We encountered an issue where the tank level unexpectedly rose due to backflow from the system it normally supplies. To prevent this from happening again, we aim to implement a logic that will automatically close the tank discharge valve when the tank level starts to increase beyond the desired threshold. Usually, the tank operates by discharging to the system while the fill pump is turned off. When the tank level reaches 45%, the fill pump kicks in to fill it up before shutting off. Subsequently, the tank discharge goes down to around 12.5% before the fill pump starts the cycle again. Our goal now is to ensure that the valve closes if the tank level rises while the fill pump is inactive. One solution I have considered is to monitor the tank level every 5 seconds and compare it with the previous reading. However, I am open to suggestions for a more efficient and effective method to achieve this task. Currently, I am utilizing RSLogix 5000 for this project.
The reliability of the level signal during regular operation plays a crucial role in avoiding false alarms. It is important to monitor any slight fluctuations to prevent inaccuracies, especially when the fill valve is not in use. The approach you outlined for checking the sensor's accuracy is thorough and efficient.
In a recent scenario, a tank experienced an unexpected increase in level due to backflow from the system it normally supplies. To prevent this, we are looking to implement logic that will automatically close the tank discharge valve when the level is rising, despite the fill pump being off. Typically, the tank operates by discharging to the system while the fill pump remains inactive. Once the tank reaches 45% capacity, the fill pump kicks in until the tank is filled, and then stops. Subsequently, the tank discharges down to about 12.5% before the fill pump resumes its operation. Currently, the strategy involves monitoring the tank level every 5 seconds and comparing it to the previous reading. However, a more efficient approach may be needed. Working with RSLogix 5000, it is vital to consider a deadband to avoid false alarms. Moreover, simply relying on the previous change in level may lead to overfilling the tank. To address this, it is recommended to calculate a rate of change value over time and set alarms based on that. This method helps account for cumulative errors and fluctuations around the current level, ensuring a more accurate monitoring system.
What is the duration of the level adjustment process? How fast does the level change in different scenarios, such as with the discharge valve open or closed and the fill pump running or turned off?
By sampling at a rate of 10Hz and averaging over a 5-second period, you can reduce noise by approximately a factor of 7, assuming it follows a Gaussian distribution. Utilizing a least squares regression or linear fit on the most recent X seconds of samples in a First In, First Out (FIFO) basis may also help diminish noise compared to simply calculating differences between consecutive pairs of samples.
Are there any underlying process issues related to material backfeeding back into the tank, such as contamination or temperature fluctuations? During normal operation, does the tank level ever exceed 45% and reach 50% when the pump is turned off? Is there a concern if the level reaches 50%? If none of these scenarios are problematic, simply close the valve when the tank reaches 50% (or any number above 45%). While implementing Rate of Change (ROC) control is beneficial, it is also advisable to have an absolute shutoff in place. The reopening of the valve should be triggered by the operator. Once the valve is closed, the ROC will return to zero, and the system will either resume normal behavior (with the level not increasing) prompting the valve to reopen, or it may appear that the tank is not emptying as expected and the valve will remain closed, depending on whether zero ROC is deemed favorable or unfavorable.
From a reliability standpoint, it's great you're considering proactive measures to avoid system backflow. Your proposed monitoring solution seems to be a good start. However, remember that the frequency of your checks (every 5 seconds) might be overwhelming depending on your processing speed. With RSLogix 5000, you might use a PID (Proportional-Integral-Derivative) control loop in tandem with the valve to maintain tank levels more efficiently. This setup automatically adjusts the valve based on changes in level and aims to keep the error (the difference between the set level and real-time level) minimum. Furthermore, I'd recommend ensuring there are alarms or prompts for abnormal level conditions to alert operators for manual intervention, if needed.
I agree that monitoring the tank levels is a crucial first step. However, continuous monitoring with short intervals like every 5 seconds might put unnecessary load on your system. You might want to consider implementing a rising-edge detection logic. With RSLogix 5000, this can be easily done with PIDs or timers. Associating these with a rate-of-change tag that triggers the valve closure when the tank level rises unexpectedly can be a potential solution. This way, you're not constantly checking, but only when significant changes occur. Nevertheless, it's important to thoroughly test this kind of solution to ensure that it reacts effectively to backflow situations without any false alarms.
Your approach sounds reasonable, but to make it more efficient, perhaps you can consider utilizing a differential pressure transmitter on the tank. It would provide a continuous reading that could be directly integrated into your RSLogix 5000 program. This reading would trigger the valve to close if the tank pressure rises beyond the specific threshold, indicating a backflow. That way, you would not need to do manual checks every 5 seconds and reduces the need for constant attention to the system. Also, include an alarm system to alert operators about the situation for immediate action. This may simplify the automation of the process and eliminate any time lapse.
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Answer: The tank level unexpectedly rose due to backflow from the system it normally supplies.
Answer: Implementing a logic that will automatically close the tank discharge valve when the tank level starts to increase beyond the desired threshold.
Answer: The tank operates by discharging to the system while the fill pump is turned off. When the tank level reaches 45%, the fill pump kicks in to fill it up before shutting off. Subsequently, the tank discharge goes down to around 12.5% before the fill pump starts the cycle again.
Answer: Monitoring the tank level every 5 seconds and comparing it with the previous reading is one method being considered. RSLogix 5000 is being utilized for this project.
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