Device for improving the flow profile in a gas line

Flow straightener for improving the flow profile in a gas line ahead of a flowmeter, comprising at least three plates with regularly distributed perforations and spaced apart parallel to each other perpendicularly to the axis of the line. The perforated plates are mounted with a circumferential sealing (preferably welded) in or on a sleeve which can be inserted in the line with a sliding fit.

The invention relates to a device for improving the flow profile in a gas 
line ahead of a flowmeter, the said device comprising at least three 
plates with regularly distributed perforations, which, spaced apart 
parellel to each other, are disposed perpendicularly to the axis of the 
line. 
A device of this type, usually called `flow straightener,` is known for 
example from U.S. Pat. No. 2,929,248. 
Flow straighteners are used in order that the straight line section to be 
installed in front of a flowmeter for ensuring proper operation of the 
meter can be shortened in those places where insufficient space is 
available for a long straight line section (min. length: 20 times the line 
diameter). If a flow straightener is installed behind a bend in the line, 
the length of the straight line section between the flow straightener and 
the flowmeter may be taken equal to approximately 5 times the line 
diameter. Other types of flow straighteners consist for example of a nest 
of pipes installed in the line, or of a number of baffles disposed 
parallel to the axis of the line; the type of flow straightener mentioned 
above in the first paragraph is simple and is used the most frequently. 
In the above-mentioned type of flow straightener of the known design the 
perforated plates are interconnected by means of spacer bolts or by means 
of bars welded to the plates in the longitudinal direction of the line; 
usually, one of the outer plates is slightly larger than the others and is 
clamped between two flanges of connecting line sections. It has appeared 
that this known design is not sufficiently effective if it is installed 
behind an element that has a strong disturbing effect on the flow profile. 
Such a situation arises in reducer stations of a natural gas grid where 
the gas pressure is brought down from 40-60 bar in the main line to 
approximately 8 bar in the local line. The quantity of withdrawn gas is 
measured at a relatively short distance behind the reducing valve which 
disturbs the flow profile in the line very strongly. The object of the 
invention is so to improve the flow straightener mentioned above in the 
first paragraph that even a strongly disturbed flow profile will be 
sufficiently corrected to ensure reliability of the measurement. 
The invention consequently relates to a device for improving the flow 
profile in a gas line ahead of a flowmeter, in which device at least three 
plates with regularly distributed perforations, spaced apart parallel to 
each other, are disposed perpendicularly to the axis of the line, and in 
which according to the invention the perforated plates are fitted with a 
circumferential sealing in or on a sleeve which can be inserted in the 
line with a sliding fit; the perforated plates are preferentially welded 
into or onto the sleeve. 
It is very surprising that the relatively small improvement produces a 
large effect. The improvement in efficiency compared with the known flow 
straighteners is larger than might be expected from the elimination of the 
bars disposed in the direction of flow and of the clearance between the 
inner wall of the line and the edges of the plates. 
Further, if the device according to the invention is fitted with three 
perforated plates, the plates may be set closer together -- preferentially 
at a distance approximately equal to half the internal diameter of the 
line -- than in the known device where this distance is preferentially 
taken equal to the line diameter, so that the flow straightener according 
to the invention can be given a smaller overall length than the known flow 
director. In the abovementioned application of the device behind a 
reducing valve, the overall surface area of the perforations in each 
separate plate should preferentially come within 15 and 25% of the free 
sectional area of the line, although this range is not limitative.

The meaning of the reference numerals in FIG. 1 is as follows: 
1 and 2 two gas line sections, 
3 and 4 flanges connecting parts 1 and 2, 
5 a sleeve which can be inserted in line section 2 with a sliding fit, 
6 a perforated plate welded onto the end of sleeve 5; the diameter of plate 
6 is so large that it can be clamped between the flanges 3 and 4 (the 
packing is not shown), 
7 and 8 perforated plate circumferentially welded into sleeve 5. 
The distances between plates 6, 7 and 8 approximately correspond to half 
the line diameter; the overall surface area of the perforations in each 
separate plate is approximately 20% of the free sectional area of the 
line. 
The reference numerals in FIG. 4 are the same as in FIG. 1 with the 
addition of the reducing valve 9. 
FIGS. 2 and 3 show flow profiles recorded along a diameter in a gas line at 
a distance of 5 times the diameter behind a flow straightener, FIG. 2 
referring to a flow director of the known perforated-plate type, and FIG. 
3 to a flow straightener according to the invention. In both cases a flow 
disturbing plate covering the entire sectional area of the line and 
provided with an excentrically disposed hole was placed ahead of the flow 
straightener. The ratio between the velocity V at a given point on a 
diameter and the average velocity V.sub.m is plotted on the horizontal 
axis, while the ratio between the distance r to the line centre and the 
radius R of the line is plotted on the vertical axis. As can be seen in 
FIG. 2, the flow profile behind the known flow straightener is still 
strongly disturbed, whereas the flow profile behind the flow director 
according to the invention (FIG. 3) substantially corresponds to that of 
an undisturbed flow.