How to eliminate false liquid level in boilers (recommended)

False levels are inevitable, but proper operation can minimize their impact on control!

There are many reasons for false liquid levels to occur during operation, such as excessive alkalinity in the boiler water. The reason for false liquid levels in electric contact level gauges may be poor insulation of the outer cylinder.

During the normal operation of a boiler, the control of the drum liquid level is an important parameter. Excessively high liquid levels can affect the efficiency of steam-water separation, leading to steam carrying liquid droplets; excessively low liquid levels can disrupt the water circulation, and in severe cases, it can damage the boiler. When the steam load increases suddenly, the boiler experiences a temporary pressure drop, and boiling of the water intensifies. This causes the liquid level to rise, resulting in a false liquid level. At this point, the water supply should be increased. However, with a simple single-parameter control system, the boiler’s water supply control valve will be mistakenly closed based on this false liquid level, thereby reducing the amount of water supplied. By the time a new dynamic equilibrium is reached between steam and water, the liquid level has dropped significantly, falling far below the set value, which can even put the boiler at risk. If the load decreases, its variation process and outcome are the opposite of those described above, resulting in large fluctuations in the drum liquid level. Other factors such as changes in fuel volume or evaporation rates can also cause false liquid levels. The main factor affecting the boiler level is the steam balance of the boiler. To overcome the large fluctuations in water level caused by load changes, eliminate the effects of false water levels, and prevent steam from interfering with the water level in advance, in addition to the main water level control loop, two additional parameters are introduced: steam flow rate (as a feedforward signal) and feedwater flow rate (as a measurement signal for the cascade secondary loop), thereby forming a three-element control system for the boiler.

Thank you! For various reasons, we removed three-shot control from this furnace ten years ago. Currently, electric contact pressure gauges are used for controlling the shutdown of the boiler at high pressures, while electric contact level gauges are used to control the start and stop of the water pumps, and both systems are functioning properly. When I took over, I intended to restore three-stage impulse control, but these furnaces were going to be phased out within a year; the management wasn’t willing to invest more money, so there was no choice but to keep things as they were. Today, I checked the electrical contacts and they were in good condition. After performing several sewage discharge operations, the results remained unsatisfactory. I asked the head of utilities, and the water softening tests showed that everything was within normal limits. I would like to ask about the causes and solutions. Thank you!

1. Strengthen the management of boiler water quality to prevent the alarm from failing due to changes in the resistance value between electrodes caused by scaling——If the water is soft, there is no major issue； 2. Leaks from the valve at the bottom can cause the water level to be low ； 3. Check whether the connection valves and pipes between the water level alarm and the furnace drum are blocked.

Thank you! Today, after checking that the valve at the top of the measuring cylinder was closed, all the electrical contacts should have indicated no water present, but the reading only dropped by two levels slowly. This phenomenon occurred again after several attempts at flushing. Moreover, during normal operation, it often happens that after the tank is filled with water, the water level stops dropping after falling by one or two levels; at the same time, the level gauge on the front and rear glass panels no longer shows any indication of the water level. (The front glass plate level gauge is installed in parallel with the measuring tube type.) This post was last edited by min.di on 2008-3-6 23:08.]

Equipment failures lead to a decline in the efficiency of steam-water separation, and the detachment of the top cap of the cyclone separator directly affects this separation efficiency. This should be the direct cause affecting the efficiency of soda separation; excluding instrument failures, I believe it is mainly due to the fact that after years of use, the internal soda separator has failed, and the top cap has fallen off, which has reduced the efficiency of soda separation and caused the liquid level to deviate from normal levels. If these factors have been ruled out, then it is possible that there is a blockage in the pipes or valves connected to the instrument; please conduct a thorough inspection and check all the connections to the steam drum carefully. You have tried adjusting it several times; it was supposed to decrease, but it hasn’t changed now, which indicates that there is a problem with the valve connected to the level gauge. By checking each of these factors one by one, the issue should be resolved soon.

The level of the front glass plate level gauge fluctuates frequently, but that of the rear glass plate level gauge remains stable

It seems to be a fault with the water level gauge itself. Please have the instrument repaired and calibrated. By controlling the steam pressure in the drum, the liquid level can remain relatively stable.

The electrical contacts have also been replaced; the level gauge on the front glass panel fluctuates up and down when little gas is in use