4 replies to this topic

[JoeWong]

It affect the measurement accuracy.

Search with Google...plenty of literature available.

[Rama]

Sunil,

The straight lengths upstream / downstream of the orifice depend on what is known as (beta) B value. (B is the ratio of the diameter of the orifice to the internal diameter of the pipe). The value you have quoted is for a very low B of less than 0.3, and that too is the bare minimum. For higher values of B, these values are even higher.

All that will happen if you reduce the straight distances is that the accuracy of measurement will be affected. There are many published data you can get by using a search engine; better still there are many books & manufacturers’ catalogues that can throw more light on the area of your interest.

An instrumentation engineer would refuse to accept these values as it, as they are very finicky about the accuracy of the measuring instrument! However if you can sacrifice the accuracy say to ± 8% or more you can reduce the straight lengths even to 5D by 2D (using your nomenclature)!

You should realise that the straight lengths especially on the upstream side of the orifice is to stabilise the flow and eliminate swirls etc. especially after fittings such as elbows, (or double elbows in some cases). If your layout does not permit such luxuries, you may fit flow straighteners or flow conditioning devices, immediately after the previous upstream fitting, as a remedial measure. However I have always seen that it is possible to improve the layout with some out of the box thinking at least to some extent, before sacrificing the accuracy.

The same reasoning applies for control valves.

[delphitt]

Hi,
I agree with Rama and Joe. My experience is that the distance is very important for the accuracy of the reading, to get reliable results, it is recommended that it is followed.

There was a flow rate i had to measure - there wasn't a flowmeter on the pipeline, nor did i have access to a portable one at the time. The minimum distance was just under 3 pipe diameters. I had to hire an instrumentation contractor to measure the flow and he, outright, refused to take any readings at that point.

The result was that i had to get scaffolding constructed at another location where the distance was sufficient. Later, when i measured the flow at both places, using a portable ultrasonic flowmeter, the values were approximately 30 m3/hr apart and it turned out that the value measured by the vendor was right.

I have also used straightening vanes just downstream bends and valves, but they, too, require placement according API codes. In the Plant i work on, one particular straightening vane was not installed according to the API code during the design in 1996 and to this day, the flow is always inaccurate.

Hope this gives a guide,
SM

[Andree]

"Coriolis" type flowmeters are not vulnerable to this as they directly measure mass flow - however they are usually much bigger and this can make a problem to fit an instument if you have limited space (my flowmeter for max flow around 10m3/h has 545mm flanfe-to-flange distance and its weight is 18kg!)