When liquid is heated it evaporates. The evaporation process is dependent on pressure, temperature and composition of the liquid and gas. Gas can also condense. In addition there is a convective heat transfer between the liquid and gas zone that must be considered. The surfaces in the gas zone also radiates from the shell to the liquid.
During the blowdown process mass is usually evacuated from the gas zone, but also liquid might be released. The rate of release is dependent on density and pressure as well as the release area.
As pressure and temperature change, the properties of all
materials change. This has to be considered in a prediction of a
blowdown process.
The main purpose of a blowdown process is as earlier
stated to maintain integrity of the equipment. The strength
properties of the shell are the key factor on that matter. The
strength is dependent on the inside pressure as well as the
support forces. If the exposing forces produce stress that
exceeds the ultimate tensile stress (UTS) in some regions, the
integrity of the equipment is no longer maintained. In the
design phase of a process plant, these aspects are crucial and
must be included as a dimensional factor. For that reason prediction of the blowdown process is essential.
Lately some new standards has been introduced to the
industry on this matter [3] and [4].
VessFire [1] and [2] is a multi physics system designed for
calculation of this kind of problems. It has been applied for
some time in the oil and process industry on many projects. The
system satisfies the requirements for predictions outlined in [3]
and [4]. It includes all aspects described above including
integrity of the shell. As part of the verification process some
experiments where performed. Some of the experiments are
presented here.
EXPERIMENTAL STUDY
The purpose of the experiments was to investigate the
evaporation process and the heat transfer to the liquid and
vapour. In a complex system it is important to reduce unknown
parameters as far as possible. Exposure from a flame is difficult
to control. Flux measurements are point values and not
necessarily representative for the average exposure. In order to
control the heat exposure it was decided to apply an electric
heating system. The system and the verification of the system is
described in [5], [6] and [8].
The furnace was built inside a supporting tube. Figure 2 shows
a general arrangement of the experimental outfit. A 0.05 mm
stainless steel foil formed as a tube, 300 mm in diameter,
generated the heat. The power supply was based on a 3-phase
alternating current system giving 48 Volt output as maximum.
The top exposure had a limit of 300 kW. The foil had a surface
of about 1 m2, giving a heat flux up to 300 kW/m2.
The power input could be continuously regulated from zero to maximum load. Each experiment was started from zero and brought up to the required load within a few seconds. After that the surface temperature of the heating foil was kept constant during the exposure period. Experiments with both dry objects as well as water filled object were performed. In this paper only water filled experiments are presented
Figure 2 General arrangements drawing of the experimental furnace including the specimen and its support
Figure 3 Illustration of the heating unit. The black part is copper conductors for the foil. The
grey part is the heating foil exposing the specimen. The foil is equipped with thermo-elements all marked H, except H5 which is the temperature in a copper ring
and H6 which is the temperature between the insulation and the supporting tube.