Pumps & Systems, July 2008
When choosing a pump for a given application, there are many factors to consider. The system's parameters dictate the pump needed, so the trick to ensuring a maintenance-free design is determining which parameters affect reliability.
The governing parameters when choosing a positive displacement pump are fluid, pressure, control and maintenance.
Of course, these are only the technical considerations. The performance of the pump company after the pump is installed is an additional consideration. If there is an operational issue, will the manufacturer be responsive with both spare parts and customer service?
Fluid
When choosing a pump for paint systems, the first thing to explore is chemical compatibility. Every part that is in contact with the fluid needs to be chemically resistant to it.
Air Operated Diaphragm pumps use multiple "wetted" parts consisting of metal and rubber parts, including a suction strainer, filter screen with check valve ball type, pump body, rubber seals, springs and diaphragms.
In a hose pump, the only piece in contact with the medium is the hose. The rubber hose is available in many different elastomers.
Some un-dissolved solids exist in the fluid. While the particle size can vary, the pump must be able to pass these solids through without incurring any damage, which is difficult. Hose pumps, designed for heavy slurries, can handle high solid content easily. Diaphragm pumps tend to clog easily, or the thin diaphragms puncture.
Outgassing can create problems with diaphragm pumps but not hose pumps. Some fluids change properties when the temperature or pressure is changed frequently. The buildup of gas inside the pump can cause the diaphragm pump to lose prime and shut down. While the hose pump is self priming, it will consistently pump liquids and gases.
The forces exerted by the pump affect some fluids more than others. These shear stresses can severely damage a fluid. While the vigorous back and forth motion of a diaphragm can beat up delicate fluids, a hose pump pushes the fluid through the pump in a similar way to squeezing a tube of toothpaste.
Pump users must be aware of the fluid temperatures. The temperature and metal and rubber components of the pump can affect the fluid. High temperatures can reduce pressure capability and suction lift, while low temperatures reduce rubber life expectancy and suction lift. Typically AOD pumps can handle higher temperature ranges by using Teflon diaphragms.
Pressure
The peristaltic hose pump is generally limited by a maximum discharge pressure of 225-psi. At the maximum pressure, the hose pump can easily self prime, pull 95 percent of vacuum and keep output volume consistent. Although diaphragm pumps are rated for and can operate at higher pressures, the reliability significantly decreases at those higher pressures.
Control
The output flow for both types of pumps can be controlled remotely. An integrated frequency inverter is typically used on hose pumps to cycle an AC gear motor. The user can also vary the speed of a DC motor. A diaphragm pump can also be run this way using a solenoid. Both pumps tend to create a pulse in the medium, but the hose pump is almost continuous even at lower speeds while the diaphragm pump creates an intermittent injection. A pulsation dampener can typically remove ~95 percent of the pulsation to eliminate the unwanted pressure spikes and system interruption.
Diagnostic systems can help monitor the overall system, including flow sensors, pressure sensors, hose leak detection, diaphragm detection, etc. All can be wired to sound an alarm and/or stop the pump to avoid further damage.
Maintenance
The maintenance personnel, typically the most familiar with the piece of equipment, perform preventative maintenance as well as repairs to achieve long service intervals. The service interval varies from application to application, but the hose pump generally provides a longer service interval because only one wearable piece is in contact with the medium.
The hose in a hose pump can be changed on a regular schedule to avoid leaks, or after a hose leak detector is triggered. For a diaphragm pump, check on the diaphragms, chemical attack on seals and the accumulation on valves. The service interval will increase if the chosen equipment is compatible with the system.
Operational Cost
The total operating cost of a diaphragm pump is usually not understood by the end consumer. Visit the Department of Energy's website (http://www.doe.gov/) and download information on the significant cost of producing air. A 2-in diaphragm pump requires ~75 SCFM. According to the DOE, the cost of producing air to run this 2-in diaphragm for 8,000 hours per year will be ~$9,000 per year. In comparison, the cost to run a hose pump of equal size will result in energy consumption of $1,500 - $3,200 per year, depending on the manufacturer.
Due to the paint's chemical nature, paint systems require hose pumps or diaphragm pumps to use either EPDM or Nitrile hoses or diaphragms. The paint type will dictate the elastomer for both diaphragm pumps or peristaltic pumps. Generally, paint is fairly to highly viscous, depending on type and intended use. Hose pumps may be a better solution since only one component is in contact with the medium. A diaphragm has ball check, springs, strainers and body components which can potentially clog, jam and fail.
All peristaltic pumps and diaphragm pumps are not created equally and have significant operational advantages or disadvantages depending on design.
Peristaltic Pump Designs
Understanding different peristaltic pump designs is critical to longevity and process availability.
While many hose pump designs exist, there are basically three means employed by designs that compress the hose. First, the shoe design has two or more fixed shoes that compress the hose twice per revolution by grinding against the hose. This type damages the hose the most because it generates a lot of heat and creates stress/damage to the hose on each revolution. It requires a large glycerin bath to lubricate and help dissipate heat from the hose and internals of the pump to the pump casing. A typical 3-in pump would require ~35 liters or roughly 10 gallons of glycerin inside the casing.
Shoe design pumps are limited in running speeds. The high drag/friction across the rubber hose significantly heats up the pump. Due to this heat and friction, these pumps cannot run at high speeds. For instance, a 3-in pump may only be capable of running at 40-rpm continuously, which limits the amount of continuous flow. Companies that produce this pump push the user to the next larger size pump to keep the RPMs lower. This is the right strategy, but you can typically use one size smaller pump with rolling design peristaltic pumps. A limiting factor on shoe design pumps is running an extremely low RPM. If you run these pumps at very low RPM, you may trip the VFD frequently due to the high drag.
The second hose pump design uses two or more rollers on the end of a rotating arm. This design is much more forgiving because it causes less damage to the hose in compression and also creates less heat. This design will typically have moderately longer hose life than a grinding shoe design since it causes less stress and heat to the hose.
The third and most recently available hose pump design uses a cam shaft and a roller on the end of the shaft. This radical design achieves a longer hose life because it only compresses the hose once per revolution. Also Since it rolls over the hose, it is also much more forgiving when it compresses the hose. This design also generates virtually no or very little heat.
While this rolling design pump requires glycerin for light lubrication, it requires a fraction of the glycerin a typical pump needs. For instance, a 3-in sliding shoe pump would require 35 liters of glycerin, and a 3-in rolling design would only require 8 liters. The reduced consumption of glycerin aids in the overall cost effectiveness Rolling pump hose usually lasts 4 to 5 times longer than shoe design pumps. Rolling design pumps do not have heat buildup problems and can usually run at very high rpm without overheating and hose damage.
This type of pump often requires one pump size smaller than conventional shoe design pumps since it can run at higher RPMs without overheating. Even when running at higher RPMs, this pump has significantly longer hose life than conventional shoe design pumps running at much lower RPMs. Rolling design hose pumps have also eliminated the hose barbs in the end of the hose which makes the hose replacement much simpler, especially on the larger diameter pumps.
Every pump has advantages and disadvantages, so plants must research and determine the best pump for the application. Too many plants pick one pump for one application and then continue to buy and use the same pumping technology for every application because it is "easier." This approach can ultimately cost more money because of needed repairs. Some pumps are excellent for certain processes, but do not work well in other processes.
Homework and constant trial will often result in operational savings, so make sure to research all available options.