ater . ▁ ▁Dis adv antages ▁of ▁high ▁pressure ▁ ▁Complex ity ▁High - press ure ▁locomot ives ▁were ▁much ▁more ▁complicated ▁than ▁conventional ▁designs . ▁It ▁was ▁not ▁simply ▁a ▁matter ▁of ▁building ▁a ▁normal ▁fire - t ube ▁bo iler ▁with ▁suit ably ▁increased ▁strength ▁and ▁st oking ▁harder . ▁Struct ural ▁strength ▁requirements ▁in ▁the ▁bo iler ▁shell ▁make ▁this ▁imp ract ical ; ▁it ▁becomes ▁impos s ibly ▁thick ▁and ▁heavy . ▁For ▁high ▁steam ▁press ures ▁the ▁water - t ube ▁bo iler ▁is ▁univers ally ▁used . ▁The ▁steam ▁drums ▁and ▁their ▁inter connect ing ▁tub es ▁are ▁of ▁relatively ▁small ▁diameter ▁with ▁thick ▁walls ▁and ▁therefore ▁much ▁stronger . ▁ ▁Sc ale ▁de position ▁The ▁next ▁difficulty ▁is ▁that ▁of ▁scale ▁de position ▁and ▁cor ros ion ▁in ▁the ▁bo iler ▁tub es . ▁Sc ale ▁depos ited ▁inside ▁the ▁tub es ▁is ▁invisible , ▁usually ▁in access ible , ▁and ▁a ▁dead ly ▁danger , ▁as ▁it ▁leads ▁to ▁local ▁over he ating ▁and ▁failure ▁of ▁the ▁t ube . ▁This ▁was ▁a ▁major ▁draw back ▁with ▁the ▁early ▁water - t ube ▁bo ilers , ▁such ▁as ▁the ▁Du ▁Temple ▁design , ▁tested ▁on ▁the ▁French ▁Nord ▁network ▁in ▁ 1 9 0 7 ▁and ▁ 1 9 1 0 . ▁Water ▁tub es ▁in ▁Royal ▁Navy ▁bo ilers ▁were ▁checked ▁for ▁block age ▁by ▁carefully ▁dro pping ▁number ed ▁balls ▁down ▁the ▁cur ved ▁tub es . ▁ ▁Saf ety ▁concerns ▁A ▁sudden