Patent Application
20050121461
This invention relates to control devices used in conjunction with heating pads when heating barrels and tanks, particularly those that are pressurised and which may contain hazardous materials.
In industry and life in general there is frequently found the necessity for heating the contents of a tank that is located in a place subject to the ravages of the outdoors and the associated elements. A practical method to warm the contents is by utilising an electric heater strapped to the tank. There are a number of such systems available, some of which are controlled by temperature sensing elements and some of which have no controls and they do heat tanks as required.
These systems in general include a heating element, a means by which to attach it to the tank and a power supply cable. In certain applications it is imperative that a limited amount of heat be applied to the tank so as to prevent overheating or over-pressurisation of the contents of the tank. To ensure that the tank contents do not go above a pre-determined safe set-point some systems limit the amount of power of the heater while other systems utilise a fixed point, temperature limiting thermostat as a safety cut-out while other systems use a combination of the two. There follows a list of problems associated with these types of systems when utilised for heating liquid pressurised gas (LPG) in tanks or cylinders.
LPG is used as a fuel over a broad range of temperatures in many areas of the world ranging from the arctic to the tropics. The mix of LPG is particular to each region and depends on the local requirements and each mix has its own heating requirements.
LPG is stored in pressurised tanks in the form of liquid and gas. The liquid portion being heavier fills the lower portion of the tank cavity and the gas being lighter fills the upper portion of the tank cavity. When the need arises, gas is withdrawn from the upper region of the tank and obeys the laws of chemical thermodynamics. As the gas is withdrawn the LPG temperature drops and the pressure within the tank drops thus making it difficult to obtain sufficient quantities of gas from the tank. By applying heat to the liquid contents of the tank the pressure within the tank is increased and greater quantities of gas may be withdrawn from the tank. If too much heat is applied to the tank the pressure could increase to the point where a safety release valve would be activated, releasing explosive gas into the atmosphere thus creating a dangerous situation. It is therefore desirable to heat the tank in a controlled fashion.
In the case of a heater that relies on a thermostat for safety there are three possible common configurations for the thermostat. In the first configuration the thermostat senses and limits the temperature of the heater. In the second configuration the thermostat senses and limits the temperature of the tank. In the third configuration the thermostat senses the ambient air temperature and in combination with a lower power of heater, ensures that the tank contents will not over-heat or become over-pressurised. In the case of the heater that has no thermostat the heating capacity is reduced so as not to overheat or over-pressurise the tank.
The problem with the systems that utilise the thermostats is that they are limited to pre-set operating temperatures which are not functional over a broad range of ambient temperatures. The problem with the systems that have no thermostats is that they have lower heating capacities and thus do not provide sufficient heating capacities in the warmer climates.
To address these problems, there is therefore provided according to an aspect of the invention a tank heater control that will control a heater used for heating pressurised tanks over a broad range of temperatures. The system uses a sensor for providing a signal to a processor, in which the signal is indicative of flow of gas from the tank. The processor energizes the heater with power from an adequate power source when gas flows from the tank. In various embodiments, the sensor may be a flow sensor on an outlet from the tank, temperature sensors that detect a difference in temperature of the tank from ambient temperature, or a pressure sensor on an outlet from the tank.
Further features and advantages of the invention will appear from the description that follows.
There will now be described preferred embodiments of the invention, with reference to the drawings, by way of illustration, in which like numerals denote like elements and in which:
In this patent document, the word “comprising” is used in its inclusive sense and does not exclude other elements being present. The indefinite article “a” before an element of a claim does not preclude other instances of the element being present. “Tank” as used in this patent document includes any cylinder or tank used for the storage of gas.
Referring to
A first control mechanism according to the invention uses the two temperature detecting devices 24, 26. One sensor detects the ambient air temperature while the second detects the temperature of the tank next to the gas. Before gas is withdrawn the temperature of the tank 10 is equal to the ambient air temperature. As gas is withdrawn from the tank 10 the temperature of the LPG drops and consequently the temperature of the tank 10 drops. In particular, the tank surface 30 adjacent to the gas drops significantly and the thinner metal fittings attached to the top of the tank 10 are the most sensitive to this change in temperature. By measuring the difference between the ambient and the tank temperature it can be determined when gas is being withdrawn from the tank 10.
A variety of electronic circuits or electromechanical devices acting as processor 22 can be used to detect the difference in the two temperatures and control when the heater 34 is energised.
A second control mechanism uses the flow sensor 18 inserted in line with the hose 16 though which the gas flows out of the tank 10. The same processor 22 may be used for control of the heater 34. As indicated in the
A third control mechanism uses pressure sensor 42 inserted in line with the hose 16 through which the gas flows out of the tank 10. Processor 22 also may be used to process the signal from pressure sensor 42 to detect the drop in pressure of gas and control when the heater is energised. As shown in
The three control mechanisms operate independently of ambient air temperature and thus can be used in any location regardless of local conditions. One or more of the control mechanisms may be used, although in practice only one of the control mechanisms will usually be required. The electromechanical or electronic controls of the processor 22 can be located at the sensing point or mounted remotely or be an integral part of the heater. The processor 22 may a simple electric circuit made of hardware parts, or may be part of a programmed microprocessor, using hardware, firmware or software.
Some advantages of the present invention are that the control is independent of the ambient temperature and simply depends on gas being withdrawn form the tank 10. The processor 22 or whatever control mechanism is utilised can be mounted on the tank 10, incorporated as part of the heater or can be located away from both in a remote location. The tank system disclosed here has particular utility at remote oil industry sites, such as well sites.
A person skilled in the art could make immaterial modifications to the invention described and claimed in this patent disclosure without departing from the essence of the invention.
