The increasing demand on pressure and displacement capabilities puts high stress on the pumps’ power-end components. This is partly caused by the shale gas revolution, which requires pumps to deliver highly pressurized water and chemicals. These increased loads result in short mean time between failures. One of the components that is highly loaded is the crankshaft. Although some incremental innovations have been introduced to resolve this problem, manufacturers have not found a complete solution because the key weaknesses of the common crankshaft have not been addressed. This may change, however, because of a recently invented mechanism that resolves these weak spots and provides other advantages.
The Need for New Technology
In virtually all mechanical devices in which rotary motion is converted into reciprocating motion or vice versa, an eccentric mechanism or a crankshaft mechanism is used. However, based on mechanical strength, the current mechanisms have two key weaknesses. The first is the occurrence of stress concentrations anywhere a change in diameter in the crankshaft or eccentric occurs. These stress concentrations could lead to fatigue failures, mainly because of high shock loading, which occurs in reciprocating pumps.
The second weakness is the wrist pin bearing. The limited available wrist pin bearing surface area makes this the highest loaded bearing in the pump. This eventually restricts the maximum allowable connecting rod load. Although these dominant mechanisms have worked well during the past decades, today’s excessive loads in the field require a different mechanism without these limitations. One company that develops new energy conversion systems required a light crankshaft that was able to withstand extremely high loads. The solution for this requirement is a new patented mechanism, the efficient motion converter (EMC), which can be beneficial in handling heavy loaded reciprocating pumps as well.
The heart of this new motion converter consists of a modular eccentric sheave that is mounted on a splined shaft. Unlike in a traditional crankshaft, the eccentric movement is not transferred through a connecting rod. Instead, this movement is transferred via a bearing housing to two rods. The other outer end of the hinging rods is connected to a reciprocating member that moves in a pure reciprocating manner. On one or both ends of the reciprocating member, a crosshead or other component can be connected, which requires it to move in a reciprocating way.
The two rods that transfer the plunger load to the bearing housing point in opposite directions. This means that the radial forces, to a certain extent, cancel each other and reduce the resulting crosshead load significantly. To illustrate, normally the maximum radial load on the crosshead is approximately 20 percent of the plunger load. In an EMC mechanism, the maximum is 7.1 percent of the plunger load and can be reduced even further. This is obviously beneficial because the frame load and crosshead friction losses are also proportionally reduced. The most important feature of this mechanism, however, is its unprecedented mechanism strength. Initial strength analysis shows that similarly weighted EMC alternatives for existing pumps reduce peak stresses significantly. The maximum allowable load is predominantly determined by the maximum allowable bearing load, not by the mechanism itself. Peak stress concentrations are eliminated because of three fundamental differences compared to traditional mechanisms. The first is that no critical edges exist in which stresses concentrate and which are prone to fatigue failures under a high shock load. Second, the modularity of the mechanism allows for the main bearings to be placed between the cylinders whereby the bending moment is greatly reduced. To illustrate, placing the bearings in this way can reduce the maximum bending moment by a factor of 2.5. Stresses caused by the bending moment are reduced proportionally. The third strength improvement, which is only valid compared to a crank, is that torsional rigidness is improved because the EMC mechanism does not have webs, which normally reduce torsional rigidness.
In the cases under investigation by the company, the maximum allowable connecting rod load is mainly limited by the maximum allowable bearing load. Normally, this load would be at the bearings at the pivotal points. However, much flexibility is available for increasing the bearings up to a desired load level. To illustrate, the smallest bearing surface area is easily 50 percent greater than the smallest bearing surface in a normal crank. While the wrist pin bearing in a normal crank is restricted by the diameter of the crosshead, these wrist pin bearings can be chosen for the desired safety level. While the expectation may be that the added bearings will introduce additional friction, which would lead to a low mechanical efficiency, they do not. Extensive calculations and simulations show that the mechanism is 0.4 percent more efficient compared to a traditional crankshaft while it is 1.06 percent more efficient compared to a conventional eccentric mechanism. This is mainly because the circular crosshead is loaded far less and because a linear bearing is used instead of crosshead guides. Linear ball bearings have lower friction losses compared to a normal crosshead configuration and allow for more effective lubrication.
The new mechanism allows for some interesting configurations which were not previously possible with a traditional crankshaft. Increasing flow rate in current reciprocating pumps can be accomplished by adding more plungers in a row or by increasing the plunger diameter, the plunger stroke or pump speed. All these options are limited because of hydraulic, mechanical or physical size limitations and often result in an excessively large and heavy pump.
The new mechanism, however, allows for a boxer configuration, which means that an engineer can double the displacement with a relatively small increase in system size. This increase is not reached by adding more plungers in line, which means that crankshaft length is not increased and, therefore, bending problems do not occur. Another interesting configuration is the use of the mechanism in a dual role between a combustion cycle and a pump cycle. This dual role means that on one side of the mechanism work is added by an internal combustion engine cycle while on the other side work is retracted by a reciprocating pump configuration. In this configuration, the forces induced by the combustion pressure are directly transferred to the piston/plunger that is being used for pump purposes. Therefore, a minimal residual force is transferred to the rotating members. A more efficient work transfer is impossible.
The main benefit of this configuration is that, because of the direct transfer of forces, the residual forces on the bearings and frame are decreased significantly. For example, combining a two-stroke Otto cycle with a pump cycle can (theoretically) reduce the loads on the bearings by as much as 40 percent. Also, no drive motor and transmission are required anymore, which has a significant positive influence on the overall efficiency, simplicity and reliability. In addition, the total size and mass of the system decreases.
The mechanism provides other advantages as well. One of these advantages is the lower acceleration during top dead center. A traditional crankshaft has significantly higher acceleration during the top dead center (TDC) compared to the acceleration at bottom dead center, which is about 60 percent of the TDC’s acceleration. In the EMC, the TDC acceleration is equal to the acceleration at bottom dead center. This means that acceleration during TDC is lower with an EMC than with a traditional crank (15 percent measured). This might be beneficial when powering a reciprocating pump because rotational speed is partly limited by the potential for cavitation at the inlet valve opening. A lower acceleration during TDC lowers the risk of cavitation and allows for a higher rotational frequency, which leads to a higher displacement.
Another positive feature of the EMC is ease of maintenance and repair. The modularity of the mechanism allows for rapid and targeted disassembly. The whole mechanism does not need to be dismantled if one part, such as the crosshead, must be replaced. Also, the EMC’s production process is less time and capital intensive compared to that of a traditional crankshaft. The intensive production process of a traditional crankshaft involves the main and rod/bearing surfaces being ground to a high surface finish because the crank pin sleeve bearings are mounted on the crankshaft. Therefore, the crankpin needs to have a certain wear resistance and surface finish. This requires heat treatments and careful grinding. This process is not required during EMC production because no bearing surfaces are present.
In the EMC, no critical stress concentrations exist that require a high-precision and high-level surface finish. As mentioned above, the EMC also does not have bearing surfaces that require a certain hardness and/or surface finish. Only a standard sleeve is used on the crosshead. The additional benefit of this configuration is that wear only affects the bearings, not the core components. This means that these components can be used for an infinite time. Only bearings must be replaced. First analysis indicates that the patented EMC is beneficial for heavy loaded reciprocating pumps. Further testing and investigation is required regarding topics such as lubrication (for example, can we use for-life greased bearings, which prevent the need for oil?) and vibration. The current testing/investigation has focused on the energy conversion processes.