The science of moving a fluid from point A to point B is vast, and knowing when to use one type of pump or another often requires expert engineering assistance like the kind we provide at West View Cunningham. In this article, we discuss some basics of where and how to apply different technologies to help you start your search for the right pump and achieve best results from your pumping systems.
Positive Displacement Pump
Centrifugal Pump
Positive Displacement (PD) Pump Basics
The main varieties of Positive Displacement pumps are:
Positive displacement pumps come in a wide variety of configurations. Each works in slightly different ways, but at their core they all make a fluid move by trapping a fixed amount of liquid and forcing (i.e., displacing) the trapped fluid into the discharge pipe. This type of actuation can be controlled precisely. As a result, the vast majority of metering pumps are of the positive displacement variety, the most common being electrically actuated diaphragm and hose pumps.
Gear pumps are designed to transfer higher viscosity fluids of >100cP at low to medium flow rates. They are most often used for bulk transfer applications between chemical processes/tanks moving heavy fluids like resins, paints, or tars. Gear pumps have poor solids handling capabilities, particularly if they are set up to run on thinner fluids with a tight gear mesh.
AODD pumps are unique because they are powered by compressible air or gas, so many of the considerations noted below are not a factor when using them. AODD pumps will never exceed the pressure ratings on their curves and are generally a 1:1 ratio of air supply pressure to maximum discharge pressure. If the system becomes pressurized to the limit of the pumps discharge pressure, it will simply stall until the system pressure drops again.
In this regard they are very forgiving in the field – no pressure relief is required and they can be dead headed or dry run almost indefinitely without failure. For this reason they make excellent filter-press pumps. AODD pumps can be used for almost any application and make excellent portable back-up pumps. They are not often considered as a first choice for permanent installation due to the cost of compressed air versus electricity.
Progressive cavity pumps, also known as progressing cavity, generally refer to the eccentric screw pump, though there are several pump types in this category. They are often used as a “pump of last resort.” They have middling performance characteristics and generally poor efficiencies, but those shortcomings are offset by their ability to pump nearly anything. This is particularly true of “fluids” that are otherwise not pumpable, such as spent grain at a brewery or whey powder and oil mixtures at a food processing plant. They require regular maintenance because the rotor and stator are both wear items and may require frequent replacement depending on the service.
Piston pumps are generally used in high-pressure injection or spraying applications, such as pressure washers for car washes or main pumps at small injection wells. They work best in low to medium flow applications, as scaling them up beyond a couple hundred gallons per minute becomes prohibitively expensive. Generally, that is a range where a multi-stage centrifugal pumps will be a more reliable option. Piston pumps have very strict requirements for inlet pressure and drive torque which must be catered to in the system design, otherwise, they have reliability problems. They also create rapid water hammer pulses in the discharge line, so pumping into a pressurized closed system is not recommended without adequate pulse dampening.
Sliding vane pumps are more specialized pieces of equipment, typically used on light hydrocarbon applications like propane transfer. They create excellent suction and require very little NPSHa to operate, so they are perfect for liquids with high vapor pressures. The vanes themselves are a regular maintenance item and require replacing on a schedule.
The benefits of using PD pumps include:
Some engineering considerations/drawbacks when using Positive Displacement pumps are:
An Overview of Centrifugal Pumps
The performance of a centrifugal pump, unlike that of PD pumps, is intrinsically tied to the system in which the pump is installed. Pumps must be properly sized and selected based on the system parameters to run in a stable hydraulic condition. The pressure differential created by the pump will only ever match the pressure required to induce a flow at a single point along the pump curve, as shown in the graphic below. Missing the mark in sizing and installing a pump into a system that isn’t capable of matching the curve will lead to myriad problems. We have several articles discussing these issues in detail, see our article on sizing and our article on system modeling.
These pumps are made in several impeller configurations to handle a broad range of fluids and materials. Those varieties are:
The rest of the pump can be configured in a nearly endless variety of styles. We highly recommend looking up the different configurations covered in API-610 as a good place to start.
To broadly cover what is available, centrifugals can be broken into these categories, and this list is not all-inclusive (API designations in parentheses):
The advantages of centrifugal pumps include:
Some engineering considerations/drawbacks with centrifugals:
Everything described in this article reflects general rules, and there are exceptions to nearly every rule. Therefore, it’s best to consult the experts before choosing a pumping solution. We’ve seen our share of mistakes and can help you avoid them.
To determine which type of pump or pumping system is best for your application…