Seal gas emission from dry gas seal

Abstract

Seal gas recycling system for a gas-lubricated non-contact mechanical DGS 9, having a seal gas booster 13 and seal gas recycling system. The system may have seal gas conditioning system 7 to prevent c

Description

The invention relates to centrifugal compressors and other similar rotating equipment and, more particularly to systems for controlling the emissions of gases from dry gas seals used with such equipment Background to the Invention Centrifugal compressors and other similar rotating equipment are used to transport gases from one place to another or through process systems containing gases. These devices utilise rotating shafts containing impeller devices (or similar) which, due to their motion, impart energy into the gas thereby raising the gas pressure. In order to sustain the pressure, the rotor has to be housed in a pressurised casing where some sort of sealing device is required at the point of entry / exit where the rotary shaft passes through the casing. A consequence of the speed and pressure that the rotor, and hence the seals, are subject to is that a fluid is necessary in order to lubricate the interface between rotating and stationary components within the seal arrangement. In situations where dry gas seals (DGS) are used for sealing purposes, gas is used as the primary means of lubrication. In common with all forms of lubrication, there is a requirement that lubrication fluid is conditioned to be clean. However, in the case of a DGS, the lubrication gas must also be kept dry at all times. The necessity for conditioned seal gas is largely because the lubrication film supports the entire sealing loads even though the film is only a few microns thick. In situations where the lubrication gas film becomes contaminated, the operation and reliability of the DGS is compromised. When the compressor is in operation, gas can be tapped off from a high pressure region of the compressor through a conditioning system where it is cleaned and dried before entering into the seal cavity. Once inside the seal cavity, some of the seal gas is used for lubricating the DGS faces while the remaining gas flows back into the compressor through a restrictive device. The flow through the restrictive device ensures that unconditioned gas contained downstream within the compressor cannot enter the seal cavity thus ensuring that there is no contamination of the conditioned lubrication gas. The lubrication gas which flows though the sealing interface is usually referred to as leakage where it emerges from the seal assembly into a vent system where it is either flared oremitted directlyto atmosphere. Irrespective of how the vent gases are removed, they are not only wasteful but also damaging to the environment. DGS seal leakage produces small amounts of damaging gas being released to atmosphere at all times so, even though the amounts are small, the cumulative effect of constant leakages adds up to a large amount of gas being emitted to atmosphere over a given period. When the compressor is taken offline, the shaft rotation ceases thus disabling its pressure generating ability. The lack of pressure also dictates that gas no longer flows through the conditioning system and therefore clean and dry lubricating gas no longer flows into the seal cavity. Despite the compressor being offline the seals will still leak statically due to the gas pressure within the compressor. The gas leaking across the seal faces will dictate that a small amount of gas will leave the seal cavity where the leaked gas is then replaced. A further consequence of the lack of conditioned gas flowing through the seal cavity is that unconditioned gas reverse flows from the compressor through the restrictive device thus contamination is present in the seal cavity. A further issue relating to the compressor being taken offline is that the compressor and it contents begin to cool due to energy no longer being imparted into the process gas. As the gas cools it becomes increasing wet as it nears vapour conditions where again it has an adverse effect on the seal gas. In the absence of having a means of preventing gas contamination when a compressor is offline most operators elect to allow the gas to discharge from the compressor thus discharging its contents to atmosphere either directly or via a flare arrangement. This process is commonly referred to as a blowdown, an event which produces large amounts of damaging gas being released to atmosphere over a short period of time. The associated issues are twofold- 1) DGS gas leakage being emitted through the vent system when gas is contained within the compressor. 2) Evacuating process gas (blowdown) when the compressor is offline. Statements of the Invention According to the present invention, there is provided a gas recycling system for use with a gas compressor having an inlet for gas at a relatively low pressure and an outlet for gas at a relatively high pressure, a gas-lubricated, non-contacting mechanical seal to which relatively high pressure gas may be fed from the compressor, the system comprising both a seal gas booster and a vent recycle system. The prevention of DGS gas leakage can be achieved by firstly collecting the leaked gas after it emerges from the DGS and then utilising a booster device so that the gas can be re-injected back into a low pressure section of the compressor. Collecting and recycling the gas leakage in the manner prevents atmospheric emissions and flaring. Blowdown prevention can be prevented by introducing a booster device in the seal gas supply system such that flow is generated when the compressor is offline so that conditioned gas is supplied to the seal cavity at all times. The supply of conditioned gas dictates that there will be no contamination of the DGS’s. Atmospheric emissions during blowdowns are therefore prevented. Accordingly, the invention provides a system in which the booster operates in vent recycling mode during normal operation and primarily as a seal gas booster when the compressor is offline. The system prevents fugitive emissions being exhausted to atmosphere or flare systems which damage the environment. Preferably, the system comprises means for feeding seal gas from the compressor to a conditioning system and from said conditioning system to said mechanical seal. More preferably, the system further comprises a collection vessel and means forfeedingvent gas leaked from the mechanical seal into said collection vessel. Most preferably, the system comprises means for feeding the vent gas from the collection vessel to the compressor via said inletfor gas. Preferably, means are provided to cause the booster, while the compressor is not operating, to feed the seal gas through the conditioning system. More preferably, means are provided to monitor the pressure difference between the compressor inlet and outlet and for activating the gas booster when the pressure difference falls below a predetermined level. Most preferably, means are provided to activate the gas booster when the pressure difference exceeds a predetermined level. Preferably, means are provided to monitor the pressure in the collection vessel and for activating the booster when the pressure increases above a predetermined level. The present invention also provides a method for recycling gas leaking from a gas-lubricated, non-contacting mechanical seal, the method comprisingsubjectingthe leaked gas to the action of a gas recycling system of the invention. Brief Description of the Drawings The accompanying drawings are as follows: Figure 1 is a diagram of a compressor system of the invention shown in normal operation; and Figure 2 shows the compressor system of Figure 1 in offline operation. Detailed Description of the Invention Normal Operation- Referring to Figure 1 of the accompanying drawings, the process gas pressure is increased as it passes through the compressor 1 from the compressor inlet (suction) 3 to discharge outlet 5. At the high pressure discharge end 5 of the compressor some gas is taken and routed through a conditioning system 7 which is used as the seal gas for the compressor dry gas seals 9 fitted to the compressor. The seal gas has to be conditioned so that the DGS does not get contaminated and subsequent failure occurs. Since the compressor is operational there is sufficient seal gas pressure for it to flow through the conditioning system without the need for a booster. So during normal operation the seal gas flows through a bypass line and not through the booster. The seal gas inlet and outlet valves are closed during normal operation. The conditioned seal gas then enters the seal cavity where some of the seal gas leaks through the faces and into a vent. The vent gas is then collected in collection vessel 11 and fed into the booster 13 which then allows the vent gas to flow back to the compressor suction. The vent gas inlet and outlet valves are open during normal operation. Offline Operation- During offline operation the compressor 1 does not generate pressure and therefore the seal gas cannot flow through the conditioning system. This necessitates the need for a booster 13 to provide sufficient flow of seal gas through the conditioning system . During offline operation the booster seal gas inlet and outlet valves are opened and the correspondingvent gas inlet and outlet valves are closed. The pressure difference between the compressor suction and discharge is monitored. In general terms the pressure difference (or head generated) increases with speed and 5 duration of operation. When the pressure difference falls to around 0.5bar the compressor is considered to be offline. At this point a signal will be sent which starts the seal gas booster and once the compressor is back in service (i.e. the pressure difference goes above 0.5bar) a further signal is produced which will stop the seal gas booster. The vent collection vessel pressure is normally monitored by a pressure transmitter. 10 When the pressure increases to a predetermined pressure a signal is sent to start the booster which will cause gas to flow out of the vessel and the pressure to fall. When the pressure reduces to a predetermined pressure a further signal is sent to stop the booster. The logic adopted in both the above can also be used to open / close valves on the inlet and outlet pipework such that seal gas or vent gas recycling flows can be selected. 15

Claims

1 A gas recycling system, for use with a gas compressor having an inlet for gas at a relatively low pressure and an outlet for gas at a relatively high pressure, a gas-lubricated, non-contacting mechanical sealto which relatively high pressure may5 be fed from the compressor, the system comprising both a seal gas booster anda vent recycle system, the system further comprising means for feeding seal gas from the compressor to a conditioning system and from said conditioning system to said mechanical seal and means being provided to activate the gas booster once the gas difference exceeds a pre-determined level.10 2 A gas recycling system according to Claim 1, wherein the system furthercomprises a collection vessel and means for feeding vent gas leaked from the mechanical seal into said collection vessel.

3. A gas recycling system according to Claim 2, wherein the system comprises means for feeding the vent gas from the collection vessel to the compressor via 15 said inlet for gas.

4. A gas recycling system according to Claims 2 or Claim 3, wherein means are provided to cause the booster, while the compressor is not operating, to feed the seal gas through the conditioning system.

5. A gas recycling system according to Claim 4, wherein means are provided to 20 monitor the pressure difference between the compressor inlet and outlet and foractivating the gas booster when the pressure difference falls below a predetermined level.

6. A gas recycling system according to any of Claims 2 to 5, wherein means are provided to monitor the pressure in the collection vessel and for activating the25 booster when the pressure increases above a predetermined level.

7. A method for recycling gas leaking from a a gas-lubricated, non-contacting mechanical seal, the method comprising subjecting the leaked gas to the action of a gas recycling system as claimed in any of the preceding claims.IntellectualPropertyOfficeApplication GB2502016.5Search report under Section 17 of the Patents Act 1977Date search completed: 01 July 2025Claims searched: 1-9International classificationSubclass and subgroup Valid from F01D11 / 04 01 / 01 / 2006 F04D29 / 12 01 / 01 / 2006 F16J15 / 34 01 / 01 / 2006 F16J15 / 40 01 / 01 / 2006Field of searchWorldwide search of patent documents classified in the following areas of the IPC:F04D, F01D, F16JDatabases used in the preparation of this search report:SEARCH-PATENTDocuments considered to be relevantPatent literatureCategory Relevant Document of relevanceclaimsIntellectual Property Office is an operating name of the Patent Officewww.gov.uk / ipoX 1-6 and 9 EP 1128101 A2 (CRANE JOHN UK LTD), See especially figure 1 and paragraphs 20-26. X 1-5 and 9 WO 2022 / 117227 A1 (NUOVO PIGNONE TECNOLOGIE SRL), See especially figure 1 and paragraphs 59-66. X 1-5 and 9 WO 2010 / 056408 A1 (EXXONMOBIL UPSTREAM RES CO), See especially figure 3 and paragraphs 22, 25-27 and 40. X 1,2, 5 and 9 WO 2024 / 088558 A1 (NUOVO PIGNONE TECNOLOGIE SRL), See especially figure 1 and paragraphs 17, 27 and 28. X 1,2, 5 and 9 WO 2024 / 104608 A1 (NUOVO PIGNONE TECNOLOGIE SRL), See especially figures and paragraphs 23, 30-33. Non-patent literature Category Relevant claims Document of relevanceCategories Letter or symbol Description X Document indicating lack of novelty or inventive step. Y Document indicating lack of inventive step, if combined with another document of the same category. & Member of the same patent family. A Document indicating technological background. P Document published on or after the priority date but before the fling date of the present application.Letter or symbol Description E Earlier application published on or after the filing date of the present application.

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