Mach Number Through an RV

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cingold Guru Join Date: Nov 2007 Posts: 519 Good Answers: 11	Mach Number Through an RV 06/04/2009 1:28 PM
	I was working on a project the other day with a process engineer when he told me that the program he was using to evaulate relief valve scenarios gave him a Mach number of 0.9 for one of our relief valves. We both thought this was incorrect since the vessel is at 70 psi pressure and ambient temperature. The orifice is an "M" sized orifice (I think this is about a 1" diameter opening). It did make me wander what size orifice we would need to result in a Mach 1 speed through the orifice. I did similar problems in college so I thought I would treat it as a converging nozzle with a choked area. I quickly found out that I didn't remember as much about the topic as I thought I did so I thought I would defer to CR4. Do I have enough information here to find the area needed to choke the flow? Assume we use air for the fluid.
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tcmtech Guru Join Date: Jun 2009 Location: South of Minot North Dakota Posts: 8376 Good Answers: 775	#1 Re: Mach Number Through an RV 06/04/2009 4:03 PM
	Just factor what the area of the jet opening is and what volume you are sending through. High enough pressure though a small nozzle often works out to multi mach numbers!
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sb Guru Join Date: Mar 2007 Posts: 2550 Good Answers: 103	#2 Re: Mach Number Through an RV 06/04/2009 9:34 PM
	The orifice will have a 'Orifice constant' depending on the shape and that will provide the velocity on the form Q=k_oAV There is a link which explains this Ko for a few common shapes. For others the Fluid Mechanics text book may be consulted http://www.mcnallyinstitute.com/13-html/13-12.htm __________________ Fantastic ideas for a Fantastic World, I make the illogical logical.They put me in cars,they put me in yer tv.They put me in stereos and those little radios you stick in your ears.They even put me in watches, they have teeny gremlins for your watches
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nick name Guru Join Date: Mar 2007 Location: City of Light Posts: 3943 Good Answers: 183	#3 Re: Mach Number Through an RV 06/05/2009 3:30 AM
	The velocity is determined NOT by the orifice (M<=1) but by the pressures ratio. For M>1 the nozzle (one cannot any more use the word "orifice") profile plays a very important role.
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srini Power-User Join Date: Mar 2008 Location: Houston, Texas Posts: 101 Good Answers: 6	#4 In reply to #3 Re: Mach Number Through an RV 06/05/2009 8:54 AM
	Agree with Nick name.Pressure drop is what determines velocity. Roughly when P2/P1(outlet pressure/inlet pressure in PSIA) is 0.5 we have choked flow. All normal relief valve sizing formulaes are based on choked flow. That is why when the set pressure of relief valve is very low and there is a chance for back pressure to exist we need to be careful not to use the the ASME/API standard formulae __________________ If you don't know where you are going,you are bound to get there
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Gannet Power-User Join Date: Mar 2008 Location: Baltimore, MD Posts: 148 Good Answers: 5	#5 Re: Mach Number Through an RV 06/18/2009 11:26 PM
	I use the following online calculator requires Java, it works great for determining flow properties through an orifice. http://www.flowmeterdirectory.com/flowmeter_orifice_calc.html
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Gannet (1); nick name (1); sb (1); srini (1); tcmtech (1)

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