Over several decades, variable speed drive (VSD) technology has undergone major improvements in how it controls and monitors pumps. From pump flow to efficiency, VSDs are essential in improving the performance of centrifugal and positive displacement pumps. In the past, the short life cycle and reliability of VSDs were major concerns. Today, VSD manufacturers estimate a mean time between failures to be greater than 10 years. With the reliability of this technology nearly perfected, it was time VSDs became smarter — adding more functionality and value for customers.
In recent years, VSDs have added sensorless functions, giving users the ability to control the process and provide critical information on pump efficiency and pump health. VSDs have added multipump control to allow for more accurate, safe and reliable pump operations while operating multiple pumps in series or parallel. These technologies allow for improved diagnostics, allowing operators to understand how pumps are operating along with other key performance indicators (KPIs) such as percent of best efficiency, pump flow output and flow economy. With these improvements and the introduction of smart VSDs, the pump industry is seeing improved pump control, more reliable operation and increased accuracy in pump monitoring.
Operators can identify where a pump operates on the pump curve with or without sensors. With suction pressure, discharge pressure and flow transmitters, the user can know whether the pump is within its operating range or in an upset condition based on feedback from external instruments. Without sensors, the user can monitor pump speed and torque, correlate them to pump performance data and achieve similar results. Pump load information is transferred from the pump shaft to the motor shaft, interpreted by the VSD and used to calculate pump performance data. This information can be used to provide sensorless pump control to match process demands in some applications. The relationship between speed, torque and power can be used to monitor pump performance.
Pump-specific algorithms available in today’s smart VSDs can accurately predict where a pump operates on its curve. This helps users identify upset conditions, protects pumps from premature failure and provides information on the pump and pump system’s overall efficiency. Depending on what part of the curve a centrifugal pump is operated, its efficiency changes. If it operates around the sweet spot (a.k.a. best efficiency point), it is highly efficient. If it goes left or right of that sweet spot, the efficiency starts to decrease. Smart VSDs use speed, torque and power data to know where the pump operates on the curve and can be set up to take action based on those factors. However, the drive technology can also vary. For a typical pump running off a less-sophisticated drive, the torque, speed and power feedback may not be as reliable as a more sophisticated drive. For example, scalar drives may command a motor to run at a specific speed but typically do not offer high accuracy levels on speed and torque feedback. On the other hand, sensorless vector drives or direct-torque-controlled drives will command the motor to run at a particular speed and onboard algorithms accurately control speed and torque with high accuracies. VSDs’ high levels of accuracy and repeatability enable translation of VSD data to pump-specific information.
Improved pump control
The latest improvements in smart VSDs factor in the effects of speed on the pump’s hydraulic performance to better control processes. This is accomplished through embedded pump-specific algorithms that utilize information such as torque, speed and power received from the VSD to escalate KPIs specifically related to pumps and pump systems. An example is smart VSDs controlling processes using torque rather than speed. This can result in significantly improved process control compared to traditional speed control when controlling pumps with relatively flat performance curves. Because of the significant advancements in drive technology and the information’s accuracy and reliability, industry professionals can better estimate pump performance parameters such as flow and head solely by utilizing drive output torque and speed data.
Identifying and protecting against upset conditions is essential. If properly set up, traditional VSDs can offer protection against upset conditions like dry run, minimum flow and cavitation with feedback from external instruments. Extrapolating speed and power data, smart VSDs can calculate pump health information and protect against upset conditions without external process instrumentation. By determining the pump operating state, smart VSDs can quickly recognize and respond to upset conditions to prevent catastrophic failure. As an example, seal failures caused by dry run are some of the most common failure modes in centrifugal pumps. They are often a result of changing system conditions, operation outside the recommended range or failures of instruments like level transmitters or flow switches. Smart VSDs embedded with pump-specific logic can provide protection against such failures independent of these instruments.
Balancing multiple pumps in a system can be a challenge. No two pumps are identical and a slight difference is inevitable because of manufacturing tolerance variations. These variations can include wear ring clearances, impeller geometries, volute throat area, surface finishes, etc. Traditional multipump systems can be difficult to control and are often manually controlled, leading to uneven loading and wear. For example, when multiple pumps are present in a system, they can be run at the same speed but they may not deliver at the same flow and pressure. This often causes pumps to work against each other, causing inefficient or unreliable operation.
To reduce initial investment, multipump systems are at times controlled with a VSD on one pump while operating the other pumps in the system at fixed speeds. This is not ideal since the pumps are not truly balanced. A pump at full speed generates higher pressure and flows than the pump running and controlled by the VSD at reduced speed.
The next most common way of controlling multiple pumps is running pumps at the same speed. To work optimally and efficiently, all pumps must have similar performance and be operated in a symmetrical system where each one experiences similar system resistance and back-pressures. In a practical environment and with a dynamic pump system, balancing these factors can be difficult.
The third way is by using smart VSDs that can balance the flow output of each pump regardless of pump performance or system symmetries. This allows multiple pumps to work together optimally and decreases excess energy in the system. Ultimately this balance helps prevent failures such as seal damage, mechanical shaft damage, vibration, high temperatures and other issues that are often a result of pumps operating against each other. Multipump control on smart VSDs automatically sequences pumps to match the demand and balance load evenly. It also provides automatic staging and destaging to only run the pumps necessary to most efficiently meet demands while ensuring pumps systems are balanced, leading to increased operating efficiencies, better reliability and lower costs.
What is next in VSD technology?
With the many benefits of VSDs, many challenges must be solved. Retrofitting VSDs to existing pumps and motors is one of these challenges. Harsh environments, hazardous conditions, installation constraints and motor suitability can prevent electrical equipment like VSDs from being used. With new technology and upgrades to VSD systems, the industry should expect more cost-effective, safer and easier solutions. Greater adoption of VSDs on different types of pumps and applications and further computation of VSD output data to optimize overall system performance, predict equipment behavior and improve process control can also be expected.
Adarsh Iyengar is ITT PRO Services‘ global director for monitoring and controls. In 14 years with ITT, he has focused on developing and implementing variable speed pump solutions for pump applications across many industries including water, oil & gas, power, mining, pharmaceuticals and general industry.