OpenFlows | Water Infrastructure Forum - Effect rupture disk hammer
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Question

Effect rupture disk hammer

by
Bentley Communities Legacy Member
Expert

Hello,

I have a pump pressured pipe system with vapor pressures along the whole line (aprox. 2 km) when there is no water hammer protection.

I have a type of fluid that cannot have hidroneupmatic tanks. I am told the typical way to solve hydraulic transient problems with this fluid is with a rupture disk and assuring the pipe will not suffer deformation with vapor pressure. The goal is that the pipe will resist the negative pressures and once the wave returns with the positive pressures, the disk will rupture and the flow will be emptied before there can be an implosion due to the vapor in the pipe (that was origined by the vapor presssure of the negative wave).

I am putting a rupture disk with a typial presure equivalent ot my permanent pressure and flow, and a rupture pressure slightly above the permanent pressure. 

As a result my positive pressures dissapear but my negative pressures do not change (have vapor pressure al through the pipe as I did before). The negative pressure wave continues to go back and forth even after the rupture disk has been opened (supposedly) when there should be no flow in my pipe...!

Does Hammer have the hability to model this fenomenon? Should I put more rupture disks?

Thank you,

Teresa


Accepted Solution


Hello Teresa,

Typically the pressure threshold would be much higher than the normal operating pressure, since the intent is to rupture only during transient conditions (when there is an upsurge). At the moment that the rupture disk reaches the pressure threshold, the pressure drop between the inside of the rupture disk and the atmosphere would indeed be equal to the threshold pressure, but typically that pipeline pressure would drop after the transient and settle on some "typical" pressure (after the surge), which would be the "pressure drop (typical)", and the corresponding flow out of the ruptured disk (to the atmosphere) at that pressure drop would be set as the "Flow (Typical)".

If you set the "pressure drop (typical)" equal to the pressure threshold, then the outflow results may not be as accurate after the transient settles down and the pressure is significantly different from the "typical pressure drop". This is because the pair of typical flow and typical pressure drop are used to compute an orifice coefficient that then provides HAMMER with a curve of pressure vs outflow. The further you diverge from that "typical" point, the less accurate the results will be, although they should still be within reasonable range, especially if your opening acts as an orifice. Though, in your case, if you are certain that you want to set the pressure threshold near the typical pressure, then it may not make a difference, since there would not be as much shifting on that orifice rating curve once the disk ruptures.

Remember to check the units for both of these pressure fields. If set to "Ft H2O" for example, the value should be the pressure above the pipe elevation measured in length units. You can change the units to something else such as PSI or KPa if needed. The values represent pressures (above the reference physical elevation), not hydraulic grades.

I am not quite sure I understand the part about adding elevation difference.

Just to make sure you are using the right element, here is a schematic of what the rupture disk element represents, as taken from the HAMMER help topic. It does not rupture between two pipes but rather to the atmosphere.