Pump Selection Guide
The purpose of this guide is to help clients in selecting the right pump for their application. In this guide we will discuss all the parameters that are required for pump selection and will also focus on generally accepted industrial principles when selecting a pump. Following factors will need to be known prior to pump selection
Fluid Concentration
This is the most important aspect. Material metallurgy is selected on the basis of fluid and its concentration. A dilute acid fluid might work on Stainless steel pump, but a strong acid will need a special duplex material or titanium material pump. Similarly, several fluids react with the material therefore, we need to be very sure what the fluid is? and its concentration as well to ensure good chemical resistance. MSDS (Material Safety Data Sheet) is a good source of fluid properties and helps in material selection. Chemical resistance of the fluid varies with fluid temperature and concentration
Specific Gravity/Density
This is a measure of density of the fluid. Water have a specific gravity of 1. It is basically the weight of the fluid for a same volume of water. A fluid with higher specific gravity/density will require a high power pump to make it flow through the same pipe as compared to water. For example Phosphoric Acid have a specific gravity of 1.6. This means that if we have same volume of water and phosphoric acid, then the weight of phosphoric acid will be 1.6 times more than the water even though they have the same volume. For your info: the densest material on earth is Osmium with a specific gravity of 22.1 (22.1 times heavier than water)
Viscosity
This is a measure of fluids resistance to flow. A fluid with high viscosity will require more force to push it through a pipe as compared to fluid with low viscosity. This can be seen as honey having a very high viscosity and water having very low viscosity. As the viscosity of the fluid increase, it becomes inefficient to use a centrifugal pump and its better to use a positive displacement pump instead
Flammability
This is an important factor to consider. A fluid with high flammability will require a stronger material of construction (preferably stainless steel or better, depending on fluid concentration, temperature and reactivity). Also an important aspect to consider is the pump design criteria. For example for a highly flammable material example gasoline, ethylene, etc it would be better to use seal less construction so that in no case the product is spilled or even drop wise leakage is acceptable. (in case of pump with mechanical seal, first signs of failure starts with drop wise leakage from the mechanical seal)
Temperature
It is required for proper material selection of the pump. Minimum, maximum temperature of the fluid and environment needs to be known for a proper pump selection
Flowrate
This determines the size of the pump which will be required. Flowrate can be liters/min or meter cube/hour
Head/Pressure
This is the actual pressure the pump should produce, usually measured in Meter of head. There are 3 types of heads in pumping world.
Suction Head: When the supply of fluid is above the level of the pump, this is also known as flooded suction
System Head: It is the total pressure which is required to make the fluid flow in the system
Discharge Head: This is the height (maximum) which will be achieved by discharge side of the pump
Suction Lift
This is the scenario when the pump has to suck the liquid from a tank which is placed below the below so that the pump has to suck up the liquid to itself and then push it further in the system. This is an important concern in pump selection as pumps have very limited capabilities of suction lift
Net Positive Suction Head (NPSH)
The concept of NPSH is little bit tricky. Before explaining NPSH a concept of Cavitation needs to be understood. Cavitation occurs when pump is unable to suck 100% liquid and sucks in vapor as well. When the vapors collapse in the pump housing, it causes damage to the impeller and pump parts. It is important to know the altitude at which the pump will be installed as it will be required to calculate the NPSHa
There are two types of NPSH:
Net Positive Suction Head Available (NPSHa)
This is a function of the system where the pump needs to be installed and must be calculated by the client.
To calculate NPSHa following is the formula
Term
Definition
The absolute pressure on the surface of the liquid in the supply tank
Notes
- Typically atmospheric pressure (for vented tanks) but can be different for closed tanks
- Atmospheric pressure varies with height
- It is always positive (even in vacuum conditions)
The vertical distance between the center line of the pump and the bottom of the tank
- The value is negative in case of suction lift (when the tank is below the pump)
- The value is positive in case of flooded suction (when the tank is above the pump)
Friction losses in suction piping
- Losses due to friction in the suction piping
- Always ensure to keep suction piping of as small length as possible and with minimum possible bends
- There are several friction calculators available on internet that can be used in this case
Velocity head at the suction port
- Very small value and can be neglected
Absolute vapor pressure of the liquid at the pumping temperature
- Absolute vapor pressure decreases with rise in temperature
Net Positive Suction Head Required (NPSHr)
This is a function which will be provided by the pump manufacturer. In order to avoid cavitation.
NPSHa must always be greater than NPSHr
Atmosphere/Environment
This is very important factor and is often overlooked primarily as the atmosphere it decides the motor specs of the pump and not the pump specs itself. Therefore, it is fairly possible for a mechanical engineer to overlook the issue.
For a hazardous environment where flammable gases a present an Explosion proof certified motor is required. Ask your vendor for Ex rated motor for hazardous area application
For rainy, water showering atmosphere, a more stringent Ingress Protection motor is required (IP rating). Usually vendors quote only with standard IP54 or IP55 ratings. However, for tropical, humid and water showering places an IP65 motor is required. It would be good to use the following guide for motor IP selection.
This is worth mentioning that selecting and Ex rated motor increases the cost of the pump by twice or more. Similarly, having IP65 motor raises the cost as well. If client required both (Ex rated and IP65 moto then it will be expensive and difficult to source as well)