Lab Station: Buffer tank
University of
South-Eastern Norway (campus Porsgrunn) has 10 laboratory buffer tanks, see
Figure 1. The tanks are used in control courses in both bachelor and
master programmes in technology.
Figure 1: Buffer tank
Applications
Averaging level control is an important
part of several process systems. Some important practical examples are:
·
The equalization
or buffer magazine at the inlet of a wasteater treatment plant.
·
Oil/water
separators in the oil industry.
·
Water magazines
in hydropower systems.
Both the equalization magazine and the
separator can here be regarded as liquid buffer tanks. In both examples, the
level should be compliant to flow variations so that variations in the inflow
are damped through the tank, making the outflow considerably smoother than
the inflow. Smoother outflow is advantageous for the subsequent processes,
e.g. for the biological treatment processes and the oil production. The level
controller must be tuned for compliant (or soft, or sluggish) level control
so that the volume of the tank can absorb the inlet variations.
Technical description
Figure 2 shows a piping and instrumentation diagram (P&I D) of the
tank including a level control system.
Figure 2: Piping and instrumentation
diagram (P&I D) of the buffer tank a level control system.
The measurement signals (analog inputs) from the sensors LT1, FT1 and FT2
are voltage signals in the range 0 5 V.
The control signals (analog outputs) to the actuators pump P1and pump P2
are voltage signals in the range 0 5 V.
A built-in NI USB-6001 IO device handles the IO. This IO device can be
connected to a PC with a USB cable.
As an alternative to using the NI USB-6001 IO device, there are terminals
on the rig for direct analog IO.
Technical information
To appear.
Video presenting the buffer tank
To appear.
Level controller
The control signal generated by the level controller is a demanded
outflow:
u = F_out
The level controller may be a PI controller or some other controller.
Mathematical model
Mass balance of the water
in the tank leads to the following “volume balance” assuming water density is
the same all over:
h’ = (F_in - F_out)/A
where:
·
h [cm] is the water level in the tank.
·
F_in [cm^3/s] is water inflow.
·
F_out [cm^3/s] is water outflow.
·
A [cm^2] is cross-sectional area of tank.
Nominal values and ranges:
·
h_nom = 15. Range: 0 - 30.
·
F_in_nom = 50. Range: 0 - 100.
·
F_out_nom = 50. Range: 0 - 100.
·
A = 56.7.
About
The project has been
initiated by Olav Vangen and Finn Aakre Haugen. The construction as been
accomplished by Olav Vangen, Cecilie Gløsmyr, Fredrik Hansen, Marius Bergflødt,
and students at Skogmo high school.
Updated
22 October 2023 by Finn Aakre Haugen.
E-mail: finn.haugen@usn.no.
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