2 replies to this topic

[rsk]

An orifice to be used for flow measurement in a 40mm NB pipe line with a gate valve, whether orifice to be install upstream of valve or downstream.
regards

[hollerg]

Usually one installs a flow orifice upstream rather than downstream of the valve. When a valve is the upstream obsticale requires a longer than usual length of pipe. Two elbows out of plane is a usual physical configuration which requires something like 30 up and 5 down stream pipe diameters. (Need the AGA paper to double check that data as well as to look up the proper settling distance for an upstream valve)

Some practical issues may limit you from downstream placement depending on reynolds number and or material properties:

-- With liquids, dp must be low enough to avoid flashing in the meter vena contracta.
-- with gases, the changing pressure drop changes the inlet pressure and thus the density, altering the meter reading.

Edited by hollerg, 26 August 2009 - 09:20 PM.

[papaya]

It would be better to install the orifice downstream of the Valve. This is due to pressure drop across the Orifice might create Joule Thompson effect which would lower the temperaure, depending on pressure drop it may drop to really low temperature.

As such there should be a 600mm distance between Valve & Orifice, this way the cold temperature would have less chance of transfering back to the Valve.

If you install the Orifice before the Valve, the cold temperature would most likely make it to the Valve. This case you need to take into the low temp design for that valve.

Sources:
http://webwormcpt.bl...ce-is-some.html

But then again this applies to restriction orifice. For Orifice Meter it would need a straight flow, so if pressure drop is not big you should install it upstream of valve, if downstream you may need a longer distance. But whether it's Upstream or Downstream, I believe you still need certain distance for both Upstream & Downstream. Typical is 10D upstream & 5D downstream?

Edited by papaya, 16 March 2010 - 01:50 AM.