1. Field of the Invention
The present invention relates to the liquefaction of natural gas, and more particularly, the present invention relates to monitoring moisture within natural gas prior to the natural gas proceeding to liquefaction.
2. Discussion of Prior Art
Various reasons exist for liquefaction (i.e., the process of making liquid) of gases and particularly of natural gas. One example reason for the liquefaction of raw natural gas is the separation/liquefied into different hydrocarbon components. Another example reason for the liquefaction of natural gas is that the liquefaction reduces the volume of a gas by a factor of about 1/600, thereby making it possible to store and transport the liquefied gas in containers of more economical and practical design. In one example, when gas is transported by pipeline from a source/supply to a distant market, it is desirable to operate under a substantially constant high load factor. Often the capacity will exceed demand while at other times the demand may exceed the capacity of the line.
In order to moderate the peaks where demand would exceed supply, it is desirable to store the gas when the supply exceeds demand, whereby peaks in demand can be met from gas held in storage. For this purpose it is desirable to provide for the storage of gas in a liquefied state and to vaporize the liquid as demand requires. In another example, liquefaction of natural gas is useful in making possible the transport of gas from a source of plentiful supply to a distant market, particularly when the source of supply cannot be directly joined with the market by pipeline. This is particularly true where transport must be made by ocean going craft. Transportation of natural gas in the gaseous state would be uneconomical unless the natural gas was highly compressed, and then the system would not be economical because it would be impractical to provide containers of suitable strength and capacity.
In order to reduce natural gas to a liquefied state (i.e., liquefied natural gas or LNG) cooling is require to a temperature of about −240° F. to −260° F. (−151° C. to −162° C.) at atmospheric pressure. Numerous systems exist for the liquefaction of natural gas or the like in which the gas is liquefied by passing it sequentially through a plurality of cooling stages, to cool the gas to successively lower temperatures until the liquefaction temperature is reached. For example, cooling can be accomplished by indirect heat exchange with one or more expanded refrigerants such as propane, propylene, ethane, ethylene, and methane.
In general, gases, and specifically raw natural gas from a well, need to be processed before such gases can be liquefied. Such processing can include processing referred to as “sweetening,” wherein hydrogen sulfide (H2S) removal occurs and carbon dioxide (CO2) removal occurs. Mercury (Hg) removal and moisture (water, H2O) removal occur after “sweetening.”
Dehydration is the removal of moisture content from the gas (i.e., natural gas). One example is that elevated moisture in natural gas conduit (e.g., LNG pipeline) can form methane-hydrate at high pressure and blocks conduit. Another example is that moisture condensation can corrode equipment. As yet another example is that elevated moisture can cause ice formation that can severely damage cryogenic equipment if the amount of ice is significant. Generally, moisture content in natural gas has to be reduced to below 1 ppm (part per million) before it can be cryogenically processed further.
Within a liquefaction system (e.g., a LNG plant) a moisture analyzer is usually provided to monitor the gas after a gas dehydration process is performed upon the gas but before a liquefaction process is preformed upon the gas. The purpose of the moisture analyzer is to monitor moisture content and make sure it is below a desired amount (e.g., 1 ppm) before the gas proceeds to the liquefaction process. When the moisture analyzer indicates moisture content higher than the desired amount (e.g., an action level amount), the cause can be from various sources. For example, the action-level indication can be a result of system leakage or a saturated dryer that is due for regeneration. Before any action is to be taken concerning remedying the cause of the action-level indication, sometimes it is useful to confirm that an action-level moisture indication by the moisture analyzer is correct. In other words, it is sometimes useful to check/test the operation of the moisture analyzer.
A moisture provision system may be used to provide a known moisture content to gas within a moisture provision and analyzer arrangement. One example of a moisture provision system can include a permeation tube with water provided therein. Such gas of known moisture content can then be supplied to the moisture analyzer. If the moisture indication (e.g., sensed reading) from the moisture analyzer for the known-moisture gas is within a certain tolerance of an expected indication, the moisture analyzer is deemed to be reliable. With the reliability of the moisture analyzer confirmed, investigation of other possible reasons for the action-level moisture indication can be logically pursued. For example, a technician can proceed to trouble-shoot for a possible leakage or a dryer could be in need of being switched-off to allow for regeneration. If the moisture indication from moisture analyzer for the known-moisture gas is not within the certain tolerance of the expected indication, the moisture analyzer and verification system can be re-examined carefully before any other action might be undertaken.
As can be appreciated, provision of the known moisture content to the gas and the supply of such known-moisture gas to the moisture analyzer is a selective process. In other words, such provision of known-moisture gas to the moisture analyzer is only performed at some, select times during overall operation of the liquefaction system (e.g., a LNG plant). Accordingly, the moisture provision and analyzer arrangement has components to selectively provide moisture to the gas and selectively supply the known-moisture gas to the moisture analyzer.
It should be appreciated that it may not be prudent to completely cease flow of gas through the moisture provision and analyzer arrangement when the moisture analyzer is selectively not operating to sense moisture content. If gas flow through the moisture provision and analyzer arrangement were to completely cease, is possible that moisture could start to accumulate within the moisture provision and analyzer arrangement. Such accumulation of moisture could adversely affect components of the moisture provision and analyzer arrangement. Also, such accumulation of moisture could adversely affect the operation/accuracy of the moisture analyzer within the arrangement. In one example, such moisture accumulation could occur in dead space(s) within/near the moisture provision and analyzer arrangement and when gas flow toward the moisture provision and analyzer arrangement is initiated/occurring, such moisture accumulation could the contaminate gas flow. The contamination would be in the form of added moisture above the level that is otherwise present in the gas.
In order to avoid undesirable moisture accumulation within/near the moisture provision and analyzer arrangement, another flow is utilized that proceeds through the arrangement but which does not proceed through the moisture analyzer. This other flow is referred to as a bypass flow because the flow is by-passing the moisture analyzer within the arrangement. One or more switching valves are provided within the moisture provision and analyzer arrangement for gas flow path selection of either a path through the moisture analyzer or the bypass.
It has been noted that during switching of gas flow path selection (i.e., through the moisture analyzer or the bypass) discontinuity of flow rate can occur. It has also been noted that discontinuity of flow rate can cause some disruption of the ability of the moisture analyzer to properly function and provide an accurate indication of perceived moisture. For example, the moisture provision system (e.g., a permeation tube) may provide an incorrect/inconsistent/fluctuating moisture level at/following switching of gas flow path selection (i.e., through the moisture analyzer or the bypass). As one specific example, an overshoot of moisture provision may occur. As a result, an extra amount of settling time may be needed before the moisture analyzer can make an accurate/reliable measurement. As such, there is a need for improvements in moisture provision and analyzer arrangements. There would be a technical advantage provided by such improvements in that operation time improvements would be obtained thus permitting improvements in a determination time frame concerning the flowing gas.