the desired flow rate of 20 gpm, the suction line diameter must be increased to 2 inches. This increase reduces the suction line loss from 34 psi to 2 psi, well within the operating capabilities of the AODD pump.
Table 1. Frictional pipe loss
The frictional line loss due to a 1-inch line exceeds the maximum operating pressure of most AODD pumps (120 psi). It becomes necessary to increase the discharge line diameter to reduce the losses to a level within the range of the AODD pump.
Increasing the discharge line diameter from 1 inch to 1-1/2 inches reduces the discharge line loss from 135 psi to 24 psi, a comfortable level for AODD pumps.
In the example system the static head is a simple calculation (10 ft-H20 to 15 ft-H20) x 1.2 S.G., or 6 ft-H20. Expressed in psi, the total static head is approximately -2.6 psi. Therefore, the system’s TDH is 31 psi—the sum of the static head and the frictional pipe losses.
Table 2. Frictional line loss
Figure 2. Typical published performance curve
In Figure 2, the red lines represent air consumption in standard cubic feet per minute (SCFM) and the blue lines represent air inlet pressure in psi. Reading the curve for our sample system (23 gpm and 31 psi TDH) shows that air-inlet pressure should be set at approximately 55 psi and the pump will consume 22 SCFM of air while operating. It is important to note that exceeding the required air pressure of 55 psi can result in cavitation as suction line losses may exceed the capabilities of the pump.
In this simplified system analysis, many important factors have been ignored. However, this example demonstrates the impact system factors have on an AODD pump’s ability to process viscous fluids.
Viscosity and pipe diameter play a significant role in suction line loss and must be considered during evaluation. The system capabilities are limited by the pump’s suction lift capabilities and air-inlet pressure.