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1) I'd like to get some advice from flow meter specialists. I am to install a flanged dP flow meter on a DN450 pipeline measuring hydrocarbon gas. I need to know the upstream and downstream straight length requirements. I am more confused after reading the ISO standard. Can someone give me some rule of thumb on the upstream and downstream straight length requirements, i.e. 20D upstream and 10D downstream (assume no bends alone the DN450 pipeline)? Any calculations that you can provide?

2) Has anyone connected a motorized operated valve (MOV) to a DCS system using analog or potential free contacts for remote control? Please share your experiences and advice me of any points to take care during installation.

1/ 20 & 10 is the minimum I would aim for but I don't claim to be a specialist. i.e. for custody transfer you may need more.

2/ There are many options for motorized valve depending on what you need to do.
If it's one of the AC capacitor run style I like to power it from 2 discrete outputs (preferably solid state) bypassing the 4-20 mA option. I think it's more reliable that way. The PID loop if it has one can be configured to output forward / reverse pulses based on deviation from setpoint. The process variable provides the feedback.

Using an analog output is also OK. and might have some advantage in slow loops like temperature.

One thing about motorized valves, if you need fail action on power failure you might have to provide a UPS.

Tell us a bit more about your application.
Roy

There are a number of factors when it comes to straight run requirements. The ASME has some guidelines that relate to elbows, restrictions, valve placement, etc. I suggest you go to their website. Another good source is R.W. Miller's book "Flow Measurment Engineering Handbook" (McGraw Hill publisher). Miller's book has a whole section on flow abnormalities and corrective approaches. There are also calculations for accomodating pulsation.

John Catch
www.inflowinc.com