The "Energy Concept for an Environmentally Sound, Reliable and Affordable Energy Supply" stipulates that the energy productivity of pump systems in the industrial sector is to be increased by 2.1 percent per year and the CO2 emissions are to be minimized. In addition to using energy-efficient motors, precise selection of the respective technologies and matching these technologies to the power ranges and fields of application are critical to satisfying these requirements. Factors that can influence the energy efficiency of the pumps must also be taken into consideration during the processes of selection, optimization and installation planning. Such factors primarily include taking into consideration the operating point and its variability, the flow rate and pressure range as well as the gap and friction losses in the entire pump.
Centrifugal pumps versus displacement pumps as well as their advantages
However, to find out which system makes sense for the application in question, particularly with respect to its energy efficiency, it is first necessary to make an exact comparison of the efficiencies of the various pump types. Centrifugal pumps and rotary displacement pumps usually have a rather low efficiency and therefore stand out as "energy hogs" in many industrial applications. By contrast, the conversion of drive energy into hydraulic pressure energy can be carried out at a much higher efficiency level using hydrostatic diaphragm pumps in comparison to what are known as hydrodynamic centrifugal pumps due to the predominant hydromechanical laws. The advantages of reciprocating positive displacement pumps compared to rotating units can be seen primarily in partial load operation, at high pressures and at small flow rates.
In practice, the following three pump types are usually considered for operating points in ranges from 2 to 150 m3/h and pressures between 20 and 200 bar: multi-stage centrifugal pumps, high-speed centrifugal pumps and process diaphragm pumps.
Multi-stage centrifugal pumps are used in cases where one impeller alone cannot build up enough pressure Higher pressures can be achieved by connecting multiple impellers in series. However, this results in pumps that are more complex and thus more expensive.
High-speed centrifugal pumps create large pressure increases through the high peripheral speeds of the extremely fast-moving impeller. To ensure that these pump systems operate reliably, various factors such as pressure conditions, temperatures and surrounding conditions must be taken into account and strictly adhered to during startup as well as ongoing operation. If the required basic conditions are not adhered to when these pump types are used, the machine is at risk of suffering substantial damage or even total loss, resulting in a corresponding system failure.
For pumping applications with the described requirements, the use of (reciprocating) process diaphragm pumps instead of multi-stage centrifugal or high-speed centrifugal pumps is normally the more energy-efficient and cheaper solution in the long term.
Efficiency of reciprocating positive displacement pumps
Units of this type achieve a high efficiency for quality grades (primarily a measure for the gap losses) between 0.98 and 0.99, whereby the efficiency loss through friction is usually negligible. When designed properly, these pumps can have an overall efficiency between 82% and 93%. This very high value can be directly traced to the conveying principle on which the pumps are based: The movement of the plunger, which acts hydraulically on the diaphragm, creates an alternating positive and negative pressure in the pump head. In the intake phase, the inlet valve opens and the fluid is drawn in. In the discharge phase, the outlet valve opens and the fluid is pumped into the discharge line. As a result, the energy needed for moving the plunger goes almost entirely into pumping the fluid. The resulting losses due to (internal) leakage and friction are minimal. The operating principle of reciprocating positive displacement pumps can be compared to that of the human heart. The heart also uses intake and discharge phases and heart valves that function as inlet and outlet valves, not to mention positive and negative pressure.
Pulsation analysis as an essential aid
The pulsations that frequently occur in the piping of reciprocating positive displacement pumps often pose challenges to planners and system operators, but can be reliably overcome by taking targeted action. If such a reciprocating process diaphragm pump has been put on the shortlist for an application, then what is referred to as a pulsation analysis should be carried out. This provides information about the proper design of a system with respect to the pressure pulsations and losses in the piping and fittings. Qualified manufacturers of reciprocating positive displacement pumps are on hand with help and advice to support this process as early as the planning phase. Consequently, the pumps and system are coordinated to match each other long before they are purchased, so that ideal operation is guaranteed right from the first startup. In most cases, it is enough to perform a simplified check of the system consisting of the pumps and plant. For complex systems and detailed analyses, numerical computation programs are available that enable complete analysis of the system in accordance with API 674, design approach 2. Incorporating these system analyses early on can ensure that the entire pump system operates reliably from the start.
Numerous system operators base their decision to purchase a new pump primarily on the Capex (capital expenditures). That is why operators looking into the overall life cycle costs of a metering pump system should take into consideration not only the initial investment costs, but in particular also the energy consumption and Opex (operating expenditures) as well as maintenance, potential downtime and resulting production losses.
For example, diaphragm pumps are extremely long-lasting and designed for extremely long periods of operation and minimum downtimes. If the pump is of high enough quality, it can be expected to last multiple decades. Even wear parts such as PTFE diaphragms or valves can—depending on the design and application—be used for up to 20,000 hours, and sometimes much longer. The same is true regarding potential downtime. Depending on the manufacturer and application, diaphragm pumps have an availability of up to 99 percent. This is partially due to the fact that process diaphragm pumps can function without elaborate and expensive dynamic seals such as mechanical seals. Process diaphragm pumps have only one static seal, which is extremely reliable, rugged and durable.
Accordingly, these pump systems feature both relatively low investment costs as well as durability and optimized energy efficiency.