Patent Description:
The manufacture of semiconductor devices, flat panel displays and solar panels involves various process steps (e.g. etching, deposition and cleaning) typically performed under vacuum conditions. To achieve such conditions, one or more vacuum pumps are connected to the outlet of each process chamber. During operation, the vacuum pumps receive unused process gases and/or by-products exiting the process chamber. The unused gases and by-products are usually corrosive, toxic, pyrophoric and/or hazardous gases that cannot be released directly into the environment. Thus, each vacuum pump exhausts into one or more gas abatement systems. Manufacturers commonly install two abatement systems in parallel, one system operating in an "on-line" mode and the other system operating in an "off-line" mode. Together, the dual systems provide enhanced uptime should an abatement system fail or require preventative maintenance. During such failure or maintenance, an isolation valve isolates the off-line system from the on-line system. Each inlet line to each abatement system includes an isolation valve that alternately switches between the on-line and the off-line abatement systems. Because of unused process gases and by-products flowing through the isolation valve, the valve's o-ring seals or valve seats can fail. Such failure can cause the pressurized gases to continue flowing into the off-line abatement system even after the isolation valve is "closed. " Thus, when a technician is servicing the off-line system, corrosive, pyrophoric, toxic and/or hazardous gases may be released into the environment, detrimentally harming the surrounding people and property.

A conventional gas isolation valve according to the preamble of claim <NUM> is known from <CIT>.

To minimize release of gases, manufacturers often include a second, manually operated isolation valve in both the on-line and off-line abatement systems' inlet lines. Prior to servicing the off-line abatement system, the technician must also close the manual valve to ensure that pressurized gas no longer flows into the system. However, technicians sometimes forget to close or reopen the manual valve. Thus, the addition of a manual valve can have several disadvantages. First, the addition of a second valve adds cost to the system. Second, even with the second manual valve, there remains a small risk of accidental exposure, should the ball or seals of the manual valve become damaged. Third, if a technician forgets to reopen the manual valve, for example, then when the primary system goes off-line and switches to the secondary system, a catastrophic failure in the process system would occur.

Thus, there is a need for a single, reliable isolation valve that can isolate an off-line abatement system from an on-line abatement system and substantially reduce or eliminate release of corrosive, toxic, pyrophoric and/or hazardous gases. Similarly, there is a need for a processing system that provides cost-effective redundancy and enhanced safety.

According to a first aspect of the invention, there is provided a valve comprising: a housing with at least one inlet and at least one outlet; a valve member located within the housing and being moveable between different positions for controlling, in use, the flow of a fluid from an inlet to an outlet of the valve; wherein the valve further comprises: at least two spaced-apart valve seats in which the valve member is seated so as to form a cavity bounded by the valve seats, an exterior surface of the valve member and an interior surface of the housing; a first conduit extending between the exterior of the housing and the cavity; and a second conduit extending between the cavity and a bore of the valve member, through which conduits, in use, a purge gas can be introduced into the cavity and bore.

Preferably, pressurized gas or purge gas can optionally, but preferably be introduced into the cavity to ensure that process gas cannot escape the housing. In certain circumstances, this may involve providing a continuous flow of purge gas into the cavity or bore to maintain a positive pressure within the cavity or to inhibit backflow or the escape of process gas out of the housing via either aperture.

One envisaged advantage of the invention is that it may provide protection to people and the environment surrounding the valve in the event of dysfunction or failure. This can be achieved by providing a way to purge potentially harmful gasses from the valve and replace them with a (preferably) inert or harmless purge gas such that, should the valve leak, fail or be incorrectly operated, it is more likely that inert or harmless purge gas will escape than potentially harmful process gasses. This is achieved, in practice, by surrounding the moveable valve member with a continuous supply of purge gas, and by allowing the purge gas to enter the valve member's bore.

According to the invention, the second conduit extending between the cavity and a bore of the valve member allows, in a first ("on") position, i.e. when the bore aligns with an inlet and an outlet of the valve, a purge gas to enter the cavity and the bore of the valve member. Thus, the risk of a process gas escaping the housing may be reduced whilst the valve is in the first position. This is because purge gas may be allowed to flow from a flow path and into the cavity and, from the cavity, the purge gas may flow through the second conduit into the bore of the valve member where the purge gas combines with a process gas before exiting the valve. The purge gas may be supplied at a greater pressure than the normal maximum pressure of the process gas so that the purge gas may flow into the process gas stream.

The valve can be an isolator valve or a diverter valve, as may be used in a vacuum system with redundant gas abatement systems. In the case of an isolator valve, the valve member may comprise a bore such that the bore aligns with an inlet and an outlet of the valve when moved or rotated to the first position, and/or such that the bore does not align with either an inlet or an outlet when it is moved or rotated to the second position. Of course, when the valve member is moved to the second ("off") position, the purge gas may be dead-headed.

In the case of a diverter valve, the valve member may be moveable or rotatable between first and second positions such that the bore aligns with an inlet and a first outlet in the first position and an inlet and a second outlet in the second position.

To inhibit or prevent purge gas from flowing back out of the housing, a non-return valve is preferably provided. The non-return valve may comprise a spring, whose tension may be adjustable, and which spring is preferably manufactured from an alloy having a high nickel content, such as a mnemonic (or "shape-memory") alloy, such as Nitinal™.

In order to facilitate the introduction of the purge gas into the valve, a manifold may be provided to enable the purge gas to be introduced into the purge gas conduit from outside the housing.

A heater, such as a suitably specified cartridge heater, may be provided to heat the manifold and hence the purge gas within the manifold prior to entering the cavity. Such an arrangement may inhibit or prevent condensation within the manifold or any part of the valve.

The manifold, where provided, may function as a heat exchanger for transferring heat from the heater or cartridge heater to the purge gas within it. In order to maximize the efficiency of heat exchange, the manifold is preferably manufactured from a high thermal conductivity material, such as copper or aluminum alloy. The heat transfer from the heater or cartridge heater may be maximized by engineering the flow path for purge gas within the manifold to have a large surface area and to follow a non-linear path. As such, the flow path of purge gas through the manifold is preferably disrupted, which can be accomplished by increasing turbulence of the purge gas flowing through it, a tortuous flow path for purge gas flowing through the manifold, baffles in the flow path and the appropriate use of a packing material.

In most practical situations, there is preferably a purge gas supply, which is connected to an inlet of the manifold.

As alluded to previously, it may be possible to check the integrity of the valve by monitoring the pressure or flow of the purge gas in any one or more of the group comprising: the cavity; the first and second conduits; the manifold; and the purge gas supply. This can be achieved, in certain situations, by using a pressure transducer or a flow transducer. In a most preferred embodiment of the invention, the pressure transducer is positioned within the purge gas supply, and a valve and a pressure regulator are provided upstream of the pressure transducer along with a valve for isolating the purge gas within the cavity and the manifold, the pressure transducer being adapted to monitor the pressure of the isolated purge gas.

In order to be compatible with the manufacturing processed described previously, the wetted components are preferably selected for compatibility with the process gasses flowing through the valve, such as fluorine, chlorine and hydrogen bromine. In a similar manner, the valve seats are also preferably manufactured from materials that are resistant to chemical and physical attack by the process gasses, such as, for example, stainless steel, Hasteloy™, Viton™ and Kalrez™.

A second aspect of the invention provides a system comprising: a vacuum pump having an exhaust; and a pair of abatement systems teed into the exhaust and a valve as described herein positioned upstream of each of the abatement systems.

Preferred embodiments of the invention shall now be described, by way of example only, with reference to the accompanying drawings in which:.

The isolation valve of the present invention may be a ball valve. <FIG> shows an embodiment of a gas purged ball valve according to the present invention. The ball valve <NUM> has a housing <NUM> with an inlet 104a and an outlet 104b. The valve <NUM> further includes a rotatable ball <NUM> seated between a pair of valve seats 110a, 110b. The valve seats 110a, 110b are positioned in the inlet 104a and the outlet 104b of the valve <NUM>, respectively.

The ball <NUM> has a bore <NUM> through it, and the ball <NUM> may rotate between first and second positions. The bore <NUM> aligns with the inlet 104a and outlet 104b in the first position (See <FIG>) and misaligns or is perpendicular to the inlet 104a and outlet 104b in the second position (See <FIG>). The aligning of the bore <NUM> with the inlet 104a and the outlet 104b forms the "open" process flow path through the valve <NUM>. The ball <NUM> is spaced apart from the interior surface of the housing <NUM> to form a cavity <NUM> therein as shown in <FIG> and more particularly shown in <FIG>. The cavity <NUM>, or "void" between the valve seats 110a, 110b and the housing <NUM>, is purged with an inert gas as will be discussed in further detail below. As shown in <FIG>, the ball <NUM> further includes a key slot <NUM> that engages with the drive dog <NUM> of the valve <NUM>. A small opening <NUM> in the ball <NUM> proximate the key slot <NUM> enables the bore <NUM> to be in fluid communication with the cavity <NUM>.

Close coupled to the housing <NUM> is a manifold <NUM> having a flow path <NUM> with an inlet 120a and an outlet 120b. A pressurized source of inert gas <NUM>, for example nitrogen, argon or helium, is connected to the manifold inlet 120a. The outlet 120b of the manifold <NUM> is in fluid communication with a port <NUM> in the housing <NUM> as shown in <FIG>. The port <NUM> passes through the housing <NUM> and into the cavity <NUM> thus connecting the flow path <NUM> with the cavity <NUM>. Thus, the manifold <NUM> enables the cavity <NUM> to fill with the inert purge gas. While the manifold <NUM> can be constructed of a variety of solid materials, it is preferably constructed of a material having a high thermal conductivity such as aluminum alloy or copper.

A one-way (non-return) valve <NUM> is positioned in the port <NUM> so that inert gas can flow from the manifold <NUM> into the cavity <NUM>, but not in the reverse direction. In one embodiment, a spring (not shown) is positioned in the port <NUM> between the ball (not shown) of the non-return valve <NUM> and the ball <NUM> of the isolation valve <NUM>. The spring establishes a minimum pressure at which the purge gas must enter the port <NUM> and cavity <NUM>.

Certain process steps require heat to prevent the formation of solid by-products in the pipework and components (e.g. valves, vacuum pumps, etc.) downstream from the process tool. For example, the condensable solid, aluminum chloride (Al<NUM>Cl<NUM>) is a by-product of an aluminum etch process. In another example, ammonium hexaflurosilicate ((NH<NUM>)<NUM>SiF<NUM>)) is a condensable by-product of a silicon nitride chemical vapor deposition process using a fluorine-based chamber clean. Accordingly, the purge gas supplied to the cavity <NUM> is preferably heated in order to minimize condensation within the ball <NUM> and housing <NUM> of the valve <NUM>.

As shown in <FIG> and <FIG>, the manifold <NUM> includes a heater <NUM>, for example a cartridge heater, that is sized to maintain the temperature of the purge gas so as to minimize condensation within the ball <NUM> and housing <NUM> of the valve <NUM>. The heater <NUM> should maintain the temperature of the purge gas at or above about <NUM>, and preferably at or above about <NUM>. If the manifold <NUM> is constructed of a material having a high thermal conductivity, such as aluminum alloy or copper, then the heater <NUM> can be positioned at any convenient location within the manifold <NUM>. However, preferably the heater <NUM> is positioned closer to the manifold outlet 120b than to the manifold inlet 120a. As shown in <FIG> and <FIG>, the heater <NUM> is positioned within the manifold <NUM> proximate the flow path <NUM> and the manifold outlet 120b.

In addition, the flow path <NUM> preferably optimizes heat transfer from the heater <NUM> to the purge gas flowing through the manifold <NUM>. Thus, in one embodiment the flow path <NUM> is tortuous, where the purge gas must flow back-and-forth through the manifold <NUM> before it exits into port <NUM>. In another embodiment the flow path <NUM> may include baffles to increase turbulence or may be a packed bed to enhance heat transfer.

As discussed above, the isolation ball valve <NUM> has a first position and a second position. <FIG> shows the valve <NUM> in the first "open" position and <FIG> and <FIG> show the valve <NUM> in the second "closed" position. When the valve <NUM> is "open," process gas flows into the valve <NUM> through the valve inlet 104a, through the bore <NUM> of the ball <NUM> and out through the valve outlet 104b. While the process gas flows through the "open" valve <NUM>, heated purge gas flows from the manifold's <NUM> flow path <NUM> and into the cavity <NUM>, thus heating the ball <NUM> and housing <NUM>. From the cavity <NUM>, the heated purge gas flows through the opening <NUM> into the bore <NUM> of the ball <NUM> where the heated purge gas combines with the process gas before exiting the valve <NUM>. Preferably, the pressure of the heated inert gas supplied to the cavity <NUM> is higher than the normal maximum pressure of the process gas stream so that the inert gas can flow into the process gas stream.

When the ball valve <NUM> is "closed," as shown in <FIG> and <FIG>, and there are no leaks in the valve <NUM>, the heated purge gas continues to flow into the cavity <NUM> and bore <NUM> until the pressure of the purge gas inside the valve <NUM> reaches the pressure of the inert gas source <NUM>. Thus, under normal circumstances, the purge gas is "dead-headed" by the "closed" valve <NUM>. However, if the isolation valve <NUM> is damaged, for example from a scratch on the ball or a corroded valve seat and/or o-ring, harmless inert gas rather than the harmful process gas will leak from the cavity <NUM> and through the damaged area.

To detect a leak or damage in the isolation valve <NUM>, pressure decay of the heated inert purge gas can be monitored. In one embodiment, a solenoid valve <NUM> is installed in the inert gas source line <NUM> upstream from the manifold inlet 120a together with a pressure regulator <NUM> to regulate the pressure to the manifold <NUM> as shown in <FIG>. A pressure transducer <NUM> is also positioned in the inert gas source line between the manifold inlet 120a and the solenoid valve <NUM> to monitor the pressure in the manifold <NUM>. A heater <NUM> is positioned within the manifold <NUM>, upstream of a check valve <NUM>. Under normal operating conditions, the pressure of the inert gas should remain at a constant, pre-determined value.

As discussed above, the pressure of the heated inert gas in the cavity <NUM> should be higher than the maximum operating pressure of the process gas stream. The maximum pressure of the process gas stream is in turn determined by the characteristics of equipment, such as an abatement system, located downstream from the process chamber. For example, if the abatement system is a burner (e.g., See <CIT> and assigned to Edwards Limited) or a wet scrubber, then the pressure of the process gas stream may be about ±<NUM> inH<NUM>O (or about <NUM> psi, or <NUM> Bar). If, however, the abatement system is a gas reactor column (e.g., See <CIT> and <CIT>), then the pressure of the process gas stream may be as high as about <NUM> psi (i.e. about <NUM> Bar). Thus, in the former example, the pressure of the purge gas supplied to the valve <NUM> should be about <NUM> to about <NUM> psi (i.e. about <NUM> to <NUM> Bar). In the latter example, the pressure of the purge gas supplied to the valve <NUM> should be about <NUM> to about <NUM> psi (i.e. about <NUM> to <NUM> Bar).

During operation, shortly after the isolation (ball) valve <NUM> is rotated to the second "closed" position and the pressure of the heated inert gas in the valve <NUM> has had a chance to dead-head, then the solenoid valve <NUM> is also "closed. " Thus, the cavity <NUM> becomes charged with the inert gas at a certain pressure as discussed in the preceding paragraph. Thus, if there are no leaks in the valve the pressure of the inert gas measured by the pressure transducer <NUM> will remain constant. If, however, the pressure transducer <NUM> measures a decay (or a decrease) in the pressure of the inert gas, then such decay is an indication that there is a leak in the isolation valve <NUM>.

In another embodiment, a flow transducer <NUM> is positioned in the purge gas line <NUM> to monitor the flow rate of the purge gas as shown in <FIG>. A pressure regulator <NUM> is also positioned in the purge gas line <NUM>, upstream of the flow transducer <NUM>. A heater <NUM> is positioned within the manifold <NUM> upstream of a check valve <NUM>. Under normal circumstances, the purge gas is dead-headed within the cavity <NUM> as discussed above with respect to measuring pressure decay. However, if the flow transducer <NUM> detects flow of the purge gas, then such flow is an indication that there is a leak in the isolation valve <NUM>. Notably, both flow and pressure decay of the purge gas line <NUM> can be monitored in order to detect failure of the isolation valve <NUM>. To accomplish this, the flow transducer <NUM> could be installed between the pressure transducer <NUM> and the manifold inlet 120a.

In another embodiment (not claimed), the isolation valve is a diverter valve <NUM> as shown in <FIG>. In this embodiment, the diverter valve <NUM> has a housing <NUM> with an inlet 204a (shown in <FIG>) and two outlets 204b and 204c. The diverter valve <NUM> also has a rotatable ball <NUM> seated between valve seats 210a, 210b, 210c, 210d. The valve seats 210a, 210b, 210c, 210d are positioned about the ball <NUM> as shown in <FIG>.

The ball <NUM> has a bore with two limbs 214a, 214b that are arranged to form a single "L" configuration as illustrated in <FIG>. Notably, the bores 214a, 214b are positioned within the plane represented by the horizontal dashed line in <FIG>. However, the axis of rotation of the ball <NUM>, represented by the vertical dashed line in <FIG>, is perpendicular to this plane. This perpendicular configuration is necessary to isolate the cavity <NUM> (described below) from the process fluid flow path through the bores 214a, 214b of the valve <NUM>.

The ball <NUM> is rotatable between a first position and a second position. In the first position, bore 214a aligns with the inlet 204a and bore 214b aligns with the outlet 204b. In this first position, the process gas flows from inlet 204a and through outlet 204b. In the second position, as shown in <FIG>, the ball <NUM> is rotated so that bore 214b aligns with inlet 204a and bore 214a aligns with outlet 204c. In this second position, process gas flows from inlet 204a and through outlet 204c.

The ball <NUM> is spaced apart from the interior surface of the housing to form a cavity <NUM> therein as shown in <FIG>. The cavity <NUM> is purged with an inert gas as will be described in detail below. As shown in <FIG>, the ball <NUM> includes a key slot <NUM> that engages with the drive dog <NUM> of the valve <NUM>. A small opening <NUM> in the ball <NUM> proximate the key slot <NUM> enables the cavity <NUM> to be in fluid communication with the bores 214a, 214b. The small opening <NUM> must be sized so as to provide the necessary pressure drop thus allowing the cavity <NUM> to operate at a higher pressure than the process fluid flow.

As shown in <FIG>, close coupled to the housing <NUM> is a manifold <NUM> having a flow path <NUM> with an inlet 220a and an outlet 220b. A pressurized source of inert gas <NUM>, for example nitrogen, argon or helium, is connected to the manifold inlet 220a. The outlet 220b of the manifold <NUM> is in fluid communication with a port <NUM> in the housing <NUM> as shown in <FIG>. The port <NUM> passes through the valve housing <NUM> and into the cavity <NUM> thus connecting the flow path <NUM> with the cavity <NUM>. Thus, the manifold <NUM> enables the cavity <NUM> to fill with pressurized inert purge gas. While the manifold <NUM> can be constructed of a variety of solid materials, it is preferably constructed of a material having a high thermal conductivity such as aluminum alloy or copper.

A one-way (non-return) valve <NUM> is positioned in the port <NUM> so that inert gas can flow from the manifold <NUM> into the cavity <NUM>, but not in the reverse direction. In one embodiment, a spring (not shown) is positioned in the port <NUM> between the ball <NUM> of the non-return valve <NUM> and the ball <NUM> of the isolation valve <NUM>. The spring establishes a minimum pressure at which the purge gas must enter the port <NUM> and cavity <NUM>.

As shown in <FIG>, the manifold <NUM> preferably includes a heater <NUM>, for example a cartridge heater, which is sized to maintain the temperature of the purge gas so as to minimize condensation within the ball <NUM> and housing <NUM> of the valve <NUM>. The heater <NUM> should maintain the temperature of the purge gas at or about <NUM>, and preferably above about <NUM>. If the manifold <NUM> is constructed of a material having a high thermal conductivity, such as aluminum alloy or copper, then the heater <NUM> can be positioned at any convenient location within the manifold <NUM>. However, preferably the heater <NUM> is positioned closer to the manifold outlet 220b than to the manifold inlet 220a. As shown in <FIG>, the heater <NUM> is positioned within the manifold <NUM> proximate the flow path <NUM> and the manifold outlet 220b.

In addition, the flow path <NUM> preferably optimizes heat transfer from the heater <NUM> to the purge gas flowing through the manifold <NUM>. Thus, in one embodiment the flow path <NUM> is tortuous as shown in <FIG>, where the purge gas must flow back-and-forth through the manifold <NUM> before it exits into port <NUM>. In another embodiment the flow path <NUM> may include baffles to increase turbulence or may be a packed bed to enhance heat transfer.

As discussed above, the isolation diverter valve <NUM> has a first position and a second position. When bores 214a and 214b are aligned with inlet 204a and outlet 204b, respectively, process gas flows into the valve <NUM> through the inlet 204a, through the bores 214a, 214b of the ball <NUM> and out through outlet 204b. While the process gas flows through the bores 214a, 214b, heated purge gas flows from the manifold's <NUM> flow path <NUM> and into the cavity <NUM>, thus heating the ball <NUM> and housing <NUM>. From the cavity <NUM>, the heated purge gas flows through the opening <NUM> into the bores 214a, 214b of the ball <NUM> where the heated purge gas combines with the process fluid before exiting the valve <NUM>. Preferably, the pressure of the heated inert gas supplied to the cavity <NUM> is higher than the normal maximum pressure of the process gas stream so that the inert gas can flow into the process gas stream.

Similarly, when bores 214a and 214b are aligned with outlet 204c and inlet 204a, respectively, process gas flows into inlet 204a and through outlet 204c. As in the first position, heated inert purge gas flows into the bores 214a, 214b to combine with the process fluid. Moreover, the pressure of the inert purge gas is preferably higher than the operating pressure of the process gas.

Thus, during operation, when the valve <NUM> is in either the first or second position, the heated inert purge gas flows constantly into the cavity and bores 214a, 214b. As discussed above, the port <NUM> is sized to ensure that the pressure of the purge gas exceeds the pressure of the process gas and to control the flow of the purge gas into the bores 214a, 214b. Should the valve <NUM> fail, for example, from corrosion of a valve seat, the flow rate of the inert purge gas will increase. Thus, using the same configuration shown in <FIG>, a flow transducer can be positioned in the purge gas line to monitor the flow rate of the purge gas. If the flow transducer detects a relative increase in flow rate, then this would be an indication of a valve failure.

The wetted components of the isolation valve <NUM>, <NUM>, such as the housing, ball <NUM>, <NUM>, and valve seats 110a, 110b, must be compatible with gases such as fluorine, chlorine, hydrogen bromide and other gases used in semiconductor, flat panel display and solar panel manufacturing processes. Similarly, the wetted components of the non-return valve <NUM>, <NUM>, such as the ball <NUM>, spring (not shown), washer (not shown) and sealing rings (not shown), must also be compatible with the aforementioned gases. Ball <NUM>, <NUM>, and ball <NUM> are preferably constructed of stainless steels (for example, <NUM>, <NUM>, etc.) that are corrosion resistant to the aforementioned gases. The spring (not shown) should be constructed out of an alloy having a high nickel content, or a Mnemonic material, such as those manufactured by Inco Alloys. The washer and sealing rings (not shown) should be constructed of stainless steels (e.g. <NUM>, <NUM>, etc.), Hastelloy, Viton® or Kalrez®. The manifold <NUM> can be constructed of a relatively inexpensive material such as aluminum.

Also provided is a system <NUM> having an isolation valve <NUM> according to the present invention. <FIG> shows a system <NUM> according to the present invention. The system <NUM> has redundant abatement systems 302a, 302b to receive an exhaust <NUM> from one or more vacuum pumps <NUM> connected to the outlet <NUM> of a process chamber <NUM>. The exhaust line <NUM> tees into each abatement system 302a, 302b and in the embodiment shown in <FIG>, an isolation valve <NUM> is installed in each line of the tee. In another embodiment <NUM> (not claimed) shown in <FIG>, a diverter valve <NUM> is installed at the tee upstream from the abatement systems 302a, 302b. In both embodiments <NUM>, <NUM>, the isolation valves <NUM>, <NUM> are constructed and function as described above.

Claim 1:
A gas isolation valve (<NUM>) comprising:
a housing (<NUM>) with at least one inlet (104a) and at least one outlet (104b);
a valve member (<NUM>) located within the housing and being moveable between between first open and second closed positions for controlling, in use, the flow of a fluid from the inlet to the outlet of the valve;
at least two spaced-apart valve seats (110a, 110b) in which the valve member is seated so as to form a cavity (<NUM>) bounded by the valve seats, an exterior surface of the valve member and an interior surface of the housing;
a first conduit (<NUM>) extending between the exterior of the housing and the cavity; and
a second conduit (<NUM>) extending between the cavity and a bore of the valve member, through which conduits, in use, a purge gas can be introduced into the cavity and bore; wherein the valve member is rotatable between the first and second positions such that the bore (<NUM>) aligns with the inlet and the outlet in the first position; characterised that the purge gas is dead-headed by the closed valve (<NUM>) when the valve member is in the second position and wherein if the isolation valve (<NUM>) is damaged, for example from a scratch on the ball or a corroded valve seat and/or o-ring, harmless inert gas rather than the harmful process gas will leak from the cavity (<NUM>) and through the damaged area.