Dear All,
As you are water hammer analysis experts, kindly transfer some of your knowledge and experience to me in the following issue:
in our project, we have made a transient analysis and we found a point over hill have a negative pressure during transient.
so we decided to use anti-slam air-vacuum Breaker as a protection device to allow air to enter the pipeline during pump power failure to prevent negative pressure.
and to allow air to be released slowly again after pump restart to prevent water hammer damages.
the total air enters the system during transient is approximately 100 m3.
but the project consultant insists to use one way surge tank instead of using anti-salm air valve.
he think that one way surge tank has an advantage of preventing this amount of air from entering the system.
the question is:
From a technical point of view, what is the better solution from the two above mentioned protection devices?
Is any of the two devices is better for our case?
What is the advantages and disadvantages of each one?
Is our consultant decision is right?
Thanks in advance
Hope to get reply
Hello Mohamed,
A vacuum breaker option will add air to the system at negative pressures, but assumes the outflow diameter is very small (effectively zero) so it doesn't let air out. You will see the air volume change as the air pocket is compressed, but the mass of air in the pipe doesn't reduce. There is probably a very limited number of applications for this type valve. However, it could be used for a draining pipeline.
Please note that any air pocket left in the system due to a vacuum breaker valve is assumed to be expelled out of the system by some other means. HAMMER currently cannot track the behavior of these trapped air pockets (the underlying assumption is that the air must exit the system where it came in). This information, and more, can be found in the air valve technote. Basically, while it is assumed that the air will leave the system, HAMMER doesn't model this explicitly.
A one-way surge tank is a relatively small conventional surge tank, with a check valve in the connecting pipe, or riser, that only allows flow out of the tank. The tank water level is maintained by an altitude valve bypassing the check valve. The tank is located at the high point to supply water and prevent water-column separation. However, one-way tanks provide no upsurge protection to the system because no flow is allowed back into the tank.
As for which is better, this could vary from system to system. I would recommend modeling each case and checking the results. For instance, the surge tank option may help main the hydraulic grade so that there is no downsurge. If this can help maintain a positive pressure, this may prove to be the better option. But if the tank doesn't add the necessary head, it will be possible that negative or vapor pressures occur. In such a case, it is possible the air valve option is better.
Regards,
Scott
Answer Verified By: Expert