The resistance to the liquid is its difference in potential energy( which is based on height from center of gravity). When potential energy of water is greater in one place than than other place water tries to move towards lower potential energy. Though the pipe is straight through which the water flows. The flow of water is only possible when one end of pipe is connected to the water at some height than that of other end or having force( force may come from potential energy of water itself or some machine may provide). The motion of water is due to its kinetic energy( gained by the difference in potential energy) soon the kinetic energy finishes the water stops to flow. Yes there is some other factors that makes the kinetic energy of flowing water to diminish including viscosity of water itself.
While in case of Helium, the super-fluid, though there is no viscosity at all but the problem is same again the kinetic energy provided by difference in potential energy. As there is no significant amount of viscosity the time to flow through horizontal circular pipe may take long time to stop this may be due to the long time taken by the helium to finish its kinetic energy to come to rest.