This is the second of eight articles in the series Sizing & Selecting the Right Chemical Metering Pump. Read the previous article “Sizing your chemical metering pump and determining injection rate.”
Unlike most other decisions, this is the most subjective decision to make (unless you have tried various types and brands of pumps). This is also the most consequential decision in determining the level of success with your pump applications. There are many pump types and pump brands available in the market today. Each type and brand of pump has its unique positives and negatives. It is up to the user to choose the right type and brand for their unique application.
It is important to first know that all pump types are divided into two broad types: (1) Centrifugal and (2) Positive Displacement. While centrifugal pumps are the most common types of transfer pumps, they are not suitable in applications where the flow needs to be accurate and needs to overcome high pressures. In the context of chemical metering / dosing applications, a positive displacement pump is used almost all the time. Within the positive displacement pumps, there are many types. Below, we are going to discuss only the most common types used for chemical metering / dosing applications.
1. Gear Pump: As the name suggests, these pumps function by trapping liquid between gear teeth and pushing it from the suction to the discharge port. These are often used for hydraulic pressure developing. However, gear pumps are not suitable for accurate metering applications due to the leak back of liquid depending upon the clearances. These clearances will also tend to increase with wear.
2. Air Operated Diaphragm Pumps:This is a very versatile kind of pump and perhaps the most popular positive displacement pump due to its simplicity and versatility. However, the best accuracy and repeatability you can expect is around 10% to 15% variance in flow consistency. So these are great transfer pumps, but not suitable for metering / dosing applications.
3. Peristaltic Pumps: These have been gaining more and more acceptance recently, mostly due to its ability to push viscous liquids, and especially after making improvements in its Achilles heel: the short life of the tube, which is the main element of this design. The other major issue beyond the possible failure of the tube and chemical spill, is its inability to develop even moderate pressures. This pump design depends upon the tube, and the more pressure it pushes against, the more it is likely to balloon. If you reinforce the tube too much then it takes an enormous amount of energy just to roll on the hard tube.
4. Solenoid diaphragm metering pump: This is a popular pump for dosing at low pressures and with very intermittent (infrequent and low duty cycle) use. The greatest advantage of this pump is that it is relatively cheap. While these are good for pumping residential swimming pool chemicals sitting on top of a barrel, it is not suitable for any process application where the duty cycle is more than just a few minutes every two days. Additionally the pressure limitation of this pump is usually low.
5. Mechanical diaphragm metering pump:This type is an improvement over the solenoid pump design in terms of heavier duty, but it does not address the main issue which is the inability of the diaphragm to push without stretching and ultimately failing against high pressures. Due to the fact that the diaphragm is unbalanced with liquid pressure on one side and no back up pressure on the other side, this pump will also provide less accurate flow while being susceptible to failure at low pressures as well.
6. Hydraulic Diaphragm Metering Pump: The main issue of an unbalanced diaphragm in all above pump designs is addressed here with a hydraulically balanced design. The key here is that the process liquid pressure is balanced by the hydraulic oil on the other side, which is the force driving the pump. Also, because of the fact that the hydraulic diaphragm design relies on incompressible hydraulic oil, the accuracy and repeatability of this pump is the best available out there. This is the type that meets and exceeds all the requirements of a demanding industry such as Oil & Gas with API 675 standards. Additionally, the components all have a long life, including the diaphragm as there is no mechanical stress which leads to failure as the balanced diaphragm does not stretch.
As for the many brands of pumps available, those need to be carefully examined for each application. It does not behoove for us to write critically about other manufacturers so we will only present some of the unique advantages our pumps offer:
1. Designed to be easy to install, use and service. For example, capacity of the pump can be adjusted while the pump is running. Oil level and liquid end parts are easily accessible and located in the front.
2. Each component is designed to be heavy-duty, long-lasting and quietly-operated. Larger gears for better wear resistance and bigger bearing for smoother operation.
3. Check Valves not integral to diaphragm head and easily accessible to clean and service.
4. More complete range in flow capacity (up to 3,500 GPH) and pressure up to 3,000 psi.
5. Choice of corrosion resistant materials like 316 Stainless Steel, Alloy 20, Hastelloy C, PVC, PVDF and PTFE.
6. Manufactured, assembled and tested in USA.
7. Not a part of a large corporate conglomerate which is answerable to shareholders. AquFlow is a small company run by engineers who take pride in their work, product and service. We are only answerable to our customers.
If you are interested in a one on one comparison with major competitive brands please check out this Special Presentation.