Restriction Orifice

I have a query related to restriction orifice. Since, restriction orifice is not bevelled at downstream of it.So, does it work in both directions i.e bidirectional. For eg if we are using RO for preventing reverse flow. If downstream pressure exceeds because of back flow, will it kill that extra pressure?
Plz reply
 
munny07,

This is a topic which can cause much consternation at many plants.... There are basically two functions for an orifice: One is the create a small differential pressure which can be measured and used to calculate flow; this one usually has a beveled edge and is, again, primarily used for measuring flow through a line/pipe.

The second function is to create a larger pressure drop, similar to a pressure reducing or -regulating valve, but at a much cheaper cost and often, easier installation. The pressure drop comes at the "expense" of flow-rate--there can only be a pressure drop when there is a flow-rate restriction.

In the first orifice function, the flow restriction is very small, and the differential pressure across the orifice is small, usually measured in inches of water or mm of Hg (mercury), etc. And, because it is a measurement that's the main point of interested (the differential pressure) one edge of the orifice plate is usually beveled so as not to cause too much turbulence in the flow downstream of the orifice. So, the difference between the inlet pressure to the orifice and outlet pressure of the orifice is small, and therefore the resulting flow-rate restriction is also small. The flow-rate through the orifice is restricted more as the flow-rate through the orifice increases--but in this case it's not the downstream pressure or flow-rate that's being "regulated." We are only interested in the differential pressure across the orifice, in order to measure the flow-rate of the fluid through the line/pipe the orifice is installed in.

In the second orifice function, it's most often the flow-rate through the orifice that's being restricted, or "regulated," by the orifice restriction. In some cases, it may be the downstream pressure that is being monitored or restricted or "regulated" by the orifice restriction. And, usually, the difference between the inlet pressure to the orifice restriction and the downstream (or "outlet" pressure) of the orifice is on the order of psig or barg or the like. And, the restriction to flow through the orifice is much greater; again, often the orifice serves as flow reducer--the pressure differential, if it's even measured, is just an indication of the amount of the flow restriction.

I have seen some crude flow measurement systems (in very large lines/pipes) which use an orifice plate without a beveled edge--the accuracy of the measurement isn't very important and the magnitude of the total flow is usually very high (thousands of liters or gallons, etc., or tens-of-thousands, etc.). Usually these are on very large water pumping systems.... Maybe like yours?

I'm not really clear on your question or problem. In my understanding of your question of using an orifice I am presuming you are using the orifice in the supply of the fluid to the RO (Reverse Osmosis) system to reduce the pressure of the fluid entering the RO system (and also restricting the flow-rate of the fluid entering the RO system). I could imagine a situation where the RO system gets "plugged" on the inlet side and therefore the flow through the RO system is reduced, which means the flow into the RO system is also reduced. In this case, as the flow through the inlet restriction decreases because the flow through the RO system decreases the pressure differential across the orifice plate in the inlet line/pipe also decreases, and therefore the pressure drop across the orifice plate in the inlet line/pipe also decreases. I can't imagine a back-flow of fluid from the RO system in this case; but sometimes I don't have a very good imagination (I are en enguhnear after all!).

Maybe you are referring to an orifice plate in the outlet of the RO system, and the function of the orifice plate is to serve as a restriction to flow (flow-rate) to keep the RO system operating a high efficiency. I could envision a scenario where the downstream system receiving the outlet flow from the RO system might not be able to accept the normal flow-rate output of the RO system--OR because the RO system is plugged the flow-rate through the RO system is reduced, thereby reducing the flow through the orifice. In this case, the pressure across the orifice will also start to decrease, and the flow through the orifice will also decrease. If there is little or no "forward" flow through the orifice (from the RO system to the downstream process) the pressure drop across the orifice will decrease to zero or near zero, as the flow-rate decreases to near zero or zero (either because the downstream system can't handle the flow or the RO system is plugged/plugging). If the situation were allowed to get to the point that the downstream process pressure is higher than the outlet pressure of the RO system then flow would start in the reverse direction, to the RO system. And as the flow increases, the differential pressure across the orifice would also increase and at some point the flow through the orifice restriction (in this case in the reverse direction) would start to be limited.

Which is it? Is the orifice restriction you are referring to in the inlet to the RO system, or the outlet of the RO system?

If it's in the outlet, are you worried about restricting the reverse flow/pressure into the RO system?

Are you sure the orifice you are referring to is not being used to measure flow-rate? (It would most likely have pressure taps on the both sides of the orifice flange(s) and they would be connected to a differential pressure gauge or electronic transmitter.)

I searched and searched the World Wide Web for information on orifices (or, is it orifeces???) and this was about the best of twenty or so results I surveyed:

https://instrumentationtools.com/difference-between-orifice-restriction-orifice/

Anyway, if you can be of more help in describing your situation we might be able to be of more help. An orifice restricts flow through it--always. And that flow restriction also results in a pressure drop on the downstream side of the orifice plate. But, it's the magnitude of the restriction, and in some cases the magnitude of the flow restriction OR the pressure reduction that's important. If it's the differential pressure that's being measured, then the flow restriction and the pressure on the downstream side of the orifice isn't important.

(Can you see why this causes so much animated discussion at plants? There are all manner of calculators available on the World Wide Web for calculating pressure drops and flow restrictions; but most of them were designed for particular applications and that isn't very well explained in the description of the calculator.)
 
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