US3079760A - Liquefied petroleum gas pressure and low temperature storage system - Google Patents

Description

March 5, 1963 MAHER 3,079,760

LIQUEFIED PETROLEUM GAS PRESSURE AND LOW TEMPERATURE STORAGE SYSTEM Filed June 21, 1960 2 Sheets-Sheet 1 March 5, 1963 J. B.

LIQUEFIED PETROLEUM GAS PRESSURE AND MAHER LOW TEMPERATURE STORAGE SYSTEM Filed June 21, 1960 2 Sheets-Sheet 2 QMMEMQEQQ mmwmmmmsu U x wk INVENTOR. James B. Maher Merriam, Smith 8 Marshall ATTORNEYS Unie 3,679,7tih Patented Mar. 5, 1953 nols Filed June 21, 1960, Ser. No. 37,673 3 Claims. (Cl. 62-55) This invention relates to the storage of liquefied, normally gaseous products, especially hydrocarbons or mixtures thereof. It more particularly relates to coordinating the transportation of liquefied, normally gaseous petroleum products through pipelines and the subsequent storage of the transported product in terminal facilities.

The pressure-temperature relationship of liquefied, normally gaseous petroleum products, which are generally termed LPG. requires that special handling and storage techniques be employed for maintaining the L.P.G. in the liquid state. For example, at a liquid temperature of 120 F., propane must be stored under a pressure of approximately 225 p.s.i.g. If storage at atmospheric pressure is desired, propane must be held at temperature of -44 F. For commercial n-butane, the corresponding pressure-temperature relationship is approximately 56 p.s.i.g. at 120 F. and atmospheric pressure at about |3l F.

Storage facilities used for L.P.G. include injection of the L.P.G. into depleted subterranean oil reservoirs or other permeable geological strata which are sealed by an impervious overlying rock cap, storage in salt cavities or mined caverns, or in conventional above-ground storage tanks such as spheres, spheroids, fiatbottom storage tanks, etc.

The selection of the storage facility will be general depend upon the service requirements as well as operating economics. Flexible and economic storage of LPG. at substantially atmospheric pressure can be effected by the use of fiatbottom cylindrical tanks which are modified by slightly dishing the roof so as to withstand 1-2 p.s.i.g. working pressure. At this Working pressure, a vapor compression refrigeration system or the like, is used in cooperation with storage tank in order to maintain the stored product in the liquefied state. Such a system includes conventional refrigeration equipment such as comressors, condensers, insulation, etc., and, if necessary, auxiliary heaters employed in order to prevent freezing of the earthen tank foundation. The slight atmospheric pressure, e.g., 12 p.s.i.g., in the flatbottom tanks is used to provide sufiicient operating range for control equipment and to prevent air from being drawn into the system. The cost of storing L.P.G. in such tanks at about 1 p.s.i.g. and 41 F., is much less than the storage costs incurred in the use of high pressure tanks; however, L.P.G., as delivered from the pipeline transporation system, is not refrigerated and initial refrigeration at high rates of delivery is expensive.

The following variables are generally considered for any proposed L.P.G. storage installation: (1) tank size, (2) tank type, (3) tank pressure, (4) inslulation thickness, (5) size of refrigeration equipment, (6) filling rate and temperature of incoming stream, and others.

Using these variables, the designer then determines the best combination for the most economic storage. The selection of the equipment depends upon the storage conditions to be satisfied. Product throughput and size of product tender to the storage facility will determine the total volume of the storeg required. Filling rate and temperature of the incoming stream determines how much refrigeration is required to economically effect the desired cooling, which in turn determines the tank design pressure and insulation requirements. Local climatic condi tions and tank design determine the size and type of tank to be used, and the most economical size of refrigeration equipment by its effect on operating costs.

In the handling of L.P.G. directly from the pipeline transportation systems the important consideration must be given to product throughput and size of product tender. Although a refrigerated storage system can be designed to handle any desired rate of product throughput from the pipeline systems, the cost of investment in the operation may be too great to be economically attractive if the filling rate is high in proportion to a relatively low storage temperature, particularly when cooling a hot product down to a relatively low storage temperature. If the filling rate capacity of the storage facility is low, then the incoming stream or tender must be small. While this does not present a serious problem where the product is received in relatively small tenders, such as from a barge, railroad tankcar, or a truck, pipeline tenders are usually in lots of 20,000 barrels, or more, above-ground refrigeration storage facilities with low pressure tankage of a desirable size to handle the combined tender size and filling rate required for pipeline use are not available. The instant invention, however, provides a storage system adapted to receive pipeline tenders of liquefied, normally gaseous products for storage at conventional rates of tender for subsequent storage in a low pressure refrigerated storage facility.

FIGURE 1 is a schematic flow diagram of one embodiment of the storage system of this invention.

FIGURE 2 is a simplified diagrammatic flow plan of an alternative embodiment of this invention.

In accordance with the instant invention, the storage of liquefied, refrigerated, normally gaseous products such as L.P.G., or the like, intermittently delivered through a pipeline at conventional pipeline rates is effected by using a combination of conventional high pressure storage vessels to receive the original tender of the product and hold this shipment at received conditions of temperature and pressure until the product can be subsequently transferred to a separate, fully or partially refrigerated, low pressure storage system. Normally, shipments of LPG. are made at relatively high pressures and temperatures in a pipeline, e.g., propane is delivered at a temperature of about F. and a pressure of about 133 p.s.i.a., and the entire tender must be received in a short period of time in order to avoid traffic problems in the transportation of different fluids through the same pipeline. While it is desirable to store the product in a fully or partially refrigerated state, the amount and cost of the refrigeration system required to handle and process the incoming stream would be very expensive and commercially undesirable. To alleviate this problem the instant invention uses an aboveground, refrigerated storage tank adapted to receive and store the product at the elevated temperature and superatmospheric pressure delivery conditions, viz., at pressures in excess of about 130l40 p.s.i. and temperatures within the range of about to F. The received, liquefied, normally gaseous product is then transferred over a period substantially greater than the delivery period to a storage facility wherein the product is refrigerated and stored at about atmospheric pressure and a subatmospheric tem perature. In this storage system, a refrigeration system having a smaller capacity than would be required to initially refrigerate the product tender to the desired storage conditions immediately upon delivery of the product from the pipeline is used for filling and storage refrigeration demands.

In the accompanying drawing there is shown, in FIG- URE 1, a complete specific embodiment of the storage system of this invention schematically illustrated. For the purposes of simplicity the drawing does not show en trainment separators, oil separators, float valves, control valves, pumps, bypasses, etc., where such equipment is employed in an auxiliary capacity. However, where such equipment is used in the operation of the storage system of this invention, diagrammatic representations are used. It is apparent, however, that the placing of other auxiliary equipment necessary to. enhance the operating efficiency of the, operation, as schematically shown in FIGURE 1, will be evident to those skilled in the art.

In the storage, system illustrated in the drawing, product tender is received through fill line from a suitable source such as a pipeline (not shown)- The product, which in thecase of propane, is received at a temperature of about 75 F., and a pressure'of about 133 p.s.i.a,, is received in a suitable above-ground, refrigerated storage vessel 11 which is constructed temporarily to store and maintain the tender at delivery conditions of temperature and pressure. In tank 11, the difference in operating and design pressure (about p.s.i.) is to take care of back pressure resulting from rapid compression of vapors during filling, The compressor can aid in this function, however, its main function is to maintain storage conditions in the tank 11 at 75 F. after the filling operation, i.e., after filling, the pressure and temperature are 148 p.s.i. and :85 F. The refrigeration system hereinafter discussed will bring the temperature back down to 75 F. and hold it until next filling. By storing the tender at inlet stream temperature little refrigeration is required, except that amount of refrigeration which is needed to remove the heat of compression. This amount of refrigeration is adequately taken care of by removing gaseous 'efiiuent product from storage vessel '11-, by means of line '12, and passing it through a refrigeration system such as vapor compression refrigeration system comprising single stage compressor 13 to air cooled condenser 14. The cooled liquid at a saturation pressure in excess of the saturation pressure of the stored product in vessel 11 is returned through line 12 and flash vaporized through an expansion valve 15 into storage vessel 11. Any uncondensibles present in the refluxing effluent are removed by means of a conventional purger 16. The product from storage facility '11 is then transferred to the final storage facility which consists of a low pressure'storage tank 29constructed' to retain the transferred product in a liquefied state at substantially atmospheric pressure and subatmospheric temperature, e.g., for propane 44 F., and 1 p.s.i.g.

In order to sufficiently reduce the temperature and pressure of the product received in storage vessel 11 to the desired'storage conditions of temperature and pressure, the product is slowly transferred from storage vessel 11 through line 21; The flow conditions within the transfer line 21 are controlled by pressure regulator 22 and valve 23. In the illustrated embodiment the desired low storagepressure and temperature is economically obtained by employing a flash vaporization vessel such as economizer 24 which is placed in the system in order to employ an economical compression ratio in the refrigeration system used in the filling phase of the storage cycle. In utilizing this expedient the liquid product from storage vessel 11 is flash-vaporizedin the economizer 2 4 to a pressure corresponding to the intake pressure of the second stage of the compressor '25 employed in the refrigeration system utilized in refrigerating the product during the filling phase of the storage cycle of storage tank 20. The gaseous eflluentproduct removed from economizer 24 is transferred by means of line 26 to the intake side of the second stage of compressor'25. The liquefied productis transferred from the econoniizerby means oflines 27 and 28 to storage tank 20. Automatic flow control is employed such that the entire liquid output from economizer 23} during the filling phase is, transferred to tank 20. In the economizer a substantial reduction in temperature and pressure is eifected. The remaining decrease to desired storage conditions, of temperature and pressure is a cqmr shed by h equ l br m h vaporization of the liquid into tank 20. In the reduction in temperature and pressure to the desired storage conditions a larger capacity refrigeration system is used in the gaseous eifluent refluxing system during the filling phase than is necessary to maintain the desired low temperature and low pressure storage conditions of the product once they have been reached. The required refrigeration is produced by removing the gaseous effluent product from storage facility 20, by means of line 29, through which it passes to the first stage of a two-stage compressor 25 employed in the illustrated vapor compression refrigeration system required to effect the desired conditions of low temperature and low pressure. Where propane is being stored, a ton compressor Will sufiice to handle a product transfer rate of 148 barrels per hour at 75 F. The compressed product is delivered from the first stage through line 30 to a point of confluence with line 26, through which the gaseous effluent product from economizer 24 is delivered. The resultant mixing provides interstage cooling and thus reduces the required horsepower. The vapors from the economizer and from the first stage compression are mixed and passed through the second stage compression. The discharge from the second stage compression is cooled in a suitable condenser 31 to. produce a liquid having a saturation pressure exceeding the operating pressure of economizer 24. The liquid is transferred back to economizer 24 through line 32, and flash vaporized into economizer 24. The liquid product in the econornizer 24 is transferred through lines 27 and 28' to intermediate storage vesel 2% as hereinbefore discussed. During the unload.- ing phase of the storage cycle, the liquid product discharged from condenser 31 is employed as a heat transfer fluid and directed through line 33 to heat exchanger 34, which is placed [in line 35 employed in transferring the stored product to loading facilities in order to heatthe product to about 50 F for consumer use. The fluid product used as heat exchange medium is passed through line 35 and flash-vaporized into storage vessel 20. If desired, the hot vapor from compressor 25 could be used as the heat transfer medium and thereafter returned to economizer 24 through suitable transfer lines.

Although one storage tank ofsufiicient size could handle the storage of the product transferred from initial storage vessel 11, it is generally desired that several low pressure storage vessels be employed in order to improvethe flexibility of the storage system of this invention as well as permit a build-up of stored product during ofi peak seasons. In the illustrated embodiment, after the product has been reduced to desired conditions of temperature and pressure, portions are transferred to storage vessels 4!! using gravity transfer as Well as assisted transfer by meansof pump 37' and line 35 The product is introduced into the tank 41 through lines 35 around pumps 38, by Ineansof bypasses 39. Suitable valving is, of course, provided to effect this transfer. In order to maintain the desired refrigerated conditions within storage tanks 20- and 40 during the storage phase of the storage system a reflux; ing system isemployed. These refrigeration systemsemployed in the refluxing system only handle the gaseous product evolved during the storage phase and therefore the refrigeration systems are not required to have as large a capacity as the. oneused duringthe filling phase. The gaseous eflluent products from the storage vessels 20 and 40 are transferred by means of lines 41, 42 and 43' to a pointof confluence with line 44. The total product transferred by line '44 is divided and each portion compressed and liquefied in a suitable two-stage vapor compression refrigeration system. One portion is delivered through line 45 to two-stage compressor 46'. The com.- pressed product delivered from the compressor 46 is liquefied in condenser 47-. Similarly, the other portion of the gaseousproduct delivered through line- 44 is passed through line 48, and processed in compressor 49. The product from compressor 49' is sent to condenser 54} and the liquefied product, therefrom combined with the liquefied product discharged from condenser 47 and the com bined product flash vaporized in economizer 24. The liquid product from economizer 24 is then returned to the lower pressure storage tanks 20 and 40 by means of lines 27 and 28. Suitable automatic control such as pneumatic operated valves 32 [is employed to permit the liquid product to flash vaporize into the tank requiring refrigeration during the storage phase of the storage system. The gaseous efiluent evolved from the economizer necessary to operate the system has been omitted for the purposes of simplicity. Its placement, however, will be obvious to one skilled in the art. Tank capacities and other storage system details for the terminal shown in FIGURE 2 having facilities to receive 50,000 barrels in 24 hours (2,080 barrels per hour), at a temperature of 75 F., and a pressure of 20 p.s.i.g are tabularly summarized in Table I. In this system, product is transferred from the receiving vessel at the rate of 416 barrels per 24 is processed in the second stage of compressors 45 and hour.

Table 1 Tanks Thicknew ofiusula- Refrigera- Required Storage attion fibertion filling holding Dia., Ht., Design glass, (tons) (tons) Number Capacity, barrels feet feet press, inches p.s.i.g. Temp, Press, F. p.s.i.g.

2 Spheres 30,000 69 40 75 2 0 2 at 8.5=17

Flat Btm. 100,000 105 65 0 +31 0 4% 169 21.5 Flat Btm. 100,000-- 105 65 0 +31 0 4% 0 4at21.5=86

49 in the same manner employed in processing the gaseous effluent produced during the filling phase. It will be noted that in the lines returning the cooled liquefied product from the compressors to the economizer, purgers 53 are installed to remove any incondensibles.

in a 200,000 barrel refrigerated storage system for the storage of liquefied propane, in order to handle a weekly tender of 25,000 barrels per day, one 30,000 barrel spherical storage vessel such as a Hortonsphere manufactured and erected by Chicago Bridge & Iron Company is employed to receive the initial tender. Three 60,000 barrel flat bottom cone roof tanks are used for low pressure, low temperature storage in order to provide adequate long term storage. The respective vessels and tanks are constructed such that the operating conditions existing within the refrigerated storage vessel could be maintained at 75 F. and 133 p.s.i.a., and the storage conditions existing in the fiat bottom cone roof tanks could be maintained at -44 F, and 15.2 p.s.i.a. In transferring the liquid product from the refrigerated, above-ground storage vessel to the economizer and low pressure, low temperature storage units, a transfer rate of 148 barrels per hour at 75 F., is employed. In order to hold the liquid in the refrigerated storage vessel at the desired conditions a 10-ton refrigeration system is employed, whereas, a 150- ton refrigeration unit is required in order to provide sufficient refrigeration for the filling cycle. On the other hand, 40-ton refrigeration units are required to maintain the desired storage conditions during the storage phase.

The equipment employed in the construction of the storage facility of this invention is conventional, readily available equipment. Standard storage tanks of a conventional type and design are employed in storing the liquid during the several phases of the storage system. Mechanical equipment in the refrigeration system is of a conventional type, commercially proven to render dependable service with minimum operating and maintenance expense.

While the instant invention has been described with reference to the above embodiment, it will be apparent to those skilled in the art that other storage systems employing the features of this invention can be used. For example, in FIGURE 2, is shown a schematic representation of an alternative embodiment of this invention Wherein the liquid product (n-butane) from the initial aboveground refrigerated storage facility 60, is transferred to an intermediate storage facility 61. In order to effect a desired cooling with a maximum efiiciency the product is transferred at a slow rate and refrigerated in intermediate storage vessel 61. From this refrigerating tank the cooled liquid is transferred to other flat bottom low pressure storage vessels 62, where the product is held at the low temperature and low pressure. Control equipment If desired, an adequate storage system can be employed wherein the final storage vessels are to be eliminated and there is only utilized a simple system consisting of an above-ground, refrigerated storage tank of a sufficient capacity to receive the tender of liquefied, normally gaseous product at delivery conditions of temperature and pressure and a flat bottom cone roof refrigerated tank constructed to receive the liquid at a slow rate from the initial storage facility. This tank is designed to operate at sufficiently atmospheric pressure and at sub-zero temperatures of about +30 to 50 F., depending upon the liquid being stored.

Adequate refrigeration systems are employed in conjunction with each of the storage vessels employed in the storage systems of this invention in order to maintain the desired conditions of temperature and pressure. To eifect desired economies in the storage system of this invention, product stream conditions are selected such that compression ratios in the refrigerator compressor stages do not exceed about 6 to l with compression ratios of about 4 to 1 being preferred.

It is apparent that the type of insulation which is employed in insulating the various storage vessels employed in the storage system of this invention will have a direct effect on the refrigeration required to hold the tank at the desired operating conditions. For example, in employing a 4" thickness of rigid foam polyurethane on the roof and sides of the storage tank facilities, in one installation, 7 tons of refrigeration is required to maintain a desired storage temperature, whereas for the same installation, using 3" thickness of glass fiber insulation, 23 tons per tank is required for maintaining the desired conditions.

The instant invention is employed for storage systems wherein large tenders of product, eg from a pipeline source, are received at high fill rates in excess of about 300 barrels per hour. The storage system is adapted for use in the storage of a variety of liquefied, normally gaseous products such as propane, propylene, butanes, L.P.G. mixtures, ammonia and the like. Modification in the storage facilities will be required for each system. However, such changes will be apparent to those skilled in the art to which this invention pertains. It is evident that automatic controls, indicating lights and alarms should be provided in order to increase the safety of the storage system. Refrigeration equipment, if possible, should be arranged in multiple units and sized for maximum conditions by manifolding the machines to permit interchangeable operations, and maintenance can be facilitated if one of the units is shut down. Also, standby compressor capacity and electrical generating equipment should be provided in the event of normal electrical power failure. In the event that the refrigeration equipment becomes 7 completely inoperable, the temperature rising in the insulated tanks would be quite slow. With a full tank, several days of complete breakdown would be necessary before pressure would cause tank venting through vents 54 and relief valves 55. Vapors would be flared as they come off thetank vents and such vents would produce the required amount of venting With very little product loss.

It is therefore apparent that the storage system of this invention provides storage for liquefied, normally gaseous materials at a maximum efficiency With a minimum installation and operating cost, with the cost of storage being less expensive than the storage of comparable product in mined or existing earthen caverns or full pressure storage in above-ground systems, The description and examples hereinbefore mentioned are for the purposes of illustration only. Any changes and modifications will be apparent to those skilled in the art without departing from the scope of this invention and no undue limitations in the claims should be implied because of the specific needs of the disclosures described above.

What is claimed is:

1. A storage system for the storage of an intermittently delivered tender of a liquefied, normally gaseous product, including an initial product receiving facility comprising an above-ground refrigerated storage vessel adapted to receive the complete tender of said product and store it at conditions not substantially in excess of delivery contions of temperature and pressure, a first vapor compression refrigeration system cooperating with said storage vessel and maintaining the desired temperature and pressure in said vessel, and a storage facility for retaining said product at not greater than about atmospheric pressure and at a subatmospheric temperature comprising a low pressure storage tank, conduit means for transferring said product at super-atmospheric pressure from said initial storage vessel to said storage tank, an intermediate flash vaporization vessel, means for controlling the flow of product through said conduit means to said intermediate flash vaporization vessel, means for instantaneously reducing the pressure of transferred product into said flash vaporization vessel, conduit means for transferring liquid product from said flash vaporization vessel, means for flash vaporizing said liquid product into said storage tank, and a second vapor compression refrigeration means associated with said tank for processing gaseous product effluent therefrom whereby a cooled liquid product is produced and returned to said tank and flash vaporized therein, said second refrigeration system being a two stage system.

2. A storage system in accordance with claim 1 in which means is provided for transferring vapor from said intermediate flash vaporization vessel to the second stage of said second vapor compression refrigeration means and transfer means is provided for returning the refrigerated product from said second vapor compression refrigeration means to said intermediate flash vaporization vessel.

3. In the filling of a storage system for the storage of an intermittently delivered tender of a liquefied normally gaseous product which comprises receiving an initial tender of a liquefied normally gaseous product at subatmospheric temperature and pressure, storing said tender in an above-ground refrigerated storage vessel, maintaining said product under refrigerated conditions to produce a cooled liquid product at an elevated temperature and pressure, transferring said product from said refrigerated storage vessel over an extended period of time substantially greater than the time required to receive said tender, flash vaporizing said product in an intermediate flash vaporization stage to produce a gaseous efiiuent and a liquefied normally gaseous product at conditions of subatmospheric temperature and a pressure greater than the desired storage conditions, flash vaporizing the liquid product from said intermediate flash vaporization stage into a low-pressure, low-temperaturestorage tank to produce a liquefied normally gaseous stored product at substantially atmospheric pressure and subatmospheric temperature, and a second gaseous eflluent, storing said liquefied stored product and maintaining it at said atmospheric pressure and subatrnospheric temperature, transferring the gaseous effluent from said intermediate flash vaporization stage to the second stage of a two-stage compressor, refluxing the second gaseous effluent from said storage tank to the first stage of said two-stage compressor, compressing said second gaseous effluent in said first stage to produce a compressed eflluent, admixing said compressed eflluent with the gaseous eflluent from said intermediate flash vaporization stage and compressing said admixture in said second stage to produce a second compressed eflluent, condensing said second compressed efiluent to produce a liquid product, and flash vaporizing said liquid product into said intermediate flash vaporization stage.

References Cited in the file of this patent UNITED STATES PATENTS

Claims (1)

1. A STORAGE SYSTEM FOR THE STORAGE OF AN INTERMITTENTLY DELIVERED TENDER OF A LIQUEFIED, NORMALLY GASEOUS PRODUCT, INCLUDING AN INITIAL PRODUCT RECEIVING FACILITY COMPRISING AN ABOVE-GROUND REFRIGERATED STORAGE VESSEL ADAPTED TO RECEIVE THE COMPLETE TENDER OF SAID PRODUCT AND STORE IT AT CONDITIONS NOT SUBSTANTIALLY IN EXCESS OF DELIVERY CONTIONS OF TEMPERATURE AND PRESSURE, A FIRST VAPOR COMPRESSION REFRIGERATION SYSTEM COOPERATING WITH SAID STORAGE VESSEL AND MAINTAINING THE DESIRED TEMPERATURE AND PRESSURE IN SAID VESSEL, AND A STORAGE FACILITY FOR RETAINING SAID PRODUCT AT NOT GREATER THAN ABOUT ATMOSPHERIC PRESSURE AND AT A SUBATMOSPHERIC TEMPERATURE COMPRISING A LOW PRESSURE STORAGE TANK, CONDUIT MEANS FOR TRANSFERRING SAID PRODUCT AT SUPER-ATMOSPHERIC PRESSURE FROM SAID INITIAL STORAGE VESSEL TO SAID STORAGE TANK, AN INTERMEDIATE FLASH VAPORIZATION VESSEL, MEANS FOR CONTROLLING THE FLOW OF PRODUCT THROUGH SAID CONDUIT MEANS TO SAID INTERMEDIATE FLASH VAPORIZATION VESSEL, MEANS FOR INSTANTANEOUSLY REDUCING THE PRESSURE OF TRANSFERRED PRODUCT INTO SAID FLASH VAPORIZATION VESSEL, CONDUIT MEANS FOR TRANSFERRING LIQUID PRODUCT FROM SAID FLASH VAPORIZATION VESSEL, MEANS FOR FLASH VAPORIZING SAID LIQUID PRODUCT INTO SAID STORAGE TANK, AND A SECOND VAPOR COMPRESSION REFRIGERATION MEANS ASSOCIATED WITH SAID TANK FOR PROCESSING GASEOUS PRODUCT EFFLUENT THEREFROM WHEREBY A COOLED LIQUID PRODUCT IS PRODUCED AND RETURNED TO SAID TANK AND FLASH VAPORIZED THEREIN, SAID SECOND REFRIGERATION SYSTEM BEING A TWO STAGE SYSTEM.

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