Metering Pump Selection
Both diaphragm and peristaltic metering pumps (injectors) can provide years of dependable and efficient service. When deciding which to use, it is important to know how each type of pump operates, and what regular maintenance each requires. This will help determine the metering pump that best suits your installation needs.
If you are currently using diaphragm pumps successfully, you are undoubtedly aware of the importance of basic pump maintenance. The more you know how these pumps operate and their maintenance requirements, the better the pumps will perform. Although today’s diaphragm pumps are more sophisticated—better valves, diaphragm design and material options, and improved drive mechanisms and housing—they still require proper operating procedures and regular service for a successful program.
Peristaltic pumps, on the other hand, are initially simpler to use and require less experience to operate; however, they do require an understanding of the installation requirements and regular maintenance procedures.
Diaphragm Metering Pumps
There are a number of factors that should be considered when selecting diaphragm metering pumps.
1. When properly maintained, a diaphragm metering pump will cost less to operate over time.
2. Diaphragm metering pumps are more energy efficient, using more motor torque on the forward (power) stroke, but far less on the back stroke. Overcoming line pressure is easi with a properly sized diaphragm metering pump.
3. There is less danger of leakage; if poorly maintained, a diaphragm metering pump will lose its prime, but seldom leak.
1. Diaphragm metering pumps operate best when the solution being pumped is clean and free from particulates. This is because diaphragm metering pumps have check valves in the suction and discharge side of the pump head. If either set of check valves becomes fouled (dirty), the pump will first lose its ability to meter accurately, and then it will lose its prime.
2. Diaphragm metering pumps are difficult to prime against pressure. These pumps prime best when there is little to no back pressure. Some pumps are fitted with a bleed valve to aid in this challenge.
3. Diaphragm metering pumps are difficult to prime with dirty check valves. They prime best when the valves (check balls) are clean and there is little to no back pressure.
4. Diaphragm metering pumps are difficult to prime when the stroke (feed rate) adjustment is on a low setting. Most have a diaphragm stroke adjustment, and some also have a motor speed adjustment. Priming is best achieved when the stroke adjustment is above the 60% range. These adjustments can be confusing, so it is best to minimize variables as much as possible. Avoid adjusting the diaphragm stroke length too low; the pump will lose efficiency. Keep your diaphragm stroke above 40% if possible; most pumps are simply more efficient with longer stroke lengths.
In summary, a diaphragm metering pump will require operators to have a good understanding of pump valves, proper priming, and adjustment characteristics and their importance. Once there is a good understanding of the pump, and it begins operating within its normal parameters, it will provide dependable and efficient service.
Peristaltic Metering Pumps
Peristaltic metering pumps are initially easier to use because they are more forgiving than diaphragm metering pumps under less than ideal conditions, but they have some disadvantages as well.
1. Peristaltic pumps work well even with high levels of particulate in the solution (undissolved solids) being pumped. There are no check balls to foul within the pump tube.
2. Feed rates are generally not affected by pressure or the nature of the chemical being pumped. Peristaltic pumps are easy to prime even with moderate back pressure because they have excellent suction.
3. They rarely lose prime, and can even pump air (gases).
1. The constant peristaltic (squeezing) action eventually weakens the pump tube, and the feed rate is slowly diminished. This happens lightly at first and increases as the pump tube wears out. Modes of failure include: lack of pressure capability; complete loss of pumping capability; loss of suction lift; reduction in output; and tube rupture.
2. Squeezing the pump tube requires the drive motor to be under a constant load, using more power.
3. If regular service is neglected, such as the pump tube is not changed, or the injection point not serviced, the pump tube may rupture and could damage the pump, or worse. Some manufacturers now offer models with sensors that automatically turn off the pump and trigger an alarm in the event of a tube rupture. These new features protect the pump and pump area from damage.
4. Tube life expectancy will vary based on many factors including the out- put amount, back pressure and the chemical being used. Most manufacturers rate their pumps’ tube life expectancy in hours; therefore, users must be aware of the total number of hours the pump has operated. A common problem with peristaltic pump users is underestimating how many hours the pump has operated.
In summary, peristaltic pumps are great at pumping dirty solutions, and those that tend to accumulate gases. Generally, priming problems are nearly eliminated; however, peristaltic pumps will require regular changing of the pump tube and regular service at the injection point.
The following are three main factors that affect both peristaltic and diaphragm metering pumps.
1. Make sure the pump’s wetted parts are compatible with the chemical being pumped. A diaphragm pump’s diaphragm, pump head, valves, injection fitting and tubing are commonly referred to as the “wetted end.” All of these components require attention. The wetted end on a peristaltic pump includes the pump tube, injection fitting and connec- tion tubing. Be certain these components are compatible with the chemical being pumped. Manufacturers will list the materials that make up wetted parts. The customer must do some research on chemical compatibility. No single material works successfully with all chemicals.
2. Read the pump curve. A diaphragm pump’s output will not be the same at atmospheric pressure as it will be at, for example, 50 psig. As the line pressure increases, the feed rate will decrease. A pump curve will help you, but remember the pump curves provided from the manufacturer are determined in a laboratory, pumping pure water. If the solution being pumped has a different viscosity and specific gravity than water, it will affect the pump’s output.
3. It is best to check output by measuring with a graduated measuring cup from suction tubing. This way, you will learn how the chemical, with the elements considered including viscosity, specific gravity and line pressure, is actually feeding. This is the best way to calibrate the pump.
Diaphragm metering pumps excel at pumping clean, aggressive chemicals into high-pressure systems, and require very little maintenance. A variety of wetted parts are available for chemical resistance. Diaphragm pumps, however, can lose their prime and can be difficult to prime, especially if the fluid is dirty or contains trapped gases.
Peristaltic metering pumps excel at pumping dirty fluids that contain trapped gases or particulate matter into lower pressure systems. Newer peristaltic pump designs are capable of pressures to 100 psig. Pump tube material options, however, can be limited, and chemical resistance can be a factor. In addition, peristaltic pumps will require periodic changing of the pump tube.
Research and a good understanding of both the installation requirements, and the pump’s operating parameters and maintenance requirements, are vital to choosing the best pump for your application.