NO343438B1

NO343438B1 - Vent system

Info

Publication number: NO343438B1
Application number: NO20171630A
Authority: NO
Inventors: Bård Malstenbråten
Original assignee: Aker Solutions As
Priority date: 2017-10-13
Filing date: 2017-10-13
Publication date: 2019-03-11
Also published as: NO20171630A1

Abstract

A vent system (100) for receiving vent gases from at least one source (A-C), the vent system (100) comprising: a vent stack (1) fluidly connected to the at least one source (A-C); a discharge line (2) extending from the vent stack (1) to a vent discharge (4); and a liquid lock (3) arranged in the discharge line (2).

Description

VENT SYSTEM

The present invention relates to a vent system, and more particularly to a system for handling vent gases from sources which require low and/or stable back pressure.

BACKGROUND

Various industrial systems require venting of gases, for example boil-off gases from tanks, blanketing gas of storage tanks, or blow-by gases from industrial equipment such as compressors. In some cases, such vent gases contain harmful components which are desirable (or necessary) to process in some way, and not simply discharge to the atmosphere. This may, for example, be the case for boil-off gases from tanks containing e.g. hydrocarbons. At the same time, such systems and equipment may have requirements in relation to the back pressure in the vent line, where a too high back pressure may lead to deteriorating performance or even equipment damage. It is a challenge to provide a vent system which is capable of capturing and handling vent gases effectively, while at the same time ensuring that the risk of overpressure of the vent stack and the source equipment is minimised.

Documents which may be useful for understanding the background include WO 8202496 A1, which describes a deck drainage system for removing oil containing fluid from an offshore facility; US 2005115248 A1, which describes an offshore liquefied natural gas structure; and WO 2015183072 A1, which describes a low pressure separation system comprising a separator for receiving fluid from a well, and a surge vessel for receiving liquid from the separator, arranged such that when the liquid reaches a predetermined high level in the surge vessel, a valve connecting the surge vessel to the separator is closed and high pressure gas is directed into the surge vessel by opening a valve or a gas pipe, forcing the liquid out of the surge vessel to a production header via a liquid outlet.

The present invention has the objective to provide a vent system which provides advantages over known solutions and techniques in the above mentioned or other areas.

SUMMARY

In an embodiment, there is provided a vent system for receiving vent gases from at least one source, the vent system comprising: a vent stack fluidly connected to the at least one source; a discharge line extending from the vent stack to a vent discharge; and a liquid lock arranged in the discharge line.

Further embodiments are described in the appended dependent claims and in the detailed description below.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments of the present invention will now be described with reference to the appended drawings, in which:

Figure 1 shows a vent system according to an embodiment.

DETAILED DESCRIPTION

In an embodiment, there is provided an atmospheric (ATM) vent system suitable for serving process facilities such as low pressure tanks, compressor seals and other sources that is dependent on a very low and/or a stable back pressure. Examples of such sources may be systems which handle hydrocarbons, and these may have restrictions with respect to back pressure in the vent system, in order to avoid deteriorating performance and/or damage to components. The maximum allowable back pressure may, for example, be restricted to a value in the order of 7.000 Pa (0.07 bar). At the same time, emissions of vent gas, e.g. methane, CO2 and VOCs, may need to be controlled to avoid environmental and/or health and safety impacts.

Conventional atmospheric vent systems commonly operate as open systems which relieve vented gas to a safe location, for example in a flare stack. The gas is at this location relieved and dispersed into the atmosphere. This solution can cause an environmental problem with respect to emissions, and potentially also health-and-safety risks. To control such emissions it is desirable to operate the ATM system in a closed loop, however by closing the venting directly to air there are many effects that could create too high pressures in the ATM system. Examples could be heating from the sun, over-pressurisation by the blanketing gas, increased relief rate from a source, etc. A challenge with an ATM vent is the low operating pressure, therefore there are no obvious solutions on how to close such a system while at the same time preventing over-pressure of the ATM system. The low design pressure for the ATM vent complicates the use of traditional pressure protection solutions, such as a rupture disc, in a reliable manner.

According to an embodiment, illustrated schematically in Fig.1, there is provided a vent system which is suitable for use in a closed loop, with reduced risk of over-pressurising the low pressure side of the equipment venting to the ATM vent system.

Figure 1 illustrates a vent system 100 for receiving vent gases from at least one source A-C. In this embodiment, three sources A-C vent to the vent stack 1, which is designed to receive vent gases from the sources A-C. As noted above, for many applications it may be desirable (or required) to accurately control the pressure in the vent stack 1, and to avoid temporary or permanent overpressure in the stack. For this purpose, a discharge line 2 extends from the vent stack 1 to a vent discharge 4, and a liquid lock 3 is arranged in the discharge line 2.

The liquid lock 3 comprises two fluid columns 3a,3b which are connected in a lower section of each column (for example in a U-shaped structure) and wherein and upper section of the first column 3a is fluidly connected to the vent stack 1 and the upper section of the second column 3b is fluidly connected to the discharge 4. The discharge 4 may be atmosphere, or it may for example be a flare stack or a pipe leading to a flare stack. In this embodiment, the pressure acting from the discharge 4 on the discharge side of the liquid lock 3 is therefore equivalent to atmospheric pressure.

Vent gas can be removed from the vent stack 1 via a compressor 5 fluidly coupled to the vent stack 1 via compressor supply line 5a. The vent gas removed by the compressor 5 can, for example, be regenerated back into a main process operating at higher pressure, returned to a tank, or sent to a processing unit for treatment. Alternatives to a compressor 5 for removing gas from the vent stack 1 may be e.g. an ejector, a piston or a screw compressor, or any other equipment suitable for the purpose. The compressor 5 (or equivalent component) may be configured to control the pressure in the vent stack 1, to maintain the pressure at a desired level by adjusting the rate of fluid being removed from the vent stack 1. This can be done, for example, by means of speed control of the compressor 5, or the control of valves in the supply line 5a or in a recirculation loop 5c. In some embodiments, it may be desirable to maintain a small over-pressure in the vent stack 1 and upstream the liquid lock 3, in order to prevent atmospheric air (and oxygen) from entering the ATM vent system due to leakage from the surroundings and into the system. The pressure may for example be controlled by the compressor 5 to be no more than 3.000 Pa (0.03 bar), 2.000 Pa (0.02 bar) or 1.000 Pa (0.01 bar) higher than the surrounding atmospheric pressure. These values may, for example, be suitable with a ATM vent system with 7.000 Pa (0.07 bar) maximum operating pressure. However a wide range of other design values is possible, depending on the type of equipment connected to the vent stack 1 and their pressure tolerances.

The pressure control can be a closed loop control, for example using a pressure sensor arranged in the vent stack 1 to control the compressor 5. In the embodiment shown in Fig.1, a level transmitter 6 is arranged in the liquid lock 3, whereby the controller 5 can be controlled in response to a change in liquid level in the first fluid column 3a and/or the second fluid column 3b. A change in the fluid level in the columns 3a,3b indicates an increasing or decreasing pressure in the vent stack 1, and can consequently be used for such pressure control.

In the event of an excessive pressure rise in the vent stack 1, gas blow by will occur in the liquid lock 3. The liquid lock 3 thus effectively acts as an overpressure protection. The liquid lock 3 can be engineered and installed in such a way that the pressure needed to cause a gas blow by in the liquid lock 3 is lower than a maximum allowable back pressure for the sources A-C. The design and orientation of the liquid lock 3, the height of the fluid columns 3a,3b and the amount of liquid in the liquid lock 3 are among the parameters which can be adjusted to achieve a desired blow-by pressure. The liquid lock 3 can, for example, be designed to produce a blow-by if the differential pressure across it is more than 6.000 Pa (0.06 bar), 8.000 Pa (0.08 bar) or 10.000 Pa (0.10 bar). However a wide range of other design values is possible, depending on the type of equipment connected to the vent stack 1 and their pressure tolerances.

This arrangement thus ensures that the equipment that is attached to the ATM vent system will not be over-pressurised, while vent gases at the same time are prevented from escaping to atmosphere during normal operation. In this way it is possible to achieve a closed loop ATM vent system while at the same time maintaining a reliable secondary pressure protection solution.

In one embodiment, the fluid column 3b on the discharge side of the liquid lock 3 is arranged to comprise a larger fluid volume than the fluid column 3a on the vent stack side. This can be arranged by, for example, providing a larger pipe cross-section in a pipe defining the column 3b, and/or to design the liquid lock 3 asymmetrically. Examples of both these can be seen in the embodiment shown in Fig.1.

In this embodiment, gas blowby in the opposite direction (from the discharge 4 / the atmosphere) can better be avoided, by designing the liquid lock 3 in such a way that gas blow by from the vent stack 1 to the discharge 4 requires a lower differential pressure than gas blow by in the opposite direction. In the embodiment shown in Fig.1, this is achieved by utilising the effect of an uneven liquid volume and layout across the lowermost section of the liquid lock 3.

The vent stack 1 may be arranged to serve only one source / one component, or it may be arranged to serve a plurality of sources. In the embodiment shown, three sources A-C utilise the vent stack 1 for discharge of vent gases. The sources may be, for example, one or more tanks, one or more compressors, other types of equipment or components, or a combination of these.

According to embodiments of the invention, it is therefore possible to obtain a vent system 100 which rapidly and reliably protects against overpressure, also in low differential pressure environments. If desired, vent gases may be extracted and recycled or treated, for example boil-off gases from tanks may be recycled to a tank or used in a separate process. In such a case, one can avoid discharging such vent gases to the environment, and one can better meet environmental and pollution regulations, if applicable. The sensitivity of the system can be improved, in that the liquid lock will react immediately on small pressure changes from the ATM system and the liquid level will change accordingly.

If using a compressor 5 or other gas recycle device which is controlled by the liquid level in the liquid lock 3, improved control sensitivity can also be achieved. Based on e.g. MEG (Mono Ethylene Glycol, ρ=1110 kg/m<3>) and suitable dimensioning of the pipes, a pressure variation of e.g.2.000 Pa (0.02 bar) in the vent stack 1 can for example be designed to cause a change in liquid level in the order of 18 cm. Conventional, commercially available level sensors will be able to provide accurate feedback in this operating range. Thus, the liquid lock 3 allows for a solution that regulates the compressor 5 based on a robust liquid level measurement. If the operational window is, for example, 2.000-7.000 Pa (0.02-0.07 bar), the corresponding liquid level range can be in the order of 46 cm. The liquid level can thus provide a robust method for controlling the speed of the compressor 5. A level transmitter for such an application does not have to be very complex or accurate to control the gas recirculation device or compressor 5 in a satisfactory manner.

The vent system 100 according to embodiments described herein can, for example, be used on installations which today vent low pressure sources to air.

The gases from such a closed loop vent system 100 can be fed into a process system via the illustrated compressor 5. This may, for example, be desirable on a cargo ship where the gas from the compressor 5 is fed into the ship’s engine system. A wide variety of other applications may, however, be possible, where the use of a vent system 100 with a liquid lock 3 to segregate the vent stack 1 from open air may provide advantages and make the design less complex and more cost effective than conventional solutions.

Claims

1. A vent system (100) for receiving vent gases from at least one source (A-C), the vent system (100) comprising:

a vent stack (1) fluidly connected to the at least one source (A-C) and a discharge line (2) extending from the vent stack (1) to a vent discharge (4), characterized by a liquid lock (3) arranged in the discharge line (2).

2. A vent system (100) according to claim 1, comprising

a gas removal unit (5) fluidly connected to the vent stack (1) and operable to remove fluid from the vent stack (1).

3. A vent system (100) according to the preceding claim, wherein the gas removal unit (5) is configured to control a pressure in the vent stack (1) by adjusting the rate of fluid removed from the vent stack (1).

4. A vent system (100) according to the preceding claim, wherein the gas removal unit (5) is configured to control the pressure in the vent stack (1) to a pressure higher than a surrounding atmospheric pressure.

5. A vent system (100) according to the preceding claim, wherein the gas removal unit (5) is configured to control the pressure in the vent stack (1) to a pressure which is:

- not more than 3.000 Pa higher than the surrounding atmospheric pressure;

- not more than 2.000 Pa higher than the surrounding atmospheric pressure; or

- not more than 1.000 Pa higher than the surrounding atmospheric pressure;

6. A vent system (100) according to any of the three preceding claims, wherein the gas removal unit (5) is configured to control the pressure in the vent stack (1) in response to a signal from a sensor (6), the signal being indicative of:

the pressure in the vent stack (1), and/or

a liquid level (7) in the liquid lock (3).

7. A vent system (100) according to any of claims 2-6, wherein the gas removal unit (5) is a compressor, a pump, or an ejector.

8. A vent system (100) according to any preceding claim, wherein the liquid lock (3) has first fluid column (3a) on a vent stack side of the liquid lock (3) and second fluid column (3b) on a discharge side of the liquid lock (3), wherein second fluid column (3b) comprises a larger fluid volume than the first fluid column (3a).

9. A vent system (100) according to any preceding claim, wherein the liquid lock (3) is configured to produce a blow-by in the event that a pressure differential across the liquid lock (3) is higher than:

- 6.000 Pa,

- 8.000 Pa, or

- 10.000 Pa.

10.A vent system (100) according to any preceding claim, wherein the vent stack (1) is fluidly connected to more than one source (A-C).

11.A vent system (100) according to any preceding claim, wherein the at least one source (A-C) comprises a fluid tank, a compressor, a gas sampling station or a riser annulus vent.