FIELD OF APPLICATION
The present invention relates to a method for measurement of flow in gas line
using an improved flow transmitter impulse line system with safe drainage of
accumulated water from the impulse lines.
The invention also relates to a flow transmitter impulse line system for
measuring the flow in a gas line.
BACKGROUND OF THE INVENTION
The flow in the impulse system is proportional to square root of differential
pressure ie Q = k* √ (P1 - P2), where Q = flow, k = constant, P1 = pressure
before orifice, P2 = pressure after orifice plate. This differential pressure is
tapped and transmitted through a tube and connected to a differential pressure
transmitter and this tube further extended and connected with a drain valve.

In a flow impulse system the impulse lines are transmission media of differential
pressure created across an orifice plate to the flow transmitter. This is usually
made of high quality steel tube and the diameter varies from 10 to 25 mm. A
positive impulse line and a negative impulse line are connected to the gas line to
carry the created differential pressure across the orifice plate.
The positive impulse line is connected before the orifice plate at a distance from
the orifice plate which is equal to the diameter of the gas line in which the fluid
flow takes place.
The negative impulse line is connected after the orifice plate at a distance of half
the diameter of the gas line.
A manifold unit is used for equalizing pressure across the transmitter. This is a
safety device which prevents damages to the diaphragm of the transmitter and is
used during calibration, removing or fixing of transmitter.
The orifice plate used is a primary element for flow measurement. It is made of
steel plate; the differential pressure is created across this orifice plate.

The flow transmitter used is a secondary element of flow measurement, basically
it converts pressure signal to electrical signal and retransmits this flow signal to
remote location without losing signal. The signal is usually of 4 to 20 mA and
the remote location may be upto 200 m away.
Heavy quantity of moisture contained in coke oven, blast furnace or LD gas or
mixed gas is gradually condensed in the gas line resulting in accumulation of
water in the impulse lines.
When the accumulated water in the impulse line rises to transmitter level, it
blocks the line from transmitting actual pressure to the transmitter. Thus
transmitter gets erratic pressure and acts accordingly. This may affect any one
or both the positive and negative line of the impulse lines. This will depend upon
the gas line layout and design of the impulse lines. For example if the negative
line gets blocked during low flow the transmitter will show high flow because the
differential pressure across the transmitter will be high. Such erratic flow
measurement may result in improper combustion inside the furnace with chances
of over or under temperature, or even chances of explosion.

In the existing method, in the event of water accumulation the operator should
be informed before draining water from impulse line. The control system is put
on manual mode from auto mode after braking automation level-II to level-I.
The pressure across the transmitter is equalized with the help of manifold unit.
Adequate safety precaution has to be taken when water is drained out from both
the impulse lines since this is a very poisonous gas. The manifold unit is
normalized. The operator is informed after job is over and the control system is
normalized.
Thus there exists a need to develop an improved flow impulse system for
overcoming the disadvantages of the existing system and safe drainage of
accumulated water.
SUMMARY OF THE INVENTION
Thus the main object of the present invention is to provide a totally safe
drainage system for accumulated water from the impulse line.
Another object of the present invention is to reduce the time required to drain
out the accumulated water.

Yet another object of the invention is to reduce the manpower requirement as
only one person can do the job safely.
These and other objects of the invention can be achieved by providing a pair of
small drift pots and a pair of additional drain valves in both the impulse lines.
Thus the present invention provides a method for measurement of flow in gas
line using an improved flow transmitter impulse line system, comprising the steps
of: providing a positive impulse line and a negative impulse line on either side of
an orifice plate arranged in said gas line, for creating a differential pressure
measurable at a remote location, using a flow transmitter; providing a drift pot
arranged between a pair of drain valves in each impulse line; collecting
accumulated condensed water in said inpulse lines in said drift pots by opening
the upstream drain valves, keeping the downstream drain valves closed; and
draining out the condensed water from the drift pots opening the downstream
drain valves keeping the upstream drain valves closed.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention can now be explained with reference to the figures of the
accompanying drawings, where
Figure 1 shows conventional impulse line design of flow measurement.
Figure 2 shows the improved design of flow transmitter
impulse line of the present invention.
DETAILED DESCRIPTION
The known arrangement for draining the accumulated water in the impulse lines
as a result of condensation of moisture in the gas line 1 is shown in Figure 1.
The arrangement shows the positioning of the gas line 1, orifice plate 2, the
positive and negative impulse lines 3, manifold unit 4 for equalizing pressure
across a transmitter 5 and two drain valves DV. The drain valves DV can be
used for draining out accumulated water in the impulse lines. This can be done
only when the control system is in manual mode.

Figure 2 shows the improved design of the flow transmitter impulse line of the
present invention. The positive impulse line 3 is connected at a distance D from
the orifice plate 2, where D is equal to the diameter of gas line 1. The negative
impulse line 3' is connected at a distance D/2 from the orifice plate 2.
In the present invention, two small drift pots 6 are provided in the impulse lines
each between a pair of drain valves DV1, DV2 and DV3, DV4.
The small drift pots 6 can be of, for example 50 mm diameter and 100 mm in
length. It can be made from MS or SS material. The size of the drift pot should
be as small as possible to fulfill the purpose. As the gases are compressible
more gases would try to be accommodated in the impulse line to maintain
required pressure generated across the orifice, causing time lag if extraordinary
large drift pots are used, as the flow is very much dynamic. Thus the flow
sensing by transmitter will also lag. For smaller drift pot nothing will effect the
system for the industrial application.

The draining procedure is described hereunder.
Drain valve DV1 of the positive impulse line is closed and drain valve DV2 is
opened for allowing water from the positive impulse line to drain out. Valve DV2
is then closed and valve DV1 is opened for about one or two minutes. The whole
process is repeated for the positive impulse line till all accumulated water is
drained out.
Similarly the entire process is repeated for the negative impulse time of the
transmitter 5.
Drift pots 6 are preferably as small as possible other wise measuring response
may effect. In case of any problem valves no DV1 and DV3 may be in closed
position in normal time.

Before draining, accumulated water in the line can be transferred from line to
drift pot 6 by opening valves DV1 and DV3 and keeping valves DV2 and DV4
closed. After transferring water it can be drained out by using above-mentioned
procedure. The frequency of water draining may be decided after gaining
experience in system in few days.
Some of the advantages of the improved flow impulse system of the present
invention over conventional system in case of water draining from impulse line
are:
Here there is no need of braking of auto mode to manual mode and
automation Level-II to Level-I control.
Prior information to operator is not mandatory.
100% safety during draining water, no need to take extra precaution
as required in earlier case.
Total process time is low compared to conventional system.
Only one person can do these jobs safely.
No expertise is required for controlling furnace during this period as
require in conventional system.

WE CLAIM:
1.A method for measurement of flow in gas line using improved flow transmitter
impulse line system, comprising the steps of:
- providing a positive impulse line and a negative impulse line on either side
of an orifice plate arranged in said gas line, for creating a differential
pressure measurable at a remote location, using a flow transmitter;
- providing a drift pot arranged between a pair of drain valves in each
impulse line;
- collecting accumulated condensed water in said impulse lines in said drift
pots by opening the upstream drain valves, keeping the downstream drain
valves closed; and
- draining out the condensed water from the drift pots opening the
downstream drain valves keeping the upstream drain valves closed.

2. The method as claimed in claim 1, wherein said positive impulse line is
connected to the gas line before the orifice plate at a distance equal to the
diameter of the gas line and said negative impulse line is connected to said
gas line after the orifice plate at a distance equal to half the diameter of the
gas line.
3. The method as claimed in claim 1, wherein said flow transmitter is used for
generating an electrical signal from the differential pressure of the positive
and negative impulse lines and transmitting said signal to a remote location
for measurement of flow.
4. The method as claimed in claim 1, wherein for large unequal pressure
across the diaphragm of the transmitter, a manifold unit is used for equalizing
the pressure across the flow transmitter.

5. The method as claimed in claim 1, wherein said drift pots are of small size
depending on the line flow capacity and the moisture content in the gas line.
6. An improved flow transmitter impulse line system for measuring gas flow
in a gas line comprising:

- an orifice plate arranged in a gas line for creating differential pressure
across the plate;
- a positive impulse line and a negative impulse line connected to said gas
line respectively before and after said orifice plate;
- a flow transmitter for transmitting electrical signals to a remote location
representing the differential pressure;
- a drift pot provided in each one of said impulse lines arranged between a
pair of drain valves for automatically collecting accumulated condensed
water in said impulse lines and draining out the water using the drain
valves.

7. The system as claimed in claim 6, wherein said orifice plate provided in said
gas line for creating differential pressure across the plate is made of steel plate.
8. The system as claimed in claim 7, wherein said positive and negative impulse
lines are made of high quality steel tubes having diameters between 10 and 25
mm.
9. The system as claimed in claim 6, wherein said flow transmitter is used for
converting pressure signal into electrical signal of 4 to 20 mA and transmitting
the same to a remote location upto about 200 metres away.
10. The system as claimed in claim 6, wherein a manifold unit is provided for
equalizing the pressure across the flow transmitter.

11. The system as claimed in claim 6, wherein said drift pot size is 50 mm in
diameter and 100 mm long.
12. A method and system for measurement of flow in a gas line, substantially as
herein described and illustrated in the accompanying drawings.

ABSTRACT

TITLE:- METHOD FOR MEASUREMENT OF FLOW IN GAS LINE
USING IMPROVED FLOW TRANSMITTER IMPULSE
LINE SYSTEM.
The present invention relates to a method for measurement of flow in gas line
(1) using improved flow transmitter (5) impulse line system, comprising the steps
of providing a positive impulse line (3) and a negative impulse line (3 ) on either
side of an orifice plate (2) arranged in said gas line (1), for creating a differential
pressure measurable at a remote location, using a flow transmitter (5); and
providing a drift pot (6) arranged between a pair of drain valves (DV1, DV2 or
DV3, DV4) in each impulse line; and collecting accumulated condensed water in
said impulse lines in said drift pots (6) by opening the upstream drain valves
(DV1 & DV3), keeping the downstream drain valves (DV2 & DV4) closed; and
draining out the condensed water from the drift pots (6) opening the
downstream drain valves (DV2 & DV4) keeping the upstream drain valves (DV1 &
DV3) closed.