Orifice FT and CV

It is to do with making sure that the disturbances in the flow streamlines have enough pipe length to dissipate, so that the flow has returned to normal after that point. Looking at most flow instruments in catalogues, they will have upstream-of-fittings and downstream-of-fittings criteria that the manufacturer needs to state to ensure rated instrument accuracy under the user's conditions.

It has little to do with the type of instrument, though the criteria for different instruments and different manufacturers may vary.

Fully agree with previous comment. Its important to fully understand the process medium and range of concern. Additionally, the term "Snubber" as used in process line intrumentation, is a fancy yet misleading name for an orifice-which application and placement result in a steady reading--instead of an oscillating and depending on process medium, sensor type and affects on flow in a line with many elbows and such with resultant pertabations of flow in or around area of sensor placement. Hope I didn't confuse the issue.

Control valves provide a large disturbance to flow and usually meters had to be located a significant length downstream of them to allow for the flow to have settled down (unless flow straighteners are used). This is one reason for locating meters upstream of the valve. Also by locating the meter upstream you can be more certain that there will be no flashing across the meter which will almost certainly invlaidate the readings. My personal preference would be to have the meter upstream for this reason.

As flows change the pressure downstream of a control valve will change more than the upstream pressure (hydraulic losses vary with the square of flow at 50% flow the hydraulic losses are 25% and so the pressure drop on the CV must pick up this lost hydraulic losses). By locating the meter upstream you maintain a more constant condition and one closer to that for which the meter was specified so its reading should be more reliable for liquids this isnt an issue but for gases where density (which is proportional to pressure) is key to the metering it is a factor.

You also mention that having the meter downstream allows you to measure the controlled flow - surely if there are no branches in the piping it doesnt matter whether you measure up or downstream?

Thank you for your answer sir. it was a good explanatory one. once again thankyou

With either the oriface plate or the mag flow meter you have to have a full pipe. You can measure either upstream or downstream from the control valve if you can guarantee that the pipe will always be full, even at low flow conditions. Different straight pipe upstream/downstream lengths are recommended for the different flow meters. Mag flow meters are pretty forgiving if these recommended lengths are not adhered to. Generally mag flow require 3 upstream and 5 downstream pipe diameters of straight pipe; oriface plates are generally stated as 5 upstream and 10 downstream.

Hi there,

All very good answers.

Remember a control valve will cause turbulant flow downstream. You can locate the FE downstream of the control valve - but make sure you have enough pipe diameters to do this so that by the time the flow gets there it has recovered.

The same applies to putting the FE upstream of the control valve. The FE will cerate turbulant flow as it passes through the element. You then need to make sure that the control valve is placed with sufficient pipe diamteres away from the element. If not you could end up with flashing or cavitation in the valve.

This applies to all flow measurement types. A good way to get an idea of this is to take your process data, both for the valve and the flow element/meter and calculate the Reynolds number for each. This will help you determine the flow characteristic. With these numbers in hand you will have a better picture.

Regards,

Craig

we can use DP FT before control vale but 8 diamter of pipe upsteam orifice and 12 diameter of pipe downstream of orifice should be straight. and flow measure against orifice of gas is better than othe meter. by ranjha