Air vacuum relief calculations for hydro penstock 1

[DavidCR]

Hi.

We have a 2.9m (114.2") hydroelectric penstock that we are refurbishing.

The original design included 4 air vacuum/releif valves with a flanged connection of 300mm (12"). The valves are very old (italian brand) and we cannot find tecnical info about them and we want to know the design capacity to replace them. The objetive is to prevent vacumm collapse is case something happens to the pipe or system.

I´d appreciate a lot if somebdy can help to find information on big capacity air valves and criteria/formulas on sizing. I´ve found some info (Valmatic, GA, Apco) but they give me some very big numbers and I suppose the formulas provided are not appropiate for this applcation.

Thanks a lot in advance.

 

[cvg]

you might find some useful information in this publication with respect to sizing and air requirements. For what its worth and without specifics of your hydraulic conditions, 4 - 12 inch valves sounds a bit small. Can you get by without valves and just install a goose neck vent?

http://www.usbr.gov/pmts/hydraulics_lab/pubs/EM/EM41.pdf

 

[gerhardl]

You might make some different and very rough approximations, based on some assumptions:

1. You can 'guesstimate' the necessary air out when filling the pipeline, if you assume (and control) the filling from empty pipeline at a given, controlled maximum filling speed. A given, conservative speed at filling could be limited to 0,5m/s at the normal pipeline diameter. Calculate air out (volume) equal to water in (volume).

2. Air out at normal pressure will be only fractions of this. Could be estimated as % equal to that of a 'normal' multipurpose air outlet valve at normal pressure.

3. Necessary air in has to be estimated by 'someone' and will vary. Largest capacity necessary is by breakdown or rupture of pipeline. Thisis a difficult thing to estimate, but often estimated wrongly higher than necessary. Real breakdowns is normally damped and imposed by surrounded earth if pipeline is buried, and limited anyway by pipelines structure, buried or not. Normal conditions and maintenance is assumed.

4. Remember that it is not the flanged connections nominal diameter that gives the air capacity of an air-valve, but the smallest area the air has to pass. If you assume the originals are correctly dimensioned, check smallest areas for the three separate cases (air in, air out, and air out under pressure) inside the valves (orifices? restrictions?). Assume a normal air flow speed (for instance 30- 40 m/s) at smallest areas.. This is conservative. Forget about compressions and simply calculate voulume of air.

5. You can do this far more complicated, but not necessarily better, as all hangs on the accuracy of para. 1, 2 and 3.

6. Do not forget that suppliers often gives capacity in volume, but tend to 'forget' to give the essentials: the relevant areas and the airspeed they calculate by. You need this to compare products. Some suppliers use calculating airspeeds for capacity at 60m/s and higher.

7. There exists on the market special 'plateformed' weighted air inlet and outlet valves for high capacities. Largest I have supplied was an 800mm air inlet valve/vacuum breaker for a hydroelectrical powerplants' vertical turbine-feeding watertunnel (rock).


7. Check that constructions selected do not slam shut at 'overspeeds' for air out by fast filling.