Gas compressor cleaning

Abstract

A method of cleaning deposited solid material from a fouled portion of a gas compressor whilst the gas compressor is in situ in a natural gas processing system is provided. The method comprises the steps of supplying a liquid cleaning agent to a gas inlet of the gas compressor, the liquid cleaning agent being capable of removing the deposited solid material; passing the liquid cleaning agent through the gas compressor to a gas outlet of the gas compressor, wherein at least a portion of the cleaning agent remains in a liquid state as it passes through the fouled portion of the gas compressor; and recovering a fluid containing removed material that is output from the gas compressor so as to prevent the removed material reaching one or more gas processing stages of the gas processing system downstream of the gas compressor.

Claims

1. A method of cleaning deposited solid material from a fouled portion of a gas compressor whilst the gas compressor is in situ in a natural gas processing system, the method comprising: supplying a liquid cleaning agent to a gas inlet of the gas compressor, the liquid cleaning agent being capable of removing the deposited solid material; passing the liquid cleaning agent through the gas compressor to a gas outlet of the gas compressor in order to remove deposited solid material from the compressor, wherein at least a portion of the cleaning agent remains in a liquid state as it passes through the fouled portion of the gas compressor; and recovering a fluid output containing the removed deposited solid material from the gas compressor so as to prevent the removed deposited solid material reaching one or more gas processing stages of the natural gas processing system downstream of the gas compressor, wherein recovering the fluid output from the compressor comprises supplying the fluid output from the gas outlet of the gas compressor to a separator that outputs a gas phase output and a liquid phase output, the liquid phase output comprising the removed deposited solid material output from the compressor, wherein the separator is a second separator downstream of the gas compressor and separate from a first separator located upstream of the gas compressor, and wherein, during a cleaning operation, the liquid phase output from the second separator is supplied to the gas inlet of the gas compressor.

2. The method according to claim 1, wherein the cleaning agent is a hydrocarbon liquid or other produced liquid from the natural gas processing system.

3. The method according to claim 1, wherein the liquid cleaning agent is mixed with a gas stream being supplied to the gas inlet of the gas compressor.

4. The method according to claim 1, wherein the gas processing system comprises a cooler located downstream of the gas compressor and upstream of the second separator that cools the fluid from the gas compressor causing liquid to condense.

5. The method according to claim 1, wherein the first separator outputs a gas phase output and a liquid phase output, wherein the gas phase output is supplied to the gas compressor during normal operation of the gas processing system.

6. The method according to claim 5, wherein the gas phase output from the second separator is not re-combined with the liquid phase output from the first separator during normal operation of the gas processing system.

Description

(1) Certain preferred embodiments of the present invention will now be described in greater detail by way of example only and with reference to the figures, in which:

(2) FIG. 1 illustrates a first arrangement for cleaning a gas compressor in a fluid processing system;

(3) FIG. 2 illustrates a second arrangement for cleaning a gas compressor in a fluid processing system; and

(4) FIG. 3 illustrates a third arrangement for cleaning a gas compressor in a fluid processing system.

(5) With reference to FIG. 1, there is shown a first arrangement for cleaning a gas compressor 6 in a fluid processing system 1 for processing fluids from one or more well(s). In hydrocarbon wells, such fluids may include oil, gas, water, and gas condensate.

(6) The system 1 includes the gas compressor 6 through which gas from the well is passed. The compressor 6 operates to compress the gas, to facilitate transport of the gas onward for further processing downstream of the compressor 6. The compressor 6 has an inlet for intake of the gas to be compressed, and an outlet fluidly connected to the inlet to output compressed gas (not shown). The compressor 6 may have a compressor body extending between the inlet and outlet and defining a flow channel for conveying gas therebetween (not shown). During normal operation, a gas stream is passed into the inlet, through the compressor body, where it is compressed, and out of the outlet.

(7) In this example, the system 1 has a first separator 3 located upstream of the compressor 6. The first separator 3 receives a multiphase fluid from a hydrocarbon well via conduit 2 comprising liquid and gas. The first separator 3 acts to separate gas and liquid from the conduit 2 into a gas stream 4 and a liquid stream 5. The gas phase 4 is supplied to the gas compressor 6.

(8) The system 1 additionally uses a cleaning agent injection apparatus to mix a liquid cleaning agent with the gas stream 4 from the first separator 3. The cleaning agent injection apparatus has a controllable supply valve 8 which may be opened, when required, to fluidly connect a liquid cleaning agent supply line 7 with the gas stream 4, so that liquid cleaning agent from the liquid cleaning agent supply line 6 can be injected into the gas of gas stream 4 so that the gas contains liquid. The liquid cleaning agent may be any cleaning agent suitable for removing the type of solids deposited in the compressor 6 by the vaporisation of the multiphase fluid. Examples of suitable cleaning agents may include liquid hydrocarbons, condensed hydrocarbon gas, a glycol, an alcohol, water, and mixtures thereof.

(9) At the point of mixing with the liquid cleaning agent, the gas stream 4 may be provided with an ejector to accelerate the flow of gas. This may facilitate mixing of the gas with liquid to help control the composition of the fluid entering the compressor 6.

(10) The liquid cleaning agent is supplied at a pressure higher than the gas in the gas stream 4, such that additional pumping is not required. However, a non-return valve 9 may also be located between the liquid cleaning agent supply line 7 and the gas stream 4 to prevent back-flow of the multi-phase mixture.

(11) Downstream of the compressor 6 is provided a second separator 10, such as a gas scrubber or the like. The second separator 10 receives fluid output from gas compressor 6. The second separator 10 acts to separate any liquid from the fluid flow to produce a gas stream 11 and a liquid stream 12, such as liquid cleaning agent injected into the compressor 6 by the cleaning agent injecting apparatus. The second separator 10 thus acts to remove any remaining liquid cleaning agent, together with the solid deposits removed from the compressor, which are carried in the liquid.

(12) The gas stream 11 from the second separator 10 should be substantially free from liquid such that the gas may be processed downstream. In particular, the gas stream 11 from the gas processing system 1 is not re-combined with the liquid stream 5.

(13) The liquid stream 12 has a controllable drain valve 13 which may be opened, when required, to drain separated liquid cleaning agent (containing the removed solids) from the second separator 10. The separated liquid cleaning agent may be re-circulated back to the inlet of the compressor 6, or may be drained via the liquid cleaning agent supply line 7 or optionally discharged via a drain line 14 into the liquid stream 5 from the first separator 3. A three-way valve 15, or an equivalent valving configuration, may be provided on cleaning agent supply line 7 and drain line 14 to switch between injection and draining.

(14) During normal operation of the system 1, the supply valve 8 is closed so that liquid cleaning agent is not introduced in to the gas stream 4. The gas stream 4 is received by the compressor 6 and the compressor 6 compresses the gas. The drain valve 13 may also be closed during normal operation.

(15) To initiate a cleaning operation of the system 1, the supply valve 8 is opened and liquid cleaning agent is introduced in to the gas stream 4. The multiphase stream 4 is received by the compressor 6 and the compressor 6 compresses the mixture. A flow measurement device (not shown) may be provided on the injection line downstream the supply valve 8, and preferably downstream of the non-return valve 9, to measure the quantity of liquid cleaning agent supplied to the gas stream 4. The supply valve 8 is controlled so as to supply sufficient liquid cleaning agent to the compressor 6 such that a portion of the liquid cleaning agent remains in liquid form at the outlet of the gas compressor 6.

(16) Typically, the condition of the gas stream upstream and downstream of the compressor 6 and/or the performance of the compressor are monitored. The condition of the gas (e.g. a wet, liquid-containing gas) may be the flow rate, temperature, pressure and/or composition of the gas stream. The performance of the compressor 6 may be calculated based on the increase in pressure or temperature between the inlet and outlet of the compressor. In this example, the monitoring of conditions or performance can be carried out by applying measurement apparatuses (not shown) upstream and downstream of the compressor. The measurement apparatuses may each comprises a multiphase flow meter and/or a temperature sensor and/or a pressure sensor. The amount of liquid in the gas stream 4 can determined from flow meter measurements. A change in condition of the gas and/or performance of the compressor 6 may indicate that a deposit has formed on a surface inside the compressor 6. For example, this change may be a drop in pressure of compressed gas downstream of the compressor 6. The measured conditions or performance may be compared with previous or expected (modelled) performance.

(17) Detection of fouling may be performed by detecting that the compressor efficiency is reduced compared to the reference value. This is because the compressor's ability to create a pressure increase at a given speed will be reduced by the fouling. This is especially observed on higher volumetric flow rates. If the presence of a deposit on a surface inside the compressor 6 is detected from measured data, the cleaning operation is initiated as described above.

(18) It will be appreciated that fouling will often occur when the liquid in the gas stream is very low, e.g. when liquid is measured in the gas upstream but not downstream of the compressor. Liquid cleaning agent is then injected into the gas of gas stream 4, such that the gas stream passed into the compressor 6 comprises gas with an amount of liquid entrained therein. As the gas stream 4 passes through the compressor 6, the gas with liquid contained therein acts to remove the detected deposit. Thus, the gas with liquid acts to clean or wash the internal surfaces of the compressor across which the gas is passed. Such surfaces may be surfaces that define the flow channel of the compressor body that come into contact with the gas. In a rotating compressor, these surfaces may include those of a rotating blade.

(19) In order to provide cleaning upon detecting the deposit, the amount of liquid in the gas is made sufficiently great that complete vaporization of the liquid does not occur upon passing the gas through the compressor 6. In other words, the gas needs to remain as a two-phase gas, i.e. a gas with liquid entrained therein, as it enters and exits the compressor 6. If there is insufficient liquid in the gas stream as it enters the compressor, the liquid may vaporise away and deposits may form inside the compressor. The amount of liquid cleaning agent injected is controlled using the supply valve 8. The amount of liquid at the inlet and outlet of the compressor may be monitored using the measurement apparatuses described above.

(20) During the cleaning operation, the drain valve 13 is also open to drain the liquid cleaning agent from the second separator 10. The liquid cleaning agent will continue to circulate from the second separator 10 back to the inlet of the gas compressor 6 because of the pressure difference between the inlet and the outlet of the compressor 6.

(21) Once the deposit has been removed, the valve 8 may be closed to reduce the liquid content in the gas stream, and the compressor 6 can continue to perform at previous or improved performance level, e.g. with no or with the original very low amount of liquid contained in the gas.

(22) Draining of liquid and solids from the second separator 10 can occur by switching valve 15 so the liquid flows under pressure along drain line 14 and is combined with the liquid in liquid line 5 from the first separator 3.

(23) With the deposit removed, the compressor 6 may perform close to an ideal level of performance or of compression. The removal of the deposit may be detectable as an increase in performance, or change in the conditions of the gas upstream or downstream of the compressor back to previous values. Alternatively, removal of the deposit may be assumed to be complete after a predetermined period of cleaning operation. Similar cycles of cleaning may be performed as and when further deposit build-up is detected or suspected.

(24) Upon inserting liquid into the gas stream 4 via valve 8, the system 1 is moved from a condition in which scaling occurs to one in which cleaning occurs. Typically, in order to provide cleaning, the system 1 is arranged such that the liquid content in the gas stream 4 upstream of the compressor 3 is up to around 20 times greater than the liquid content in normal operating conditions where deposits form. Typically, this may be 2 to 20 times greater, but higher amounts may also be feasible.

(25) Gas having a liquid content in an amount of up to around 5% by weight, may result in deposits forming inside the compressor. For example, a typical content of liquid of 0.2% to 0.6% by weight may result in a deposit. In general, it will be appreciated that the amount of liquid required in order to remove deposits from surfaces inside the compressor 6 is dependent on how much liquid evaporates from the gas as it passes through the compressor 6. This is in turn dependent upon the pressure and temperature conditions of the gas.

(26) Computer modelling packages are commercially available to allow processing systems 1 such as that shown in FIG. 1 to be modelled. Such packages can be used to determine the amount of liquid required in the gas supplied to the compressor 6 at the inlet for purposes of cleaning. Flow measurements downstream may verify that the amount supplied is sufficient, and that full vaporisation is not occurring. The models may define relationships between parameters for different parts of the system, including relationships between temperature, pressure and liquid content for a given configuration of processing components and fluids.

(27) With reference to FIG. 2, there is shown a second arrangement for cleaning a gas compressor 27 in a fluid processing system 21 for processing fluids from a well.

(28) The system 21 includes the gas compressor 27 through which gas from the well is passed and first separator 24 located upstream of the compressor 27. The compressor 27 and first separator 24 are structurally and operationally equivalent to the compressor 6 and first separator 3 shown in FIG. 1. The gas compressor 27 is also monitored by equivalent measurement apparatuses to detect fouling.

(29) This system 21 again uses a cleaning agent injection means to mix a liquid cleaning agent with the gas stream 25 from the first separator 24. However, the cleaning agent injecting apparatus in the system 21 shown in FIG. 2 is different from that in the system 1 shown FIG. 1.

(30) In system 21, a cooler 28 is provide downstream of the compressor 27 to cool the gas output from the compressor 27. This causes liquid hydrocarbons to condense, providing a multi-phase gas stream. This multi-phase gas stream is supplied to a second separator 29. The second separator 29 receives the multiphase fluid and acts to separate gas and liquid from the fluid into a gas stream 30 and a liquid stream 31.

(31) The gas stream 30 from the second separator 29 is passed onwards for downstream processing. For example, as illustrated, the gas stream 30 may be compressed by a second compressor 32. In FIG. 2, the second compressor 32 is illustrated including a cleaning agent injection apparatus similar to that shown in FIG. 1, and the details thereof are not repeated.

(32) The liquid stream 31 from the second separator 29 is drained for disposal or other processing. However, a liquid supply line 33 connects to the liquid stream to divert at least a portion of the liquid stream for use as a liquid cleaning agent to clean the first compressor 27 during a cleaning operation.

(33) In system 21, the cleaning agent injection system has a controllable supply valve 34 which may be opened, when required, to fluidly connect the liquid supply line 33 to the gas stream 25 from the first separator 24, so that liquids from the liquid supply line 33 can be injected into the gas of gas stream 25 so that the gas contains liquid.

(34) At the point of mixing with the liquid, the gas stream 25 may be provided with an ejector to accelerate the flow of gas. This may facilitate mixing of the gas with liquid to help control the composition of the fluid entering the compressor 27.

(35) The liquid from the liquid supply line 33 is at a pressure higher than the gas in the gas stream 4 because it is downstream of the gas compressor 27. However, as described above, a non-return valve 35 may also be located between the liquid supply line 33 and the gas stream 25 to prevent back-flow of the multi-phase mixture.

(36) During normal operation of the system 21, the supply valve 34 is closed so that liquid from the liquid stream 31 is not introduced in to the gas stream 25. The gas stream 25 is received by the compressor 27 and the compressor 27 compresses the gas.

(37) To initiate a cleaning operation of the system 21, the supply valve 34 is opened and liquid is introduced in to the gas stream 27. The multiphase stream 25 is received by the compressor 27 and the compressor 27 compresses the mixture. The supply valve 34 is controlled so as to supply sufficient liquid cleaning agent to the compressor 27 such that a portion of the liquid remains in liquid form at the outlet of the gas compressor 27.

(38) A flow measurement device (not shown) may be provided on the injection line 33 downstream the supply valve 34, and preferably downstream of the non-return valve 35, to measure the quantity of liquid cleaning agent supplied to the gas stream 25. The cleaning operation of the compressor 27 in FIG. 2 is monitored and controlled in the same manner as the cleaning operation of the compressor 6 in FIG. 1, and details thereof are not repeated. As with the system 1 shown in FIG. 1, the system 21 shown in FIG. 2 is an online cleaning process.

(39) With reference to FIG. 3, there is shown a third arrangement for cleaning a gas compressor 47 in a fluid processing system 41 for processing fluids from a well.

(40) The system 41 includes the gas compressor 47 through which gas from the well is passed and first separator 44 located upstream of the compressor 47. The compressor 47 and first separator 44 are structurally and operationally equivalent to the compressor 6 and first separator 3 shown in FIG. 1. The gas compressor 47 is also monitored by equivalent measurement apparatuses to detect fouling.

(41) This system 41 further comprises a cleaning agent injection means to mix a liquid cleaning agent with the gas stream 45 from the first separator 44. The cleaning agent injecting apparatus in the system 41 shown in FIG. 3 is structurally similar to the cleaning agent injecting apparatus in the system 1 shown FIG. 1. That is to say, in system 41, the cleaning agent injection system has a controllable supply valve 49 which may be opened, when required, to fluidly connect a liquid cleaning agent supply line 48 to the gas stream 45 from the first separator 44, so that liquids from the liquid supply line 33 can be injected into the gas of gas stream 25 so that the gas contains liquid. As above, the liquid cleaning agent may be at a higher pressure than the gas stream 45 and an ejector may be used to facilitate mixing of the gas with the liquid.

(42) The liquid phase output from liquid stream 46 may be supplied to the cleaning agent supply line 48 via a liquid supply line 54. Optionally, a pump (not shown) may be used, e.g. in the liquid supply line 54, to pressurise the liquid phase from stream 46. In such an embodiment external source of cleaning fluid may not be required.

(43) The system 41 shown in FIG. 3 differs from the system 1 shown in FIG. 1 in that it does not use a second separator 10 downstream of the compressor 44 to remove the liquid in the fluid output from the compressor. Instead, during the cleaning operation, the system 41 in FIG. 3 fully opens an anti-surge valve 52 in an anti-surge line 51 associated with the compressor 47. When (fully) opened, substantially all of the fluid output from the compressor 47 is re-circulated back to a location within the system 41 that is upstream of the first separator 44.

(44) During normal operation of the system 21, the supply valve 34 is closed so that liquid cleaning agent from the liquid cleaning agent supply line 48 is not introduced in to the gas stream 45. The gas stream 45 is received by the compressor 47 and the compressor 47 compresses the gas. The anti-surge valve 52 may be operated during normal operation of the compressor so as to regulate the output from the system 41. Anti-surge lines 51 are commonly used in most compressor systems to account for varying demand. As such, the system 41 shown in FIG. 3 requires less substantial modification than the systems 1, 21 shown in FIGS. 1 and 2 to retrofit into an existing fluid processing system.

(45) Optionally, the system 41 may be provided with a valve on the outlet line 53. If such a valve is included, it may be closed during the cleaning operation to ensure that all of the fluid from the compressor 47 is recirculated. Optionally, a valve on the inlet line 42 may also be provided, which may similarly be closed during the cleaning operation.

(46) To initiate a cleaning operation of the system 41, the supply valve 49 is opened and liquid cleaning agent is introduced in to the gas stream 45. At the same time, the anti-surge valve 52 is opened sufficiently such that substantially all fluid output from the compressor 47 is re-circulated to upstream of the first separator 44.

(47) The multiphase stream is received by the compressor 47 and the compressor 47 compresses the mixture. The supply valve 49 is controlled so as to supply sufficient liquid cleaning agent to the compressor 47 such that a portion of the liquid remains in liquid form at the outlet of the gas compressor 47. The multi-phase fluid output from the compressor 47 is re-circulated to upstream of the first separator 44, thus preventing the liquid cleaning agent from affecting any gas processing steps downstream of the compressor 47.

(48) The cleaning operation of the compressor 47 in FIG. 3 is monitored and controlled in the same manner as the cleaning operation of the compressor 6 in FIG. 1, and details thereof are not repeated.

(49) In the system 41 of FIG. 3, all of the fluid from the compressor 47 is re-circulated and thus gas production is temporarily suspended during the cleaning operation. However, since the cleaning operation can be carried out in situ, the down time is relatively short compared to removing the compressor from the system 41.

(50) In practice, a gas compressor 6, 27, 47 will often comprise multiple stages. Since the liquid content of the gas received at the gas inlet to the gas compressor 6, 27, 47 is typically maintained at very low levels, any liquid will often evaporate in the first few stages of the gas compressor 6, 27, 47. As a result, fouling due to evaporation of the liquid often occurs predominantly in a portion at the inlet to the gas compressor 6, 27, 47

(51) The preferred embodiments describe a full clean in which the liquid cleaning agent is supplied in sufficient quantity such that it remains in a liquid state at the gas output of the gas compressor 6, 27, 47. However, this may not always be necessary and partial cleaning of the gas compressor 6, 27, 47 may be sometimes be sufficient. To achieve this, only sufficient liquid cleaning agent needs to be added such that it remains in a liquid phase as it passes through the fouled portion of the gas compressor 6, 27, 47. The removed solids which have been displaced will then be carried in the gas stream.

(52) When the gas and vaporised cleaning agent leave the gas compressor 6, 27, 47, a downstream cooler (cooler 28 or cooler 43 in FIGS. 2 and 3, or optionally a cooler may be added upstream of separator 10 in FIG. 1) will condense a portion of the gas and the removed solids will become entrained in this condensate, which is removed by the respective separator.