GB2398874A

GB2398874A - Cryogenic vessel boil-off monitoring system

Info

Publication number: GB2398874A
Application number: GB0301909A
Authority: GB
Inventors: Antonis Chris Ioannides
Original assignee: Magnex Scientific Ltd
Current assignee: Magnex Scientific Ltd
Priority date: 2003-01-28
Filing date: 2003-01-28
Publication date: 2004-09-01
Anticipated expiration: 2023-01-28
Also published as: GB2398874B; GB0301909D0

Abstract

A cryogenic vessel boil-off monitoring system comprises a chamber 5 connectable to a cryogenic vessel 1, and electrically operated valves 2 and 3 positioned within conduits 2A and 3A linking the exhaust outlet of the cryogenic vessel 1 to an inlet of the chamber 5 and an outlet of the chamber 5 to atmosphere 7. A pressure transducer 6 is provided within the chamber 5, and a programmed controller 4 serves to control the opening and closing of the valves 2 and 3 in dependence on a preset programme and an electrical output signal from the pressure transducer 6 indicative of the pressure within the chamber 5. The controller 4 also provides an electrical output signal indicative of the flow rate of the exhaust gas from the cryogenic vessel 1 that may be used for monitoring the system performance and for providing an alarm in the event that system parameters are exceeded. Such a monitoring system is not only more accurate and more easily controllable than known monitoring arrangements, but is also less costly to produce and has the advantage that it produces an output signal indicative of the flow rate of the exhaust gas

Description

"Cryogenic Vessel Boil-Off Monitoring Systems" This invention relates to

cryogenic vessel boil-off monitoring systems, and is concerned more particularly, but not exclusively, with systems for pressure control and gas flow monitoring of the cryogenic vessels of superconducting magnet systems.

Superconducting magnet systems, such as are used in nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging ( MRI) and Fourier- transform mass spectroscopy (FTMS), incorporate a cryogenic vessel for containing the cryogenic fluid (normally liquid helium) to maintain the system at the required very low temperature. During normal operation of the system heat losses will cause the cryogenic liquid to boil-off thereby increasing the pressure within the cryogenic vessel.

This change in pressure in turn causes the temperature of the liquid helium to change. . It is beneficial in many high resolution applications to operate the cryogenic arrangement at constant pressure (or temperature) in order to provide optimum resolulion/stability of the system. It is known that pressure can be controlled using expensive one way checkvalves that can attempt to maintain absolute pressure in the cryogenic vessel. Alternatively, a linear valve can be used to control the pressure with a control signal from an absolute pressure transducer. Such an arrangement also does not provide an indication of the flow rate of the exhaust gas such as is desirable in such a system, for example to trigger an alarm in the event of an upper or lower flow rate limit being exceeded.

It is an object of the invention to provide a cryogenic vessel boil-off monitoring and pressure control system which is relatively simple and accurate and monitors the flow rate of the exhaust gas.

According to the present invention there is provided a cryogenic vessel boil-off momLoring system comprising a chamber connectable to a cryogenic vessel to he monitored, inlet valve means for the flow of exhaust gas from the cryogenic vessel to the chamber, pressure sensing means for monitoring the pressure of the exhaust gas, and control means for operating the valve means in dependence on the pressure monitored by the pressure sensing means to control the Dow of exhaust gas from the cryogenic vessel and for providing an output signal indicative of the flow rate of the exhaust gas.

Preferably the pressure sensing means is positioned within the chamber to monitor the pressure of the exhaust gas within the chamber.

It is also preferred that outlet valve means are provided for controlling the flow of exhaust gas from the chamber to atmosphere, and that control means are arranged to operate the inlet and outlet valve means in dependence on the pressure monitored by the pressure sensing means to alternately release exhaust gas from the cryogenic vessel to the chamber and release exhaust gas from the chamber to atmosphere.

Such a monitoring system is not only more accurate and more easily controllable than known monitoring arrangements, but is also less costly to produce. It also has the advantage that it produces an output signal indicative of the flow rate of the exhaust gas, for example for triggering of an alarm in the event that the exhaust gas flow rate is above or below a predetermined flow rate range (which may be indicative of a leak In the cryogenic system).

The invention also provides a cryogenic vessel boil-off monitoring system comprising valve means for controlling the flow of exhaust gas from the cryogenic vessel to atmosphere, pressure sensing means for monitoring the pressure of bol-off gas within the cryogenic vessel, and monitoring means for operating the valve means in dependence on the pressure monitored by the pressure sensing means to control the flow of exhaust gas from the cryogenic vessel and for providing an output signal indicative of the flow rate of the exhaust gas.

In a development of the invention the control means is arranged to effect pulse width modulated operation of the valve means in which the length of time for which the valve means Is opened Is varied to control the flow of the exhaust gas.

In order that the nvcntion may be more fully understood, several embodiments of cryogenic vessel boil-off monitoring system in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, In which Figures 1 to 6 are schematic diagrams of the systems.

Each of the cryogenic vessel boil-off monitoring systems shown in the drawings arc designed to be connected to the cryogenic vessel of a superconducting magnet system, which may be, for example, a high field NMR spectroscopy system. However it will be well understood that similar systems may also be used in other arrangements utlising cryogenic vessels.

The embodiment of Figure 1, which is shown connected to a cryogenic vessel 1, comprises a chamber S of a defined volume which is small in relation to the "dead" volume of the cryogenic vessel 1, electrically operated valves 2 and 3 positioned within conduits 2A and 3A lmking the exhaust outlet of the cryogenic vessel 1 to an inlet of the chamber 5 and an outlet of the chamber 5 to atmosphere 7. A pressure transducer 6 is provided within the chamber 5, and a programmed controller 4 serves to control the opening and closing of the valves 2 and 3 in dependence on a preset programme and an electrical output signal from the pressure transducer 6 indicative of the pressure within the chamber 5. The controller 4 also provides an electrical output signal indicative of the flow rate of the exhaust gas from the cryogenic vessel 1 that may be used for monitoring the system performance and for providing an alarms in the event that system parameters are exceeded.

In operation of such a system the release of the exhaust gas from the cryogenic vessel 1 proceeds in accordance with an operating cycle controlled by the controller 4 as follows. At the beginmng of the cycle the inlet valve 2 is maintained in the open position by the controller 4 so that exhaust gas flows along the conduit 2A from the outlet of the cryogenic vessel 1 to the chamber 5 where it is held by virtue of the outlet valve 3 being in the closed position. The pressure of the exhaust gas within the chamber 5 is monitored by the controller 4 by means of the electrical output signal received from the pressure transducer 6, and when the monitored pressure excocds a set pressure value (programmed Into the controller 4), the controller 4 closes the mlet valve 2 and opens the outlet valve 3 so as to release the exhaust gas within the chamber 5 by way of the outlet conduit 3A to atmosphere 7 (or an intermediate pressure environment.

When the gas has been released from the chamber 5, the outlet valve 3 is closed by the controller 4 as a result of the lowered pressure sensed by the pressure transducer 6. The inlet valve 2 Is then again opened by the controller 4 to allow further exhaust gas from the cryogenic vessel 1 to enter the chamber S until the pressure sensed by the pressure sensor 6 again rises above the set pressure, and the operating cycle is repeated.

The cryogenic fluid within the cryogenic vessel l, which may be helium for example, is caused to boil due to insulation losses, and the resulting boil-off causes the pressure in the "dead" volume in the chamber 5 above the liquid to slowly Increase.

The volume of the chamber S is small in relation to the "dead" volume in order to minmise pressure perturbations in the cryogenic vessel l during the opening and closing cycles of the valves 2 and 3. The effect of this operating cycle is to maintain the pressure within the cryogenic vessel l substantially constant and substantially independent of any pressure changes within the atmosphere 7.

The amount of exhaust gas expelled from the cryogenic vessel 1 can be calculated on the basis of the gas expelled during each operating cycle, where: V1 is the volume of the gas space in the cryogenic vessel 1.

PI is the pressure in the cryogenic vessel 1.

V2 is the volume of the chamber S. P3 is the pressure in the combined volume of V1 and V2 after a volume of gas has boon expelled from V2.

P4 is the pressure at which the volume of gas is measured after expulsion from V2.

VTRUE is the true volume of gas expelled as measured at a pre-spccified pressure V4.

VOUT is the effective added volume after the cryogenic vessel I is linked to the chamber S. The starting pressure of P] in the cryogenic vessel 1 will be changed to the pressure P3 after the valve 2 has been opened. The relationship will be: Pi *Vl=P3*(Vl+VOUT) The true volume of gas (VTRIJ) expelled at a pre-specified pressure P4 can also be expressed as: P1 *VOUT=VTRUE*P4 From the above equation the volume of gas expelled can be calculated as: VTRUE=Vl *(P l-P3)*P1/(P3*P4) Other corrections can also take place based on the effective change of temperature of the expelled gas and other parameters.

As the outlet pressure 7 can vary relative to atmospheric pressure, the amount of gas expelled from the chamber 5 can increase or decrease during such cycling operation.

To maintain the average pressure in the cryogenic vessel 1 correct, it is possible for the set pressure maintained by the controller 4 to be varied with time in dependence on other detected values. If required, multiple pressure transducers may be used to measure the pressures directly in the cryogenic vessel l, in the chamber 5, and also the outlet pressure 7. To further minimise pressure variations, a flow restrictor may be positioned in series with the outlet valve 3, and the outlet valve 3 may be pulsed so as to intermittently open the valve 3, thus ensuring that the pressure in the chamber 5 is kept as high as possible before the inlet valve 2 is opened. This will have the effect of increasing the number of operating cycles and reducing the amount of gas expelled per cycle.

The embodiment of Figure 2 is a variant of the embodmcnt of Figure 1 with the cryogenic vessel 13, the chamber 14, the valves 12, 16, the pressure transducer 15 and the controller 18 serving the same function as the corresponding parts in Figure 1 for effecting controlled release of exhaust gas to atmosphere 17. However, in this cmbodment, a further chamber 10 is coupled to the chamber 14 by a conduit 11A incorporating a further valve 11. Such an embodiment is advantageous in cases where the volume of gas to be expelled per operating cycle may at times exceed the volume of the chamber 14. In this case the pressure monitored by the controller 18 by way of the pressure transducer 16 may be used to open the valve 11 with the view to increasing the volume available for receipt of the exhaust gas flowing through the valve 12. The combined volume of the chambers 14 and 10 can be calculated so as to allow measurement of the maximum flow rate from the cryogenic vessel 13. In cases where the valve 11 is opened to increase the available volume for receiving exhaust gas, the valve 11 is maintained opened when the valve 16 is opened (and the valve 12 is closed) so as to enable the full volume of gas to be expelled from the chambers 14 and 10 to atmosphere 17.

The embodiment of Figure 3 is a modification of the embodiment of Figure 1 with the cryogenic vessel 30, the valve chamber 33, the valves 32 and 35, the pressure transducer 34 and the controller 37 serving the same function as the corresponding parts in Figure 1 for effecting controlled release of exhaust gas to atmosphere 36. However, in this case, gas flow restrictors 31 and 38 are provided in series with the valves 32 and with a view to minimising pressure fluctuations when the valve 32 is opened lo connect the cryogenic vessel 30 lo the chamber 33. Thus, in cases where the outlet pressure 36 is far too low, the controller 37 can calculate (based on expected flow rate) what pressure of gas Is required in the chamber 33 prior to opening of the valve 32. In order to obtain the required gas pressure in the chamber 33 prior to opening of the valve 32, the valve 35 is closed when the required pressure is reached. When the valve 32 is opened, the required gas flow from the cryogenic vessel 32 to the chamber 33 is obtained. Furthermore the valve 32 can initially be pulsed in dependence on the detected pressure in the chamber 33 so that the small pressure variation in the cryogenic vessel 30 is further decreased.

The embodiment of Figure 4 effectively corresponds to the embodiment of Figure 3 but with multiple chambers for receiving the exhaust gas. In this case, the cryogenic vessel 40, the chamber 50, the valves 42 and 44, the pressure transducer 52, the gas flow restrictors 41 and 56 and the controller 55 perform the same function as the corresponding parts in Figure 3. However, this embodiment further includes a further chamber 51, further valves 43 and 45 and a further pressure transducer 53 arranged in parallel with the chamber 50 and valves 42 and 44. In this case the controller 55 controls the four valves 42, 43, 44 and 45 in dependence on the pressures sensed by the pressure transducers 52 and 53 such that, when gas is being expelled from the chamber by way of the outlet valve 44, the chamber 51 is being filecd with exhaust gas by way of the inlet valve 43, and, when gas is being expelled from the chamber 51 by way of the outlet valve 45, the chamber 50 is being filled with exhaust gas by way of the inlet valve 42. With this approach, a substantially continuous flow of gas from the cryogenic vessel 40 can be provided.

Furthermore the restrictor 41 serves to further regulate the gas flow from the cryogenic vessel 40, particularly if the inlet valve 42 is subjected to pulsed operation for transfer of exhaust gas to the chamber 50 and the inlet valve 43 is subjected to pulsed operation when exhaust gas is transferred to the chamber 51. The pulsing of the inlet valves 42 and 43 may be effected by the controller 55 so as to minimise the pressure fluctuations in the cryogenic vessel 40. In addition the outlet valves 44 and 45 can be pulsed as already described with reference to Figure 3 so that the gas pressures in the chambers 50 and 51 can be accurately set to minimise pressure fluctuations when the inlet valves 42 and 43 open during their respective cycles.

The embodiment of Figure S is a further recodification of the embodiment of Figure 3 in which the cryogenic vessel 60, the chamber 63, the valves 62 and 66, the pressure transducer 64, the gas flow restictors 61 and 65 and the controller 68 serve the same function as the corresponding parts in Figure 3 for effecting controlled release of exhaust gas to atmosphere 67. However the embodiment of Figure 5 additionally includes a further pressure transducer 69 connected to the cryogenic vessel 60, so that, when exhaust gas is expelled From the vessel 60 by way of the restrictor 61 and the mlet valve 62 to the chamber 63, the valve 62 can be pulsed in order to ensure that the pressure m the vessel 60 is not reduced significantly when the valve 62 is opened. The pressure in the chamber 63 can also be controlled by pulsing the outlet valve 66 and by restricting the gas flow out of the chamber 63 by means of the restrictor 65. Both of the pressure transducers 69 and 64 and the valves 62 and 66 are controlled by the controller 68, which may be a computer or microprocessor controller, so as to provide an output signal indicative of the gas flow rate and so as to ensure accurate pressure control within the cryogenic vessel 60.

The embodiment of Figure 6 is a simplified system in which a separate chamber is dispensed with, and instead the pressure in the cryogenic vessel 80 is controlled by opening and closing of a valve 83 to allow gas to escape along a conduit 82A and by way of a gas flow restrictor 82 to atmosphere 84. In this case the pressure within the vessel 80 is monitored by a pressure transducer 81 which supplies an electrical output signal to a controller 85 for controlling operation of the valve 83. In this case the valve 83 can be pulsed by the controller 85 so as to effect accurate control of the pressure within the vessel 80, for example by varying the rate at which the valve 83 is pulsed or by applying a pulse width modulation technique in which the length of time for which the valve 83 is opened is varied depending on the amount of gas to be expelled. The controller 85 may open the valve 83 for a shorter period of time if the atmospheric pressure is much lower than the pressure in the vessel 80.

A similar valve pulsing arrangement may be used in a variant of the embodiment of Figure 3 so that the volume of gas expelled from the cryogenic vessel is controlled in such a way as to mimmise the pressure reduction in the vessel. In this case the inlet valve 32 may be kept permanently open and pulse width modulation control may be applied tom the outlet valve 35, the presence of the intermediate chamber providing an additional pressure reduction/slap change..

In each of the above described embodiments special measurements are taken at different points during the operating cycles so that the expelled volume of gas can be measured, and an electrical output signal indicative of the gas flow rate can be supplied for performance monitoring purposes or for triggering an alarm. Although each of the above described embodiments is described In terms of venting of the gas to atmosphere, it should be understood that it would also be possible to arrange to vent the gas to an evironment having a pressure other than atmospheric pressure prodded that this pressure which is lower than the controlled pressure in the cryogenic vessel.

In addition to the measurement of the gas expelled from the cryogenic vessel, embodiments of the invention can also be used to measure the volume of gas in the cryogenic vessel. Such measurement can be effected in two different ways. In one arrangement, considering the embodiment of Figure 1, when valve 3 is closed and the valve 2 is open, the pressure rate of increase in the chamber 5 and the cryogenic vessel 1 will be dependent on the combined volume of gas space in these chambers. If the gas flow rate is known, then the combined gas volume can be measured since the starting and finishing pressures are known and the gas volume increase is also known for a specific period of time. By evaluating the volume of gas in the chamber 5 and the cryogenic vessel 1, the gas volume in the cryogenic vessel 1 can be measured. Since the total volume of the cryogenic vessel 1 is known, the volume of the liquid in the cryogenic vessel 1 can also be calculated.

An alternative arrangement for measuring the gas volume is based on measuring the pressure reduction when the inlet valve 2 in Figure 1 is opened. This pressure change is measured by the pressure transducer. The volume of the chamber 5 is known and therefore the gas volume of the cryogenic vessel 1 can be calculated. Some correction needs to be applied to these calculations due to the time delay in the measurements whilst gas is boiling off m the cryogenic vessel 1.

Claims

CLAIMS: 1. A cryogenic vessel bol-off monitoring system comprising a

chamber connectable to a cryogenic vessel to be monitored, inlet valve means for the flow of exhaust gas from the cryogenic vessel to the chamber, pressure sensing means for monitoring the pressure of the exhaust gas, and control means for operating the valve means in dependence on the pressure monitored by the pressure sensing means to control the flow of exhaust gas from the cryogenic vessel and for providing an output signal indicative of the flow rate of the exhaust gas.

2. A monitoring system according to claim 1, wherein the pressure sensing means are positioned within the chamber to monitor the pressure of the exhaust gas within the chamber.

3. A monitoring system according to claim 1 or 2, wherein outlet valve means are provided for the flow of exhaust gas from the chamber to an environment having a pressure less than the monitored pressure.

4. A monitoring system according to claim 3, wherein the control means are arranged to operate the inlet and outlet valve means in dependence on the pressure monitored by the pressure sensing means to alternately release exhaust gas from the cryogenic vessel to the chamber and release exhaust gas from the chamber to said environment.

5. A monitoring system according to claim 4, wherein the control means are arranged to open the inlet valve means to release exhaust gas from the cryogenic vessel to the chamber where the pressure of the exhaust gas is monitored by the pressure sensing means, and to open the outlet valve means and close the inlet valve means in response to the pressure monitored by the pressure sensing means exceeding a pressure threshold in order to release exhaust gas from the chamber to said environment.

6. A monitoring system according LC) claim 3, 4 or 5, wherein the chamber is of a defined volume, and the control means are arranged to provide the output signal indicative of the flow rate of the exhaust gas in dependence on the change of pressure monitored by the pressure sensing means.

7. A monitoring system according to any preceding claim, wherein the control means are arranged to effect pulsed operation of the mlet valve means to provide controlled release of exhaust gas from the cryogenic vessel.

8. A monitoring system according to any preceding claim, wherein a further chamber is connected to the first-mentioned chamber, and further valve means are provided for selectively placing the further chamber in fluid communication with the frst-mentioned chamber to provide increased capacity for exhaust gas.

9. A monitoring system according to any preceding claim, wherein restrictor means are provided to restrict the flow of exhaust gas from the cryogenic vessel to the chamber and/or from the chamber to atmosphere.

10. A monitoring system according to any preceding claim, wherein a further chamber is connectable to the cryogenic vessel, and further inlet valve means are provided for controlling the flow of exhaust gas from the cryogenic vessel to the further chamber, the control means being operable to effect operation of the first-mentioned valve means and the further valve means such that exhaust gas from the cryogenic valve is supplied alternately to the first-mentioned chamber and to the further chamber.

ill. A monitoring system according to claim 10, wherein further pressure sensing means are positioned within the further chamber, and further outlet valve means are provided for controlling the flow of exhaust gas from the further chamber to atmosphere. I 12. A monitoring system according to any preceding claim, wherein auxiliary pressure sensing means are connectable to the cryogenic vessel to monitor the pressure of bol-off gas within the cryogenic vessel.

13. A momtoring system according to any preceding claim, which is adapted to measure the volume of gas in the cryogenic vessel by measuring the pressure rate of increase in the chamber and the cryogenic vessel when the inlet valve means is closed.

]4. A monitoring system according to any preceding claim, which is adapted to measure the volume of gas m the cryogenic vessel by measuring the pressure reduction when the inlet valve means is opened.

15. A cryogenic vessel boil-off monitoring system comprising valve means for controlling the flow of exhaust gas from the cryogenic vessel to said environment, pressure sensing means for monitoring the pressure of boiloff gas within the cryogenic vessel, and control means for operating the valve means in dependence on the pressure monitored by the pressure sensing means to control the flow of exhaust gas from the cryogenic vessel and for providing an outpost signal indicative of the flow rate of the exhaust gas.

16. A monitoring system according to any preceding claim, wherein the control means is arranged to effect pulse width modulated operation of the valve means in which the length of time for which the valve means are opened is varied to control the flow of the exhaust gas. I 17. A cryogenic boil-off monitoring system substantially as hereinbefore described with reference to any of the accompanying drawings.