Quick question regarding the Crane 410 manual and reduced port globe valves. Is there a limit to the resistance coefficient (K factor)? Using A-26, 27 I'm getting some very large K values and can find no examples anywhere of K values over 100. Also, if my K value is over 100, is my expansion factor (Y) limited to .718? Is my Dp/P1 also limited to .920?

Dealing with a choked flow conditions, determining max gas blowby rates in the event of an upset through a globe-style LCV. The globe valve is a 2" body with a 3/4" port...the numbers make sense as i'd expect a large pressure drop through the reduced port of the valve, but looking for more assurance.

Did you remember to multiply the K₂ value by the friction factor on the top of page A-26*.

I did a quick calc, following Example 4.20 (specifically step 4).  I assumed your K₁ was 340 ft (the largest value for globe or angle vales), that d₁ was 0.75" and d₂ was 2".  This gave me a K₂ of 319.8 ft, which I then multiplied by 0.019 from the "K" factor table, resulting in final K value of 6.06 ft for the valve.

*If your pipe isn't clean steel, you would need to go back to the graph on page A-24 and figure out the friction

factor.

Assuming I read the equation correctly (it is late for me here in New Jersey, USA), and assuming the full port valve with K=340ft, then I get a final K value (call it K2 if you wish) of 348.8. The reason you don't see high K values in any examples is simply Crane doesn't use any reduced port valves.

You would be better off obtaining the full open Cv of your valve and use the ISA control valve sizing equation to determine the actual pressure drop.

Be that it may, your expansion factor maxes out at 0.718 at K=15 (Cp/Cv = 1.3). As far as the pressure ratio goes, you can't do any better than 0.92; you're getting close to 1 any ways.

The better way to do this calcuation is to us the ISA control valve formula (as I state above) for the valve and use the Isothermal gas equation overall. But following CRANE TP40 Example 4-21 with my comments above should be OK.

MORE NEWS