Did you get the difference? High pressure always flows to low pressure, so we now have two separate methods of restricting internal recirculation that can lower the efficiency of your pump and generate a lot of unwanted heat to the product you’re moving.
- For the open inmpeller design you have to set the inpeller to casing clearances
- For the losed impeller you will need wear rings.
There are advantages and disadvantages to each design:
|CLOSED IMPELLER||OPEN IMPELLER|
|Can compensate for shaft thermal growth, but if there is too much axial growth the vanes may not line up exactly with the discharge nozzle.||The impeller to volute or back plate clearance must be adjusted when the pump is at operating temperature and all axial thermal growth has occurred|
|Good for volatile and explosive fluids because the close clearance wear rings are the parts that will contact if the shaft displaces from its centerline||You would have to use soft, non-sparking materials for the impeller and that is not very practical.|
|The impeller is initially very efficient, but looses its efficiency as the wear ring clearance increases||Efficiency can be maintained through impeller clearance adjustment.|
|No impeller adjustment is possible. Once the wear ring clearances doubles they have to be replaced. This means the pump had to be disassembled just to check the status of the wear rings.||The impeller can be adjusted to compensate for wear and stay close to its best efficiency. No pump disassembly is necessary.|
|The impeller can clog if you pump solids or “stringy material”. It’s difficult to clean out these solids from between the shrouds and vanes.||The open impeller is less likely to clog with solids, but if it does, it is easy to clean.|
|The impeller is difficult to cast because the internal parts are hidden and hard to inspect for flaws||The open impeller has all the parts visible, so it’s easy to inspect for wear or damage|
|The closed impeller is a more complicated and expensive design not only because of the impeller, but the additional wear rings are needed.||The pump is less costly to build with a simple open impeller design.|
|The impeller is difficult to modify to improve its performance.||The vanes can easily be cut or filed to increase the capacity.|
|The specific speed choices (the shape of the impeller) are limited||You have a greater range of specific speed choices.|
My experience in Europe has been that about 85% of the pumps used in the process industry are of the close impeller design. Here in the United States it is the opposite, with the exception of oil refineries.
At one of my International seminars I quizzed a couple of KSB Pump Company application engineers about this difference and was told they used closed impellers more often because the German mechanic will not make the proper impeller clearance adjustment.
Oil refineries choose the closed impeller design because their products are often explosive or a fire hazard. If you use open impeller pumps in these applications there is always the danger of the impeller contacting the volute and causing sparks. This means that the impellers would have to be manufactured from a non-sparking material, which is often too soft for the abrasives in an oil refinery application.
If you want to get a feel for the thermal growth involved in a typical pump, be aware that a stainless steel shaft grows both radially and axially at the rate of 0.001 inch, per inch of shaft, for each 100°F (0.001 mm/millimeter of shaft length or diameter/50°C) rise in temperature.
Let’s take a look at a typical heat transfer oil pump running at 600°F (300°C) and see what type of expansions we are talking about. We will start with the inch version and assume a 20-inch long, 1.875-inch diameter shaft. If the 1.875 diameter shaft measured twenty inches from the end of the impeller to the thrust bearing and you heated the shaft to an average of 400°F over ambient, it would grow 0.080 inches in length and 0.0075 inches in diameter.
- This would be enough axial growth to allow the impeller to contact the volute because a typical impeller to volute clearance would be between 0.015 and 0.020 inches. The volute is often manufactured from a different material than the shaft and we have no evidence that both the shaft and volute will grow in the same direction and at the same rate.
- The radial growth is enough to allow the shaft to contact the low expansion metal vibration dampers frequently used in metal bellows seals specified for this service.
If a heat transfer oil pump in the metric system had a 48mm shaft, 450mm long heated to 200°C over ambient, it would grow 1.8mm in length and 0.20mm in diameter
This is the reason both seal and pump manufacturers recommend turning the shaft by hand prior to start up, but be careful, it’s hot!
All of this means that all impeller clearance must be set when the pump is at operating temperature. It also means that you’re going to have to specify cartridge mechanical seals in these applications because their operating length must be set when the pump is at operating temperature, or anytime after the open impeller has been adjusted to compensate for vane or volute wear.