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Flow Measurement Process Fundamentals

Difference between Orifice & Restriction Orifice

This article discusses the differences between the orifice and the restriction orifice of the physical form, functionality and applications in the oil and gas facilities

Orifice vs Restriction Orifice

preliminary

Do you want to measure the flow of a fluid or if you want to reduce the pressure of a fluid drastically and permanently? Both of these can be done by objects whose names both contain the word Orifice, the first and the second Orifice Restriction Orifice. Generally, the orifice has the shape as a plate that has a hole in the middle. Examples of the orifice as shown below:

orifice

Approximately what are the differences between the Orifice for flowmeter and Restriction Orifice? Let us examine the figure below is an example of the image for the flowmeter and Restriction Orifice Orifice.

orifice-restriction-orifice

The shape of the orifice flowmeter and orifice above restriction is a common form and in general there is a difference in the profile of thehole of the second orifice. Orifice flowmeter generally have a profile for the first pit straight, but then notched (bevel) with a slope of about 45°. While the restriction orifice hole has a straight profile.

Differences Orifice and Restriction Orifice

Whether from the shape of the hole will make the difference between them? Yes, generally so. To find out let’s look at the difference between the pressure profile of a fluid passing and Restriction Orifice Orifice on their design conditions of each of the image below.

orifice-restriction-orifice-flow-profile
Do you see the difference? If we look carefully, the amount of lost press (pressure loss) caused by narrowing of the fluid at the orifice area was not as big as in restriction orifice.

At orifice flowmeter, because the shape of the hole that has notches – which means reducing the mileage of flow are experiencing differences in cross-section – then the pressure profile which occurs after passing through the orifice will decrease, but then tried to get back into the pressure was and there is a little lost tap a permanent ( permanent pressure loss) so that the difference between the pressure upstreamand downstream pressure is not too great.

While on the Restriction Orifice, because the shape of the hole is straight and long enough – depending on the thickness of the plate – then the amount lost permanent press is large enough so that the difference between the pressure upstream and downstream pressure is quite striking. In this case, the fluid flow in a state of being strangled (choked flow).

In the matter of design, orifice for flowmeter always designed to flow in a subsonic velocity in order to ensure the accuracy of flow measurement, while the restriction orifice always designed to flow in the sonic velocity to ensure choked flow.

Indeed RO main function is to limit the flow (flow-limiting). Function pressure restriction (limiting pressure) of the RO is essentially a consequence of the relationship between pressure drop and flowrate.The phenomenon of choked flow itself is the mass flowrate constant downstream pressure despite its decline due to sonic velocity. In a real application in the plant, the function of flow restriction and pressure restrictions can equally be applied to the use of RO.

Flow restriction between the pressure vessel system with a flare system

There is a new pressure vessel to be installed and the existing flare system that has a certain capacity. The capacity of the existing flare system actually has been designed to not take into account the presence of a new pressure vessel, while additional flare to create a new system to increase the capacity of the existing flare system is not economical. On one hand, the capacity of the flare is certainly not to be missed and one solution is to install RO which has a function as a flow restrictor.

Installation in the downstream RO BDV is generally done on the advice of study over pressure protection and blowdown study, in order to flare its capacity is not exceeded.

Simple sketch pressure vessel, RO and flare system

Simple sketch pressure vessel, RO and flare system

Press a different division of the control valve

There are a recycle line of a high pressure system (discharge of areciprocating compressor) and into the low-pressure system(separator vessel). It is desirable to regulate the pressure of the fluid which you want to recycle by using a control valve that is not a special design. Control valve essentially has the function of regulating a fluid flow by creating a pressure difference between upstream and downstream. So the fact that there will be a high enough pressure drop in the control valve.

With such high pressure difference between upstream and downstream, control valve has a load large enough to reduce the fluid pressure. Slightly deviated from the main topic, there are some considerations when designing a control valve that has a high compressive lost:

  1. At high pressure drop which means you can develop erosion, abrasion or cavitation on the trim of the control valve.
  2. In the high pressure drop hydrate / liquid droplets / solid material makes it possible to form within the valve (depending on the fluid and other materials that shipped).
  3. High pressure drop will cause the outlet temperature of the control valve is very low (a decrease of pressure is usually followed by a decrease in temperature = Joule-Thomson effect), so in some valve materials can become brittle / brittle which can lead to rupture valve
  4. High speed due to high pressure drop that can be eroded downstream piping.
  5. High pressure drop which means the amount of energy dissipated in turbulence is quite large and cause noise (noisy).

Alternative application lost control valve for high pressure is to use special design of the control valve or the division of the difference between the control valve tap with another device, namely Restriction Orifice.

To reduce the load can be mounted Restriction Orifice after the control valve. Installation restriction orifice as shown below.

Simple sketch control valve and pressure vessel

Simple sketch control valve and pressure vessel

For applications in the downstream RO control valve, basically used the principle / correlation that for a given flow rate and Bore certain size there are differences in pressure (differential pressure) specific.Generally, Differential Pressure needed by those in the control valve between the control valve and Restriction Orifice. By division Differential Pressure (DP share) the expected effects of the vibrations that would occur in the pipe was not so great due to the change in pressure differential is too great in one device.

It should be noted carefully that the actual control valve is the main tool that bear different press, so that the portion of the largest division of the differential pressure control valve is not in the RO. DP-sharing applications such as described above can use a special design of a control valve that is capable of working in high pressure differential, of course taking into account that there is a regulated flow of critical line due to the special design of the control valve will certainly have a special fee as well.

Also Read: Calibration of DP Capillary Seal Level Transmitter

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3 comments

Johanna November 13, 2015 at 1:26 pm

Are there sources for your article?
Especially about the behaviour of pressure through restriction orifice and orifice?
Many thanks in advance, interesting article!

Reply
fawad April 7, 2016 at 7:13 am

kindly tell me installation parameters of orifrice plate what is the exact point to fix orifice

Reply
Thomas Sebastian February 17, 2017 at 6:01 am

shall install the restriction orifice ( RO ) in any direction even though it is marked upstream on the tab. Pls.clarify.

Reply

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