9 replies to this topic

[Steven McIntosh]

I'm looking for a definition of flutter as it relates to storage tank vents. My Protectoseal ® pro-flo II ™ software says that for certain combinations of flow, design pressure and set pressure all choices of vent style and size are in the "flutter zone". So, I'm looking for some info on just how bad is it to be in the "flutter zone". I've discovered one way out is to keep lowering the set pressure, but this isn't always desirable.
thanks,
Steven McIntosh

[proinwv]

Thanks Steve,

All of the common pallet loaded vents will flutter when operated close to the seat, or at very low flow rates as compared to the vent capacity. What happens is that the vent opens at the "setrpoint" and if oversized for the application or the actual flow to be vented, then it immediately vents and drops the pressure back to a reseat pressure. This is followed by a rise in pressure and the vent reopens, and the cycle repeats itself.

Using a smaller vent will avoid this, but there may be times when it will still happen.

The result is additional wear on the seat and stem or bearings due to the additional seating and wear cycles.

Paul

[Steven McIntosh]

Thank you Paul. I'm going to have to get back with you after a little more study. Everything you wrote makes perfectly good sense, but it's not exactly in-line with the results of the program from Protectoseal ®. (good program by the way) For example, without changing the size of the vent I simply had to lower the set pressure to get out of flutter. I suppose that could be considered to be taking the vent from over-sized down to proper sized.
I hope to have more to write by next Monday. In the mean-time, if you come across any "literature" definitions of flutter they might be helpful. I'm also wanting to learn more about inlet and exit dP and their impact on vents.
thanks again.
Steve

[djack77494]

Steve
You may find it easier to locate information on safety valve "chatter". I believe that the two (chatter and flutter) are very similar, though you must keep in mind that safety valves tend to "pop" while conservation vents (e.g. Protectoseal) are more like modulating valves.

[proinwv]

Steven

Yes, by dropping the pressure, you require more vent to pass the same volume. I.e. less delta P more Cv required for the same Q.

[sheiko]

Steven
Yes, by dropping the pressure, you require more vent to pass the same volume. I.e. less delta P more Cv required for the same Q.

Sorry to interject myself in this discussion but i believe that by dropping the pressure (and thus the density), you require more vent to pass the same mass because the volume increase (mass= density*volume).

Please correct me if i am wrong.

Oupss...this thread is 1 year old...

Edited by sheiko, 27 March 2010 - 05:21 PM.

[fallah]

The same volume (less mass) is correct, because the volume enclosed by liquid level and tank roof is almost fixed.

Edited by fallah, 29 March 2010 - 12:30 AM.

[fallah]

Steven

Yes, by dropping the pressure, you require more vent to pass the same volume. I.e. less delta P more Cv required for the same Q.

I think same Q wouldn't be achieved by the same size in less delta P.It could be achieved by larger size.

[Dazzler]

An alternative way to move away from flutter or chatter zone for vent valves (tank vents or vessel pressure relief) would be to have say two valves (or multiple valves), one smaller at a lower set pressure and then a second valve set at a slightly higher pressure. Smaller flow scenarios are therefore likely to only open the first valve and it would stay open longer rather than fluctuate as a larger single valve may be expected to.

I believe this is why it is important for relief valves to have all relief scenarios identified (for flowrate), incase there is a frequently expected scenario of smaller flow that needs to be considered specifically so that a single valve does not chatter.

Dazzler

[fallah]

An alternative way to move away from flutter or chatter zone for vent valves (tank vents or vessel pressure relief) would be to have say two valves (or multiple valves), one smaller at a lower set pressure and then a second valve set at a slightly higher pressure. Smaller flow scenarios are therefore likely to only open the first valve and it would stay open longer rather than fluctuate as a larger single valve may be expected to.

I believe this is why it is important for relief valves to have all relief scenarios identified (for flowrate), incase there is a frequently expected scenario of smaller flow that needs to be considered specifically so that a single valve does not chatter.

Dazzler

Just to add my comment on above nice statements:

Most PSVs to be designated such that can tolerate smaller flow than rated flowrate which those PSVs sized based on,in order to cover different scenarios without chattering.The limitation in most cases would be 25% of rated flowrate.

Edited by fallah, 05 April 2010 - 01:31 AM.