flow rate through a leaky ball valve cavity.

I've got a 900mm ball valve with about 60 bar in the line. The fluid is Methane. I've got a problem in that it doesn't seal - I suspect the seal is shot. I want to quantify the leakage rate across the valve seat.

xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx The valve has a 25mm nominal bore vent pipe attached to the ball cavity with a valve{no1} to close it off. Above valve 1 there is 150 mm of pipe with a gauge connected via a tee with another valve {no2} above it. Above this valve i have 2m of vent pipe (25mm) open to atmosphere. imagine the attempt at a sketch below orientated thru 90 degrees.

! ! O gauge ! line pressure!

----------------- atmos==valve2===="=====Valve1======[ ball valve ] --

--------------- ! line pressure! ! !

If I open valve 1 I get line pressure - 60 bar- at the gauge. when I then open valve 2 the pressure at the gauge drops almost instantly to to 27 bar. At the vent I see 'stacked diamond' shapes in the gas jet. [Underexpanded?? Sonic velocity?? Could I assume the Velocity to be about 330 m/s?? ] after about 2 minutes of constant venting the pressure at the gauge falls to

14 bar and the jet takes on a sharp cone shape. I never decrease below 14 bar at the gauge and maintain 60 bar in the line outboard of the valve seats. If I close valve 2 I regain line pressure at the gauge in about 10 minutes.

I presume that the hole in the valve seat is going to be proportional to the bore of the vent pipe and is less than the area of the pipe bore because the pressure falls off. Can I establish a (rough) flow rate through the pipe from the info given? ============================================================================ ================= supplementary info... I cannot remove the valve from the line to test and cannot reduce the line pressure significantly in any way. Valve 1 is fixed and has a port 1"NPT on the discharge side. I have no equipment other than pressure gauges and standard pipe fittings to hand to construct a device for measurement. I dont have an orifice plate or any similar equipment I can connect into the vent line. ============================================================================ =================

any suggestions??


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Trying again with the sketch

" " " " X Valve 2. " " "===O Pressure Gauge. " " " X Valve 1. " " " Line pressure [BV] Line pressure

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Dear SS

If we know the volume (Vc) of the cavity in the main valve, then from what you said the pressure goes from 14 bar to 60 bar in 10 min. So the initial gas volume that was in the cavity was (14+1)Vc and the final gas volume is (60+1)Vc, so there has been a gas volume of (60-14)Vc that entered in 10 min. Of course the flow rate in these 10 min is not constant. A rough estimation is that the initial flow rate is 4 to 6 times the average (46Vc in 10 min ).

Just to get an idea lets say that Vc=0.08 m3 So average flow rate is 46*0.08/(10/60) = 22 m3/h The initial (I mean at the time when valve 2 closes) flow rate is approximately 5 times the average, so it is 110 m3/h

Verification: For a flow rate of 110 m3/h in pipe of 25 mm we get a velocity of 62 m/s. Can you verify (perhaps using an air speed meter or anemometer at the outlet of the pipe) this velocity?

Finally remember that all of the above is based on knowing the Vc.

There is another way of calculating the flow rate at the time when valve 2 closes. It is based on calculating the flow rate that has a pressure drop of 14 bar for a length of aprox 2 m with diameter 25 mm taking into consideration the gas compressibility. But I don't have the formula.

Hope this helps


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