Load Cell for Total Valve
The flow of high pressure and extreme temperature fluids in a system is often controlled by a series of total valves that ensure that the flow rates are maintained to the required specifications. However, over time of continued use, the valves may experience tear and water that would cause them to malfunction and endanger the low processes in the systems in which they have been installed. Often, valves malfunction when their either allow fluid leakages or their opening and closing runs out of synchronization to the flow system they are intended to control (Osterman 621). To this end, companies undertake regular services of their valve systems as part of their asset management strategies to ensure that the systems remain in optimum operation and the maintenance downtimes are scheduled as well. Unfortunately, the costs incurred on maintenance and replacement of valves is high, reaching over 6 billion dollars annually in the oil and gas industry for example. In addition, valve malfunctions can cause plant downtimes that are unplanned in addition to lengthen the period of outages that have been scheduled. Therefore, the common problem has continued to be the inability to detect the defects and malfunctions of the valves in a system early enough to ensure that the operations and efficiency of the systems in which the valves operate are not compromised. Therefore, valve-monitoring mechanisms that can detect valve defects and malfunctions well in advance are necessary because of their potential for enhancing the safety of the fluid systems and reducing the operations costs associated to valve failure. Various approaches to the problem include load cells, solenoid valves, and servo valves. This discussion dwells of the load cells as the solution to the problem related to untimely breakdown and malfunction of a total valve, and justifies why the choice was preferred over the proportional control and servo versions of valve control systems.
The two alternative approaches of controlling valve systems include the proportional control valve and the servo valve systems. The proportional control valves have the advantage of being infinitely variable and cost effective because of their lower prices in relation to those of other valve mechanisms. However, their limitation is in their inability to be responsive of feedback related to flow parameters or handle miniscule changes in flow in a prompt and accurate manner. Contrastingly, servo valves have the advantage of being responsive and capable of operate within rapid changes in flow accurately (Minh Quyen). However, their cost is prohibitive compared to that of proportional control valves. The two valve systems also differ in the methodology employed for provision of feedback, which aims at ensuring that the actuator is operating under the instructions of the controller although they use a solenoid actuator (Omega Engineering).
Load cells, also known as load sensors, have been preferred in this case because of their ability to provide accurate and precise measurements of fluid forces, thus triggering the valve body to open and close optimally while providing predictive information that can be used to activate valve maintenance proactively before the system becomes compromised (Moreau 587). A load cell is an electrical device that is able to convert mechanical force into an electrical signal as the output, which in this case can be used to provide real time information regarding when the valve operations deviate away from their expected operational parameters. As such, a load cell acts as a transducer for converting a mechanical property to an electrical signal that can be obtained remotely over a signal readout (Loadstar Sensors). The basic design of a load cell is illustrated in figure 1.
Figure 1. The basic structure of a load cell
The load cell fits in the flow system with the valve and electrical components being housed therein. The load cell of often powered by 10 volts and provides a readout in the range of millivolts. In addition, resistance generated by the load cell is usually 350 ohms although it may range between 120 and 1000 ohms depending on design (Loadstar Sensors).
The signal output of the load cell can be in form of current change, voltage change, or frequency change, which depends on the design and type of the load cell and its electrical circuitry. On the other hand, the load cell is designed to sense and respond to compressional and tensional forces as well. As such, the two main classes of load cells include the resistive load cells and the capacitative load cells. The operation principle for the load cells is based on piezo-resistivity in which the load force, in form of a stress, strain or shear, is converted to a change in resistance when the load is applied to the sensor. Contrastingly, the working of a capacitative load cell is based on the principle of changing capacitance depending the amount of charge when a sensor has a voltage applied to it (Loadstar Sensors). In both cases, the sensitivity to changes in resistivity or capacitance is used to provide an electrical feedback that can be used to control the valve system.
In addition, the load cell enables the management of pressure and fluid systems that contain numerous valves that would challenge the monitoring and maintenance efforts. Further, load cells tend to be cost effective operationally because they do not contain any moving parts therein, thus eliminating the problems caused by wear and tear over time while their detection mechanism can be automated, further improving the efficiency of the system.
Moreover, the load cell can be configured to provide an audible or visible alarm as an early warning system that can provide information related to the malfunctioning of the valve in good time thus allowing for synchronized and well-managed implementation of valve maintenance or replacement. The sensitivity of the load cell is pertinent for the protection against cavitation in the flow system, which can compromise its integrity and shorten its lifespan, thus increasing operational costs in the long term. The sensitivity of a valve can be able to detect low amounts of cavitation and through its feedback, adjust the flow parameters accordingly to minimize these detrimental effects without compromising the operations of the flow system in terms of its efficiency and accuracy (Osterman et al. 622).
Load cells comes in five major configuration , which include the S-type, the button load cell, the canister configuration, the sheer type and the beam type. The differences in design of the load cells underpin their configuration. Those that are based on direct stress are mounted either longitudinally or transversely to the incident load while others employ a cantilever design of a binocular design (Sherborne Sensors).
The load cell has been preferred for its high level of safety compared to the other alternatives available in industry. Specifically, the hardiness and longevity of the load cell contributed to its safety and the safety of the valve systems it controls. As such, it can be employed in rugged flow systems that are at a high risk of compromising the safety of operators and the flow systems in which they have been installed.
Conclusions and Recommendations
Valve systems are vital components of flow systems because of their ability to vary flow depending on the requirements of the equipment and it operations. Of the valve controls systems available, the load cell valve control system was preferred to the proportional control valve and the servo valve systems due to its hardiness, high sensitivity, and simple operation mechanism. This renders the load cells cost effective and able to provide automated feedback that can promptly inform about valve integrity thus enabling the proactive and timely detection of malfunctions of a valve system. This allows for scheduling of maintenance of valve systems without leading to disruptive and expensive unscheduled down times that would compromise operational efficiency.
However, the continuous calibration of the load cells is imperative to ensure that their operation remains within the acceptable range. With the advancements in technologies, particularly wireless technologies, taking root in industry, the ability for load cells to transmit their feedback to system controllers remotely would improve their applicability in complex flow systems that contain numerous valve systems. In addition, their ability to utilize the enormous amounts of data available in the industry can be advanced by leveraging web technologies and wireless communications, which can be used to push the automation of valve systems a notch higher.
Le, Minh Quyen, Minh Tu Pham, Richard Moreau, Jean Pierre Simon and Tanneguy Redarce. “Force tracking of pneumatic servo systems using on/off solenoid valves based on a greedy control scheme.” Journal of Dynamic Systems, Measurement, and Control, vol. 133, no. 5, 2011, pp. 054505.
Loadstar Sensors. “What is a load cell? How do load cells work?” Loadstar Sensors, 2017, http://www.loadstarsensors.com/what-is-a-load-cell.html. Accessed 18 November 2017
Mcilavaine, Bob. “The valve market: the oil and gas industry will pay $6 billion for Total Valve Solutions in 2015.” Mcilvaine Company, 2 June 2014, http://www.mcilvainecompany.com/Decision_Tree/subscriber/Tree/DescriptionTextLinks/mcilvaine%201.pdf Accessed 18 November 2017
Moreau, Richard, M.T. Pham, M. Tavakoli, M.Q. Le and T. Redarce. “Sliding-mode bilateral teleoperation control design for master–slave pneumatic servo systems.” Control Engineering Practice, vol. 20, no. 6, 2012, pp. 584-597.
Omega Engineering. “Technical principles of valves.” Burkert Controlmatic Corporation, 2017,
https://www.omega.com/techref/techprinc.html. Accessed 18 November 2017
Osterman, Aljaž, Marko Hocevar, Brane Širok, Matevz Dular. “Characterization of incipient cavitation in axial valve by hydrophone and visualization.” Experimental Thermal and Fluid Science, vol. 33, no. 4, 2009, pp. 620-629.
Sherborne Sensors. “Load cells overview.” Sherborne Sensors, 2017, http://www.sherbornesensors.com/international/pages/standard/load. Accessed 18 November 2017
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