shape of restriction orifice plate

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Thread Starter

Benjamin Tsai

I have an application about the restriction orifice plate installed in the 8" (schedule 80S) cryogenic LNG pipe line for reducing the pressure (inlet pressure= 94 kg/cm2G, and pressure loss required is 13.5 kg/cm2G) based on the flow rate 120 ton/hr, -152 degreeC LNG liquid fluid. I got the calculated bore diameter is 44.65 mm with pipe inside diameter 193.68mm (beta ratio =0.23056), and the plate thickness will be 37mm. But I need to know which shape is better for this application, square edge type or bevel edge type (as like traditional orifice plate for metering)?

Any critical items shall be considered in this application?
 
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Bill Clemons

Orifice meter causes significant pressure drop due to eddies developed down stream of the plate (compared to venturi meter). By beveling the orifice, you should eliminate some of the pressure drop, akin to a nozzle meter. With a recovery ratio of about 0.1, your downstream pressure should be 82 kg/cm2. Is 82 sufficent or will beveling the orifice improve your downstream pressure?
 
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Benjmain Tsai

This restriction orifice plate which I prepare to install in the LNG pipe is for reducing the downstream pressure to 80.5 kg/cm2G. So as per your suggestion it shall not be beveled. But I don't know what noise level will be when 13.5 kg/cm2G drop ? and what distance down stream of the plate will be for the velocity back to the same velocity at upstream of plate?
 
> This restriction orifice plate which I prepare to install in the LNG
> pipe is for reducing the downstream pressure to 80.5 kg/cm2G. So as
> per your suggestion it shall not be beveled. But I don't know what
> noise level will be when 13.5 kg/cm2G drop ? <

Might be better to ask a mechanical engineer about noise level.

> and what distance down
> stream of the plate will be for the velocity back to the same
> velocity at upstream of plate? <

Conservation of mass says velocity will be basically the same downstream as upstream (I think as LNG is a little compressible AFAIR then velocity will remain higher downstream than upstream.)
 
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Matthew Hyatt

My experience in fueling large capacity vehicles with LNG at flow rates at or above 1500SCFM, there was little noise involved with the process, since it is liquid, there will be some noise as with a gas, but even with water flowing at these flow rates the noise level is not that great. I don't think I would be that concerned about the noise level. I would be more concerned with the pressure drop across the orifice plate, we used ultrasonic flow meters since they were not effected by the extermely low temp of the LNG.
 
You should try for multistage restriction orifice to reduce noise level. please try two stage which will bring less than 85db(A).
 
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