Pumps & Systems, July 2008
Air operated double diaphragm pumps (AODDP) require the correct amount of pressure (PSI, pounds per square inch) and air (CFM, cubic feet per minute) to deliver the proper amount of fluid. A typical air operated diaphragm curve shows how the correct amount of pressure and CFM are required to deliver a required flow rate against a discharge head.
Problem: Plenty of Pressure But Too Low CFM
In many Air Operated Diaphragm (AOD) applications, the end user has ample amount of pressure on the gauge reading but has no idea the available CFM. It is just as critical to have the correct amount of CFM available as it is pressure to achieve a desired flow rate. Many end users might state that they have a 100-psi gauge reading on their double diaphragm pump but they are not getting the desired GPM as promised in the flow curve.
In this situation, remember that the pressure to the pump is needed to achieve the desired flow point but the CFM determines the actual output in GPM. The pressure to the pump overcomes the discharge head, and the CFM determines the actual gallons pumped per minute.
CFM meters can be installed in the air line to determine the amount of CFM available at a given location in a facility. Note that one BHP = 5-cfm.
Problem: Air Supply Hose/Line Too Small
Every AODD pump requires a certain amount of CFM to deliver the required GPM. Properly size the supply air hose and supply line to ensure an adequate amount of CFM is supplied to the pump.
Assume your air operated pump application requires 40-cfm at 80-psi to deliver 100-gpm. You installed 10- ft of 3/8-in air hose and wondered why it never achieved 100-gpm. The maximum CFM that can pass through a 3/8-in air hose is 30-cfm. The pump will never deliver the 100-gpm because not enough CFM can be supplied to the pump to achieve the desired flow rate of 100-gpm.
If the supply air hose was increased to ½-in, which can pass 60-cfm, the desired 100-gpm can be achieved. The same scenario holds true for hard pipe. By increasing a pipe from 2-in to 2.5-in, CFM capabilities increase from 500-cfm to 2,000-cfm.
The size of the air hose and supply hard pipe is commonly overlooked and can create problems not only for air operated pumps but also for any pneumatic piece of equipment.
Problem: Incorrect Air Supply System Layout
The correct piping layout for a compressed air system is critical to achieving the maximum performance from an air operated pump. Often air operated pumps do not run properly due to poor piping designs.
Remember to keep the water and compressor oil out of the air motors of air operated pumps. The air supply pipe that supplies the air operated pump should come from an inverted tee to prevent the water and compressor oil from seeping into the air motor of the air pump. Too much water and oil in the air motor will prevent the air pump from performing and possibly stall the pump.
Problem: Air Regulator Too Small
What if an air operated pump requires 50-cfm at 100-psi to pump 120-gpm, and the pump is installed with the proper size air line to pass the 50-cfm but still does not deliver the required 120-gpm? Further investigation reveals that the installed air regulator reads 100-psi but can only pass a maximum of 25-cfm.
The pump will never deliver the required GPM because of the shortage of CFM. The solution is to install a regulator that can pass the 50-cfm achieving the 120-gpm. This potential problem is commonly overlooked when installing an air regulator to determine the rated CFM.
The main problem in the field is that the typical end user will use the regulator in line to control the speed of the pump (CFM) which determines the gallons pumped. When increasing and decreasing the pressure on an air regulator simultaneously, the CFM is increased or decreased. The regulator does have an orifice inside when adjusting the pressure, which increases or decreases the speed of the air operated pump. When using the regulator to control the speed enough pressure may not be supplied to overcome the discharge head and consequently achieve the flow the application may require.
Problem: Air Pumps Running Too Fast and Burning Up Compressed Air
Remember the CFM and not pressure controls the speed of an air pump. Installing a needle valve after the regulator is the correct installation to achieve the desired speed of the air pump.
If an air pump must run at a constant speed regardless of supplied pressure, install a needle valve out of sight and set at desired CFM to maintain a constant speed for the air pump.
New air motors can be preset to ensure that the air pump runs at a constant speed to avoid excessive costs on compressed air.
