Turbine Meter without Flow Straightener 1

[EmmanuelTop]

In AFC drawings we spotted a turbine flow meter without flow straightener upstream. Straight piping run upstream of the meter is 10D, downstream of the meter is 5D.

The fluid measured is wet Natural gas, almost pure Methane, water-saturated, at 15 barg and 50 degC.

I'm afraid it is too late for any design changes and installation of a straightening vanes, so my question is how bad the measurement accuracy will be affected in absence of the vanes? Measurement is quite important and it will serve as one of the key parameters for acceptance of the compressor (discharge flow). Contractual tolerance is +/- 5% on flow.

http://antwrp.gsfc.nasa.gov/apod/astropix.html

 

[roydm]

I would expect the turbine to do a pretty good job, it's kind of like it's own straightener in a way.

 

[rcvonbrecht]

The amount of straight run is a function of what is happening prior to the run. If you have a couple of elbows then you have trouble. You’ll get swirl which is a flow vector that is not orthogonal to the pipe axis. This will reduce your accuracy. API MPMS 5.3 allows 5D upstream for a standard system with straighteners but that jumps out to 15 to 50D depending on the upstream conditions when you don’t have them. Though intended for orifices, ISO TR 12767 can give you some insights on how much you’ll gain in uncertainty for non-ideal installations. But at 5% doesn't look too impossible since turbine meter installations target 0.5% or better full scale accuracy.

If was my money I’d go for a Zanker plate or a proprietary design that completely breakup the existing flow field and allow it to reform. Traditional tube bundles have been found not to do a good job with swirl.

 

[rcvonbrecht]

The installation of gas turbine meters is more govern by AGA Report 7 and they take a little differnet bent. They're not perscriptive as API so vendors have different recommendations for turbine meters. For smaller meter you could use a confirguraion like that shown in the attachment.