GB2206161A

GB2206161A - Secondary seals

Info

Publication number: GB2206161A
Application number: GB08714660A
Authority: GB
Inventors: Donald Peter Ketch; Nigel Keith Wood
Original assignee: E G & G Ltd
Current assignee: E G & G Ltd
Priority date: 1987-06-23
Filing date: 1987-06-23
Publication date: 1988-12-29
Also published as: FR2617261A1; GB8714660D0; FR2617261B1

Abstract

A secondary containment seal system acts between a rotary shaft (10) and a housing (12) through which the shaft (10) passes. The inside of the housing contains a hazardous fluid. The secondary seal system has a primary rotary seal continually acting between the shaft (10) and the housing, and a secondary rotary bellows seal (14, 16) spaced outwardly from the primary seal. The secondary seal (14, 16) is also continually acting but runs dry the mating seal faces (X) being constituted by a hard resin-loaded carbon and a silicon carbide material respectively. One secondary seal face is lapped to a very flat finish. The other is self-polishing. The mechanical loading on secondary seal interface (X) is relatively low. The primary and secondary seals are separated by an intermediate chamber (13) having a single venting port (18) to which monitoring arrangements (20-28) are connected. In a modification, a pump (P) and additional ports (30, 32) are provided to be activated to circulate lubricating fluid to the secondary seal (14, 16) only after failure of the primary seal is detected. <IMAGE>

Description

Title: Secondary Containment Seal System The invention relates to a secondary containment seal system. Such systems find application in pumps for toxic or otherwise hazardous fluid where leakage of the fluid into the environment is to be avoided.

The pump impeller in the hazardous environment is conventionally driven via a rotating shaft whose drive end is exterior to this environment.

Thus the shaft passes from this environment to normal ambient and a barrier has to be provided to prevent escape of hazardous fluid along the shaft. It is conventional to fit a rotary face seal, for example a bellows seal, between a shaft and its housing, the hazardous fluid existing on one side of the seal and providing a lubricant for the seal faces. Safety often requires further measures to be taken, particularly to avoid the escape of hazardous fluid should the face seal fail. Even when the seal is running normally, there is an inevitable small leakage of fluid past the seal faces which it may be undesirable to have vented to ambient in the vicinity of the shaft and which should be removed for venting or disposal elsewhere. It should be mentioned that in many cases of hazardous liquids, the liquid will vaporise if it leaks into the air.

One solution to this problem is to provide a second rotary face seal outwardly of the one contacting the hazardous environment which thus becomes the inner seal of two between which there is an intermediate chamber. Such a seal installation may be referred to as a barrier seal system. In prior systems, ports are provided to the intermediate chamber to circulate an inert barrier fluid through the chamber both to remove hazardous fluid leaking past the inner seal and to lubricate the second seal. The latter has to run at sufficient pressure between its faces to retain this circulating liquid and is thus subject to normal wear.

A barrier seal system of this kind requires an exterior reservoir for the circulating fluid, some means of venting toxic product that becomes entrained in the barrier fluid and some means of circulating the barrier fluid. The latter may be located in the seal installation to be driven by the rotating shaft.

Additionally the installation requires some form of pressure sensing that detects the change in pressure in the barrier fluid which occurs when the primary (inner) seal fails.

Examples of the barrier seal system discussed above (also known as tandem seals) are the 670 and 676 series of tandem seals offered by E. G. and G.

Sealol Inc. of Cranston, Rhode Island, U.S.A. in their Bulletin No. 670/6B.

The barrier fluid installation adds substantially to the cost of the pump installation and itself requires monitoring and maintenance. Because of the need to lubricate the second (outer) seal it has heretofore been necessary to provide a circulating liquid as the barrier fluid.

An alternative proposal has been to run the outer seal with a small gap between the faces so that they are not actually in running contact and thus do not require lubricant. Also wear is avoided in normal operation. The outer seal assembly is responsive to a build up of pressure in the intermediate chamber due to failure of the inner seal so as to move the outer seal faces into sealing contact. Such an arrangement, while providing a back-up seal in the event of a major leakage must iniierently be less efficient than a constantly active outer seal in preventing leakage of hazardous fluid to the atmosphere during normal operation. It is rare that a seal fails instantaneously. Failure is progressive during which time there may be substantial leakage to atmosphere before the secondary seal faces close.

There would be advantage in a secondary containment seal system in which the secondary seal is continuously active as a seal but in which the amount, and thus cost, of ancillary equipment associated with a barrier fluid installation of the type outlined above, could be reduced.

The present invention enables us to provide a secondary containment seal system having primary and secondary seals in which the secondary seal is run with the seal faces in contact to provide a continuous seal but is run dry, i.e. without lubricant being supplied.

According to the present invention there is provided a secondary containment seal system for a device in a hazardous environment that is driven by a rotary shaft extending out of the environment through a housing, the system comprising a primary seal arrangement acting between the shaft and the housing to prevent egress of hazardous fluid from its environment, and a secondary seal arrangement acting between the rotary shaft and the housing and separated from the primary seal arrangement by an intermediate chamber, the secondary seal arrangement serving to prevent any leakage past the primary seal arrangement escaping to ambient, the secondary seal arrangement comprising mating seal faces providing a rotary seal, characterised in that the seal faces of the secondary seal arrangement run dry and the intermediate chamber comprises a venting port out of which hazardous fluid or vapour is driven by the pressure due to its leakage past the primary seal arrangement.

A secondary containment seal system in accord with the present invention will now be described with reference to the accompanying drawing in which: Fig. 1 is a diagrammatic, partial longitudinal section through the intermediate chamber and secondary seal arrangement, the primary seal arrangement not being shown.

In the figure a rotary driven shaft 10 extends between a pump impeller (not shown) located to the left and a power source (not shown) located to the right. The impeller is used to pump a hazardous fluid that vaporises in air. The shaft 10 exits from the hazardous environment through a housing, a part 12 of which is shown. A primary seal arrangement (not shown) is mounted between housing and shaft to the left of the figure to provide a seal against leakage of the hazardous fluid along the pump shaft. The primary seal arrangement is conventionally a rotary bellows type of seal with the bellows secured on the shaft to rotate with it and having its outer surface in contact with the hazardous fluid, and with a cooperating stationary seal ring secured to the housing outwardly of the bellows.

Such sealing arrangements are entirely conventional in their construction and operation and will not be described further. Bulletin No. 670/6B referred to above exemplifies the construction. Any leakage past the primary seal passes between the shaft and housing as indicated by arrow A to a chamber 13 in the housing.

Further progress of any hazardous fluid or resultant vapour from chamber 13 to ambient B is prevented by a secondary or outer seal arrangement acting between the outer end portion 12A of the housing and shaft 10.

Thus the chamber 13 is intermediate the two seal arrangements to trap any leakage product.

The secondary seal arrangement comprises a stationary metal bellows face seal 14 that is secured to the housing portion 12A with its inner surface in contact with the ambient B.

The bellows seal is provided with an inwardly facing annular seal face insert 14a which engages a mating sealing face 16a of a seal ring 16 secured to shaft 10 to rotate with it. The seal interface thus exists at point X with the interior of the bellows in contact with ambient B. The bellows arrangement and its mounting to the housing and the securing of ring 16 to the shaft are done in conventional fashion and need not be described further. The seal interface at X is further discussed below.

The secondary seal is continuously active to isolate the chamber 13 from ambient B. The vapour of the hazardous fluid that arises in chamber 13 following leakage past the primary seal, is vented from'the chamber 13 through a port 18 in the housing connecting the chamber to an external venting and monitoring arrangement. It is noted at this point that only a single port is required for venting in contrast to the two ports required in the prior barrier seal installations in order to allow the barrier fluid to be circulated through the intermediate chamber and provide lubricant to the outer seal.

The exterior venting and monitoring arrangement includes a pressure-sensitive air solenoid which acts as a pressure-sensitive pneumatic switch and through which the vapour vented from port 18 passes to and through an orifice plate 22 as indicated by arrow Y to a flare stack/drain appropriate to the fluid or vapour being vented. The pressure sensitive device 20 is supplied with pressurized air as indicated by arrow Z and upon the pressure at port 18 exceeding a predetermined safe value, the device 20 connects the pressurized air supply Z to an air siren 24.

Preferably a visual pressure indicator 26 is connected between the switch 20 and the siren 24 and is set in a visible alarm state by the pressurized air.

If, as is illustrated in the figure, a manually-operable shut-off valve 28 is connected in the air supply to the siren, it should be connected after indicator 26 so that even on turning off the siren, the indicator will remain set as long as the excess vapour pressure is detected.

The device 20 could alternatively be sensitive to the rate of flow of the vented vapour.

The exterior venting/monitoring arrangement that has been described requires no reservoir for a barrier fluid or pump to circulate the fluid as is required by the more complex conventional barrier fluid installation.

Reverting to the operation of the outer seal 14, 16, it is necessary that the mating seal faces indicated at X be essentially dry running. To assist this purpose the bellows face seal is both balanced and lightly loaded. The balance is to external pressure and a 70/30 balance can be achieved in known fashion.

The loading is discussed more fully below. It has been found that a satisfactory dry running performance can be achieved under these circumstances by making one of the seal faces of a hard carbon material and the other of silicon carbide which is still harder. More particularly a satisfactory dry running seal has been achieved with the rotary seal face 16a constituted by a reaction-bonded silicon carbide material and the seal face insert 14a constituted by a hard resin-loaded carbon. Tests to date indicate that the harder the insert 14a, the better.

The combination of materials described provides a number of advantages. Silicon carbide is both hard, wear resistant and corrosion resistant. The face 16a should be given a very flat finish.

Preferably it is lapped to give a flatness better than one helium light band - 11.6 x 10-6 inches (0.29 microns). The face of seal insert 14a is similarly lapped. However, the combination of the hard carbon with the harder silicon carbide results in the carbon face being self-lapping so that in an initial run-in period it quickly beds itself into the shape of the silicon carbide face. The carbon is self-polishing and also polishes the silicon carbide face. In practice, it has been found that the carbon material quoted undergoes very little wear particularly at the light mechanical loading of the seal face that is proposed.

Another factor affecting the operation of the dry running seal interface is the mechanical loading. As already described a bellows seal member 14 is used to support the insert 14a. While the provision of such a member will be readily apparent to those in the art, it is to be noted that the seal is to be adjusted with significantly less than the usual mechanical loading that would be applied in a secondary seal of the kind illustrated in the above-mentioned Bulletin No. 670/6B and less than that applied in bellows seals used in high speed, low pressure applications. The lightly loaded seal interface and the hard face materials of the seal faces provide dry running at low friction. This both ensures low wear and low heat generation.

In operation of the secondary containment seal described, should the primary seal start to leak, the secondary seal has to prevent vapour or liquid escaping to ambient for a sufficient period to allow the monitoring system to give an alarm and allow the pump to be closed down. Thus the seal is intended to provide its protection for a limited period. It is not intended that the secondary seal should mitigate consequences of the failure of the primary seal for a prolonged period.

In summary although the contacting secondary containment seal described inevitably involves wear over time, and consumes power with the generation of heat, these factors are considered acceptable under the conditions described and provide significant advantages.

These are a positive seal at all pressures; contamination cannot prevent face closure as can arise in a secondary seal in which the faces are not normally in contact; and transient response is eliminated, this being transient effects occurring on closure of the faces of a normally non-contacting seal.

Fig. 1 also illustrates a modification which may be employed in some specific installations. For certain hazardous liquids, it is the practice to apply an additional pressure to the inner side of the primary seal by circulating the liquid in the vicinity of the primary seal. It may be considered desirable to treat the secondary seal in the same manner as the primary seal upon failure of the latter, that is where the secondary seal has now come into action as the one seal effectively isolating the hazardous liquid from ambient B. Consequently the modification is intended to transfer the circulation at the inner side of the primary seal to the secondary seal.

To this end, Fig. 1 shows in dot-dash line the provision of two additional ports 30 and 32 leading to intermediate chamber 13 acting as inlet and outlet respectively for the liquid (possibly port 18 could be used as the outlet port). The normal circulation to the primary valve extends over a path P through a switching valve 34. The valve 34 is switchable to disconnect the pump side of the primary path from the primary seal and to connect to the port 30 over line 36. The outlet port 32 is connected to switching valve 38 that normally closes the port but which is openable to connect the port to the suction side S of the circulating pump.

Both valves 34 and 38 are controlled as indicated in chain line 40 from the sensing device 20.

In normal operation the primary path P is completed and both ports 30 and 32 are closed. The secondary seal runs dry as described above. Upon sensing device reacting to a failure of the primary valve, it switches valves 34 and 38 to interrupt the primary seal circuit and establish a corresponding circulation on the inner side of the secondary seal.

Claims

1. A secondary containment seal system for a device in a hazardous environment that is driven by a rotary shaft extending out of the environment through a housing, the system comprising a primary seal arrangement acting between the shaft and the housing to prevent egress of hazardous fluid from its environment, and a secondary seal arrangement acting between the rotary shaft and the housing and separated from the primary seal arrangement by an intermediate chamber, the secondary seal arrangement serving to prevent any leakage past the primary seal arrangement escaping to ambient, the secondary seal arrangement comprising mating seal faces providing a rotary seal, characterised in that the seal faces of the secondary seal arrangement run dry and the intermediate chamber comprises a venting port out of which hazardous fluid or vapour is driven by the pressure due to its leakage past the primary seal arrangement.

2. A secondary containment seal system as claimed in Claim 1 in which one seal face of the secondary seal arrangement is provided by a member formed of a silicon carbide material and the other seal face is provided by a member formed of a hard resin-loaded carbon material.

3. A secondary containment seal system as claimed in Claim 1 in which one seal face is formed on a harder material than the material having the other seal face, at least the one seal face being lapped to a very flat finish, and the material having the other seal face being self-polishing and polishing the one seal face.

4. A secondary containment seal system as claimed in Claim 1 or 2 in which the mechanical loading at the seal interface is less than that employed in normal applications of bellows seals.

5. A secondary containment seal system as claimed in any preceding claim in which the secondary seal arrangement comprises a bellows seal secured to the housing and a cooperating seal ring member secured to the shaft.