Pump Impeller Designs
Screw Impeller Pumps :
In recent years screw impeller pumps have gained ground. The idea has merit but it have not proven itself to be as dependable as promised. Once again, a product with good potential has been heavily mis-applied. The screw impeller design will allow some solids handling and will handle a low to moderate amount of stringy material. On the positive side when applied properly they will do an adequate job. Because of the long over hang of the impeller the bearing loading is high. Also, since the impeller has a “cork screw” like shape the area of operation on the performance curve should be considered carefully. Too far to the left or right on the performance curve and there will dynamic bearing loads which are difficult to predict and difficult to correct.
Maintenance costs are high with this type of pump. Because wear is high in this type of pump design, manufactures have introduced suction wear liners made from hard materials. This provides a short term solution but often the liner will cost as much or more than the volute or impeller while only marginally increasing component life. When the pump wears the impeller will require axial adjustment. This is a difficult task at best, due to design constraints of this style of pump. In dry applications such as with horizontal and vertical pumps, impeller adjustments are very difficult. Methods such as shimming of the volute and axial adjustment of the suction wear liner have not proven satisfactory at all. The use of typical shaft adjusting-locknut arrangements have marginally improved the situation.
Another issue facing the end user with this style of pump is variable speed. The screw impeller design does not allow for the trimming of the impeller. This means that if the desired condition point is not on the synchronous speed performance curve then some sort of speed varying methodology must be used. Either variable speed drives or belt driven systems are the choice. Both add high additional initial cost and high maintenance costs to the pumping equipment.
A major drawback which has shown up in recent years is the space required to install a unit. Screw Impeller pumps tend to require less space. The problem is if the application does not pan out, the customer is now locked into a poor design. An end user who has been the “victim of a good salesman” and/or over zealous manufactures walk away knowing the customer is tied to them for parts and service for many years to come.
Careful consideration should be given to selecting and accepting this type of equipment by the end user or owner. While most of the claims made by the manufactures of this type equipment may be true initially, the perceived advantages quickly disappear leaving an unhappy customer. Applications with non-abrasive sludges, liquids containing non-abrasive silts, some solids and moderate amounts of stringy material may be acceptable for this style of pump.
More information coming soon :