Pumps & Systems, July 2007
Hose pumps have been around for many years, with some designs dating back more than 75 years. They are excellent devices for pumping slurries, due to their ability to handle very abrasive slurries. Hose pumps are also very good at dosing chemicals, since they are a positive placement device that can very accurately control the flow rate desired.
Hose pumps can run dry for long periods of time without damage to the pump. Typically the only wearing part is the rubber hose, which is also the only part in contact with the pumped medium. NPSHA is not a concern for hose pumps because they create their own suction on the inlet side. Finally, hose pumps will never cavitate.
The biggest challenge is manufacturing the hose itself, which is the main element and only repair part. As such, the hose is the source of the greatest MTBF.
The slower a hose pump is run, the better, because it places fewer revolutions on the hose. One school of thought suggests that the abrasiveness of the slurry is what destroys the hose in a hose pump. This is not the reality, because the number one factor in determining hose life in a hose pump is how many compressions are placed on the rubber hose. The number two factor that contributes to hose wear is the amount of stress being placed on the hose during a compression and how much heat is generated from that compression force.
In other words, the best way to maximize hose life and eliminate pump downtime is to reduce the number of compressions on the hose and compress the hose in the less damaging manor.
There are basically two types of pumps that can be considered a hose pump, but there is a clear distinction between hose pumps and tube pumps. The difference between these two types of pumps are that tube pumps typically do not have a glycerin bath, pump at very low pressures and also are very small and low flow rate devices.
A hose pump is more of an industrial piece of equipment rather than a piece of lab equipment. Hose pumps typically range is size from .500-in to 6-in and typically have a maximum pressure capability of ~230-psig, depending on manufacturer. This discussion is focused on industrial hose pumps, rather than tube pumps.
There are many designs of hose pumps, but there are essentially only three means employed by all of these designs to compress the hose. The first is the shoe design, where two or more fixed shoes compress the hose twice per revolution by grinding against the hose. This type of design damages the hose the most because it generates a lot of heat and creates a lot of stress/damage to the hose on each revolution.
The shoe design requires a large glycerin bath to lubricate, and more importantly, help dissipate heat from the hose and internals of the pump to the pump casing. A typical 3-in pump would require ~35-l (roughly 10-gal) of glycerin inside the casing.
Shoe designed pumps have a significant limitation regarding the speed at which the pump can be operated. Because of the high drag/friction across the rubber hose, the pump heats up significantly. Due to this heat and friction, these types of pumps cannot run at very high speeds.
For example, a 3-in pump may be capable of running at only 40-rpm continuously, which, in turn, limits the amount of flow that can be produced continuously. Manufacturers of this type of pump tend to push the user to the next larger size pump so the rpm is kept lower. Though this strategy is correct, the user of a rolling design peristaltic pump can typically work with a unit that is one size smaller.
Also, a limiting factor on the shoe type of pump involves running a very low rpm. The high drag created from a very low rpm may frequently trip the variable frequency drive (VFD).
The shoe design hose pump uses two or more fixed shoes to compress the hose twice per revolution by grinding against the hose.
The second design of hose pump is one which utilizes two or more rollers on the end of a rotating arm. This type of design is much more forgiving because is causes less damage to the hose while compressing it. It also creates less heat. This design typically will have moderately longer hose life than a grinding shoe design because it causes less stress and heat to the hose.
This hose pump design utilizes two or more rollers on the end of a rotating arm.
The third and most recently available hose pump design is one that utilizes a cam shaft and a roller on the end of the shaft. This type of design is the most radical in achieving a higher level of hose life because it only compresses the hose one time per revolution. Also, since it rolls over the hose it is also much more forgiving to the hose when it compresses it. This design generates virtually no or very little heat.
This type rolling design pump does require glycerin for light lubrication, but it requires a fraction of the glycerin that a typical pump would require. A 3-in sliding shoe pump, for instance, would require 35-l of glycerin and a 3-in rolling design would only require 8-l. The reduced consumption of glycerin helps to aid in the overall cost effectiveness of the rolling pump design.
A rolling design peristaltic pump. This hose pump design utilizes two or more rollers on the end of a rotating arm.
Customer experience is proving that rolling pump designs have a hose lifetime that is usually 4X to 5X longer than shoe design pumps. Rolling design pumps do not have problems with heat buildup and can usually run at very high rpm without the problems of overheating and hose damage. This type of pump, in many cases on a given application, will require one pump size smaller than conventional shoe design pumps, due to its ability to run at higher rpm without overheating.
Also, this type of pump - even when running at higher rpm - will produce significantly longer hose life than conventional shoe design pumps running at much lower rpm than the rolling design. Rolling design hose pumps have also eliminated the hose barbs in the end of the hose. This makes the hose change so much simpler that it is reduced to a one-man job on the larger diameter pumps.
Table 1 shows a real-life cost comparison of the one-year operating costs of a 3-in shoe design hose pump compared to a 2.5-in rolling design pump at a calcium carbonate manufacturer. The pumps were pumping 70 percent calcium carbonate at a pressure of 58-psig to 72-psig, with a flow rate ranging from 35-gpm to 53-gpm.
The 3-in shoe design pump required three times more floor space and a 20-hp motor. The 2.5-in rolling design hose pump was one-third the required floor space and required only a 10-hp motor.
Table 1. One-year operating cost savings using rolling design vs. shoe design hose pump = $11,388.
Based on the scenario in Table 1 that was provided by a manufacturer of calcium carbonate and user of hose pumps, the operating, maintenance and spare parts cost of using a shoe design hose pump over a 7-year period would be $134,666. And if a single roller rolling design was selected, the 7-year operating cost would be $54,950. This means the 7-year savings is just under $80,000.
This is significant and points to the fact that users should consider the operating, maintenance and spare parts costs when selecting their hose pumps. The rolling design hose pumps have significant cost savings compared to shoe designs, and also are more robust and are easier to perform maintenance on.