The article describes some problem aspects of monitoring the operational regimes of the remote equipment: stationary and transport cryogenic tanks. A functional structure is presented that demonstrates interconnection of programs for modeling heat and mass transfer processes, a computational modules and programs for monitoring the state of stationary and cryogenic tanks of various types. The main advantages of using the considered models and algorithms for remote monitoring and controlling are the possibilities of taking into account the changing of different operational regimes for cryogenic equipment, variable ambient temperature, as well as the technical condition of the screenvacuum superinsulation. An example of a decision support algorithm for controlling operational regimes of cryogenic tanks with a volume of up to 70 cubic meters is considered. The effectiveness of the proposed models and algorithms is confirmed by the results of software testing according to experimental data obtained during multimodal transportation of cryogenic products by various types of transport.
The main problem aspect need to be taken into account in monitoring and controlling
the remote cryogenic equipment is frequently changing operational regimes of
stationary and transport tanks [
Lack of adequate information about the current parameters of the cryoproduct in
the tank can lead to making the incorrect decisions by the personnel responsible for
the remote control of the equipment [
2.1
The main task in controlling the operational regimes of cryogenic tanks is to achieve
the maximum drainage free holding time, which is ensured by regulation of the
pressure in the gas phase ps with the help of discharge valves (Fig. 1), as well as switching
to product delivery from the gas phase, instead of delivery from the liquid phase, and
vice versa (if technically possible), at a given maximum working pressure in the tank
pmax. Therefore, as the objective function is considered drainage free holding time of
the cryoproduct τH (the holding time) [
Based on practical experience in operation of stationary and transport cryogenic
tanks [
Where pf, ps – current values of pressure in, respectively, liquid and vapour phase of the tank, T0m – current measured value of outside air temperature, pvac – pressure in the vacuum space (optional), τH – the holding time. For transport tanks with appropriate mechanical sensors can be taken into consideration amplitude Al, Atr and frequency fl, ftr of, respectively, longitudinal and transverse vibrations of the tank.
With such a formulation of the problem, especially given a lot of random parameters, it is impractical to compile a general analytical expression for the objective function. To solve the problem of maximizing the holding time, a decision support algorithm for predicting drainage free holding time of the cryoproduct was developed. 2.2
To improve information support of operators, it is proposed to use decision support
system for controlling the operational regimes of cryogenic tanks. The functional
scheme of the considered system support system is shown in Fig. 2. The based
information for evaluation of the pressure in the vapour phase of tank and predicted
holding time is the results of computational modeling, obtained in universal software
complex ANSYS Fluent. A several two-dimensional computer models were prepared
to calculate temperature and pressure fields for heat and mass transfer processes in
cryogenic tank [
A database of the main parameters of the simulation results is being accumulated, as new data on the geometric and operational characteristics of stationary and transport tanks (including tank containers), thermodynamic components (including mixtures) are accumulated. From this information, upon request from the computing module, an array of values of tank gas pressure and holding time data is formed, from the elements of which the required value of the predicted drainage free holding time is subsequently determined (Fig. 3).
Remote control center accumulates information from the tanks: data on the pressure and level of the liquid product, technical condition of the thermal insulation, the current storage regime (stationary or transport), the predicted holding time. 3 3.1
The algorithm described in this subsection allows one to consider stationary and 3 transport cryogenic tanks for various purposes with a volume of up to 70 m .
Optionally having a current value of the vacuum pressure pvac obtained from the mechanical vibration sensors, the calculation of the additional heat gain due to the gas in the inter-tank space may be written as follows:
k 1 18,2 pvac T0m Tc Fc Qgas k 1 T0 (2)
Where Fc – the evaluated area of the cold wall of the tank, Tc – the temperature of the cold wall of the tank, µ – molecular weight of the cryogenic product, k – adiabatic Poisson's ratio, α – the energy accommodation coefficient. The evaluated heat gain through the superinsulation Qins is calculated as follows:
Qins T0m Tc Qdb Qgas
T0 Tc (3)
Where T0=293 К (0 °C) – normal temperature, Qdb – the returned value of the heat gain through the superinsulation of the tank from the main database.
For a stationary mode the data array of time τH,i and storage pressure pj corresponding to the previously calculated heat flow is loaded into the computing module. If the obtained value of τH turns out to be less than the specified value of the critical pressure τcr, the system generates and sends an emergency message to the remote control center. 3.2
Based on current tank-state information picture (Fig. 4), the operator responsible for remote monitoring of the state of cryogenic tanks can make decisions for: ─ sending a message to the technical gases logistics service about the need to refuel the tank (when the liquid level drops below 30%, if the tank is used in stationary storage mode); ─ informing the responsible person when changing the status to "ATTENTION" (the status changes if the value of holding time becomes less than 24 hours); ─ immediately informing the person responsible for the good condition and safe operation of the tank and special services (if necessary) in case of an emergency message (the pressure in the tank exceeds 1,15 of maximum, there is no vacuum in the heat-insulating cavity, the liquid level exceeds 98 %, etc. ). For analyzing the correspondence between the calculated and passport values of the holding time, some experimental data concerning storage of cryogenic products (nitrogen, argon, liquefied natural gas and ethylene) in multimodal transport units (40 000 litres volume ISO-containers) were considered (Fig. 5). For the consideration, the total holding time for one multimodal transport unit was evaluated. The holding time from the initial minimum pressure to the maximum allowed pressure 0.7 MPa was considered .
As shown in comparison diagram (Fig. 5), the evaluated and experimental values of the holding time turn out to be significantly lower than the maximum theoretical values (passport theoretical holding time values) for ISO-containers. The results of the calculations of the operational parameters differ from the experimental values by no more than 3 ... 5 %, which makes it advisable to predict the safe holding time based on the results of computational modeling. 5
The research describes some features of estimation the key characteristics of stationary and transport cryogenic tanks, namely: the time of drainage free holding time and level of liquid – for various regimes of storage.
The advantages of the decision support system for monitoring the state of stationary tanks, cisterns and ISO-containers were presented. An algorithm for calculating the non-drainage holding time makes it possible to predict both stationary and transport operational values, which allows to make timely operational decisions on storage and transportation regimes of remote cryogenic equipment. Validation results on the non-drainage holding time by empirical data, obtained in the process of multimodal transportation of cryoproducts, showed that the calculated data differ from the empirical values by no more than 5 %. The introduction of the proposed monitoring system for a specific fleet of stationary and transport cryogenic tanks will significantly increase the safety of operation by ensuring technological processes without venting flammable gases into the atmosphere. 6
The study was carried out with state support from leading scientific schools Russian Federation, grant No. NSh-2553.2020.8.