[I]I work for Environmental Hydraulics Group in Canada and my boss is a designated Specialist in Ontario. We have worked previously with your colleagues at Hatch in Mississauga and Montreal. I am providing this info freely and you understand that only you are ultimately responsible for your design. Here goes:[/]
Should we be concerned about water hammer, or will the mining hose dampen it out?There are only two ways I (and other posters) can think of it dampening the transient energy: i) by moving or whipping about; and ii) by friction within the hose, couplings or steel system components (but this only applies to the counter-surge and subsequent cycles, so it may not prevent a burst). Expansion of the pipe material should not dissipate significant energy (within its elastic range). Change of pipe material will also not change the surge energy once generated.
You would likely not be satisfied with whipping or bursting as a surge relief mechanism, so you want to make sure the system can take it long enough (many wave travels back-and-forth, testing the limits of components each time) until friction can do its work.
Caution: if flow can begin suddenly into an empty pipe, the surge pressures from rapid filling can greatly exceed the Joukowski formula head you may be using.
The hose will have a lower modulus but I would need its material and/or spec to comment on that. It may be much lower than steel (of the order of 1/2 to 2/3). The majority of the piping is mining hose, the composite behaviour in terms of transmitting transient pressures may correspond approximately to "stiff" mining hose; but a computer program is required to really track and quantify this.
But I thought that maybe the mining hose would dampen the pressure waves before they reached the steel pipe run, in which case I wouldn't have to worry about doing a water hammer analysis?If your mining hose is rubber or plastic, 100 m long may translate to about 1/4 second travel time for the transient pressure wave assuming it is anchored and fairly straight run. There will not be much dampening on its first trip and there will be no time to "react" by getting out of the way either. You should therefore analyse this.
Do I use the P from the mining hose, because that's where the pressure wave started?You would need to use the same P for the steel pipe as for the mining hose IF the pump is the only possible cause of transients. Also, a pump will normally slow down, not stop suddenly, therefore the Joukowski equation may be an over-estimate. If a valve can close in your steel system, you need to use the larger P from the higher modulus and the Joukowski head.
To keep the same energy I would have thought that it would have to reduce in pressure or something?Once the transient pressure wave Pt starts moving, it will essentially keep the same magnitude (less friction) as it enters the metal piping. What governs the waterhammer potential is the velocity before flow stops (steady in this case), not the Pt's magnitude. Think of Pt as a signal (Volt or traffic accident) telling a transport phenomenon to slow/stop (current or vehicles slowing as the space between them is reduced): the kinetic energy (velocity) does not immediately respond to the change in potential energy (pressure); a short time is required for this response to propagate through the system. Remember, this is transient behaviour during which the fluid is elastic!
Therefore, there will not be a sharp (local) change in velocity due to the change in material from mining hose to metal system (conductor or lane reduction): rather, continuity will ensure the effect will be felt throughout the system. E.g. the mining hose's modulus does not protect the system from Pt transmission, only reduces its magnitude when generated. A computer model is normally required to analyse changes during transmission, however if the length of the mining hose is 90% of the overall piping, the system will behave in a composite manner: like a stiff mining hose.
What are your thoughts?If the hose can take it and flow cannot be valved-off in the metal system, you should be ok.
Recall that we consult in this and sell software, so..
I hope all this helps and I encourage all COADE users to reserve some time and budget for hydraulic transient issues before undertaking a pipe stress analysis with CAESAR II: after all, even the best pipe software in the world needs correct inputs (forces on anchors, thrusts on elbows) to produce correct outputs. GIGO is universal.