WO2015158469A1

WO2015158469A1 - A pressure relief valve arrangement

Info

Publication number: WO2015158469A1
Application number: PCT/EP2015/054942
Authority: WO
Inventors: Michael Simpkins
Original assignee: Siemens PLC
Current assignee: Siemens PLC
Priority date: 2014-04-16
Filing date: 2015-03-10
Publication date: 2015-10-22
Anticipated expiration: 2016-10-16
Also published as: GB201406837D0; GB2525217B; GB2525217A

Abstract

A pressure relief valve arrangement (1), in particular for a magnetic resonance imaging (MRI) magnet system is described. The pressure relief valve arrangement (1) comprises a flow channel (2) configured as vent path, a fluid chamber (3) and a piston (4). The vent path (2) comprises an inlet opening (5) and an outlet opening (6). The piston (4) is positioned within the vent path (2) and configured for opening and closing a flow connection between the inlet opening (5) and the outlet opening (6). The piston (4) comprises a centerline (7), a top surface (8) and a bottom surface (9). The bottom surface (9) faces towards the inlet opening (5) of the vent path (2). The fluid chamber (3) comprises an inner surface portion which is formed by the top surface (8) of the piston (4) and the fluid chamber (3) is in flow connection with the vent path (2) via a bleed line (30).

Description

A PRESSURE RELIEF VALVE ARRANGEMENT

Field of the invention The present invention relates to a pressure relief valve arrangement for pressure vessels, for instance for magnetic resonance imaging (MRI) systems, especially for superconducting magnet systems with a helium cooling system. Background of the invention

Magnets, which are typically used for magnetic resonance imaging (MRI), preferably comprise superconducting coils. Such MRI magnets require a pressure relief valve to enable helium gas to be vented during a magnet quench. It is desirable that this valve is closed unless venting to prevent air ingress into the system which may cause a build-up of ice. These pressure relief need to be CE approved and need TUV-SUD PED (EU Pressure Equipment Directive) approval or a burst disc must be fitted in parallel to the valve in case of the valve failing to operate.

Pressure relief valves which are maintained in the closed state with the use of springs are subject to variations in opening pressure depending on elevation of the system above sea level. For example, if an MRI system is in transit over a mountainous region the air pressure is lower at altitude and the valve has less force holding it in the closed state. Therefore, the pressure required to open the valve is lower and the valve may open inadvertently releasing helium gas and potentially allowing air to enter the system and risk of ice blocking the venting components. Also during air shipping the quench valve may require a blanking plate to be fitted as the valve will not remain closed during transit and in this instance a separate venting arrangement is required. Up to now pressure relief valves in an MRI magnet system consist of a plate directly blocking flow of gas from a cryogen pressure vessel enclosing a magnet coil structure. When the magnet quenches the plate lifts exposing an annular gap for the gas to flow around. The plate is held closed by a spring arrangement which determines the pressure at which the valve will open. The outside face of the spring plate has atmospheric pressure bearing on it. This works in addition to the springs and determines the opening pressure of the valve, therefore at various altitudes the absolute opening pressure of the valve will vary. The plate may be dual purpose: a burst disc can be installed in the plate such that, if the valve does not open or opens but the pressure is still too high, the burst disc will rupture enabling gas in the cryogen pressure vessel to escape without overpressurizing the cryogen pressure vessel. During shipping the systems may have this valve locked closed and alternative venting in place. Alternatively, if the system is on truck at high altitude the variations in pressure may be accommodated by using bespoke spring settings .

Description of the invention

It is therefore an objective of the present invention to provide an advantageous pressure relief valve arrangement and an advantageous magnetic resonance imaging (MRI) system, which especially deals with the difficulties related to changing air pressure or different atmospheric pressure situations . The inventive valve arrangement for pressure vessels comprises a flow channel configured as vent path, a fluid chamber and a piston. The vent path comprises an inlet opening and an outlet opening. The piston is positioned within the vent path and is configured for opening and closing a flow connection between the inlet opening and the outlet opening. The piston comprises a centerline, a top surface and a bottom surface. The bottom surface is facing towards the inlet opening of the vent path. The fluid chamber comprises an inner surface portion which is formed by the top surface of the piston. The fluid chamber is in flow connection with the vent path via a bleed line.

Preferably, the valve arrangement for pressure vessels is configured as a quench and/or pressure relief valve arrangement and/or as valve arrangement for a magnetic resonance imaging magnet system.

The opening and closing of the flow connection between the inlet opening and the outlet opening of the vent path may be realized by means of the piston. For example, the piston may be positioned between the inlet opening and the outlet opening . By means of the bleed line equal pressure above and below the piston, more precisely an equal pressure at the top surface and at the bottom surface of the piston is realized during normal static operation and during shipping and transit. The pressure above the piston or at the top surface of the piston or in the fluid chamber may be obtained from the magnet system and therefore is not subject to variation at altitude. This has the advantage, that the functionality of the pressure relief valve arrangement is not influenced or negatively affected by a varying atmospheric pressure of the environment. Preferably, the bleed line comprises an orifice. This has the advantage that as the pressure in the system raises the pressure in the fluid chamber, for example above the piston, also rises. The fluid chamber is also fed by the system pressure but restricted due to the orifice. This causes the piston to move when a predetermined pressure differential is reached, for example when the magnet quenches.

Alternatively the top chamber may be connected to a manual relief valve for example in the form of a ball valve fitted to the cap. It may be advantageous depending on the pressure vessel application to be able to manually or automatically vent the pressure from the top chamber thereby causing the valve to open and release system pressure.

The vent path may comprise an outer surface and the bleed line may be at least partly located outside of the outer surface, in other words external of the vent path. Alternatively, the bleed line may be located inside of the piston. This may be realized such that the bleed line is formed as a hole in the piston. For example, the hole may run through the bottom surface and through the top surface of the piston. Generally, the inlet opening of the vent path is connected to a sealed cooling fluid system and/or the inlet opening may be in flow connection with a cooling fluid system. The inlet opening may for instance be connected with a cooling fluid system or it may be part of a cooling fluid system or it may be an in integrated portion of a cooling fluid system. Preferably, the cooling fluid system is a helium cooling system.

The quench valve arrangement may comprise a burst disc, for example, a replaceable burst disc. The burst disc may comprise or may consist of a graphite or metal disc. If the pressure in the cooling fluid system increases above a predetermined pressure value, the valve opens or the burst disc ruptures, which allows the cooling fluid to escape and prevents damage of the cooling fluid system.

In an advantageous example the burst disc is located within the piston. Alternatively, the vent path may comprise an additional opening or port, which is closed by means of the burst disc.

Moreover, the piston may comprise a fluid channel with an opening in the bottom surface and a lateral opening located in a side surface of the piston. The lateral opening is in flow connection with the opening in the bottom surface. For example, the lateral opening may comprise a centerline which crosses the centerline of the piston and which includes an angle with the centerline of the piston between 10° and 90°. Preferably, the lateral opening may be connectable with the outlet opening of the vent path via the piston.

Furthermore, the quench valve arrangement may comprise a spring pressing against the top surface of the piston, especially for closing the valve, which means for closing the flow connection between the inlet opening and the outlet opening of the vent path. For example, the top surface of the piston may comprise a recess and the spring, which preferably is a coil spring, may be fitted into the regress.

Moreover, the quench valve arrangement may comprise a pin arrangement or key arrangement configured to aid a predetermined movement or displacement of the piston in the vent path and/or to ensure the burst disc is fitted in a predetermined position. A key or pin arrangement may be necessary to aid assembly and ensure the burst disc is always fitted in the correct orientation, especially if the burst disc is profiled to provide a smooth flow path when open, or to align piston entrance with flow exit if the valve remains closed and a fitted burst disc ruptures. A key way may be incorporated into the valve body with a key in the piston or a valve cap may incorporate an eccentric pin to act as a key with a corresponding offset hole or key way in the piston. For example, the vent path may comprise a groove or slot and the piston may comprise a corresponding protrusion fitting into the groove or slot.

Furthermore, the fluid chamber may comprise a means for aiding lifting the piston and/or for aiding opening, preferably for aiding fast opening, of the fluid chamber. Advantageously, the means for aiding lifting the piston and/or for aiding opening of the fluid chamber comprises a vent groove located at an inner surface of the fluid chamber. Preferably the vent groove runs parallel to the centerline of the piston, which may be oriented vertically. For example, the fluid chamber, which may be a top chamber, may have a vent groove or similar feature to aid fast opening and minimize the height of the fluid or top chamber. This groove may positioned above a first seal or upper seal and may run vertically, when the piston starts to lift due to a quench the seal moves up and expose the groove. The gas in the fluid chamber is then vented along the groove into the vent path or into a quench line thus enabling the piston to move up further and quicker. Advantageously, the vent groove may be combined with the previously described key way.

The inventive pressure relief valve arrangement may comprise a cap which may be located on top of the fluid chamber. The cap may form an inner surface portion of the fluid chamber. The cap may comprise a pin, which ensures that the burst disc is fitted in the correct position. The pin may be part of the cap or may be connected to the cap. If the burst disc ruptures then replacing the disc is a relatively quick and easy operation by removing the cap and swapping the piston. As an advantageous variant the fluid chamber comprises an inner diameter, which is larger than the inner diameter of the vent path at the bottom surface of the piston, when the piston is in a position where the fluid channel between the inlet opening and the outlet opening of the vent path is closed by means of the piston. The top surface of the piston may have larger diameter than the bottom surface of the piston. This causes larger pressure acting on the top surface of the piston than on the bottom surface of the piston. In other words, the pressure of the cooling fluid system in the fluid chamber holds the piston in a closed valve position. The larger diameter of the upper portion of the piston than the diameter of the vent path at the bottom surface of the piston may prevent that the piston is pressed too far into the vent path.

As a further option the piston may comprise a first seal and a second seal. The first seal, which may be an upper seal, is positioned at or around the circumference of the piston close to the top surface of the piston. The first seal may act for sealing the fluid chamber. The second seal is positioned at or around the circumference of the piston close to the bottom surface of the piston sealing the cooling system. The first seal may have a larger diameter than the second seal. The greater force exerted by a larger seal at the top may negate the need for a spring to be fitted. Alternatively, a spring may be fitted to the top surface of the piston and the second (lower) seal may have a larger diameter than the first seal. A larger diameter of the first seal or top seal has the advantage that this will aid the piston to stay closed under normal magnet operation as the larger diameter will generate greater force holding the piston closed. During transit the valve is unaffected by changes in atmospheric pressure due to the fluid chamber being linked to the cooling fluid pressure, for example to the helium vessel pressure, via the bleed line . Alternatively, if a spring is fitted the second seal or the lower seal may have an equal or larger diameter than the first seal or the top seal sealing the fluid chamber. Therefore, under pressure rise the piston moves and a clear flow path is enabled to vent the magnet pressure. Once the pressure has stabilized, the pressure in both chambers, which means in the fluid chamber and a vent path portion which is in fluid connection with the cooling fluid system, is balanced and with the additional spring force the piston moved and the valve is closed leaving the burst disc intact.

The fluid chamber may be in flow connection with the inlet opening or with the outlet opening of the vent path via the bleed line.

The present invention has the advantage, that the system pressure of the cooling fluid system of the MRI system is used to both open and close the valve. Moreover, the inventive provides a very reliable way to ensure that the valve maintains closed under normal operation. The diameters of the seals can be tailored to suit valve opening pressure. The connection of the fluid chamber to the cooling fluid system, for example to the helium system, provides an isolation of the valve from atmospheric effects, like pressure differences.

The addition of the bleed line either through the piston or externally into the fluid chamber provides a cheap and simple solution to obtain a reliable valve arrangement. This enables the orifice to be set to the required diameter to allow the system pressure to rise and balance during imaging but under a quench scenario the orifice restricts gas flow or more generally cooling fluid flow and the valve lifts to enable the system pressure to be vented. The addition of the vent groove in the fluid chamber allows the gas to be vented quickly and effectively enabling the piston to lift completely .

Generally, the described seals may be O-ring seals. Alternatively to using a single O-ring seal a double O-ring seal or a wiper seal may be used. Other seals may be better suited to cryogenic conditions such as Trelleborg "Turcon Variseal H" . Description of embodiments

Further features, properties and advantages of the present invention will become clear from the following description of embodiments in conjunction with the accompanying drawings. The embodiments do not limit the scope of the present invention which is determined by the appended claims.

Corresponding elements are designated with the same reference numerals in all figures and are only once described in detail to avoid repetition.

Fig. 1 schematically shows a sectional view of the inventive pressure relief valve arrangement magnetic resonance imaging magnet system.

Fig. 2 schematically shows the functionality of the pressure relief valve arrangement.

Fig. 3 schematically shows the functionality of the pressure relief valve arrangement.

Fig. 4 schematically shows a sectional view of a portion of the inventive pressure relief valve arrangement. Fig. 5 schematically shows a sectional view along V-V of the arrangement shown in Fig. 4. Fig. 6 schematically shows a sectional view of a portion of the piston and the cap.

Fig. 7 schematically shows a variant of a pressure relief valve arrangement in a closed position.

Fig. 8 schematically shows the pressure relief valve arrangement of Fig. 7 with lifted piston.

Fig. 9 schematically shows a sectional view

inventive pressure relief valve arrangement

Fig. 10 schematically shows a conventional arrangement of a cryostat including a cryogen vessel.

Fig. 11 schematically shows a typical present arrangement of a magnet within a cryostat, with a mechanical refrigerator . Figure 1 schematically shows the inventive pressure relief valve arrangement 1 for a magnetic resonance imaging magnet system. The pressure relief valve arrangement 1 comprises a flow channel configured as vent path 2, a fluid chamber 3 and a piston 4. The vent path 2 comprises an inlet opening 5 and an outlet opening 6. The piston 4 is positioned within the vent path 2. The piston is configured for opening and closing a flow connection between the inlet opening 5 and the outlet opening 6 by means of the piston 4. In figure 1 the piston 4 is positioned between the inlet opening 5 and the outlet opening 6.

When the MRI magnet system is vented, a cooling fluid, for example helium, can escape from the system via the vent path in flow direction 45.

The piston 4 comprises a centerline 7, a top surface 8 and a bottom surface 9. The bottom surface 9 faces towards the inlet opening 5 of the vent path 2. The upper surface or top surface 8 of the piston 4 faces towards the fluid chamber 3. The fluid chamber 3 comprises an inner surface portion which is formed by the top surface 8 of the piston 4.

The fluid chamber 3 is in flow connection with the vent path 2 via a bleed line 30. The bleed line 30 may comprise an orifice 31. In figure 1 the bleed line 30 is formed by a hole reaching from the top surface 8 to the bottom surface 9 of the piston 4.

By means of the bleed line 30 the fluid chamber 30 is filled with the cooling fluid from the cooling fluid system, for example with helium. This has the effect that the pressure of the cooling fluid system acts on the bottom surface 9 of the piston 4 and on the top surface 8 of the piston 4. This ensures, that the piston 4 stays in its position within the vent path 2 and the position of the piston 4 does not depend on atmospheric pressure or the particular pressure difference between the cooling fluid system and the atmospheric pressure .

In Figure 1 the fluid chamber 3 is closed by means of a cap 41. Moreover, a spring, for example a coil spring 42, is positioned within the fluid chamber 3, preferably between the cap 41 and the top surface 8 of the piston 4. In figure 1 the top surface 8 of the piston 4 comprises a recess 43 and the spring 42 is fitted into the recess 43. This ensures that the spring 42 keeps in its position. The spring may or may not be required, the cap also is advantageous for service but the cap could be a fixed feature of the valve body and the piston inserted from below prior to fitting the valve to the system.

By means of the spring 42 the pressure acting on the top surface 8 of the piston 4 is increased and prevents an unintended lifting up of the piston 4 towards the cap 41; on the other hand, if pressure in the cooling fluid system increases over a predetermined threshold value, the piston 4 lifts and the system is vented. To aid the lifting of the piston 4 in such a situation, the diameter di of the fluid chamber 3 is larger than the diameter d₃ of the piston 4 and larger than the diameter d2 of the inlet opening 5 of the vent path 2. Once the piston has lifted, the fluid, for example helium gas, in the fluid chamber 3 will quickly vent out of the fluid chamber 3 along the side surface 48 of the piston .

Moreover, the pressure relief valve arrangement 1 may comprise a key arrangement for ensuring a correct alignment of the piston 4 in the vent path 2. The key arrangement comprises a protrusion 47, which may be located at the top surface 8 and a slot or groove 44 in the inner side surface 50 of the fluid chamber 3. The protrusion 47 fits into the groove or recess 44 and acts as key. The groove or slot 44 may additionally act as vent groove in case of a lifting of the piston.

The inlet opening 5 is preferably connectable with a cooling fluid system, for example a helium cooling system, or it may be connected to the cooling fluid system or it may be part of the cooling fluid system or an integrated portion of the cooling fluid system. Furthermore, the inlet opening 5 can be in flow connection with the cooling fluid system.

The pressure relief valve arrangement 1 may comprise a burst disc 35. In figure 1 the piston 4 comprises a fluid channel 51 with an opening 52 in the bottom surface 9 and a lateral opening 37. The lateral opening 37 is located at the side surface 48 of the piston 4. The lateral opening 37 comprises a centerline 49. The centerline 49 intersects with the centerline 7 of the piston 4 and includes with the centerline 7 an angle a between 0° and 90°, preferably between 45° and 90° . The burst disc 35 is located within the opening 52 and closes the fluid channel 51 within the piston 4. The burst disc 35 may be replaceable. Moreover, the burst disc 35 may consist of graphite or stainless steel.

Figure 2 and figure 3 schematically show the functionality of the previously described pressure relief valve arrangement. The dashed line 40 designates an alternative bleed line configuration. In this variant instead of integrating the bleed line into the piston 8 or additionally to the previously described bleed line 30 the bleed line 40 is positioned at the outer surface 53 of the vent path. The bleed line 40 provides a flow connection between the inlet opening 5 and the fluid chamber 3.

In figure 2 and figure 3 the cooling fluid, for example helium, is designated as dots. It presses against the bottom surface 9 and the burst disc 35 coming from the inlet opening 5. Cooling fluid, for example helium, is guided into the fluid chamber 3 via the bleed line 40. This provides a pressure against the top surface 8 of the piston 4 and ensures that the outlet opening 6 of the vent path 2 is closed and no cooling fluid can escape. In figure 3 a venting situation is schematically shown. The piston 4 is lifted towards the fluid chamber 3, for example due to a pressure increase in the cooling fluid system. The cooling fluid flows directly from the inlet opening 5 through the outlet opening 6 of the vent path 2. The cooling fluid in the fluid chamber 4 escapes via the bleed line 40 or via the bleed line 30, not shown in figures 2 and 3, or via the space between the piston 4 and the inner surface 50 the fluid chamber 3 caused by the difference in diameter di and d₃, as shown and described in conjunction with figure 1.

A further embodiment is shown in figures 4 to 6. Figure 4 schematically shows a sectional view of a portion of the inventive quench valve arrangement. In Figure 4, the system comprises an internal bleed line in form of a hole through the piston 30 and additionally or alternatively an external bleed line 40.

The inner surface 55 of the fluid channel 51 within the piston 4 may have a rectangular shape in a sectional view. Alternatively, it may have a curved or profiled surface shape in a sectional view, as designated with reference numeral 54.

As a further difference to the previously described embodiment the piston 4 in figure 4 comprises a first portion 57 and a second portion 58. The first portion 57 is positioned close to the fluid chamber 3. The first portion 57 has a diameter d₃. The second portion 58 has a diameter d₄, which is smaller than the diameter d₃ of the first portion 57. The second portion 58 is positioned close to the inlet opening 5 of the vent path 2. The first portion 57 comprises a first seal 33 and the second portion 55 comprises a seal 34. Both seals 33 and 34 may be O-ring seals. They are positioned along the circumference of the piston 4. In figure 4 the first seal 33 has a larger diameter than the second seal 34. The fluid chamber 3 has a diameter di, which is larger than the diameter d₂ of the inlet opening 5. In figure 4 the top surface 8 of the piston 4 has a diameter, which corresponds to the diameter d₃ of the first portion 57. The bottom surface 9 of the piston has a diameter which corresponds to the diameter d₄ of the second portion 58 of the piston 4. Hence, the diameter of the top surface 8 is larger than the diameter of the bottom surface 9. This has the effect that a higher pressure is acting on the top surface 8 than on the bottom surface 9 due to the flow connection between the cooling fluid system and the fluid chamber 3. Figure 5 schematically shows a sectional view along V-V of the arrangement shown in figure 4. In the present embodiment the piston 4 comprises a recess or groove acting as key and acting as vent groove, designated with reference numeral 60.

An optional feature is shown in figure 6. Figure 6 schematically shows a sectional view of a portion of the piston 4 and the cap. In figure 6 a pin 61 is connected to the cap 62. Generally the cap 62 may correspond to the cap 41, as shown in figure 1 and as shown in figures 2 and 3. In figure 6 the piston comprises a hole 63, in which the pin 61 engages. This provides a correct alignment of the piston 4 within the vent path. The cap 41, 62 generally provides an access to the piston 4 and to the burst disc 35.

A further embodiment is shown in figures 7 and 8. In this embodiment the bleed line 70 is located along the centerline 7 of the piston 4. Moreover, the vent path comprises an additional opening or port 71, which is closed by means of the burst disc 35. The additional opening or port 71 is in flow connection with the outlet opening 6 of the vent path 2.

Figure 7 shows the pressure relief valve arrangement in a closed position. The piston 4 interrupts the flow connection between the inlet opening 5 and the outlet opening 6. In figure 8 the piston 4 is lifted towards the cap 41 and the flow connection between the inlet opening 5 and outlet opening 6 is opened, such that the cooling fluid, for example helium, can escape from the system in flow direction 45.

A further variant is shown in figure 9. Figure 9 schematically shows a sectional view of the inventive quench valve arrangement. In figure 9 the arrangement differs from the embodiment shown in figure 4 in that a bleed line 80 is positioned at the outer surface 53 of the vent path and provides a flow connection between the fluid chamber 3 and the outlet opening 6. This alternative solution shows the pressure in the fluid chamber 3 taken from a quench line downstream of the valve. In this configuration the piston 4 lifts to vent the system pressure when the cooling fluid, for example helium, exceeds a particular pressure, determined by the design of the arrangement, above atmospheric pressure. A larger diameter seal 33 will be required at the top of the piston 4 unless a spring 42 is used.

Figure 10 shows a conventional arrangement of a cryostat including a cryogen vessel 12. A cooled superconducting magnet 10 is provided within cryogen vessel 12, itself retained within an outer vacuum chamber (OVC) 14. One or more thermal radiation shields 16 are provided in the vacuum space between the cryogen vessel 12 and the outer vacuum chamber 14. In some known arrangements, a refrigerator 17 is mounted in a refrigerator sock 15 located in a turret 18 provided for the purpose, towards the side of the cryostat. Alternatively, a refrigerator 17 may be located within access turret 19, which retains access neck (vent tube) 20 mounted at the top of the cryostat. The refrigerator 17 provides active refrigeration to cool cryogen gas within the cryogen vessel 12, in some arrangements by recondensing it into a liquid. The refrigerator 17 may also serve to cool the radiation shield 16. As illustrated in figure 10, the refrigerator 17 may be a two-stage refrigerator. A first cooling stage is thermally linked to the radiation shield 16, and provides cooling to a first temperature, typically in the region of 80-100K. A second cooling stage provides cooling of the cryogen gas to a much lower temperature, typically in the region of 4-10K.

A negative electrical connection 21a is usually provided to the magnet 10 through the body of the cryostat. A positive electrical connection 21 is usually provided by a conductor passing through the vent tube 20. For fixed current lead (FCL) designs, a separate vent path (auxiliary vent) (not shown in figure 1) is provided as a fail-safe vent in case of blockage of the vent tube 20. Figure 11 shows a typical present arrangement of a magnet (not visible) within a cryostat 10, with a mechanical refrigerator 17 providing cooling to the interior of the cryostat. The refrigerator 17 is placed in a helium circuit including high pressure supply line 11, low pressure return line 13 and helium compressor 15.

A magnetic resonance imaging system will comprise further equipment (not illustrated) , such as gradient and field coils, shim coils and a patient table. One or more system electronics cabinet (s) 19 house (s) a magnet supervisory system 22 and other control and measurement equipment 24 which control operation of the magnet, and such further equipment, over communications lines 26. The magnet supervisory system 22 receives data input from appropriate system sensors attached to various components of the MRI system. Helium compressor 15 is typically an electromechanical device. It is conventionally mechanically enclosed within the system electronics cabinet (s) 20 but the helium compressor is conventionally a standalone device.

Claims

1. A pressure relief valve arrangement (1) for pressure vessels, the pressure relief valve arrangement (1) comprising a flow channel (2) configured as vent path, a fluid chamber (3) and a piston (4),

wherein the vent path (2) comprises an inlet opening (5) and an outlet opening (6),

wherein the piston (4) is positioned within the vent path (2) and configured for opening and closing a flow connection between the inlet opening (5) and the outlet opening (6), the piston (4) comprises a centerline (7), a top surface (8) and a bottom surface (9), the bottom surface (9) facing towards the inlet opening (5) of the vent path (2), and wherein the fluid chamber (3) comprises an inner surface portion which is formed by the top surface (8) of the piston (4) and the fluid chamber (3) is in flow connection with the vent path (2) via a bleed line (30) .

2. The pressure relief valve arrangement as claimed in the previous claim,

wherein the bleed line (30) comprises an orifice (31) .

3. The pressure relief valve arrangement (1) as claimed in any of the previous claims,

wherein the vent path (2) comprises an outer surface (53) and the bleed line (40) is at least partly located outside of the outer surface (53) or the bleed line (30) is located inside of the piston ( 4 ) .

4. The pressure relief valve arrangement (1) as claimed in any of the previous claims,

wherein the inlet opening (5) is connectable with a cooling fluid system and/or is in flow connection with a cooling fluid system.

5. The pressure relief valve arrangement (1) as claimed in the previous claim,

wherein cooling fluid system is a helium cooling system.

6. The pressure relief valve arrangement (1) as claimed in any of the previous claims,

wherein the pressure relief valve arrangement comprises a burst disc (35) .

7. The pressure relief valve arrangement (1) as claimed in the previous claim,

wherein the disc (35) is located at the bottom surface (9) of the piston (4) or

wherein the vent path (2) comprises an additional opening (71) which is closed by means of the burst disc (35) .

8. The pressure relief valve arrangement (1) as claimed in any of the previous claims,

wherein the piston (4) comprises a fluid channel (51) with an opening (52) in the bottom surface (9) and a lateral opening (37) located in a side surface (48) of the piston (4), the lateral opening (37) being in flow connection with the opening (52) in the bottom surface (9) .

9. The pressure relief valve arrangement (1) as claimed in any of the previous claims,

wherein the pressure relief valve arrangement (1) comprises a spring (42) pressing against the top surface (8) of the piston ( 4 ) .

10. The pressure relief valve arrangement (1) as claimed in any of the previous claims,

wherein the pressure relief valve arrangement (1) comprises a pin arrangement or key arrangement configured to aid a predetermined movement of the piston (4) in the vent path (2) and/or to ensure the burst disc (35) is fitted in a predetermined position.

11. The pressure relief valve arrangement (1) as claimed in any of the previous claims,

wherein the fluid chamber (3) comprises a means for aiding lifting the piston (4) .

12. The pressure relief valve arrangement (1) as claimed in any of the previous claims,

wherein the fluid chamber (3) comprises an inner diameter di which is larger than the inner diameter d2 of the vent path (2) at the bottom surface (9) of the piston (4), when the piston (4) is in a position where the fluid channel (2) between the inlet opening (5) and the outlet opening (6) of the vent path (2) is closed by means of the piston (4) .

13. The pressure relief valve arrangement (1) as claimed in any of the previous claims,

wherein the piston (4) comprises a first seal (33) and a second seal (34), the first seal (33) is positioned at the circumference of the piston (4) close to the top surface (8) of the piston (4) and the second seal (34) is positioned at the circumference of the piston (4) close to the bottom surface (9) of the piston (4), the first seal (33) having a larger diameter than the second seal (34) or a spring (42) is fitted to the top surface (8) and the second seal (34) having a larger diameter than the first seal (33) .

14. The pressure relief valve arrangement (1) as claimed in any of the previous claims,

wherein the fluid chamber (3) is in flow connection with the inlet opening (5) or with the outlet opening (6) of the vent path (2) via a bleed line (80) .

15. A magnetic resonance imaging magnet system comprising a pressure relief valve arrangement (1) as claimed in any of the previous claims.