CA1115745A

CA1115745A - Mechanical seal assembly

Info

Publication number: CA1115745A
Application number: CA315,798A
Authority: CA
Inventors: Angel C. Martinez
Original assignee: Borg Warner Corp
Current assignee: Borg Warner Corp
Priority date: 1977-11-28
Filing date: 1978-11-03
Publication date: 1982-01-05
Also published as: DE2849256C3; DE2849256B2; DE2849256A1; CH633624A5; GB2008690A; JPS5482554A; GB2008690B

Abstract

MECHANICAL SEAL ASSEMBLY

Abstract:

A balanced mechanical seal between two zones having different fluids, at least one of which may be corrosive, abrasive, of different lubricity, of elevated temperature and with other undesirable properties or char-acteristics. A buffer fluid is introduced into one zone at a constant rate so as to flow to a location (68) radially inwardly of contacting seal faces (34,70) and then radially outwardly of said seal faces. The flow serves to cool and lubricate the seal faces and substantially prevent the flow of fluid from the other of the zones radially inwardly across the seal faces. The ratio of areas which determines the seal balance is such that in the direction of buffer fluid flow it is of relatively low value. However, in the opposite direction of fluid flow, the balance is a much higher value. Thus in the event of buffer fluid failure, flow of the undesirable fluid across the seal faces is substantially prevented, thus preserving the faces from substantial damage from m that undesirable fluid.

Description

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This invention relates to a balanced mechanical seal assembly of the type which may be used for separating different fluids in first and second zones.
In known high pressure, high temperature systems where a pump is operating with a fluid which has undesirable characteristics especially to motors, mechanical seal assemblies are employed to separate two fluid zones. A
buffer fluid, generally the same fluid as used in the motor environment having a pressure above product pressure and a temperature generally less than product temperature is supplied from a constant flow source to the O.D. of the seal.
The supply of the buffer fluid being substantially constant, a by-pass means is generally provided in parallel with the seal. The constant flow source is employed because with a product pressure, as on the order of 3000 p.s.i., pressure control is difficult, expensive and a source of unreliability.
Upon failure of the buffer fluid source, the seal is pressurized at the I.D., potentially at full product pressure, such that the abutting seal faces of the seal are subjected to tensile stresses which could lead to fracture of the seal rings. This leads to failure of the seal and the flow of product with its undesirable characteristics to the motor.
This invention relates to a meci~anical seal especially for use with a shaft connecting a product pump and its driving motor and for separating two zones with different fluids.
Motors used for driving such pumps are immersed in a fluid, such as an oil which can be circulated to maintain the motor at a substantially uniform temperature, generally below that ., ;.

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of the product. The product may be caustic, abrasive, of certain lubricity or have other undesirable properties or characteristics which could seriously affect the operation and even destroy the motor if permitted to flow to the motor.
The present invention resides in a balance mechanical seal assembly for separating different fluids in first and second zones and includes a pair of seal rings one of which is rotatable and the other of whlch is stationary such that they are relatively rotatable with respect to one another and have contacting relatlvely rotatable radial faces. There is provided means to inject a buffer fluid at a relatively substantially constant rate to a location radially inwardly of the seal faces in one of the zones and at a pressure in excess of the fluid in the other of said zones. The buffer fluid flows radially outwardly across the seal faces toward the outer of the zones to cool and lubricate the relatively rotatable and contacting seal faces and substantially prevent the flow of fluid from the other of the zones to the one of the zones. The seal is so constructed and arranged that the seal balance in the direction of buffer fluid flow is less than the seal balance in the opposite direction of fluid flow.

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"~~ 075079-BJ-V 13L~L5 m e mechanical seal of the invention is constructed with the usual contacting seal faces; however, the balance (a ratio of areas) of the seal in a ra~;~lly outward direction, i.e., from the I.D. to the O.D., is materially different and lower than in a reverse direction, i.e., O.D. to the I.D. A
buffer fluid - generally the same fluid as the tor fluid - is supplied or injected at a constant rate, as in the order of one gpm or less, and at a te~perature generally less than that of the product, to a zone adjacent the I.D. of the seal. me injection 10 pressure is not a controlled pres Æ e; it is the sum of the vessel pressure plus the pressure differential across the seal.
me latter is determined by the seal design involvLng such features as seal balance, configuration to control pressure deflection, spring loading, materials of construction and others. Some of the buffer fluid flows across the seal faces from the I.D. to the O.D. and toward the product pump; the remainder is recirculated in the system. In the event of failure of the buffer fluid supply, the probability of a concentration of product fluid at the O.D. of the seal is remote because of the constant supply of buffer fluid beforehand. m e seal will then act as a ncrmal seal with SQme flow across the se faces from the O.D. to the I.D.
Hcwever, the seal balance in this direction is different fm that in the I.D. to O.D. direction, such that flow across the faces is much less than in the opposite direction. With these factors, product fluid may not flow to the mDtor, giving time for repair without hann to the motor. This, also, is effectively controlled by constructing the seal balance frcm the O.D. to the I.D. at a different value from that in the opposite direction.

In the æ companying drawdng:
The single figure of the drawlng is a partial sectional view of a mech nical seal assembly accor~ing to this ~ ention.

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- . - , .-~ ~L~L~'7 The single figure of the drawing illustrates a mechanical seal asse~bly, generally identified as 10, which is associated wqth a shaft 12 and a housing 14. The shaft 12 is illustrated as constructed in tw~ parts 16, 18, joined by a curvic ccnnector 20.
5 The shaft 16 is connected to a driven product pump PP. The shaft 18 is connected to a motor M. The housing 14 may be in multiple parts, such as the housing part 22 surrounded by a jacket 24.

m e mechanical se 1 assembly 10 ccnprises a rotating seal assembly 26 in a cavit~ 28 in the housing 22, and a non-10 rotating or stationary seal assembly 30. The rotating se 1assembly 26 com4rises a generally cylindrical sealing ring 32 spaced from the shaft 16 and h~ving a seal face 34 and with a rearwardly extending cylindrical portion 36 su=~ounling a cylindrical portion 38 of a spring holder 40 of generally cylindrical shape attached by a drive screw 42 to the shaft 16. Ihe drive screw 42 has a tang 44 received in a correspondingly shaped opening 46 in the shaft 16. A cylindrical sleeve 48 surrounds the spring holder 40, and the holder has a p~urality of generally equally spaced spring pockets 50 therein, each of which receives a coil spr m g 52 which resiliently urges the seal ring 32 tcward the right, as viewed in the drawing. The seal ring 32 is provided with an axially disposed kegway 54 to receive a drive pin 56 connected to the sleeve 48. m e sleeve 48 is attached by a drive mEans, such as the upper end of the set screw 42 to the spring holder 40.
This co.nstruction insures non-rotation of the seal ring 32 relative to the spring holder 40. Suitable 0-ring seals 58 and 60 in grooves 62, 64, respectively, in the spring holder 40 and the seal ring 32, seal the spring holder 40 to the shaft 16 and the seal ring 32 to the cylindrical portion 36 of the spring holder 40.

m e no.n-rotatable seal asse~bly 30 c~mprises a generally cylindrical seal ring 66 having an inside diameter such that it defines a cylindrical passage 68 with the shaft 16. Ihe saal .

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ring 66 has a seal face 70 engaging the seal face 34 of the seal ring 32. m e seal ring 66 is connected to the hDusing 22 by means of a generally cylindri~l seal flange 72 bolted to the housing 22 and a ring nYmber 74 bolted to the seal flange 72. An 0-ring seal 76 located between a flange 78 of the seal flange 72 and an annLlar recess 80 of the seal ring 66 seals aga mst fluid passage along the outer periphery of the seal ring 66.

The jacket 24 is provided with at least one inlet passage 82 connected to a source of a buffer fluid such as from a 10 pumping ring located ~n the motor. A positive displacemYnt pump P pressurizes the tor. A one way valve 84 is provided between the pump P and the motor M to prevent the back flow of fluid to the pum~ P. The housing 22 is formed with an annular groove 86 communicating with the passage 82 and one or re ~YnAl commu-nicating passages 88 which in turn comm~nicate with coaxialpassages 90 in the seal flænge 72. Ihe source of buffer fluid provides a constant flow of fluid at a pressure above that at the left side of the seal, i.e., the product being pumped by the product pumD PP.

Buffer fluid at a constant rate, as on the order of one gpm or less, _nd at a temperature generally less than that at the product pump PP, is injected by the positive dispLacement pu~p P from a source through the check valve 84, into the tor and the pumping ring supplies the buffer fluid to the passage 82.
The fluid flows into the annular groove 86, and through the communicating passages 88, 90 into an annular cavity 92. Some of the buffer fluid flaws to the inner diameter of the seal faces 34, 70. Because of the seal balance, spring force, and other design features, a baclk pressure is created and some of the buffer fluid flows a~ross the seal faces and toward the product pump PP, as indicated by the arrows. The flow of buffer fluid across the seal faces cools, lubricates and preserves the faces ;' ,. . . ': . . .

' with a nnn;T~ of buffer fluid, and above all, substantially prevents product fluid from the product pu~p PP flowing to the n~tor M, thus preserv m g the motor. Also there is a cooling effect on the seal by the buffer fluid in the groove 86. Excess 5 buffer fluid flows through a passage 94 in the shaft 18 and back to the pumping ring to be recirculated.
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In the event of a pressure reversal, i.e., when the pressure of the fluid at the product pump PP is above that of the buffer fluid (due to operaticnal difficulties of the pump P or 10 buffer fluid supply failure), the seal becomes pres Æ ized in a conventional fashion, i.e., from the outer dLameter. Back flow of buffer fluid to the pump P is normally prevented by the valve 84.

Looking again at the drawing, the sealing surface is identified as S, one diameter of the rotating seal ring is identified as BD (also known as balance diameter) and the opposite pressure sides of the seal are identified as A and B. A large percentage of the sealing surface S is radially ou dly of the balance diameter BD and the remainder is rad;~lly inwardly of BD.
Thus, if the first percentage is 75% then the seal balance ratio in a radially inwardly direction at B is 75~/O while the seal balance ratio in a radially outwardly direction at A is 25%. Thus, flow of buffer fluid ra~i~lly outwardly from A to B easily oc s;
in the event of buffer fluid failure, flow of pump fluid does not easily occur; it is substantially prevented.

m e buffer fluid supply is integrated with the tor fluid supply and a heat exchanger E is located in the systen as shown. The heat exchanger is usable not only to re~Dve heat fram buffer fluid which has contacted the motor ~ut also to condition fluid to the I.D. of the seal.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a balanced mechanical seal assembly for separating different fluids in first and second zones comprising a pair of seal rings one of which is rotatable and the other of which is stationary such that they are relatively rotatable with respect to one another and having contacting relatively rotatable radial faces, the improvement which comprises:
means to inject a buffer fluid at a substantially constant rate to a location radially inwardly of said seal faces in one of said zones and at a pressure in excess of the fluid in the other of said zones, said buffer fluid flowing radially outwardly across said seal faces toward the other of said zones to cool and lubricate said relatively rotatable and contacting seal faces and substantially prevent the flow of fluid from said other of said zones to said one of said zones; said seal being so constructed and arranged that the seal balance in the direction of buffer fluid flow is less than the seal balance in the opposite direction of fluid flow.