I have a flowmeter (variable area) and am deciding where to put the needle valve to regulate the flow for a) the sample line and b) water cooing line. I have had discussions with various engineers with some recommending pre and some post needle valve location for both.

In this particular example for the sample flowmeter and needle valve  it is hydrocarbon condensate (think gasoline) with a line pressure of 11 Barg and a flow rate of 3 Litres a minute.

For the cooling water it is a similar flow rate but a reduced pressure at 3Barg nominal.

Where would you put it? and why?Safety valves

All flow meters will have a functional description regarding working range and and installation to give minimum variation performance variation(within guarantee).

The description will normally give flow limit variation, art of fluid, temperature, pressure, direction for mounting orientation (and insertion depth in pipeline if relevant) and requirement of straight pipeline stretch before and after the flowmeter.Plug Valves

Very often one or the other of theese parameters are not fulfilled or broken in practical installations, straight stretches often the most likely to be neglected.

If all can be fulfilled its more a pure practical question if you place the valve upstream or downstream the flowmeter.Reducing Valves

If one or several are broken the flowmeter should be placed at the strech (upstream or downstream the valve) where there is least disturbance or influence on parameters.

A general rule of thumb that I have seen used is install the needle valve upstream of the rotameter for gas, and downstream for liquid.  

Basically, installing the valve downstream for liquid simply keeps you away from your bubble point in your rotameter tube.  Obviously, the needle valve acts as a considerable pressure drop, so keeping that downstream of your rotameter tube will help maintain a higher pressure of liquid within the tube - thus stopping any clogging from bubbles causing a foam.  

Installing the needle valve upstream in gas applications does two things.  First, it helps keep you away from the dew point within the rotameter tube (opposite logic as mentioned above in the liquid applications).Balance Valves  Second, if you are flowing to vent pressure with little downstream pressure drop, your rotameter will be measuring flow rate at atmospheric pressure.  This is usually desired.API Cast Steel Valves  Otherwise, the flow rate will be measured given the compression of gas at higher pressure, and unless your rotameter was calibrated at a higher pressure, your measurement can have significant error.

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