First the types:
The open impeller is nothing more than a series of vanes attached to a central hub for mounting on the shaft without any form of side wall or shroud. This design is much more sensitive to vane wear than the semi or closed impeller.
The semi-open impeller incorporates a single shroud at the back of the impeller. This is the most common design used in the United States and the one you find on most ANSI standard pumps.
- The shroud often has “cast in” pump out vanes that will help circulate lubricating liquid from the lantern ring connection through the packing ahead of the lantern ring.
- Most modern pump designs allow you to adjust the semi- open impeller without disassembling the pump. This is a tremendous advantage if you want to maintain the pump efficiency by adjusting the impeller to volute clearance for thermal expansion and volute/impeller wear. Remember that if there is a mechanical seal in the stuffing box any impeller adjustment can interfere with the seal face loading. Those designs that adjust to the volute (Goulds type) will unload the seal faces and those that adjust to the back plate (Duriron type) will increase the seal face loading.
- A typical volute or back plate clearance for a semi open impeller would be 0.015 to 0.020 inches (0,4 to 0,5 mm). For each 0.002 inches (0,05 mm) you increase this clearance, the pump will lose about 1% of its capacity.
The closed impeller has a shroud on either side of the vanes. This is the most common design found with ISO standard pumps, oil refinery applications and the design you see on double ended pumps.
- To maintain impeller efficiency you are required to replace the wear rings after the original clearance has doubled. The first problem is to determine when it has doubled, and then you have to take the pump apart to replace them. The result is that timely replacement is seldom done, and pump loss of efficiency with resultant vibration becomes the rule.
- The general rule of thumb is that the pump will lose about 1% of its capacity for each excessive 0.001 inches (0,025 mm) of impeller clearance.
- Since the wear ring clearance is usually smaller than the area of the balance holes drilled through the impeller, you will lose the advantage of suction recirculation as stuffing box pressure is very close to suction pressure.
The impeller specific speed number describes the shape of the impeller
- The shape of the head/ capacity curve is a function of specific speed, but the designer has some control of the head and capacity through the selection of the vane angle and the number of vanes.
- The pump with the highest specific speed impeller, that will meet the requirements of the system, probably will be the smallest and the least expensive. The bad news is that it will run at the highest speed and be subject to maximum wear and damage from cavitation.
Radial flow impellers (low specific speed numbers)
- They should be specified for high head and low flow conditions.
- They seldom exceed 6 inches (150 mm) in diameter and run at the higher motor speeds
- The casing is normally concentric with the impeller as opposed to the volute type casings normally found in the industry..
- These impellers exhibit a flat head/capacity curve from shut off to about 75% of their best efficiency and then the curve falls off sharply.
- Radial flow impellers are normally started with a discharge valve shut to save start up power.
Axial flow impellers (high specific speed numbers)
- They run at the highest efficiency
- They have the lowest NPSH requirement.
- They require the highest power requirement at shut off, so they are normally started with the discharge valve open.
Impellers can be manufactured from a variety of materials:
We would like a combination of a hard material to resist wear and a corrosion resistant material to insure long life. This is often a conflict in terms because when we heat treat a metal to get the hardness we need, we lose corrosion resistance. The softer metals can have corrosion resistance, but they lack the hardness we need for long wear life. The best materials that combine these features are called the “Duplex Metals”. These duplex materials are now in their second generation. They can be identified by letters and numbers such as Cd4MCu
If a new impeller is required because of cavitation, the new design should incorporate those features we have learned that will increase impeller performance:
- The use of large fillets where the vanes join the shrouds to lessen stress.
- Investment castings so that you can design in the compound curves that produce less wear.
- The latest design iteration to help reduce radial thrust.
- Sharpened leading edges of the vanes to reduce losses.
- A reduction of shroud to cutwater clearance to lessen internal recirculation.
- A conversion to the newer duplex metals.
Impellers can be designed for a variety of applications:
- The ideal impeller would have an infinite number of vanes of an infinitesimal size.
- The conventional impeller design with sharp vane edges and restricted areas is not suitable for handling liquids that contain rags, stringy materials and solids like sewage because it will clog. Special non-clogging impellers with blunt edges and large water ways have been developed for these services.
- Paper pulp impellers are fully open and non-clogging. The screw conveyer end projects far into the suction nozzle permitting the pump to handle high consistency paper pulp stock.
- Vortex pump designs have recessed impellers that pump the solids by creating a vortex (whirl pool effect) in the volute and the solids move without ever coming into contact with the impeller. You pay for this feature with a greater loss of pump efficiency.
- An axial flow impeller called an Inducer (it works like a booster pump) can be placed ahead of the regular pump impeller, on the same shaft, to increase the suction pressure and lessen the chance of cavitation. In some instances this can allow the pump to operate at a higher speed with a given NPSH. The inducer will contribute less than 5% of the total pump head, and although low in efficiency the total efficiency of the pump is not reduced significantly. The total reduction in NPSH required can be as much as 50%.
- People often inquire about forward curved vanes. They have been tested.
- Both the capacity and efficiency were reduced.
- There was a slight increase in head.
- The impeller exhibited unstable characteristics at the low end of capacity range.
- The impeller exhibited steep characteristics at high end of the range.
- Increasing the number of vanes tends to flatten out the curve and steady the flow.
Impellers can be single or double suction designs.
- Because an over hung, single suction impeller does not require an extension of the shaft into the impeller eye it is preferred for applications handling solids like sewage. The suction eye is defined as the inlet of the impeller just before the section where the vanes start. In a closed impeller pump the suction eye is taken as the smallest inside diameter of the shroud. Be sure to deduct the impeller shaft hub to determine the area.
- Double suction pumps lower the NPSH required by about forty percent.
- Most double suction impellers are constructed so that the stuffing box is at suction pressure. This causes you to lose the advantage of suction recirculation to prevent seal failure when handling solids. You are going to have to flush many of these seals with a clean, compatible liquid that will dilute your product to some degree.
Looking at the axial thrust in single stage pumps.
Single stage, single suction, semi-open impeller.
- The axial thrust generated is higher than in closed impellers because of the hub. Pump out vanes and balance holes are a common solution to this problem.
- A mechanical seal can add to this axial thrust. The amount is dependent upon the design of the seal. Balanced designs create less thrust.
Single stage, single suction, closed impeller:
- Balancing holes are not desirable with closed impellers because leakage back to the impeller inlet opposes the main flow creating disturbances. A piped connection to the pump suction can replace the balance holes
- Theoretically there shouldn’t be any thrust in a double suction closed impeller, but:
- An elbow with the inlet piping running parallel to the shaft will cause an uneven flow into the impeller eyes. This uneven flow will cause thrusting of the impeller in one direction depending upon the flow difference. The eye is taken as the smallest inside diameter of the shroud. Remember to deduct the area occupied by the impeller hub.
- The two sides of the discharge casing may not be symmetrical causing an axial thrust.
- Unequal leakage through both sets of packing can upset the axial balance. Leaking seals can do the same thing.
Impellers can be cut down to keep the application close to the pumps best efficiency point :
- Theoretically up to twenty five percent of an impeller diameter can be removed, but any time you remove more than ten percent of the maximum impeller diameter the affinity laws are no longer accurate because of slippage between the impeller outside diameter and the pump volute.
- Changing the impeller diameter changes the head, capacity and power requirements.
- The capacity can be increased by under filing the vane tips, but the discharge head and the power requirement will automatically adjust to the values where the pump curve intersects the system curve.
- If you intend to cut down the impeller diameter, the impeller should be cut down in at least two steps and tested after each step.
- After cutting down the impeller diameter the discharge vanes should be reshaped to a long gradual taper to increase the pumps performance. Chamfering or rounding the discharge tips will frequently increase the losses and should never be done.
- Over filing is removing metal from the leading edge of the blade. This seldom produces any increase in the vane spacing and produces a negligible change in pump performance.
- Under filing is removing metal from the trailing edge of the blade. If properly done it will increase the vane spacing and can increase the capacity by as much as ten percent.
- If the inlet vane tips are blunt, over filing will increase the inlet area and the cavitation characteristics can be improved.
- Cutting back the tongue increases the throat area and increases the maximum capacity. The head/capacity is then said to “carry out further”.