WO2009156729A1

WO2009156729A1 - Mechanical seal support system

Info

Publication number: WO2009156729A1
Application number: PCT/GB2009/001590
Authority: WO
Inventors: John Thomas Bright; Alan James Roddis; Stephen Martin Shaw
Original assignee: AES Engineering Ltd
Current assignee: AES Engineering Ltd
Priority date: 2008-06-24
Filing date: 2009-06-24
Publication date: 2009-12-30
Anticipated expiration: 2010-12-24
Also published as: GB0811562D0

Abstract

A seal support system which tracks the process fluid pressure of one or more items of equipment and automatically maintains the barrier fluid pressure at a constant pressure differential, preferably higher, over the respective process fluid pressures specifically when the barrier fluid is intolerable to direct gas pressurisation. An example of such an application is an oil based barrier fluid. The system may also be employed with multiple pressure tracking means for use with multiple items of industrial equipment.

Description

MECHANICAL SEAL SUPPORT SYSTEM

Field of the Invention

This invention relates to seal support systems and especially systems, which are used with mechanical seals for the containment of process fluid.

Background to the Invention

A seal support system typically comprises of a pressure vessel or tank (vessel), which generally contains a volume of fluid. The vessel is piped to a sealing device on a pump, mixer or item of rotating equipment. Generally a return pipe is lead back to the vessel from the sealing device, hence closing the "loop". This allows the fluid, contained in the vessel, to enter and exit the sealing device. Typically such a seal support system is employed with a mechanical seal with two or more sets of seal faces, more commonly referred to as a double or dual seal.

The fluid within the vessel is generally chosen so that it lubricates and cools the components within the sealing device, whilst being compatible with the process fluid. The industry term for the fluid contained within the vessel, is Barrier or Buffer fluid.

It is not uncommon for the seal support system to have other items of equipment, sited around the vessel, to permit pressure being applied to the barrier / buffer fluid, or to allow additional cooling or fluid circulation around the seal.

Generally the sealing device sited on the item of rotating equipment is a mechanical seal comprising of a rotating member, which is secured to a shaft, and a stationary member which is secured to a housing. The interface, between the rotating member and the stationary member, on the mechanical seal, prevents the processed product from escaping.

It is a well-documented fact that the majority of mechanical seals have a fluid film, acting between the two sliding seal faces. This fluid film lubricates the set of seal faces. The wider the fluid film, the more the mechanical seal can be said to leak. Therefore over time, some fluid loss from the system is to be expected.

Furthermore, mechanical seals are often subjected to process upsets. Pressure surges and wide fluctuations in fluid temperature and can result in barrier / buffer fluid loss.

Yet further loss of fluid from the vessel may result due to evaporation.

Conventional dual seals should have the barrier fluid set at a higher pressure than the process pressure as this ensures clean barrier fluid lubricates the sliding seal faces instead of contaminated process fluid and prevents loss of process fluid .

Unfortunately, some items of rotating equipment, as found on batch processes, can be subject to fluctuating process pressures in the seal chamber. It is not uncommon for the process pressure, in a typical application, to be subject to pressure variations of 10 barg or more. In order to ensure that the seal chamber pressure is always higher than the process pressure the operator will set the barrier pressure higher than the maximum seal chamber pressure. When the seal chamber pressure is at its lowest value the differential pressure across the inner seal faces will be the difference between the seal chamber pressure and the barrier pressure. This can have the effect of creating a massive closing force on the two sets of seal faces. This closing force increases the heat generated by the seal which in turn breaks down the fluid film between the seal faces resulting in premature seal failure.

An innovative answer to this pressure fluctuation dilemma is to track the pressure in the equipment seal chamber thereby creating a reference pressure. This reference pressure is then referenced back to the 'valve' which is connected to an external pressure source, typically nitrogen which is in turn connected to the seal support system via the vessel. The valve will add a bias pressure, which can be set to a variable amount, to the reference pressure. The nitrogen then pressurises the barrier fluid to ensure it is greater than the reference pressure by a set amount (the bias pressure). This 'valve' ensures that the barrier fluid maintains a constant pressure above the process pressure, irrespective of the process pressure fluctuations.

There are perceived problems associated with nitrogen absorption into the barrier fluid especially at pressure in excess of 10 barg. An example of this is an oil barrier fluid where it is generally accepted that nitrogen acts to aerate the oil, effectively absorbing small gas bubbles into the oil. If these gas bubbles are released, usually due to rapid pressure loss (depressurisation), the mechanical seal faces may experience a foaming or frothing effect which can have a serious impact upon the fluid film and cause damage to the seal faces and premature failure of the seal.

Another issue is that the seal support systems may have to operate in a variety of ambient temperatures due climate effects, as a result of geography or even time of day. The effect of high temperature fluctuations on the barrier fluid causes volume variation which results in changes in the system pressure constant between the barrier and process fluid.

A further issue which occurs in systems which employ a piston type accumulator which is a conventional systems designed to manage pressure fluctuations is the issue that the process fluid reference pressure, taken from the seal chamber, is directly connected to the accumulator. This can lead to chemical compatibility issues between the process fluid and the accumulator parts as well as seizing the accumulator when used with viscous process media.

It is deemed to be advantageous if a seal support system can be developed whereby the piston accumulator is not in direct contact with the process media.

Furthermore, it is advantageous if the seal can accommodate pressure fluctuations in the barrier fluid as a result of barrier temperature without changing the pressure differential constant between the reference pressure of the process chamber and the barrier fluid pressure.

It is deemed to be a further advantage if the gas, used to pressurise the accumulator, is not in direct contact with the barrier fluid media therefore can not aerate said fluid. Preferably a method of automatically detecting the barrier fluid level in the vessel is desirable, specifically if said detection can be used to trigger an alarm in low/high barrier ϋ'jid level conditions.

Bladder accumulators are frequently employed in industry as they are generally a lower cost solution than a piston accumulator. The problem with bladder accumulators if they are employed on a self-pressurising barrier system, is that the amount of fluid contained within the barrier vessel is difficult to automatically detect/sense. Therefore, as fluid is lost from the mechanical seal and the accumulator bladder inflates to maintain the barrier pressure, a scenario can be reached where all the fluid from the vessel is lost, yet the pressure in the accumulator remains at the constant level above the reference process pressure. This could lead to the mechanical seal running dry and prematurely failing. It is deemed to be an advantage if the barrier fluid level in the seal support vessel can be automatically sensed or determined when the seal support system employs a bladder accumulator

Statements of the Invention

According to the present invention there is provided a seal support system for use with a sealing device employed to seal a piece of industrial equipment. Said seal support system is characterised by - an isolator connected to the seal chamber by suitable piping means

- a differential pressure valve connected to the isolator

- an accumulator or pressure vessel connected to the differential pressure valve wherein, said vessel contains a fluid and is connected to the sealing device in a closed loop system and said differential pressure valve is connected to a suitable pressure source.

The first embodiment is that the seal support system of the invention is a pressure tracking means in that the system tracks the process fluid pressure and automatically maintains the barrier fluid pressure at a constant pressure differential, preferably higher, over the process fluid pressure, specifically when the barrier fluid is intolerable to direct gas pressurisation. An example of such an application is an oil based barrier fluid.

Preferably, the differential valve has a venting means so that indirect increases in barrier fluid pressure, separate to the tracking means, can be automatically vented so to maintain said constant pressure differential between the process fluid and barrier fluid. An example of such is where the barrier fluid, within the barrier fluid vessel, expands due to changes in temperature and said expansion creates an undesirable increase in barrier fluid pressure. Preferably the seal support system of the invention has a vessel containing fluid. Inside the vessel is a floating member preferably of a material and/or construction which is buoyant in a fluid. Preferably, said floating member has one or more magnet or magnetically attractable members on the outer most surface substantially adjacent to the inner most surface of the vessel. Preferably the floating member is an annular flat plate with maximum surface area contact with the fluid.

Preferably the vessel is made of a material which is substantially not magnetic. A preferable non-magnetic vessel material is stainless steel.

Preferably, on the outer most surface of the vessel, there is a level indicator in the form of a sliding magnet which is attracted and hence corresponds with the floating member. Preferably said level indicator has an upper and lower limit switch thereby when, for example, a low fluid level condition is reached the system provides a signal for more fluid to be added.

Preferably the vessel fluid level can be increased by a suitable manual re-fill means, such as a hand pump connected to a fluid reservoir.

Preferably, the signal from the limit switch automatically activates the fluid re-fill operation without any human intervention.

The second embodiment is that the seal support system of the invention that has a vessel containing fluid, the fluid level of the vessel is automatically monitored by a magnetic level indicator which is substantially out of contact with the fluid.

Preferably the floating member has a ring construction sited inside the vessel. Said ring has an inner and outer diameter and is characterised in that the radial clearance between the outer diameter of said ring and inner diameter of said vessel is a clearance fit thereby allowing fluid to pass from one longitudinal side of the ring to another. Preferably said radial clearance is relatively small.

Preferably the vessel contains a cooling coil, which is considered as a spiral tube, the assembly of said cooling coil has an effective outer diameter, which is substantially smaller than the inner most diameter of the ring thereby permitting the ring to longitudinally slide inside the vessel without being constrained by the cooling coil.

Preferably the accumulator charges the fluid in the vessel to the desired pressure. Said accumulator is preferably connected yet independent of the vessel and of a bladder construction.

According to the present invention there is provided a seal support system for use with a sealing device employed to seal a piece of industrial equipment. Said seal support system is characterised by

- two or more isolators connected to two or more seal chambers by suitable piping means - two or more differential pressure valves each connected to the isolators

- two or more accumulators or vessels connected to the differential pressure valves

- one central fluid re-fill means wherein, said vessels contain fluid and are connected to the sealing devices in a closed loop system on two or more items of industrial equipment. Said differential pressure valves are connected to a suitable pressure source.

The third embodiment is that the seal support system of the invention incorporates multiple pressure tracking means for use with multiple items of industrial equipment. The system tracks the process fluid pressure of two or more items of equipment and automatically maintains the barrier fluid pressure at a constant pressure differential, preferably higher, over the respective process fluid pressures specifically when the barrier fluid is intolerable to direct gas pressurisation. An example of such an application is an oil based barrier fluid.

Preferable the respective vessels in the seal support system of the invention have a fluid separating diaphragm device, permitting different types of fluid in the upper and lower sections of the vessel.

Preferably the respective vessels in the seal support system of the invention have level indicators so that when an appropriate level condition is sensed, the system provides a signal for more fluid to be added.

Preferably, the signal from the limit switch automatically activates the fluid re-fill operation without any human intervention. Preferably, the multi- application re-fill operation is activated from a central reservoir re-fill system.

Preferably, as additional fluid is introduced into the respective vessels, the pressure within the respective vessels is regulated by an appropriate venting means so that an over-pressurisation condition of the barrier fluid can not be encountered.

The reader will relate to various benefits of the seal support system of the invention which will now herewith be described with the aid of the following drawings. Description of the drawings

The accompanying drawings are as follows:

Figure 1 is a schematic view of a prior art Plan 53C seal support system as found in the international standard API682 Ed.3 Figure 2 is a schematic view of the system of the invention

Figure 3 corresponds to Figure 2 and is a schematic view of the system of the invention with a piston type accumulator.

Figure 4 corresponds to Figure 2 and is a schematic view of the system of the invention with a bladder type accumulator. Figure 5 is a schematic view of multi-application seal support with an automatic re-fill system

Figure 6 corresponds to Figure 5 and is a schematic view of multi- application seal support with an automatic single re-fill system with a duty and stand-by motor/pump system

Detailed description of the Invention

The invention will now be described, by way of examples only, with reference to the accompanying drawings.

Figure 1 is a schematic view of a prior art Plan 53C seal support system 10 as illustrated in the international standard API682 Ed.3 From Figure 1 , a item of industrial equipment such as a pump 11 is sealed with a sealing device such as a mechanical seal 12.

The pump 11 is processing a fluid 13 herewith called process fluid typically operating at a fluctuating pressure. A communication path 14 is taken from the seal chamber 15 to one side of an accumulator 16. The barrier fluid in the seal 12 and/or seal support system 18 is connected to the other side of the accumulator. Figure 1 and the above description describes a typical piston type accumulator. As the pressure fluctuates in the process chamber 15, the barrier pressure in the seal 12 fluctuates also. The result is that the pressure differential between the process chamber 15 and seal chamber 12 can be kept to a desired level.

The problem with this prior art system is that the process media contaminates the communication means 14 and the accumulator 16 and the communication means 14 must be chemically compatible with the process media as well as being pressure and temperature compatible.

Figure 2 is a schematic view of the seal support system 20 of the invention.

From Figure 2, a item of industrial equipment such as a pump 21 is sealed with a sealing device such as a mechanical seal 22.

The pump 21 is processing a fluid 23 herewith called process fluid typically operating at a fluctuating pressure. A reference communication path 24 is taken from the seal chamber 25 to an isolator 26. The isolator 26 is connected to a differential pressure valve 27.

The purpose of the isolator 26 is to transmit reference pressure to the differential pressure valve 27.

The process fluid 23 reference pressure is taken by the isolator 26 and fed into the differential pressure valve 27, which is also connected to a gas supply source 28 such as nitrogen. The differential pressure valve 27 creates the desired barrier pressure and charges an accumulator 29 with said desired pressure. The accumulator 29 contains barrier fluid used to lubricate and cool the mechanical seal 22.

As the pressure fluctuates in the process chamber 25, the barrier pressure in the seal 22 fluctuates also. The result is that the pressure differential between the process chamber 25 and seal chamber 22 can be kept to a desired level. However, because the system of the invention does not have the gas supply 28 in direct contact with the barrier media, it can be used in barrier fluids such as oil, which overcomes gas absortion of the barrier fluid.

The design of the invention is also advantageous as it has no moving piston accumulator and/or elastomeric parts in contact with the process fluid.

Figure 2 also illustrates a re-fill device 31 connected to a non-return valve 32. If the level in the barrier fluid reduces, the re-fill device can be activated to apply additional barrier fluid into the seal system 30.

Figure 3 corresponds to Figure 2 and is a schematic view of the system of the invention 40 with a piston type accumulator 41.

The system of Figure 3 operates in a similar manner to as previously described other than the accumulator 41 has a piston 42 which moves longitudinally inside the accumulator 41 depending on the pressure applied by the differential pressure switch 43. A magnetic level switch 44 mounted on the outer most surface of the accumulator senses the position of the accumulator so to automatically determine the level of fluid in the accumulator 41. Once the barrier fluid level in the accumulator 41 reaches a critical level, the level switch 44 may automatically activate the refill device 46 to add further barrier fluid into the seal system 45. Figure 4 corresponds to Figure 2 and is a schematic view of the system of the invention 50 with a diaphragm type accumulator 51.

The system incorporates a diaphragm 59, preferably secured between the upper and lower parts of the accumulator 51.

The system 50 incorporates a level switch 52 on the gas side 57 of the accumulator 51. The level switch is mounted on the outer most surfaces of the accumulator 51 and is activated by a floating member 54. Said floating member is buoyant on the fluid 58 and has a magnet or magnetically attractive outer most surface 55 which correlates with the level switch 52.

Again, when a predetermined level is reached, the level switch 52 sends a signal to the re-fill device 56 so that additional barrier fluid 53 can be added. This invention of the device is particularly advantageous because the barrier fluid 53 is not in contact with the gas 57 and therefore has all the advantages of bladder accumulators but also allows direct monitoring of the barrier fluid level.

An experienced reader will relate to the fact that the barrier fluid could be cooled with a cooling coil in the accumulator as shown in Figure 5, item 80.

Figure 5 is a schematic view of multi-application seal support with an automatic re-fill system 60.

From Figure 5, the reader will note that a central, common re-fill system 66 is employed to re-fill one or more sealing applications 62 and 63.

Again, the level switch 61 is preferably electrically connected 64 to the pump/motor 65 on the forced circulation re-fill device 66. The level switch 61 is also preferably electrically connected 67 to a electrically activated ball valve 68. On a low level 73 application, the level switch 61 on the offending vessel 70 activates/opens the appropriate ball valve 68 and activates the forces circulation device 66. The non-return valve 69 prevents any loss of fluid or pressure from the sealing system 70.

The barrier fluid, from the automatic re-fill device 66 passes through the open ball valve 68 and opens the non-return valve 69 adding fluid into the vessel 70.

In order to open the non-return valve 69, the barrier fluid pressure from the re-fill device 66 must be greater than the barrier fluid pressure in the vessel 70. This over pressure is vented by the integral vent 71 on the pressure differential switch 72. When the barrier level in the vessel 70 reaches the upper limit 74, the level switch 61 sends a signal to close the ball valve 68 and deactivate the motor/pump 65 on the re-fill device 66.

The seal support system 60 of the invention then continues to track the process pressure 75 and adjust the barrier fluid 76 pressure accordingly as previously described.

The reader will relate to the considerable advantages that this automatic, self compensating system 60 offers.

Figure 6 corresponds to Figure 5 and is a schematic view of multi- application seal support 90 with an automatic single re-fill system 91.

The system 91 comprises of a duty motor/pump 92 and a stand-by motor/pump 93. In many critical industrial applications, two circulation pumps are deemed necessary to eliminate the likelihood that a single motor/pump will not activate when required to do so.

The system of the invention in Figure 6 accommodates such an occasion by providing two pumps/motors 94 and 95 which can be linked so that if one motor fails to start, the other motor is activated to perform the required task in hand.

Clearly whilst electrical valve and pump/motor means have been detailed above, the reader will appreciate that the seal support system of the invention can employ any other suitable actuation means such as air or hydraulics.

Claims

1. A seal support system for use with a sealing device employed to seal a piece of industrial equipment, characterised by

- an isolator connected to the seal chamber by suitable piping means - a differential pressure valve connected to the isolator

- an accumulator or vessel connected to the differential pressure valve wherein, said vessel contains a fluid and is connected to the sealing device in a closed loop system and said differential pressure valve is connected to a suitable pressure source.

2. A seal support system according to Claim 1 , whereby the differential valve has a venting means

3. A seal support system according to Claim 1 and/or Claim 2, whereby the vessel is longitudinally split and incorporates a diaphragm thereby sealingly separating the upper and lower sections of the vessel.

4. A seal support system according to Claim 1 and/or Claim 2, whereby adjacent to the inner most surface of the vessel is a floating member preferably made of a material and/or construction which is buoyant in a fluid.

5. A seal support system according to Claim 3, whereby said floating member has one or more magnet or magnetically attractable members on the outer most surface substantially adjacent to the inner most surface of the vessel.

6. A seal support system according to Claim 3 and/or 4, whereby said floating member is an annular flat plate with maximum surface area contact with the fluid.

7. A seal support system according to any preceding claims, whereby the vessel is made of a material which is substantially not magnetic.

8. A seal support system according to any preceding claims, whereby the outer most surface of the vessel has a fluid level indicator substantially adjacent to the floating member.

9. A seal support system according to any preceding claims, whereby the fluid level indicator is in the form of a sliding magnet device, which is attracted and hence corresponds with the floating member.

10. A seal support system according to any preceding claims, whereby said fluid level indicator has an upper and lower limit switch thereby when, for example, a low fluid level condition is reached the system provides a signal for more fluid to be added.

11. A seal support system according to any preceding claims, whereby the vessel fluid level can be increased by a suitable manual re-fill means, such as a hand pump connected to a fluid reservoir.

12. A seal support system according to any preceding claims, whereby a signal from said limit switch automatically activates the fluid re-fill operation without any human intervention.

13. A seal support system according to any preceding claims, whereby the floating member is located in the upper segment of the vessel and floats on a fluid which is separated by a diaphragm from the fluid in the lower segment of the vessel.

14. A seal support system according to any preceding claims, whereby the vessel contains a cooling coil, which is considered as a spiral tube and is positioned in the lower segment of the vessel.

15. A seal support system according to any preceding claims, whereby the external gas source charges the accumulator with reference pressure plus bias pressure to impart equal pressure in the barrier fluid without direct contact.

16. A seal support system for use with a sealing device employed to seal a piece of industrial equipment characterised by

- two or more isolators connected to two or more seal chamber by suitable piping means

- two or more differential pressure valves each connected to the isolators

- one central fluid re-fill means wherein, said vessels contain fluid and are connected to the sealing devices in a closed loop system on two or more items of industrial equipment and said differential pressure valves are connected to a suitable pressure source.

17. A seal support system according to Claim 16, whereby the respective vessels have level indicators so that when an appropriate level condition is sensed, the system provides a signal for more fluid to be added.

18. A seal support system according to Claim 17, whereby the vessel fluid level can be increased by a suitable manual re-fill means, such as a hand pump connected to a fluid reservoir.

19. A seal support system according to Claim 17, whereby a signal from said limit switch automatically activates the fluid re-fill operation without any human intervention.

20. A seal support system according to Claim 17, whereby the multi- application re-fill operation is activated from a central reservoir re-fill system.

21. A seal support system according to Claim 17, whereby an electrically activated ball valve is connected to the fluid level switch, said ball valve is naturally biased to the closed position when the fluid level in the vessel is in normal working condition and said ball valve automatically opened when the fluid level reaches a predetermined position.

22. A seal support system according to Claim 17, whereby an positive fluid circulation means such as a pump is connected to the fluid level switch, said pump is not activated when the fluid level in the vessel is in normal working condition and said pump is automatically activated when the fluid level reaches a predetermined position thereby permitting the re-fill of fluid into the vessel.

23. A seal support system according to any preceding claims, whereby as additional fluid is introduced into the respective vessels, the pressure within the respective vessels is regulated by an appropriate venting means so that an over-pressurisation condition of the fluid can not be encountered. A seal support system according to any preceding claims, whereby the respective vessels in the seal support system of the invention have a fluid separating diaphragm device, permitting different types of fluid in the upper and lower sections of the vessel.

A seal support system as defined in claim 1 and substantially detailed by Figures 2 to 6.