To keep boiler drum water level in an acceptable range is an important safety guideline for boiler operation. In recent years, accidents have occurred frequently due to boiler drum level measurement and control problems. These can seriously affect the safe operation of coal-fired power plants.

At present, there are various security risks for water level measurement system in most power plants. There are some problems that trouble the power plants continuously, such as significant measurement errors, high deviation between different types of measurement gauges, protection systems being unable to activate in time, etc.

Qinhuangdao Huadian Measurement & Control Equipment Co., Ltd. (HDSC) has improved the traditional DP-type water level gauge and connected vessels water level gauges (include mica water level gauge and conductivity electrode type water level gauge). With our products, the accuracy has been improved and the safety risk reduced. Our products have been applied successfully in more than 400 different capacity boilers, and the effects have been remarkable.



There are two kinds of drum level gauges in theory. The one is connected vessels water level gauge, and another is DP-type water level gauge.

The traditional connected vessels water level gauge (such as mica water level gauge and conductivity electrode-type water level gauge) measures water level by leading water inside the drum to the outside. The water temperature in the water gauge column is lower than the water temperature in the drum because the temperature outside the drum is lower than temperature inside the drum. So the measurement error is significant and will change according to drum running condition and environment, etc.

The expression of measurement error for connected vessels water level gauges is as follows:


ΔH= H'–H=−   (ρa–ρw) H



H         -           Actual water level of drum

H'        -           Indicated value of water level gauge

ρs        -           Density of saturated steam in drum

ρw       -           Density of water in drum

ρa        -           Average density of water column (affected by environmental
                        temperature and drum running condition)


HDSC has developed new WDP Series bi-color water gauge glass and GJT Series conductivity electrode-type level indicators. HDSC has improved the water gauge structures of the traditional water gauge. It assembles a heating cycle system in the internal area of the recorder, which means the temperature of internal water at the gauge could be nearly equal to the temperature of internal water of boiler, and the height of water level could be nearly same as the real water level, so the measurement result would be correct and reliable.

The DP-type water level gauge measures water level by transforming the change of water level to the change of pressure to send the signal to transmitter.

The expression of water level is follows:


H=L     (ρa–ρw) g −ΔP

            (ρa–ρs) g


ρa        -           Density of water column

ρw       -           Density of saturated water
                      in drum

ρs        -           Density of saturated steam in

H         -           Actual water level of drum

The water column density of traditional DP-type water level gauge is difficult to measure, and its density changes according to environment temperature. And the water in drum is non-saturated, but it is often calculated as saturated water so that error happens inevitably.

HDSC has improved the structure of DP-type water level gauge by moving the reference water column to the internal of the drum from the exterior. This movement avoids the influence of environment temperature to water level measurement and also avoids the influence of under-saturated boiler water to the measurement. So the measurement result could be seen as accurate and reliable, and the new DP water level gauge could be seen much better than traditional DP water level gauge.


A power plant owned by The Huaneng Group has installed HDSC water level gauges. Photos 1 and 2 illustrate the water line comparison before and after improvement.

The error measured by traditional water level gauges is significant. The power plant was always operated at a higher water level status, creating hidden dangers such as an unplanned boiler stoppage, running with full water or a lack of water, etc.

The in-operation water line of the drum after improvement is nearly in accordance with the normal water level. It is obvious that the measurement of every improved drum level gauge is correct and reliable. With these gauges, the hidden dangers of drum level measurement and protection systems can be avoided.

With the technology and new gauges of HDSC, the results of different drum water level gauges are almost the same throughout the whole range during boiler operation. The following diagram is one of the cases. From Photo 3, we can see that the curve of different gauges by different principles is coincident in the entire measuring range.  TB