In power plant boilers the water quality is controlled within the range of 9-10 pH and the drum conductivity is about 10uS/cm (micro siemens). With these conditions problems due to sediment and sludge deposits do not occur on a properly designed external chamber where condensate flow is purposely stimulated to reduce density errors. With these conditions there is no necessity for periodic blow downs of the external chamber, as in a gauge glass. Due to the condensate flow the external chamber water conductivity is about 1uS/cm.

Ways of reducing density errors include insuring that the vessel is installed following the directions in the installation manual and lagging the steam and water piping, this helps to maintain the temperature inside the vessel.


Within a vessel the lower half normally contains water and the top half steam. As the steam condenses it is replaced by steam from the boiler, giving a continuous supply of constant temperature. Because of this the heat loss in the portion of the water column containing steam is minimal. Heat loss in the water section is greater. The object is to reduce this loss to a minimum, lagging of piping contributes to a reduction, but the design of the vessel can greatly reduce heat loss. Heat is transferred to the vessel walls from the steam; this in turn will help maintain the water temperature. The design of the vessel can greatly affect this heat transfer. With larger cross sectional areas the resistance to heat transfer is reduced.

It should be realised that as more heat is taken from the steam, the flow of condensate will increase. Thus if the total heat loss is constant, but the proportion taken from the steam is increased, the cooling of the water column is reduced by two factors- the lower heat abstraction and the increase in flow.
EDLI- Density Error Compensation.

By promoting more condensate flow recirculation, the temperature differential between drum & side- armed Pressure vessel is minimized and density error is thus maintained within acceptable  accuracy. This is valid for all  pressure/ temperature configurations. The error compensation due to density variations is not separately needed in the electronics/ microprocessor ¬†for the  EDLI.

Calculation for error on density variation:

                                            H2( D3- D2)
                                           ( D1 -  D3)           

H1 = Level error                  D1 = Steam  density
H2= Level in Pr. vessel       D2= Water density in Pr. vessel.

H1 + H2 =  H3                     H3= Level in drum.              D3=  Water density in  drum