All pumps have the best efficiency point or BEP. This spot is the point of fluid flow where the pump operates at its highest efficiency levels for a given impeller diameter. Making sure your pump is running as close to its BEP as possible requires keeping to a strict maintenance schedule.
A pump maintenance program incorporates four main areas. They are pump performance monitoring, vibration monitoring, bearing temperature monitoring and visual inspections.
The impeller is a component in many types of pumps, such as centrifugal pumps. It works to convert mechanical energy to pressure.
It also plays an important role during the pump’s maintenance. Below are some common problems associated with the impeller and what you can do to prevent them.
Reduced Pump Efficiency
Pumps that run at their best efficiency point are more cost-effective and consume less fuel. Therefore, you should always check your pump’s efficiency levels to get the most out of your pump.
For pumps with impellers, such as centrifugal pumps, a reduced pump efficiency could mean the centrifugal pump impeller clearance is under the pump’s design guidelines. Under the design clearance means that the space between the impeller and the wear plate is not enough for the impeller to rotate freely. This design causes excess friction as the impeller rubs against the wear plate as it spins. As a result, the pump’s motor needs to perform extra work to overcome the frictional forces, leading to inefficiencies and potential overheating problems.
Decrease in Pump Flow
A decrease in the pump flow causes a decrease in the pump efficiency. The pump can no longer pump the same amount of fluids as before. A decreased flow can be observed visibly if the decrease in volume is significant. Measuring the time it takes your pump to do the same amount of work as before is another way to detect decreases in flow.
There are many causes for a decrease in pump flow. Some of these causes include a collapsed suction hose lining, a leaking gasket or a worn impeller. If the impeller is the cause of the decreased flow, the pump’s gauge readings will decrease or fluctuate at abnormal levels.
Overheating pumps are likely caused by a restricted flow of liquid into the pump. This restriction may be caused by an improper impeller clearance or a clogged recirculation port in the volute casing. To check the volute casing, simply shut down the pump and wait until it has cooled before opening the pump up for inspection. Look for obstructed air release valves or open-ended discharge lines.
To determine if the pump impeller clearance is ideal, measure the clearance and compare it with the pump’s design clearance. The recommended clearance should be listed in the pump owner’s manual provided by the manufacturer.
Cavitation is a major problem that can occur in inefficient pumps. If left unchecked, cavitation can lead to the formation of air bubbles around the impeller. These air bubbles are very dangerous as they can implode, triggering strong shockwaves inside the pump and seriously damaging the impeller, requiring significant repairs to fix.
Cavitation occurs when the Net Positive Suction Head or NPSH is too low. The NPSH is a measure of the margin of pressure at the suction side of the pump over the vapor pressure of the fluid flowing inside the pump. This causes the formation of the air bubbles.
There are many causes of cavitation related to different parts of the pump. One of the causes related to impellers is the vane passing syndrome. This syndrome is when the space between the tip of the impeller blade and the fluid is less than 4% of the impeller diameter. As a result, the velocity of the fluid flow becomes very high, creating areas of low pressure around the impeller.
Vane passing syndrome can be avoided by using the proper impeller diameter. This process requires keeping a length of at least 4% of the impeller diameter between the tip of the impeller blade and the fluid. For example, an impeller with a 10-inch diameter will require 0.4 inches between the impeller tip and the fluid. This distance is calculated by multiplying ten by 0.04, or 4%.
Keeping your pump running at the best efficiency point can help you save on operation costs. However, it also involves adhering to a strict maintenance schedule. This schedule means making sure components of the pump, such as the impeller, are working properly.
Common problems associated with the impeller are reduced pump efficiency or cavitation. They can be prevented by making sure proper clearance is kept between the impeller tip and the fluid.