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
Hello Teresa,
The rupture disk will help relieve upsurge (high) pressures when it opens and releases water to the atmosphere, but it may not necessarily help with downsurge (low) pressures. You may have a downsurge wave front pass by the rupture disk (causing your minimum transient pressures) which may then reflect as an upsurge, and subsequently open the rupture disk.
The effect is the same as if you had a demand (consumption/outflow) at the location of the rupture disk - the outflow can help keep the water column moving in the outflow direction to prevent the pressure from increasing further. However since there is no inflow (like a tank/reservoir), the pressure can still drop and potentially form a vapor pocket. The model topography tends to be important in this case - if there is no boundary condition at a higher elevation, there would not be anything to keep the pressures positive as long as the pumps remain off. This topic is discussed in further detail in this wiki article: Surge mitigation for systems with intermediate high points experiencing negative pressure
Even if there is no flow in your pipes, transient waves will continue to propagate, and will take much longer to dampen without headloss. This is explained a bit more in this wiki article: Transient pressure wave not dampening or unexpected lack of headloss
Beyond this, as Sushma mentioned, we would need to take a look at your model to understand and explain in a more specific way.
Regards,
Jesse DringoliTechnical Support Manager, OpenFlowsBentley Communities Site AdministratorBentley Systems, Inc.
Thank you Jesse. As an input for the rupture disk there are three parameters for which i assumed the following:
- Pressure drop (typical): Permanent pressure i have in the system (before the disk ruptures)
- Flow (typical): The flow i want the disk to discharge (dependent on the diameter i choose)
- Pressure threshold: a certain % above my permanent pressure
Did I put them correctly?
When I change my flow or pressure drop I see no change in the results of my maximum pressures of the transient... I only see a difference when a change de pressure threshold. Is that supposed to be so?
The "typical" flow should correspond to the flow out of the rupture disk, at the "typical" pressure drop you enter. This establishes the pressure vs outflow relationship of the rupture disk when it is open (ruptured).
For example if you know that the outflow would be 10 L/s at a pressure (inside the pipeline) of 10 meters, this is the pair that you would enter. The following wiki article was written for the Orifice Between Pipes element but the same concept can be applied to the rupture disk, to help determine the pair of typical flow and typical pressure drop based on the orifice equation:
Defining Orifice Between Pipes element by diameter
If the pressure is not getting high enough to rupture the rupture disk, then changing the "typical flow" or "typical pressure drop" would not impact the results, because they only define the hydraulics of the outflow when the disk has ruptured. Most likely you are seeing a change in results when changing the pressure threshold because you may be changing it from a value that isn't high enough to cause a rupture, to a value that does cause it to rupture. Check your user notifications for an alert.
Yes I understood thank you.
In my case I have a very high pressures at the rupture disk location, and therefore I am also designing a pipe line downstream the rupture disk with orifice plates to dissipate some of the energy and control my flo and velocity in this pipe (and prevent erosion at the discharge point).
Still have a doubt: I dont really understand the difference between Pressure drop (typical) and Pressure threshold. Because when my rupture disk ruptures, the typical pressure drop would correpond to the pressure threshold because that is when it breaks. Thus would mean that the Pressure drop (typical) would be the same as Pressure threshold...!? Or does the pressure drop have a different meaning?
The only difference I can see is that maybe, and in my case, I should put a Pressure drop (typical) equal = Pressure threshold+ elevation difference (between the elevation of my rupture disk and the elevation of my final point of discharge). Is this correct?
So something like:
- Pressure drop (typical): 250 m + 10 m (elevation difference) = 260 m
- Flow (typical): The flow i want the disk to discharge (that considers the pressure loss generated by the orifice plates)
- Pressure threshold: 250 m
Does this make sense?
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.
Answer Verified By: Sushma Choure