108.Whether the drum level between the transmitter, direct level gauge and electronic level monitors will be same on load? If no, what is the reason and how the level will be indicating?

The DP transmitter measures the difference of force ( due to pressure + mass of the liquid/steam) applied at the two ports. When used to measure level, the same shall be corrected for any change of Density of the liquid in the legs. When drum level is measured with DP transmitter, the density of water and steam in one port and the reference coloumn water are applied deriving from drum pressure. This is a real time correction applied from 0 to rated pressure of the boiler. Hence the level indicated by the transmitter will be nearer to the actual in the drum for all load.                                   

However, direct level guages and electronic level monitors (like Hydarstep) do not provide this correction. The water in the measuring vessel will be less thatn that of drum. Typically the temperature of the drum will be 340 Deg.C and the level gauge will be 150 Deg.C. The density of drum water will be 0.6 and that in the level coloumn will be arround 0.8. This is known sub- cooling. This will cause a considerable difference in the level between the DP transmitter and the direct level gauge. It can be 120 mm for a boiler of 180 bar drum pressure. Mounting/ insulation of measurement legs on which these devices are mounted will have considerable bearing on measurements of Drum level vis-a-vis actual level in the drum. The direct mounted devices will read always lower level compared to compensated DP Transmitter measurement.