Varec Flow Curve Question

[avgguyfrom85]

Good afternoon. First off, great site and forums! I'll get right to it.

I work for a company who services low pressure relief devices (Groth, Varec, Enardo, etc). I'm fairly new, and for my own curiosity I've been reading the manufacture's literature on different models, trims, applications etc. While looking at the Varec flow curves for a 0.5 Oz setting (0.865" WC), I noticed that the flow rates for say the 8" 2010B unit are different (pressure side aprox 34,000 SCFH vs the vacuum side aprox 25,000 SCFH). Why is this if the seat size and set point are the same?

Again, great site! Thanks,

Brandon

 

[don1980]

Unlike safety valves, tank vent devices don't have an independent body that ensures uniformity in flow capacity testing. This is a recognized issue in the industry, because there are a number of anomolies like the one you point out.

Until there's a uniform flow testing and certification by an independent body, we'll continue to see questions and challenges to the vendor published capacity curves.

 

[avgguyfrom85]

Thanks for the prompt response. Having said that, it only leads to more questions I guess. For further information, I presume I'd have to call each manufacture to ask for their testing procedures if my curiosity is so inclined. And more to the point, if you were tasked with sizing the equipment for a tank, would you trust the flow curves?

 

[LittleInch]

I think it might just be a function of that valves layout and orientation. As the pressure valve lifts it has equal flow all round the piston on the exit and a straight flow path. The vacuum valve though has the main port into the valve only on one side of the piston and needs to turn 180 degrees. Normally pretty negligible stuff, but at these pressures and flow veleocities will make a small difference. Also perhaps not at these pressure, but the pressure and hence force lifting the cylinder and also the density are slightly different, again it all just addes up to the a difference for the same circualr area.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[don1980]

avgguy - Since there's no independent or standard verification of those curves, and since there are unexplainable anomolies when comparing devices, one can't trust the curves to be highly accurate. But, there's also no evidence that your tank will fail due to the use of those imperfect curves.

I think the main driver for generating uniform testing standards is to allow honest (apples-to-apples) comparison between devices from different vendors.

 

[avgguyfrom85]

"Since there's no independent or standard verification of those curves, and since there are unexplainable anomolies when comparing devices, one can't trust the curves to be highly accurate. But, there's also no evidence that your tank will fail due to the use of those imperfect curves.

I think the main driver for generating uniform testing standards is to allow honest (apples-to-apples) comparison between devices from different vendors. "

Agree! So maybe I'll learn to test and measure flow through all these guys low pressure valves .
At the shop, we have a test stand to test set point popping and leak rates, but it would be fun just to test the actual flow through the units at a given pressure and set point to see how it jives w/ the reading material.