GB2546271A

GB2546271A - A cylinder for pressurised liquefied gas and a method of calculating the liquid volume

Info

Publication number: GB2546271A
Application number: GB1600566.2A
Authority: GB
Inventors: Bayerl WILHELM
Original assignee: Linde GmbH
Current assignee: Linde GmbH
Priority date: 2016-01-12
Filing date: 2016-01-12
Publication date: 2017-07-19
Also published as: GB201600566D0

Abstract

A method of calculating liquid volume in a transportable liquefied gas cylinder 1. An upper portion of the cylinder 1 contains vaporised gas G and a lower portion contains liquid L. The method comprises determining the hydrostatic pressure by determining the difference between the vaporised gas pressure as a first pressure and the pressure within the liquid, at a known position towards the bottom of the cylinder, as a second pressure. The liquid volume is then calculated based on the hydrostatic pressure. Preferably, the pressure difference between the first and second pressures is measured directly using a differential pressure sensor (e.g. piezoresistive, capacitive, piezoelectric, electromagnetic, potentiometric sensors). Preferably, the method further comprises measuring the temperature within the cylinder and including this reading in the calculation of the liquid level. Also claimed is a transportable gas supply cylinder comprising a pressure detector to detect the hydrostatic pressure. The cylinder is preferably provided with a wireless transmitter to allow information concerning the fill levels to be transmitted to a remote location.

Description

A Cylinder for Pressurised Liquefied Gas and a Method of Calculating the Liquid Volume

The present invention relates to a cylinder for pressurised liquefied gas and a method of calculating the liquid volume in such a cylinder.

The only known method of measuring the content of a high pressure mobile gas vessel filled with pressurised liquefied gas is to weigh the cylinder and to subtract from this measurement the empty weight of the cylinder (known as the Tare weight). From this measurement the weight of the liquefied gas and hence the liquid level and content can be determined. Mechanical devices (floater principle) are known for low pressure LPG cylinders, but this principle is not feasible for higher pressures. Using thermochromatic sensors on the outside of the cylinder to detect the liquid level is only possible during the release of gas. If the gas release is stopped the indication will disappear as the temperature rises. Level detection of cylinder in the gas storage is not possible.

It is not possible to measure the remaining content by measuring the pressure as can be done for a permanent gas because the head pressure is more of less constant until the last droplet of liquid phase has vaporised. Once that happens, the pressure quickly drops as gas is released from the cylinder.

Weighing the cylinder is not particularly useful in many cases, particularly where the cylinder is connected up to piping. Disconnecting the cylinder from the piping is time-consuming and causes gas losses. On the other hand, weighing the cylinder whilst it is connected will skew the results as the pressure and tension in the connected pipes and the cylinder fixation will affect the measured weight.

The present invention aims to provide a method and apparatus for determining the liquid level in a cylinder which does not suffer from these problems.

According to a first aspect of the present invention there is provided a method of calculating the liquid volume in a transportable gas supply cylinder filled with pressurised liquefied gas, the upper portion of the cylinder containing vaporised gas and the lower portion containing vaporisable liquid, the method comprising the pressure within the vaporisable liquid at a known position towards the bottom of the cylinder as a second pressure and calculating the liquid volume based on the hydrostatic pressure.

The first pressure is the gas pressure, while the second pressure is the sum of the gas pressure and the hydrostatic pressure of the liquid above the sensor at a known location in the cylinder. Subtracting the second pressure reading from the first gives the hydrostatic pressure of the liquid at the level of the sensor. From this, the level of liquid above the sensor and hence the volume can be calculated if the density of the liquid is known. Alternatively, it can be determined from a look-up table.

This allows the liquid level and hence the volume to be determined without needing to place the cylinder on scales.

Preferably, the pressure difference between the first and second pressures is measured directly using a differential pressure sensor. Alternatively, the first and second pressures may be measured separately, and the two values can be subtracted to determine the hydrostatic pressure.

The second pressure reading should be taken as low down in the cylinder as possible. The lower down it is measured, the greater the difference between the two readings and therefore the greater the accuracy of the calculation. Ideally, the second pressure reading should be taken at the bottom of the cylinder. However, practical considerations may dictate that it is preferable to measure the pressure at a level just above the bottom of the cylinder. Under these circumstances, a distance equal to the distance between the bottom of the cylinder and the sensing position can be added in to the liquid level calculation or it can be ignored if it is insignificant. A reasonable first approximation of this calculation can be carried out without measuring a temperature by assuming that the temperature is a constant value. However, preferably, the method further comprises measuring the temperature within the cylinder and including this reading in the calculation of the liquid level as this affects the density of the liquid.

The pressure also has an effect on the liquid density. If the first pressure is measured directly, then this value can be used in the calculation. If a differential pressure sensor is used, a separate pressure sensor may be used to measure the first pressure separately.

According to a second aspect of the present invention there is provided a transportable gas supply cylinder for pressurised liquefied gas, the cylinder comprising a pressure detector to detect the hydrostatic pressure by determining the difference between the pressure of the vaporised gas as a first pressure measured towards the top of the cylinder and a second pressure measured within the liquid in a fixed position towards the bottom of the cylinder.

Preferably, the pressure detector is a differential pressure sensor to measure directly the pressure difference between the first and second pressures. Alternatively, the pressure detector is a first pressure sensor to measure the first pressure and a second pressure sensor to measure a second pressure; the cylinder further comprising means to subtract the two sensed pressures to determine the hydrostatic pressure.

The cylinder preferably also comprises a temperature sensor to measure the temperature within the cylinder.

An example of a cylinder in accordance with the present invention will now be described with reference to the accompanying drawings, in which:

Fig. 1 is a schematic cross-section through the cylinder;

Fig. 2 is a diagrammatic view of the bottom of the cylinder showing the pressure measurement; and

Fig. 3 is a graph showing the values quoted in Example A. A cylinder 1 has a wall 2 which is able to maintain the pressure of the pressurised liquefied gas to the required level. The pressurised liquefied gas comprises a gas component G and a liquid component L. The gas G maintains a constant pressure as it is withdrawn through an outlet valve 3 (shown only schematically in the Figure as this is a conventional component) because, as the gas escapes, some of the liquid L vaporises to maintain the pressure. A differential pressure sensor 4 is provided close to the top of the cylinder 1 to measure the differential gas pressure. This can be for example: piezoresistive, capacitive, piezoelectric, electromagnetic, potentiometric, etc. sensors. A dip tube 5 extends from the top of the cylinder 1 to a location adjacent to the bottom of the cylinder 1. A second tube 6 extends a short distance into the gas space and leads to the opposite side of the differential pressure sensor 4. Within the pressure sensor 4 there is a pressure detecting element, one side of which is exposed to the pressure P(tot) while the opposite side is exposed to the pressure P(gas) such that it measures the pressure differential between the two in a manner well known in the art.

Alternatively, there may be separate pressure sensors at the two locations in order to measure an absolute pressure value at each location. The two values can then be subtracted.

The volume of liquid is determined as follows using the following equations: P(tot) = P(gas) + P(liq) => P(liq) = P(tot) - P(gas) P(liq) = h(liq) * p(liq) => h(liq) = P(liq) / p(liq) => h(liq) = ( P(tot) - P(gas)) / p (liq) where the value for P(tot) - P(gas) is either measured directly by the differential pressure sensor 4, or if separate sensors are being, it is determined by subtracting the two pressure readings. P(liq) is the hydrostatic pressure at the bottom of the dip tube 5. The density of the liquid is a known quantity, but is dependent on temperature If the application is being used where the temperature is known to be constant, this constant value of temperature can be used. Alternatively, a pressure temperature sensor 7 may be used to measure the temperature. This sensor 7 may also measure the absolute pressure (particularly in the case where a differential pressure sensor 4 is used). This pressure also has an effect on the density of the liquid and can therefore be taken into account in the above calculation.

The pressure sensor 4 can be located outside the cylinder (valve), just being connected to the dip-tube and the gas space.

This allows h(liq) to be calculated. As the dip tube is not exactly at the bottom of the cylinder, the distance between the end of the dip tube and the bottom of the cylinder fn is added to provide the liquid depth. Normally this is not necessary due to a limited influence on the overall result. From this, the volume of liquid can be calculated given that the geometry of the cylinder is fixed and known. A number of examples are given below. They initially state the volume of the cylinder as well as the maximum weight of the stored gas of a full cylinder, followed by the gas being stored and the overall height of the cylinder in metres.

The table sets out values which effectively provide a look-up table. This means that, on measuring the “Delta P” which is the pressure read by the differential pressure sensor 4, the look-up table allows the filling rate, level and weight of gas to be determined. A graphical representation of the results of example A exluding the filling rate is provided in Fig. 3.

Example height: A: 13,4 ltr/1 Okg C02 cylinder [m] 0.8

Example height: B: 50ltr/37,5kg C02 cylinder [m] 1.5

Example height: C: 79ltr/33kg LPG cylinder [m] 1.25

With this method, the level of liquid in the cylinder can be determined without the need to place the cylinder on scales. This system therefore readily lends itself to being used in many environments. The only stipulation is that the readings should be taken with the cylinder in an upright position in order to obtain a true reading.

The cylinder is preferably provided with a wireless transmitter to allow information concerning the fill levels to be transmitted to a remote location, for example, to a control

station or a smart phone to provide information to a user on the fill level in the cylinder as well as alerting them, for example, when the cylinder becomes depleted.

Claims

CLAIMS:

1. A method of calculating the liquid volume in a transportable gas supply cylinder filled with pressurised liquefied gas, the upper portion of the cylinder containing vaporised gas and the lower portion containing vaporisable liquid, the method comprising determining the hydrostatic pressure by determining the difference between the vaporised gas pressure as a first pressure and the pressure within the vaporisable liquid at a known position towards the bottom of the cylinder as a second pressure and calculating the liquid volume based on the hydrostatic pressure.

2. A method according to claim 1, wherein the pressure difference between the first and second pressures is measured directly using a differential pressure sensor.

3. A method according to claim 1, comprising measuring the first and second pressures separately, and subtracting the second pressure from the first to determine the hydrostatic pressure.

4. A method according to any of claims 1 to 3, further comprising measuring the temperature and/or pressure within the cylinder and including this in the calculation of the liquid volume.

5. A transportable gas supply cylinder for pressurised liquefied gas, the cylinder comprising a pressure detector to detect a hydrostatic pressure by determining the difference between the pressure of the vaporised gas as a first pressure measured towards the top of the cylinder being, and a second pressure measured within the liquid in a fixed position towards the bottom of the cylinder.

6. A cylinder according to claim 5, wherein the pressure detector is a differential pressure sensor to measure directly the pressure difference between the first and second pressures.

7. A cylinder according to claim 5, wherein the pressure detector is a first pressure sensor to measure the first pressure and a second pressure sensor to measure a second pressure; the cylinder further comprising means to subtract the two sensed pressures to determine the hydrostatic pressure.

8. A cylinder according to any of claims 4 or claim 7, further comprising temperature and/or pressure sensor to measure the temperature and/or pressure within the cylinder.