How to Read a Generic AOD Pump Curve

Step One: Identify your required TDH and flow rate

Total Discharge Head (TDH) is the energy required by your fluid to make it through all the constrictions, bends, and appliances plus your required pressure on the outlet side. Flow Rate is your required flux of material. In the sample application above, TDH is Point A; flow rate is Point B.

 The BPH team can help you calculate TDH and flow rate.  These values are critical in choosing the best pump for your application.

Step Two: Determine air pressure requirements

AOD pumps run faster with more air pressure. In the sample application, you’ll be running at 80 PSI. Point C, the intersection of your required TDH and flow rate lies on the 80 PSI flow curve. Please note that your required air pressure will, in most cases, be higher than your TDH requirement.

Step Three: Determine your SCFM requirement and ensure your air compressor is adequately sized

More flow and TDH requires more air. As you go to the right in a flow curve, the amount of compressed air required increases. In the sample application, just less than 15 SCFM is required, (Point C).

Your air compressor is the driving force behind your pump. It provides air pressure and a flow to make your pump work at a given pressure. The table below gives the power per SFCM for rotary compressors.

Air Supply pressure (PSI)	Horsepower Per SCFM (Rotary Air Compressor in good condition)
20	0.071
25	0.085
30	0.096
35	0.108
40	0.118
45	0.127
50	0.136
55	0.145
60	0.153
65	0.161
70	0.168
75	0.175
80	0.182
85	0.189
90	0.195
95	0.201
100	0.207

In the example, you're running at 80 psi, so will need 0.182 HP/SCFM. The air consumption is just less than 15 SCFM. You'll need a compressor of at least 2.73 HP (0.182 HP/SCFM * 15 SCFM = 2.73 HP).

How to Read a Centrifugal Pump Curve

Step One: Know and identify your required TDH and flow rate

Total Discharge Head (TDH) is the energy required by your fluid to make it through all the constrictions, bends, and appliances plus your required pressure on the outlet side. Flow Rate is your required flux of material. In the sample application above, TDH is Point A; flow rate is Point B.

The BPH team can help you calculate TDH and flow rate. These values are critical in choosing the best pump for your application.

Step Two: Select the proper impeller diameter

This refers to choosing the proper flow curve for your requirements.  In the sample application shown above, TDH and flow rate intersect at Point C, indicating that a  2” impeller is the correct diameter

Step Three: Size your motor

For any given (and maintained) TDH, the power the required horsepower increases. This is because you are looking at the curve.  As you follow the curve your BHP may decrease if the TDH drops off quickly. In the sample application, a 1 HP motor is recommended, since Point C is to the right of the 0.75 HP curve.

Step Four: Consider your efficiency requirements

Each centrifugal pump curve has a set of gradients for efficiency. For most pumps, the best efficiency lies about 2/3 down the right side of a curve. The greater the efficiency, the more power is converted into flow and pressure. In the sample application, Point C indicates efficiency greater than 50%.

If several pumps appear to meet your requirements, consider how much power is needed to move your product.  The BPH team can help you choose a pump with the greatest efficiency in order to keep your long-term power consumption costs down.

Step Five: Make sure you have enough NPSH

Rule of thumb: If you have a flooded inlet and low temperature on a non-volatile fluid, you’ll have adequate NPSH the vast majority of the time.

Fluid must move freely into your pump at your required flow rate, without help from your pump. Expressed mathematically, NPSHa must be > NPSHr, where NPSHa is net positive suction head available and NPSHr is net positive suction head required. NPSH is typically expressed in units of feet of head. Running a pump with inadequate NPSH is not a good idea. The pump will cavitate and which will cause severe wear on the impeller and seal.

In the sample application, the NPSHr required is found at Point D.

Common Pump Terminology

The purpose of this area is to give you a reference for some of the terms used by pump users and manufacturers. If you don't see something you think needs to be defined, call us at 888-289-8787 or drop us a note on the "Contact us" page.

Term	Definition
Air Consumption	The flow rate of air that is required by an AOD pump or air motor.
Air Pressure	Air-Operated Diaphragm (AOD) pumps use compressed air as a power source. Required air pressure is a component of the flow curve.
AOD	Air Operated Diaphragm Pump.
Cavitation	Cavitation occurs when a localized region of pressure is below the vapor pressure of your process fluid. Your process fluid literally boils forming bubbles and those bubbles quickly collapse-causing damage to pump internals. Cavitation is not a problem as long as you have adequate NPSHa.
Flow Rate	Volume of material per unit time. This is usually expressed in units of volume per time (e.g. gallons per minute or GPM).
Head	The pressure in a column of liquid.  Pressure will increase as the height of the column increases. Head refers to the height in feet; pressure refers to the psi (pressure pounds per square inch).
NPSHa	Net Positive Suction Head Available. The head that your system has available is a available on the fluid prior to entering the pump.  This is a function of the conditions in a particular installation and is expressed in absolute units. Call us at 888-289-8787 for assistance.
NPSHr	Net Positive Suction Head Required. The head that the pump requires on the inlet side to operate properly and is a function of the pump and the position on its flow curve.
PSI	Pounds (force) per square inch. A common unit of pressure.
SCFM	Standard cubic feet per minute. A common unit of air flow.
TDH	Total Discharge Head. This is the required discharge pressure plus friction losses from bends, constrictions and appliances on both sides of your pumping system.