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Strong Acid System - Pump Tanks
September 6, 2004
Introduction
Pump tanks in a strong acid circulating system provide the
following functions:
- reservoir to hold sufficient acid from which to fill the
system on initial start up
- reservoir to contain the run back of acid when the system is
shutdown
- reservoir from which the acid circulation pumps draw
- a convenient location to add dilution water and cross flows
from other circulating systems
To adequately perform the above functions the pump tank must
be properly sized and designed. The most common types of pump tanks are vertical
cylindrical and horizontal cylindrical tanks. Materials of construction are the same
as for the towers; brick lined or stainless steel.
Pump boots are closed coupled to the tower such that the
bottom of the tower and the pump boot provide the same functions as a standalone pump
tank.
Sizing
Properly sized pump tanks will ensure trouble free operation
of the acid circulating system. The following information is required to size a pump
tank:
- Type and size of pump
- Run back volume of system
When vertical submerged pumps are use, the length of the pump
will generally set the height for vertical cylindrical tanks or the diameter of horizontal
cylindrical tanks. Pump manufacturers generally offer standard pump lengths but
custom lengths are also available. The diameter or height of the tank must take into
consideration that the pumps will require a minimum undersuction which is the distance
from the pump suction to the floor of the tank. If the minimum undersuction is no
provided the pump may not operate properly. As well, the height of the pump mounting
nozzle must also be considered when determining the height or diameter of the tank.
Once the height or diameter of the tank is set the other
major dimension must be determined. For vertical tanks the diameter is calculated
and for horizontal tanks it is the length that must be determined. The diameter or
length is calculated by setting the operating levels in the tank, knowing the required
runback volume and setting the freeboard or ullage at the top of the tank.
Vertical submerged pumps require a minimum liquid level above
the pump suction to prevent vortexing and drawing air into the pump suction. This
requirement sets the minimum operating level of the tank. The normal operating level
is typically set 150 mm (6") above the minimum level. The high level is
typically set 150 mm (6") above the normal operating level. These levels
provide sufficient flexibility in operation such that the level in the tank can vary but
alarms and nterlocks will not be activated all the time.
There must be sufficient volume in the tank between the high
operating level and the freeboard to accomodate the runback volume of the system.
Setting the operating levels in the tank fixes the height available for runback. In
a vertical tank, the diameter is set so the height of the cylinder equals the volume of
the runback. In a horizontal tank, the length is set so that the runback can be
accomodated in the required height.
Level Measurement
There are many options
available for level measurement in an acid pump tank.
The choice will depend on several factors such as cost, reliability,
accuracy, etc.
Bubbler
Bubbler type instruments
are used in many existing plants and are still specified for new plant applications. The instrument consists of a bubbler tube which is
inserted into the tank and extends nearly to the bottom of the tank. Instrument air enters the top of the tube and
bubbles out the bottom of the tube. A
certain air pressure will be required to overcome the liquid head in the tube to allow air
to bubble out the bottom of the tube. The air
pressure required is measured by a pressure transmitter and the pressure reading can be
converted to a level measurement. For the
instrument to work properly, there must be a constant flow of air or bubbles out the
bottom of the tube, hence the name bubbler.
The ‘high
side’ of the pressure transmitter measures the pressure in the bubbler tube. The transmitters ‘low side’ is piped to
the vapour space of the pump tank. This
compensates for the fact that the pump tank may be operating under a negative or positive
pressure depending on the design of the plant.
Bubblers are generally
quite reliable but problems may be encountered with the operation of the instrument.
- The instrument will stop working on loss of instrument air supply
- The bubbler tube must be corrosion resistant.
Any holes in the bubbler tube will cause a false level reading.
- Fluctuations in the pump tank vapour space pressure may cause false level
readings.
- Air flow to the bubbler must be constantly checked to ensure sufficient air
is being delivered to the bubbler tube.
Capacitance
The capacitance level
transmitter measures the dielectric constant values which are present in all materials to
determine exact changes in liquid level. The
probe is made up of two electrode plates and a guard wire which are completely
encapsulated in PTFE. When liquid rises or
falls against the probe, the dielectric material in the medium bridges the signal across
the active and ground plates to complete the circuit.
The change in capacitance value is converted by the processing electronics
into a proportional 4 to 20 mA signal output. Capacitance
level measurement works best with conductive liquids greater than 100 µS and/or
non-conductive liquid greater than 20 dielectric constant units.
Radar
Use of radar type
instruments in acid pump tanks for level measurement is relatively new. The instrument is mounted in a nozzle in the top
of the tank. A radar signal is emitted from
the instrument, travels to the surface of the liquid and is reflected back to the
instrument. The time it takes for the signal
to travel the distance from the instrument to the liquid surface and back again is
converted to a level reading.
One major advantage of
this type of instrument is that no part of the instrument contacts the liquid. The instrument should operate trouble free as long
as the part of the instrument that is exposed to the vapour space is corrosion resistant.
Actual experience with
this type of instrument shows that it is operates reliably most of the time. However, there are times when the instrument
reading would fluctuate wildly for no apparent reason.
The result would be that the level control valve would sometimes open fully
causing the pump tank level to decrease resulting in a low level and loss of pump flow and
flow over the tower. These wild fluctuations
would sometimes occur when the plant was shutdown and only acid flow over the towers was
being maintained.
It was surmised that
with the plant shutdown, the pump tanks were no longer being vented back to the inlet of
the drying tower and the accumulation of acid mist/SO3 in the pump tank vapour
space was causing interference with the instrument.
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