Frictional　pressure　drop　is　determined　by　either　subtracting　hydrostatic　head　from　total　pressure　drop　according　to　momentum　balance,　or　subtracting　potential　energy　term　from　total　pressure　drop　according　to　energy　balance.　The　divergency　and　consistency　between　momentum　balance　and　energy　balance　are　discussed　for　two-phase　flow.　In　vertical　bubbly　and　intermittent　flows　at　high　gas–liquid　ratios,　the　experimental　results　indicate　that　the　frictional　pressure　drops　from　energy　balance　are　significantly　greater　than　those　from　momentum　balance.　It　is　found　that　liquid　holdup　leads　to　the　increase　of　hydrostatic　head　in　the　momentum　equation　of　gas–liquid　two-phase　flow.　However,　the　increment　in　hydrostatic　head　turns　to　be　a　part　of　irreversible　pressure　loss　instead　of　potential　energy.　At　high　gas–liquid　ratios　from　500　to　10000,　up　to　99　percent　of　the　hydrostatic　head　does　not　turn　into　the　potential　energy　and　is　thus　not　recoverable　in　vertical　bubbly　and　intermittent　flows.　©　2021　Elsevier　Ltd