Downcomer pipework

To a good approximation, we may treat the subcooled region as having a constant volume of the order of 10% of the liquid volume in the drum. It may be treated as separate from the equilibrium thermodynamics of the steam drum. It may be regarded as an extension of the downcomer, with its volume added to that of the downcomer pipework in order to work out a downcomer time delay. 

The subcooM region is modelled separately along with the downcomer pipework. As far as the drum model is concerned, its sole contribution comes in determining liquid level, where it adds a constant term to the liquid volume. As mentioned previously, the subcooled region may make up typically about 10% or less of the water volume of the drum, and so its effect on steam drum dynamics is second-order only. 

In view of the complexity of the pipework, it was difficult to prejudge the likely accuracy of the overall prediction. Total flows were measured by Dali tubes situated in the main downcomers to the pumps over a range of thermal powers ranging up to 300 MW. The predictions were within the scatter of the measured points. At full power the comparison was as shown in Table 3. 

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