There’s a problem with the three-impulse control drum; urgent solution needed!!!!
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The steam used in my company is produced by drum boilers, but currently we use three-parameter control to regulate the liquid level in these boilers. In 2008, this control method worked quite well (I wasn’t working at the company during that time), but this year the liquid level has been fluctuating significantly – the range for the liquid level is 0-600 mm, with 300 mm being the set point. This year, the amplitude of these fluctuations is around 100 mm. The three impulses for the drum in our company are level control, control of the desalinated water flow entering the economizer, and control of the steam flow exiting the drum. The desalinated water pipeline leading to the steam drum is 30 meters long; the desalinated water is pumped in using a pump, and a pneumatic control valve is used to regulate the flow rate. However, this control valve is located more than 20 meters above the steam drum in vertical distance. This is basically how the drum is controlled, but does anyone know why there are problems with the drum? Is the problem in the system logic? Does anyone know what disturbances cause such large fluctuations in the steam drum? How to deal with it? Why is this approach taken? ? This post was last edited by Mobei Yihai on 2009-4-18 09:12.]
Since control was good last year, it is recommended that the poster look for the fault from the following aspects: 1. Check the level transmitter; large fluctuations in liquid level are likely to be caused by a problem with the transmitter； 2. Have there been any changes in operating conditions? If so, what impacts might they cause? 3. Have the system parameters been changed, for example, have the PID parameters been modified by anyone?
It’s most likely a problem with the valve. Check the valve first.
Based on years of personal maintenance experience, check the following: 1. Control valves, including cylinder leaks and positioners, etc. 2 transmitters: mainly to detect whether there are bubbles in the pressure guiding tube; first shut off the heating steam, and check whether there are heat sources near the pressure guiding tube that could cause the cold water inside the tube to evaporate. 3 PID parameters. The focus is on the scene.
1. recalibrate the level transmitter. 2. Check whether the valve position of the control valve corresponds to 4-20mA. 3. Are the PID parameters of the regulator suitable for the current operating conditions?
Boiler drum level – the measured value of the main loop in the cascade control system; this value is used as the setpoint for the secondary loop via a PID output. Feedwater flow rate – the measured value of the secondary loop in the cascade control system; this value is used to control the feedwater control valve. Steam flow rate – a feedforward signal that, depending on the algorithm used, affects the cascade loop, thus forming a three-element control system. The poster failed to mention an important aspect: does this phenomenon occur when the three-element control system is in automatic mode, or does it also happen in manual mode? 1. If the level fluctuates so greatly even when operating in manual mode (or by switching to manual control for observation), first suspect that the signal is interfered with; check the input signal to see if there is any alternating current component present – use a multimeter for this. 1.2 Observe the local two-color level gauge or electric contact level gauge installed on the steam drum. 1.3 In larger boilers, in order to prevent uneven heating, usually one level transmitter is used on each side – a total of 2 transmitters; one of them supplies the signal to the control circuit, while the other is used for the recording instruments ； Observation recorder; swap these two signal paths if necessary. 1.4 Check the dual-chamber balance container, using methods such as draining and emptying it ； If it doesn’t work, the level transmitter needs to be checked or calibrated again. 2. If it is automatic, 2.1 observe the output of the regulator – that is, the output controlling the feedwater control valve – to see if it also exhibits this periodic pattern ； Even if not, it is still necessary to go to the valve chamber to check this valve, as well as consider all possible external issues such as surge, faults in the feedback mechanism, bypass leaks, and problems with the feed water pump piping system. 2.2 Measure the signals for feed water flow and steam flow, and check whether they match the readings on the instruments on site and whether there are any fluctuations. Based on the measurement results, inspect one of the instruments installed on site ； I won’t go into detail about the inspection methods. It is still necessary to suspect a problem with the program in version 2.3; as a year has passed, it’s possible that changes were made to the process parameters and pipeline conditions during maintenance, which may have rendered the original three-parameter program unsuitable for the new operating conditions. If necessary, a signal generator can be used to replace the steam flow signal and the feedwater flow signal in order to verify one’s hypotheses
What specific impact does persistent and stable internal leakage in the bypass of the feedwater control valve have on liquid level fluctuations???
A What specific impact does persistent and stable internal leakage in the bypass of the feedwater control valve have on liquid level fluctuations? b If the feedwater pipe is located far away from the steam drum, is there a pure lag element? If so, how can it be addressed? c If the drum blowdown rate is higher than before, what impact will this have on controlling the drum liquid level? d If the steam consumption is higher than before, would it also be difficult to control the drum liquid level?
1. Check whether the PID parameters of the regulator are appropriate. 2. See if there are significant changes in the feedwater flow rate. 3. Determine if there are any changes in the flow characteristics of the control valve; a slight leak under high load is not a problem, but it’s hard to say the same for low loads. B. As long as it’s not too long, there’s no issue – there’s already some lag in the liquid level, so adding a differential term can help. C. The blowdown rate from the steam drum has increased significantly, which adds to the load. D. If the steam consumption is high but does not exceed the limit, it will not affect the liquid level in the steam drum