Without understanding the function of a slurry pump impeller, we will never understand why and how a pump is designed and functions.
The Impeller = an energy converter!
“The function of the rotating impeller is to impart kinetic energy to the slurry mass and accelerate it”.
A part of this kinetic energy is subsequently converted to pressure energy before leaving the impeller.
Apart from the strict hydraulic transformation this is, in slurry pumps, partly achieved by the special capacity of the solids in the slurry itself to convey energy by “hydraulic drag forces”. These drag forces are used in a number of hydraulic machines for wet processing (classifers, clarifers, separators etc.)
Energy conversion done?
The impeller vanes are the heart of the impeller. The rest of the impeller design is just there to carry, protect and balance the impeller vanes during operation.
Slurry pump impellers have external and internal vanes.
These vanes also known as pump out or expelling vanes are shallow and located on the outside of the impeller shrouds. These vanes aid pump sealing and efciency.
Also known as the main vanes. They actually pump the slurry.
Normally we use two types of main vane design in slurry pumps:
When to use Francis or Plain?
”As the Francis vane is more effective in energy conversion, it is used when efciency is of
prime concern, although the advantages are less clear cut with wide slurry impellers".
”The drawback of the Francis vane is that its design is more complicated to produce and also takes on more wear when pumping slurries with coarse particles!” Therefore Plain vanes are used when pumping coarse particles.
Number of impeller vanes?
”More vanes gives higher efciency. This means that the maximum number of vanes is
always used whenever practical”. (The exception is torque ﬂow.)
Limitations are created by the vane thickness required for good wear life and the need to pass a required particle size.
Maximum number of vanes in practice is fve which are used on metal impellers with a diameter exceeding 300 mm and rubber exceeding 500 mm.
The vane area relative to the impeller area becomes critical (too large vane area, giving too much friction) and effciency starts to drop and blocking can occur.
Semi-open or closed impeller?
The design of the slurry pump impeller is not related to a closed or open confguration. This is determined by production aspects and what type of applications the impeller will be used on.
Closed impellers are by nature more efcient than open impellers, due to the reduction of “short circuiting” leakage over the vanes. The efciency is less aﬀected by wear, “If you are looking for efciency - use a closed impeller whenever possible!”
Semi-open impellers are used to overcome the limitations of a closed design and depend on impeller diameter, size or structure of the solids, presence of entrained air, high viscosity, etc.
The efciency is slightly lower than for closed impellers.
“The diameter of an impeller governs the amount of head produced at any speed.” The larger the diameter of the impeller the greater the head produced. A large diameter impeller running very slow would produce the same head as a smaller impeller running much faster (key aspect when it comes to wear, see section Wear Protection.
What will be the correct diameter?
The factors that have guided Metso in this respect are:
For highly abrasive duties we want a long wear life and reasonable efciency!
For abrasive and mildly abrasive duties we want reasonable wear life and high efciency!
To make it simple:
For highly abrasive duties we use large impellers giving long life and reasonable efciencies.
So even if larger impellers are more expensive and have slightly lower efciency, they give a better pay off in highly abrasive duties.
For abrasive duties where wear is not the primary concern, smaller impellers are more
economical, and oﬀer better efciency.
This relation is known as:
Impeller Aspect Ratio (IAR) = Impeller diameter / Inlet diameter
for highly abrasive duties we use IAR = 2.5:1
for abrasive duties we use IAR = 2.0:1
for mildly abrasive duties we can use IAR of less than 2.0:1
All the above parameters have been considered when designing the Tobee slurry pump ranges, giving optimal operation economy at various duties.
"The width of the impeller governs the ﬂow of the pump at any speed.” A large width impeller running slowly could produce the same ﬂow rate as a thinner impeller running faster, but most important - the velocity relative to vane and shroud would be considerably higher.
Compared to water pumps and depending on the “wear profle”, slurry pumps normally have impellers that are not only larger, but very much wider.
Tobee® Slurry Pump Impeller Design:
Hebei Tobee Pump Co.,Limited