5 replies to this topic

[villivord]

Hello everyone,

 

I have a question about restriction orifices. I have to calculate the bore diameter of the orifice and to do this, I need to know the pressure drop across the orifice and the required flow rate (plus some other input data like density, viscosity and outer piping diameter). 

 

I know the upstream pressure (60 psig) and I desire a water flow rate of 20 gpm through the orifice. What pressure drop should I take? The flow is delivered by a centrifugal pump and the orifice discharge line is connected to an atmospheric vessel (which is connected to the centrifugal pump). The purpose of the orifice is to have a continuous recirculation of water through the system. 

 

Can I take the pressure drop = 60 psig? 

 

Kind regards 

Edited by Borpe_, 19 December 2016 - 04:05 PM.

[Pilesar]

If you know the pressure at the end of the discharge line, the flow rate, and the piping configuration, then you can calculate what the pressure has to be at the beginning of the discharge line. Use that pressure to calculate what the orifice pressure drop should be.

[Bobby Strain]

Pilesar is almost correct. You must also include elevation difference, (if any), between the tank minimum level and the minimum flow recirculation line entry to the tank. Which will result in a greater recycle flow with a higher level in the tank. If you return the recycle line below the minimum level, then there will be no change in recycle flow due to tank level variation.

 

Bobby

[latexman]

You'll also have to obtain and use the permanent pressure drop of the restriction orifice in lieu of the usual differential pressure at the taps the correlations give.

[ecas]

As the other reply say, you must take into account the piping pressure drop and the piping elevation in order to calculate the pressure drop that must be produced by the restriction orifice (RO). With the flow and the pressure drop you can calculate the hole diameter. A good reference to do this is the Crane Technical Paper No. 410.

Take into account that if the RO must provoke the majority of the available upstream pressure, you must consider that the RO may have cavitation. A RO with 1 hole has a high cavitation coefficient and in this case it's better to design a multi-hole restriction orifice.

Finally, install the RO at the beginning of the piping run in order to increase its back pressure to reduce the cavitation.

[raju.kct]

I think, one more point to be considered is the intersection point (operating point) of the system and pump characteristic curves.

Since the pump is of centrifugal type, change in system characteristics (due to the addition of orifice) will change the pressure rise and discharge by the pump. So it is to be ensured that the intersection point of the curves should be at the desired flow rate of 20pgm after adding the orifice in circuit.

 

Thank you,

 

Regards,

Raju