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Saturday, October 4, 2008

LEVEL MEASUREMENT




Three valve manifold


Bubbler method
Accurate continuous measurement of volume of fluid in containers has
always been a challenge to industry. This is even more so in the nuclear
station environment where the fluid could be acidic/caustic or under very
high pressure/temperature. We will now examine the measurement of fluid
level in vessels and the effect of temperature and pressure on this
measurement. We will also consider the operating environment on the
measurement and the possible modes of device failure.

 Level Measurement Basics

Very simple systems employ external sight glasses or tubes to view the
height and hence the volume of the fluid. Others utilize floats connected to
variable potentiometers or rheostats that will change the resistance
according to the amount of motion of the float. This signal is then inputted
to transmitters that send a signal to an instrument calibrated to read out the
height or volume.
In this module, we will examine the more challenging situations that require
inferential level measurement. This technique obtains a level indication
indirectly by monitoring the pressure exerted by the height of the liquid in
the vessel.
The pressure at the base of a vessel containing liquid is directly proportional
to the height of the liquid in the vessel. This is termed hydrostatic pressure.
As the level in the vessel rises, the pressure exerted by the liquid at the base
of the vessel will increase linearly. Mathematically, we have:
The level of liquid inside a tank can be determined from the pressure
reading if the weight density of the liquid is constant.
Differential Pressure (DP) capsules 
These are the most commonly used devices to
measure the pressure at the base of a tank.
When a DP transmitter is used for the purpose of measuring a level, it will
be called a level transmitter.
To obtain maximum sensitivity, a pressure capsule has to be used, that has a
sensitivity range that closely matches the anticipated pressure of the
measured liquid. However, system pressures are often much higher than the
actual hydrostatic pressure that is to be measured. If the process pressure is
accidentally applied to only one side of the DP capsule during installation or
removal of the DP cell from service, over ranging of the capsule would
occur and the capsule could be damaged causing erroneous indications.

 Three Valve Manifold
A three-valve manifold is a device that is used to ensure that the capsule will
not be over-ranged. It also allows isolation of the transmitter from the
process loop. It consists of two block valves - high pressure and low-
pressure block valve - and an equalizing valve
During normal operation, the equalizing valve is closed and the two block
valves are open. When the transmitter is put into or removed from service,
the valves must be operated in such a manner that very high pressure is
never applied to only one side of the DP capsule.

To valve a DP transmitter into service an operator would perform the
following steps:
1.Check all valves closed.
2.Open the equalizing valve œ this ensures that the same
pressure will be applied to both sides of the transmitter, i.e.,
zero differential pressure.
3.Open the High Pressure block valve slowly, check for
leakage from both the high pressure and low-pressure side of
the transmitter.
4.Close the equalizing valve œ this locks the pressure on both
sides of the transmitter.
5.Open the low-pressure block valve to apply process pressure
to the low-pressure side of the transmitter and establish the
working differential pressure.
6.The transmitter is now in service.
Note it may be necessary to bleed any trapped air from the capsule housing.

 Open Tank Measurement
The simplest application is the fluid level in an open tank. 
typical open tank level measurement installation using a pressure capsule
level transmitter.
Open Tank Level Measurement Installation
If the tank is open to atmosphere, the high-pressure side of the level
transmitter will be connected to the base of the tank while the low-pressure
side will be vented to atmosphere. In this manner, the level transmitter acts
as a simple pressure transmitter. We have:         
Phigh = Patm +SH
                                                                                                                 Plow = Patm
Differential pressure  
                                                  P = Phigh - Plow =SH
The level transmitter can be calibrated to output 4 mA when the tank is at
0% level and 20 mA when the tank is at 100% level.

 
Closed Tank Measurement
Should the tank be closed and a gas or vapour exists on top of the liquid, the
gas pressure must be compensated for. A change in the gas pressure will
cause a change in transmitter output. Moreover, the pressure exerted by the
gas phase may be so high that the hydrostatic pressure of the liquid column
becomes insignificant. For example, the measured hydrostatic head in a
CANDU boiler may be only three meters (30 kPa) or so, whereas the steam
pressure is typically 5 MPa. Compensation can be achieved by applying the
gas pressure to both the high and low-pressure sides of the level transmitter.
This cover gas pressure is thus used as a back pressure or reference pressure
on the LP side of the DP cell. One can also immediately see the need for the
three-valve manifold to protect the DP cell against these pressures.


Typical Closed Tank Level Measurement System
We have:
                          
   Phigh = Pgas +SH
                                Plow = Pgas
                            P = Phigh - Plow =SH
The effect of the gas pressure is cancelled and only the pressure due to the
hydrostatic head of the liquid is sensed. When the low-pressure impulse line
is connected directly to the gas phase above the liquid level, it is called a dry
leg.

Bubbler Level Measurement System in Open Tank Application

A bubbler tube is immersed to the bottom of the
vessel in which the liquid level is to be measured. A gas (called purge gas)
is allowed to pass through the bubbler tube. Consider that the tank is empty.
In this case, the gas will escape freely at the end of the tube and therefore
the gas pressure inside the bubbler tube (called back pressure) will be at
atmospheric pressure. However, as the liquid level inside the tank increases,
pressure exerted by the liquid at the base of the tank (and at the opening of
the bubbler tube) increases. The hydrostatic pressure of the liquid in effect
acts as a seal, which restricts the escape of, purge gas from the bubbler tube.
As a result, the gas pressure in the bubbler tube will continue to increase
until it just balances the hydrostatic pressure (P =SH ) of the liquid. At
this point the backpressure in the bubbler tube is exactly the same as the
hydrostatic pressure of the liquid and it will remain constant until any
change in the liquid level occurs. Any excess supply pressure will escape as
bubbles through the liquid.
As the liquid level rises, the backpressure in the bubbler tube increases
proportionally, since the density of the liquid is constant.
A level transmitter (DP cell) can be used to monitor this backpressure. In an
open tank installation, the bubbler tube is connected to the high-pressure
side of the transmitter, while the low pressure side is vented to atmosphere.
The output of the transmitter will be proportional to the tank level.

A constant differential pressure relay is often used in the purge gas line to
ensure that constant bubbling action occurs at all tank levels. The constant
differential pressure relay maintains a constant flow rate of purge gas in the
bubbler tube regardless of tank level variations or supply fluctuation. This
ensures that bubbling will occur to maximum tank level and the flow rate
does not increase at low tank level in such a way as to cause excessive
disturbances at the surface of the liquid. Note that bubbling action has to be
continuous or the measurement signal will not be accurate.
An additional advantage of the bubbler system is that, since it measures only
the backpressure of the purge gas, the exact location of the level transmitter
is not important. The transmitter can be mounted some distance from the
process. Open loop bubblers are used to measure levels in spent fuel bays.

Closed Tank Application for Bubbler System

If the bubbler system is to be applied to measure level in a closed tank, some
pressure-regulating scheme must be provided for the gas space in the tank.
Otherwise, the gas bubbling through the liquid will pressurize the gas space
to a point where bubbler supply pressure cannot overcome the static
pressure it acts against. The result would be no bubble flow and, therefore,
inaccurate measurement signal. Also, as in the case of a closed tank
inferential level measurement system, the low-pressure side of the level
transmitter has to be connected to the gas space in order to compensate for
the effect of gas pressure.
Some typical examples of closed tank application of bubbler systems are the
measurement of water level in the irradiated fuel bays and the light water
level in the liquid zone control tanks.

 
Effect of Temperature on Level Measurement
Level measurement systems that use differential pressure P as the sensing
method, are by their very nature affected by temperature and pressure.
Recall that the measured height H of a column of liquid is directly
proportional to the pressure P exerted at the base of the column and
inversely proportional to the density of the liquid.
Thus, for any given amount of liquid in a container, the pressure P exerted at
the base will remain constant, but the height will vary directly with the
temperature.

 Effect of Pressure on Level Measurement
Level measurement systems that use differential pressure P as the sensing
method, are also affected by pressure, although not to the same degree as
temperature mentioned in the previous section.
Again the measured height H of a column of liquid is directly proportional
to the pressure P exerted at the base of the column by the liquid and L
inversely proportional to the density of the liquid: H a P/L
Density (mass per unit volume) of a liquid or gas is directly proportional to
the process or system pressure Ps.a Ps
Thus, for any given amount of liquid in a container, the pressure  
P(liquidpressure) exerted at the base of the container by the liquid 
will remain constant, but the height will vary inversely with the process 
r systempressure.H a 1/Ps
Most liquids are fairly incompressible and the process pressure will not
affect the level unless there is significant vapour content.

 
Level Measurement System Errors
The level measurement techniques described in this module use inferred
processes and not direct measurements. Namely, the indication of fluid level
is based on the pressure exerted on a differential pressure (DP) cell by the
height of the liquid in the vessel. This places great importance on the
physical and environmental problems that can affect the accuracy of this
indirect measurement.

Connections
As amusing as it may sound, many avoidable errors occur because the DP
cell had the sensing line connections reversed.
In systems that have high operating pressure but low hydrostatic pressure
due to weight of the fluid, this is easy to occur. This is particularly important
for closed tank systems.
With an incorrectly connected DP cell the indicated level would go down
while the true tank level increases.

Over-Pressuring
Three valve manifolds are provided on DP cells to prevent over-pressuring
and aid in the removal of cells for maintenance. Incorrect procedures can
inadvertently over-pressure the differential pressure cell. If the cell does not
fail immediately the internal diaphragm may become distorted. The
measurements could read either high or low depending on the mode of
failure.
Note that if the equalizing valve on the three-valve manifold is inadvertently
opened, the level indication will of course drop to a very low level as the
pressure across the DP cell equalizes.

Sensing lines
The sensing lines are the umbilical cord to the DP cell and must be
functioning correctly. Some of the errors that can occur are:
Obstructed sensing lines
The small diameter lines can become clogged with particulate, with
resulting inaccurate readings. Sometimes the problem is first noted as an
unusually sluggish response to a predicted change in level. Periodic draining
and flushing of sensing lines is a must.

Draining sensing lines
As mentioned previously, the lines must be drained to remove any debris or
particulate that may settle to the bottom of the tank and in the line. Also, in
closed tank dry leg systems, condensate must be removed regularly to
prevent fluid pressure building up on the low-pressure impulse line. Failure
to do so will of course give a low tank level reading. Procedural care must
be exercised to ensure the DP cell is not over-ranged inadvertently during
draining. Such could happen if the block valves are not closed and
equalizing valve opened beforehand.
False high level indication can be caused by a leaking or drained wet leg.
A leaking variable (process) leg can cause false low-level indication.

1 comment:

Sijo Joy said...

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