[en] This paper presents two topics on the subject of steam-water slip modelling for flow in a pipe. Primarily, a new slip ratio correlation is proposed which has a special property. Namely, when the correlation is used with the two-phase mixture conservation equations for mass, energy and momentum the resulting partial differential equation set is hyperbolic for all two-phase conditions with pressures greater than or equal to atmospheric pressure. The correlation has been developed primarily for use in a two velocity model of two-phase flow of the type embodied in the RELAP-UK code for LOCA studies. The hyperbolic property is desirable for practical calculation purposes because non-hyperbolic equations are unstable as initial value problems for an infinite range of high frequency, short wave-length, disturbances. The proposed correlation slip ratio approaches unity at high flow rates, it gives a good fit with a range of experiments when used to calculate mixture density from the flow quality, and on this basis it is better than other slip correlations which do not result in consistently hyperbolic equations. The second topic is an examination showing that the steady state conservation equations with slip do not always satisfy the Second Law of Thermodynamics, i.e. that under some conditions the flow entropy decreases. The range of flow conditions for which this is also believed to be smaller for the new correlation than for flow-independent slip correlations; in particular the new correlation is compared favourably with the Smith slip correlation previously used in RELAP-UK. The new correlation appears to satisfy the Second Law of Thermodynamics for steady, two-phase, sub-sonic flow at constant area. (author)