Needle Valve Position Pre or Post Flow Meter 2

[JohnnyHS]

Ladies and Gentlemen of the engineering community your input please!

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?









Johnny H-S BSc(Hons) MInstMC

Engineers like to solve problems. If there are no problems handily available, they will create their own problems.

 

[zdas04]

Sample lines should have a stinger that sticks into the flow to keep from getting a sample too near the no-flow boundary. Consequently, a sample port creates some interesting purtubations in the flow profile and AGA calls for them to be downstream of the primary element for every type of flow meter.

I'm not familiar with a "water cooing line" so I can't help there.

David

 

[gerhardl]

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.

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.

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.

 

[JLSeagull]

Apply the needle valve to the inlet of the rotameter. You can use it to reduce the flow when the glass breaks. ;-)

For hydrocarbon, toxics or other hazards use armored rotameters.

 

[rneill]

As mentioned above, you might want to look at a sample valve a little more sophisticated than the needle valve such as those made by Oliver, Anderson Greenwood, Alcoa, or Hy-Lok (and perhaps others) which can be ordered with the sample quill that will project into the line:

http://www.valves.co.uk/valves/sampling-dbb-valves/d_type .html

http://www.alco-valves.com/dbbValvesProducts.asp

http://hylokusa.thomasnet.com/produ...valves/sample-and-injection-valves?&plpver=10

http://www.aginstrumentation.com/primary_isolation.asp

If you use a valve with a "sample quill" then this is definitely a flow disturbance and you would want to stay outside the upstream / downstream distances specified by the flowmeter manufacturer as mentioned by the folks earlier.

 

[DeePig]

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). 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. 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.

Hope this helps.

 

[JohnnyHS]

Hi Guys Thanks for your inputs. great stuff!

After a little digestion and reading your comments what do you think of putting the Neddle Valve always downstream on the Flowmeter for both liquid and gas.

My thinking being this: if the needle valve affects the state (phase) of the medium being metered (via pressure drop) then why insert it prior at all?

Or i've just thought. Deepig were you thinking of the back pressure increase caused by the restriction of the downstream needle valve on the gas system which would make it more liable to turn the gas into a liquid? (whereas in the liquid system this is not an issue as the phase is liquid anyway)

mmmmm



PS JLSeagull - like your thinking but not a very phase/state applicable solution!!

PPS for all this is a 1/2" OD line i'm using for this application



Johnny H-S BSc(Hons) MInstMC

Engineers like to solve problems. If there are no problems handily available, they will create their own problems.

 

[JimCasey]

>>Deepig were you thinking of the back pressure increase caused by the restriction of the downstream needle valve on the gas system which would make it more liable to turn the gas into a liquid?<<

Deepig was right. the gas will likely not liquefy but it IS compressible. So for a given mass flow rate, the compressed gas (Valve at outlet) will flow at a lower velocity and you will get a lower flow indication than you would with the valve at the inlet and the flow media expanded to atmospheric pressure.

Actual industrial flowmeters are available with custom scales calibrated for the specific fluid, density, pressure, and viscosity. Still, the best reason for putting the valve downstram on a gas flow is to be able to use a smaller, thus less expensive, meter. Another reason is to locate the valve so that the meter is exposed to the least pressure (density) variation. Likely either the upstream pressure OR the downstream pressure but seldom both.