by Joe Evans, Pump Ed 101
The selection of an impeller for a sewage pump application has a significant effect on the efficiency, maintenance, and reliability of the pump. So, how should you determine which impeller is the suitable choice for your application?
One way to differentiate between the various sewage pump impellers available is by examining how they accomplish the passing of solid material.
The fundamental difference between a centrifugal sewage pump impeller and the clear water impeller is the former’s ability to pass solid material that would cause a clog in the latter. Although the mathematics that define the operation of an impeller can be complex (it is the stuff of Bernoulli and Euler), its purpose is straightforward. An impeller is designed to impart energy to a fluid so that it will flow, or, if it is already flowing, undergo some increase in its elevation or pressure.
It accomplishes this by increasing the fluid’s velocity as it travels through its vanes from the leading edges, located at the eye, to their exits at the periphery. The ever increasing radius of the vanes results in an increasing rotational velocity that reaches some maximum at the periphery. The resulting linear velocity of the fluid, at the vane exit, is then converted to pressure in the volute.
If one were to set out to design a typical radial vane impeller, several guidelines would be followed quite closely. For instance, the overall diameter of the impeller would closely match the volute and cut water diameters in order to reduce slippage of the pumped fluid in these areas.
Also, depending upon the desired hydraulic characteristics, four or more vanes would be incorporated to smooth flow at the vane exit. Their leading edges would also be sharpened to reduce losses due to friction and turbulence.
Unfortunately, if one followed these same guidelines when designing a solids handling impeller, the outcome would be doomed to failure. Unlike the typical radial vane impeller, those designed to accommodate solids violate many of the standard design rules.
Small to medium-sized sewage pumps are often referred to as non-clogs and their impellers are designed to try to live up to that name. Although many factors contribute to the ability of an impeller to pass solids without clogging, one of the more important factors is its throughlet size.
The throughlet is defined as the open internal passage through the impeller that ultimately determines the largest diameter solid that can be passed. All impellers, regardless of their design, have some maximum throughlet size. In order to maximise throughlet size, solids handling impellers limit the number of vanes so that the passages between them can be as large as possible.
Let’s take a look at some of the common sewage pump impeller designs and discuss the benefits and limitations of each.
Radial flow solids handling impellers
The various members of the radial flow impeller family include the closed, open, and semi-open designs. Depending upon capacity, each design may incorporate anywhere from one to four vanes. The vanes are not straight, but describe a smooth curve that begins at the eye of the impeller and extends to its periphery. They may also be curved upward at their entry as in the Francis vane design.
The closed impeller looks very much like an exaggerated version of the clear water impeller. The shrouds of the closedimpeller enclose the impeller’s vane passages from the eye to the periphery and are designed to accommodate the largest possible diameter solids. The vanes themselves have large, rounded leading edges to prevent clogging by rags and stringy material that could become entangled at the vane entry.
On pumps with suctions up to 12”, a two-vane (often referred to as a two-port) design is typical, while larger pumps may utilise a three or four-vane design. Most closed impellers also incorporate pump out vanes on the back side of the back shroud. These small, straight vanes keep the sealing area free of debris and also reduce the unbalanced axial forces that can occur due to back shroud’s larger surface area.
The major wearing surface of the closed impeller is the area where the eye protrudes into the volute suction. Replaceable volute wear rings are used to maintain proper clearance and hydraulic efficiency. A typical rule of thumb calls for wear ring replacement when the factory set tolerance has doubled.
Very large sewage pumps often use a mixed flow impeller for low head, high flow conditions. The mixed flow design utilises a double curvature vane that provides both radial (centrifugal) and axial (lifting) flow characteristics. Also, because of their extremely large throughlets (4” and greater) these larger pumps can utilise sharpened vane leading edges for greater efficiency.
Another characteristic of the closed solids impeller is that its diameter seldom exceeds 80 per cent of the volute cut water diameter, as compared with about 92 per cent for a standard impeller.
This diameter is restricted, at the expense of slippage, in order to reduce vibration and noise, especially at lower flows. This larger than normal clearance also reduces clogging in the area where the impeller periphery is closest to the volute case.
Another closed design is the single-vane impeller. On the positive side, it allows for the largest possible throughlet.
Since there is only one vane, there is only one leading edge, and thus potential clogging at the vane entry is reduced.
Unfortunately, due to its lack of symmetry, it is inherently out of balance. Unlike the multi-vane impeller, most cannot be trimmed and must be replaced if hydraulic conditions change.
The single-vane impeller also tends to produce a rather steep head-capacity curve. Although this can be useful in some applications, the flatter multi-vane curve generally has greater utility.
By definition, the true open impeller consists of nothing more than vanes mounted to a hub that is attached to the pump shaft. They are usually seen in smaller pumps and are best suited for applications involving stringy materials. Because they are shroudless, it is less likely for material to become entrapped between the impeller and the front and rear portions of the pump case.
A disadvantage is their structural weakness and, because of this, they are often strengthened by a partial shroud on the back side. If the back shroud covers the entire vane structure, the impeller is designated as semi-open.
Since one or both shrouds are missing from each design, both are prone to wear at the vane edges and must be adjusted periodically in order to maintain hydraulic efficiency. Typical volute/vane clearances range from 0.020” to 0.030” and increases due to wear affect pump efficiency to a greater degree than the eye/volute wear of the closed impeller.
The semi-open impeller, due to its lack of a front shroud, also tends to create greater unbalanced axial forces than does the closed impeller. Both pump out vanes and balance holes are often utilised to minimise these forces and prevent potential bearing damage.
Although the radial flow impeller is the workhorse of the sewage pump industry, there are applications for which it is not well-suited. One example is low flow applications.
By virtue of its large throughlet, flow rates will always be far greater than impellers of the same diameter designed for clear fluids.
For example, even a small impeller designed to pass 2” solids will create BEP (Best Efficiency Point) flows of 80 to 120gpm. Increase solids size to 3” and the flow range increases to 400 to 700gpm.
With conventional pumps, flow can be reduced by throttling the discharge. However, such a tactic is not acceptable when solids are involved. This problem is exacerbated when a low flow application is complicated by a high head requirement.
When a centrifugal pump is operating, the pumped fluid exerts a force on its impeller both radially (perpendicular to the shaft) and axially (parallel to the shaft). When the pump is operating at its design point (BEP), relatively uniform pressures act upon most surfaces of the impeller.
An exception is the area about the periphery where pressures are rarely uniform, regardless of the operating point. As flow decreases (or increases), unbalanced radial forces increase and usually reach a maximum at or near shut off head. This radial thrust, as it is known, is a function of total head, and the width and diameter of the impeller.
Thus a high head pump with a large impeller will generate more radial thrust than a low head model incorporating a smaller impeller.
By design, the sewage pump impeller is unusually wide and the radial forces created can be extremely high as operation moves to either side of BEP. Depending upon the particular pump, as much as the first 30 per cent of the entire performance curve is considered unsuitable for normal operation.
High radial forces can damage a pump’s rotating components and can, in some cases, create enough vibration to dislodge a submersible pump from its lift out connection.
One way of reducing the effect of radial thrust is to neutralise the force itself. The double volute pump accomplishes this by adding an internal wall to the casing that, in effect, creates two volutes.
Although the double volute is found in very large sewage pumps, it is not a workable solution when small to medium-sized solids handling pumps are involved.
Another method involves modifying the standard constant velocity volute by increasing the volute volume in the area about the cut water. Although this reduces efficiency by one to two per cent, radial forces at lower flows can be reduced by as much as 25 per cent.
Vortex (recessed) solids handling impellers
So, is there another way to overcome the low flow, high head shortcomings of the radial flow impeller? The answer is yes but, since there is no such thing as a free lunch, there is a price to pay.
The vortex impeller operates quite differently than the radial flow type. Instead of imparting energy directly to the pumpage, it creates a liquid vortex (whirlpool) which, in turn, imparts its energy to the pumped fluid. And, as with any multistage process, some energy is consumed by the intermediate step (in this case creation and maintenance of the vortex) and results in a lower overall hydraulic efficiency.
Actual efficiency depends upon pump size and speed and can range from a low of 20 per cent, to better than 55 per cent.
Nonetheless, such losses can often be tolerated if the end result allows something unobtainable otherwise. And, in the case of the vortex impeller, several distinct advantages are offered.
The most obvious visual difference between the vortex pump and radial flow models is that its semi-open impeller resides completely out of the volute. This feature offers the sewage pump designer three distinct advantages.
The throughlet size can easily be made to equal that of the pump’s inlet. Therefore any solid that can enter the inlet can traverse the throughlet.
Since the pumpage traverses the throughlet via vortex action, its solids seldom come in contact with the impeller. This reduces the possibility that solids, especially stringy ones, will become entangled or clog it. For the very same reason impeller wear is minimised.
Due to its location above the volute, unbalanced radial forces are almost nonexistent. This allows the vortex impeller to run continuously at or near shut off head without damage.
An important application of the vortex impeller is the centrifugal grinder pump. The grinder pump utilises a shredder assembly to macerate large solids into a fine slurry prior to its entry into the volute. Since solids no larger than 1/8” are encountered, the volute and impeller can be designed for low flow at very high heads.
Small centrifugal grinders offer flows to 40gpm at heads to 140’, while larger units offer flows to 180gpm at heads to 170’.
These pumps are well suited for low pressure sewer systems because of their ability to vary flow dynamically depending upon conditions and to run at shut off head during periods when the system is loaded to capacity.
Centrifugal screw impellers
The centrifugal screw impeller is another very different solids handling impeller design. The centrifugal screw pump is a hybrid that combines features of the positive displacement screw and the centrifugal pump. More often seen in the process environment because of its low shear and NPSHR characteristics, it is becoming more common in the wastewater industry.
It offers relatively high efficiencies and good solids handling capability.
The head capacity curve is quite steep, but is typically non-overloading. This allows operable range from about ten to 125 per cent of BEP. The portion of the screw that resides in the suction nozzle is prone to wear and is protected by a replaceable or adjustable wearing surface.
Like open impeller pumps, these clearances must be adjusted periodically to maintain hydraulic efficiency. The major disadvantage of the centrifugal screw is that they tend to be quite costly when compared to other designs.
Which sewage pump impeller is most suitable?
So, which impeller is the best choice for any given sewage pumping application? There is no easy answer to this question. If there were, there would not be such a broad selection from which to choose.
It can probably be said that the closed, mixed flow impeller is the most efficient and trouble-free choice when large pumps are involved. In the case of small to medium-sized pumps, however, the particular application becomes an important factor in the selection process.
Although the closed Francis vane tends to be more popular, the open vane design has its own strengths under certain conditions. The vortex impeller combines the positive traits of both but does so at the expense of lower hydraulic efficiency.
Yet when it comes to high head, low flow applications, its lower efficiency is hardly factored into the equation.
Several manufacturers have recognised these differences and the advantages that different impeller designs can offer in a particular application environment.
For this reason they offer different impeller options for their more popular pump models. It is definitely worth considering these options when designing new sewage pumping systems. They can also often solve problems in existing installations that have undergone major changes in hydraulic conditions.
Joe Evans has been in the pump industry since 1986 and is passionate about the sharing of knowledge within the industry. To read more of his insights into the world of pumping, visit www.pumped101.com.