GB2422409A

GB2422409A - Process Fluid Apparatus

Info

Publication number: GB2422409A
Application number: GB0601090A
Authority: GB
Inventors: Alan Roddis
Original assignee: AES Engineering Ltd
Current assignee: AES Engineering Ltd
Priority date: 2005-01-19
Filing date: 2006-01-19
Publication date: 2006-07-26
Also published as: GB0601090D0; GB0501081D0

Abstract

The present invention is a single mechanical seal with a set of secondary containment faces adjacent to the atmospheric end, with the seal interspace, between either set of seal faces, connected to the suction chamber of a venturi with its operation inlet connected to the process fluid discharge of the pump and the exhaust of the venturi to the fluid suction side of the pump. Said closed loop system, of the invention, between the mechanical seal, venturi and rotating equipment provides an environmentally, commercially and operationally acceptable emission control system when sealing processed fluid.

Description

PROCESS FLUID APPARATUS

Field of the Invent ion

This invention relates to rotary seals and is concerned with their use in rotating equipment. It especially relates to single mechanical seal designed with a secondary containment seal, such a seal assembly is employed to seal a processed fluid.

Such devices are often referred to as secondary containment seals or emission control seals (ECS).

Background to the Invention

The purpose of a single seal with a secondary containment seal is reduce, / prevent emissions to atmosphere, and/or contain the processed fluid in the rotating.

equipment. Their use is preferred in some applications, over double mechanical seals, in that a double mechanical seal requires a complex and costly support system to function correctly.

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Secondarily containment seals, do not typically require such a complex support * system in that the outboard seal face is typically dry running and only sealing the C.::, vapour leakage across the inboard seal face.

Unfortunately, if said vapour leakage is not vented, the seal inter-space pressure accumulates and can over-pressujjse the outer containment faces leading to increased emissions to atmosphere and consequent premature seal failure.

An option used to relieve said vapour leakage pressure, in the seal interspace, is to connect said interspace via an orifice, to a flare stack. The flare stack burns off the leaked potentially toxic vapours to atmosphere. This in effect converts one type of emission pollutant to another, which could be environmentally unacceptable in certain parts of the world. In addition to this, not only is the operational costs of the flare stack an issue, but the associated pipework to connect each sealing system and maintenance of such adds further to the plant cost burden.

A mechanical seal, containment or otherwise, typically includes a "floating" component, which is mounted for axial movement about the rotary shaft of, for example, a pump and a "static" component which is axially fixed and is typically connected, directly or indirectly, to a housing. The floating component has a flat annular end seal face which is directed towards a corresponding seal face of the static component. The floating component is urged towards the static component to close the seal faces together to form a sliding face seal, usually by means of one or more spring members. The rotating component, which in practice could be either the floating or the static component, is referred to as the rotary component. The other component does not rotate and is referred to as the stationary component. A rotary seal is one whose floating component is rotary and, in a stationary seal, the floating component is stationary. If the sliding seal between the rotary and the stationary components is assembled and pre-set prior to despatch from the manufacturer, the * ,* , seal is termed a "cartridge seal". If the rotary and the stationary components are despatched in unassembled condition by the manufacturer, the seal is termed a "component seal".

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* .* S * *5S# Mechanical seals are used in all types of industries to seal a variety of different *:: process media and operating conditions. The term "inboard" defines the area adjacent to the process media and the term "outboard" defines the area adjacent the atmospheric side.

A single mechanical seal typically contains one set of counter rotational seal faces.

A double mechanical seal typically contains two sets of counter rotational seal faces, each set is either axially or radially or both axially and radially separated.

A single seal with secondary containment seal, typically contains one set of counter rotational seal faces sealing the primary processed fluid, and a secondary set of seal faces which typically seal the vapour space / inter space between the two sets of seal faces. Typically, the secondary seal face is positioned on the outboard side of the seal. Furthermore, the secondary seal face runs on a marginal fluid film and/or dry running.

The experienced reader will understand that any number of sets of seal faces could be employed for a given application, however the containjnent set of seal faces is typically the set adjacent to the atmosphere.

As stated, the major benefit of a containment mechanical seal is that it provides a simple, cost effective emission control system for rotating equipment. However, all contacting mechanical seal faces operate on a fluid film between the counter rotating seal faces. The inboard seal faces typically operate in a substantial fluid film. The outboard, contajnent seal faces operate in a marginal lubrication condition, where the fluid film is typically very thin. This causes heat generation and/or seal face deterioration. Seal face deterioration will lead to loss of the hermetic outer seal and. , increased emission rate. To combat this, the outboard contaiimient seal faces are typically lightly axially loaded, with a reduced spring rate compared to a conventional, primary mechanical seal face. aI * *

If vapour pressure is allowed to build up in the seal interspace, it will increase and * potentially axially overload the outboard containment seal faces. This will lead to increased heat generation, increased seal face wear and increased seal face deterioration with consequent increased emissions to atmosphere.

The experienced reader will note, that with the flare stack option, the typical flare stack pressure is around 2.5 bar (70 psi). This, in itself creates an unwelcome axial load onto the containment seal faces, accelerating wear et al. The invention is to relieve the interspace pressure build-up potential by simple, non- mechanical means. The invention utilises the differential pressure between the discharge (processed fluid outlet) and suction (processed fluid inlet), to create a vacuum. Said vacuum is created by the processed fluid passing through a venturi system. Said venturi contains a third orifice, which connects to the seal interspace.

Said vacuum, or negative pressure, pulls vapours and/or primary seal face leakage, from the seal interface to the venturi and back to Suction of the pump.

Venturi flow rate adjustment can control the level of vacuum created in the seal interspace. Consequently, a balance, either automatically compensating, or manual, can be maintained to ensure the correct vacuum level in the seal interspace. This will ensure compete evacuation of leaked vapours back to suction of the pump. This self contained, closed loop system, effectively creates a zero emission to atmosphere rate from the contaient seal. This is highly desirable in terms of environmental, commercial and operational issues.

Statements of the Invention

According to a first aspect of the invention there is provided apparatus for evacuation *.

of process fluid from a mechanical seal attached to process equipment, said process equipment having an inlet and an outlet for process fluid, the apparatus comprising means for feeding process fluid from said outlet to said inlet, means for lowering the: * pressure at a location on said feed means between said inlet and said outlet and means for interconnecting said mechanical seal with said location, whereby said: : process fluid is caused to move from said mechanical seal to said location and thence:: to said inlet.

In a preferred embodiment of the invention, the means for feeding the process fluid from the outlet to the inlet on the process equipment is at least one conduit, for

example a pipe.

Preferably the present invention is a single mechanical seal with a set of secondary containment faces adjacent to the atmospheric end. Preferably, the seal interspace, between either set of seal faces, is connected to a venturi. This connectioj is by means of at least one conduit, for example a pipe.

Preferably the venturi of the invention has its operation inlet connected to the process fluid discharge of the pump and the exhaust of the venturi to the fluid suction side of the pump, said suction chamber of the venturi is connected to the seal interspace.

Preferably, the invention is a closed loop system between the mechanical seal, venturi and rotating equipment.

Embodiments of mechanical seals in accordance with the present invention may be such that a rotary or stationary seal is offered. A single, double, triple or quadruple with or without a further containment seal is also embodied.

The invention may also be embodied in cartridge and component seal formats with metallic components as well as non-metallic components both single and double tandem or concentric back-to-back etc. ..

Embodiments of the pre-mentioned venturi, in accordance with the present invention may any arrangement which provides negative pressure andlor a vacuum within a device, and said device is connected to a mechanical seal interspace. :..

Embodiments of the invention include a venturi, in accordance with the present invention, where the venturi is connected to the mechanical seal, but not the interspace but to the process side adjacent to the inboard mechanical seal face, for control of the rotating equipment seal chamber pressure.

In general rotary seals in accordance with the present invention may be used not only in the case where the shaft is a rotary member and the housing is a stationary member but also the reverse situation, that is to say, in which the shaft is stationary and the housing is rotary.

According to a second aspect of the invention there is provided a method for the evacuation of process fluid from a mechanical seal attached to process equipment, said method comprising the steps of, i) providin g feed means between an outlet and an inlet on the process equipment; ii) circulating process fluid through said feed means; iii) lowering the pressure at a location on said feed means between said inlet and said outlet; and, iv) providing means for interconnecting said mechanical seal with said location, whereby said process fluid is caused to move from said mechanical seal to said location and thence to said inlet.

Description of the drawings

The accompanying drawings are as follows; Figure 1 is a longitudinal cross-section through an item of rotating equipment with a closed loop system of the first embodiment.

Figure 2 is a longitudinal cross-section of the venturi system of the first embodiment of the invention.

Figure 3 corresponds to Figure 1 and shows a partial longitudinal cross e section of a mechanical seal, of the invention, with the venturi connected to the seal interspace. :...: Figure 4 is a longitudinal cross- section through an item of rotating equipment with a closed loop system of the second embodiment.

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 longitudinal cross-section through an item of rotating equipment 10 with a closed loop system 11 of the first embodiment.

From Figure 1, the rotating equipment 10 is a centriftigal pump 12 with an inlet 13 and an outlet 14. Between the inlet 13 and outlet 14 is an impeller, not shown, which circulates and/or pumps fluid between the two orifices. The inlet 13 is often referred to as the suction side of the pump 12. The outlet 14 is often referred to the discharge side of the pump. Practically all pumps have a pressure differential between the Suction 13 and discharge 14 orifices.

The rotating equipment 10 has a mechanical seal 9, which seals the interface between the rotating 8 and stationary 7 components of the rotating equipment 10.

From Figure 1, a negative pressure source, such as a venturi 15, is sited and connected between the discharge 14 and Suction 13 orifices of the rotating equipment by appropriate pipework 16. The negative pressure chamber of the venturi 15 is connected to the space adjacent to a set of mechanical seal faces by appropriate pipework 17.

Preferred, although not essential, is a non-return valve 18 connected between the venturi 15 and mechanical seal 9. 15:..

In dynamic operation, the pump 12 creates a pressure differential to energise the venturi 15. The movement of fluid through the venturi 15 creates a negative pressure chamber inside the venturi 15. The negative pressure chamber is connected to the mechanical seal 9, thereby pulling any and all vapours andlor fluid from the area adjacent to the mechanical seal faces, to the venturi 15. This vapour and/or fluid is then contained with the energising fluid in the venturi 15 and passes through to the suction side 13 of the pump 12. This invention of the first embodiment provides a closed loop, environmentally friendly system for particular use when processing toxic or volatile fluids.

Figure 2 is a longitudinal cross-section of the venturi 15 system of the first embodiment of the invention. Fluid from the discharge 14 of the pump 12, enters the venturi 15 at the inlet 20 and subsequently flows through a radially diverging nozzle 21 to the radially converging chamber 22. The axial end 23 of the radially diverging nozzle 21, preferably but not essentially, axially extends beyond the axial shoulder 24 of the converging chamber 22. Said axial shoulder 24 is adjacent to the negative pressure chamber 25.

From Figure 2, a return pipe 26 from the mechanical seal 9 is connected to the negative pressure chamber 25 in the venturi 15.

In operation, the energised fluid passes through the radially diverging nozzle 20 and draws the vapour and/or fluid, from the mechanical seal 9, through the negative pressure chamber 25 and into the radially converging chamber 22. This fluid and/or vapour enters and is contained with the energising fluid and exhausts through the venturi 15 outlet 27 back to the Suction side 13 of the pump 12.

Figure 3 corresponds to Figure 1 and shows a partial longitudinal cross section of a mechanical seal 9, of the invention. The mechanical seal 9 shown, by way of example only, is a single cartridge seal with a secondary outboard containment seal 30.

Processed fluid 31 is contained within the seal chamber 32. The mechanical seal 9 contains a first set of inboard counter rotating sliding seal faces 33, which prevent the processed fluid 31 from escaping the seal chamber 32. Said seal faces are common to most mechanical seal designs and will therefore be no longer described.

At the outboard side 34 of the mechanical 9 is a set of containment seal faces 30 which are sited adjacent to the atmosphere 35. It is preferred, although not essential, that the floating seal face 36 is lightly axially loaded and spring-like biased to the stationary and non-floating seal face 37 as this reduces seal face wear and heat generation.

The stationary gland member 38 is connected to the venturi 15 by appropriate pipework 17 into the gland 38 orifice port 39. The radially extending communication orifice 40 connects the radially inward 41 part of the gland 38 to the radially outward 42 part of the gland 38.

In operation, from figure 3, vapours and / or fluid will pass radially inwardly across the inboard set of mechanical seal faces 33 into the seal inter-space 43. Said seal interspace 43 communicates with the communication orifice 40 and seal vapours and/or fluid is drawn off to the venturi 15 through pipework 17.

Figure 4 is a longitudinal cross-section through an item of rotating equipment 50 with a closed loop system 51 of the second embodiment of the invention. As shown in Figure 4, the closed loop system operates in a similar manner as previously described, however the venturi 52 is connected to the pump sea! chamber 53. This allows the system to control and/or reduce the fluid pressure in the pump seal chamber 53.

The experienced reader will relate to the operational advantages of such a simple yet technically effective system of the invention. Furthermore, the commercial cost of the system is considerably less than existing prior art alternates and the environmental impact on reduced / zero emission to atmosphere is something that is a further significant benefit to a user. * ** * S

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Claims

I. Apparatus for evacuation of process fluid from a mechanical seal attached to process equipment, said process equipment having an inlet and an outlet for process fluid, the apparatus comprising means for feeding process fluid from said outlet to said inlet, means for lowering the pressure at a location on said feed means between said inlet and said outlet and means for interconnecting said mechanical seal with said location, whereby said process fluid is caused to move from said mechanical seal to said location and thence to said inlet.
2. Apparatus according to claim 1, wherein said interconnecting means is connected at or adjacent to an interspace between a set of seal faces within the mechanical seal, said apparatus thereby evacuating process fluid from between the seal interface. * **
3. Apparatus according to claim 2, wherein the mechanical seal is provided with at least two sets of seal faces and whereby the interconnecting means is connected at or adjacent to the interspace between either set of seal faces.

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4. Apparatus according to claim 3, wherein a second set of seal faces is provided by a secondary containment seal located on the outboard side of the seal.
5. Apparatus according to any preceding claim, wherein the mechanical seal is housed within a seal chamber and the interconnecting means is connected at or adjacent to an inboard seal face thereby causing the evacuation of process fluid from within the seal chamber.
6. Apparatus according to any preceding claim, wherein the fluid is process fluid vapour.
7. Apparatus according to any preceding claim, wherein the means for lowering the pressure is a venturi.
8. Apparatus according to claim 7, wherein the interconnecting means is connected to the negative pressure chamber of the venturi.
9. Apparatus according to any of claims 1 to 8, wherein the interconnecting means is provided with a non-return valve to prevent the process fluid flowing back to the mechanical seal.
10. Apparatus according to any of claims 7 to 8, wherein means for adjusting the flow rate of process fluid through the venturi enables the controlled generation of a vacuum at or adjacent to the seal.
11. A method for the evacuation of process fluid from a mechanical seal attached to: process equipment, said method comprising the steps of..: i) providin g feed means between an outlet and an inlet on the process equipment; :....

ii) circulating process fluid through said feed means; iii) lowering the pressure at a location on said feed means between said inlet * and said outlet; and, iv) providing means for interconnecting said mechanical seal with said location, whereby said process fluid is caused to move from said mechanical seal to said location and thence to said inlet.
12. Apparatus or method as substantially herein before described with reference to the accompanying drawings. Il