I have these curves from vendor
Can I use these curves as a characterization of my air combination air valves?
What would be a reasonable value for the transition volume or transition pressure? In someplace in the forum I read that the transition value would be the volume of air inside the valve, is this correct?
Can I use the air valve combined with other protection equipment, ie a surge release valve in the same node? I've tried this and get really weird results; the pressure spikes are higher than if I use only the SRV.
Thank you in advance.
Denis Gonzalez said:Can I use these curves as a characterization of my air combination air valves?
You can input custom air flow curves for your air valves instead of using orifice diameters.
Denis Gonzalez said:What would be a reasonable value for the transition volume or transition pressure? In someplace in the forum I read that the transition value would be the volume of air inside the valve, is this correct?
Yes. Transition volume is the volume of air between the lowest and highest position of the float. It is usually approximated to the volume of the body of the valve.
Denis Gonzalez said:Can I use the air valve combined with other protection equipment, ie a surge release valve in the same node? I've tried this and get really weird results; the pressure spikes are higher than if I use only the SRV.
Typically air valves are located at high points in the system or areas which are susceptible to sub-atmospheric pressures. Using two protection devices at a single node is a bit unusual as the transient mitigation principles for both devices are different.
If you are modeling a air valve as a surge protection equipment here is an article which can help you for setting it up;
Modeling Reference - Air Valves
The above article also has detailed information on setting up air flow curves, transition volume and usage of air valves.
Hope this helps.
Bentley Technical Support
Thank you Yashodhan for your answer.
I'm trying to model a deep well pump and here we always use an air valve in the discharge of the pump and not far along the header a SRV or SAV.
I haven't seen this sort of calculations anywhere in the documentation.
Also, as you can read I'm trying to gain as much accuracy as possible.
As for the air valves, what would be the transition pressure to use instead of the orifice diameters? Can't find that in the technical sheets of several manufacturers I've checked.
Once again thanks a lot.
Transition pressure is the pressure at the air valve location beyond which the outflow orifice switches from the large to the small size. Please refer the article I shared in my earlier response for details.
For details on how to enter custom air flow curves, see the section called "Using a Custom Air Flow Curve" in the air valve modeling reference article that Yashodhan linked to. Note the negative vs. positive sign and other tips.
Denis Gonzalez said:As for the air valves, what would be the transition pressure to use instead of the orifice diameters? Can't find that in the technical sheets of several manufacturers I've checked.
The transition pressure vs. volume is a parameter of triple-acting air valves and is independent from the orifice sizes. The transition pressure/volume determines when the air outflow transitions from a large size to a small size. This tends to help prevent severe surge pressures that may otherwise occur if air is releases too quickly. At first you will want to expel air quickly (hence the large outflow orifice size) but then you want to cushion the release of the last bit of air to prevent the adjacent water columns from colliding at high speed.
The transition volume is the volume or air left in the system below which HAMMER transitions from the small to the large outflow orifice size, when using a triple acting air valve. The transition pressure is the pressure above which this transition occurs. You basically need to correlate one of the two to your particular air valve. That is, you'll need to determine what the pressure would normally be at the air valve location, at the point when the water level inside the air valve (that you are trying to model) reaches the float that closes the large orifice. Or for transition pressure, you would need to determine the volume of void space inside the air valve when the water surface reaches the float - as Yashodhan mentioned, this is often estimated as the volume of the body of the valve.
If in doubt, I would recommend a sensitivity analysis - try a range of estimated values based on the information you have from the air valve manufacturer along with your engineering judgment, and see if the transient results you are interested in are impacted significantly. If not, then you know that you may not need to worry about determining the exact value to use.
The combination of air valves and other protective equipment is indeed common, as air valves alone would typically not be enough to protect a system, and you would normally have air valves at local high points anyways, to expel air that accumulates during normal operation. Air valves are helpful for preventing negative pressures at local high points but will only protect in the immediate vicinity of the air valve - see the related notes on this in the air valve modeling reference article.
If the transient pressures are worse with air valves added, it is possible that the air is being released too quickly, causing the adjacent water columns to collide at high speed, resulting in severe upsurge pressures. This is mentioned in the air valve modeling reference article and also highlighted in the following additional article: Transient pressure worse with air valve added
To visualize this better, be sure to enable profile animations, and animate the profiles in the transient results viewer: Using Transient Results Viewer animations for visualizing a transient simulation
Jesse DringoliTechnical Support Manager, OpenFlows ProductsBentley Communities Site AdministratorBentley Systems, Inc.
Answer Verified By: Sushma Choure