VAPOR PRESSURE MEASUREMENT APPARATUS AND METHOD

Abstract: Vapor pressure of liquid compositions is measured by a system having a cylinder member in which a reciprocal piston is disposed for movement to contain a sample of liquid and expand the chamber while measuring the pressure in the chamber until a substantially constant pressure is reached with increasing chamber volume. Liquid is circulated from a source such as a petroleum pipeline to the apparatus and continually through the expansible chamber in a retracted position of the piston. When a sample vapor pressure measurement is to be taken, the piston closes off a passage leading to the chamber and displaces liquid out of the chamber until a minimum volume is reached. The movement of the piston is then reversed to increase chamber volume while monitoring pressure and temperature to detect the onset of cavitation. Different samples of the same liquid may be taken while permitting the maximum volume of the chamber to vary with each sample so that a plot of maximum or equilibrium pressures can be obtained as a function of chamber size whereby extrapolation may be used to determine true vapor pressure of the composition.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention pertains to an apparatus for measuring the vapor pressure of various liquids, particularly complex hydrocarbon mixtures. Background It is often necessary to determine the vapor pressure of a liquid or liquid mixture to assist in controlling transport conditions of the liquid or certain process conditions involving the liquid or its vapor. In many crude oil pipelining operations, it is desirable and often necessary to transport other liquids mixed with the crude oil, such as, natural gasoline liquids. Accordingly, a relatively complex liquid mixture is formed under such transport conditions and the vapor pressure of such a mixture cannot be readily predicted but can only be determined by frequent measurements of the fluid being transported through the pipeline. Moreover, the true vapor pressure of a complex liquid mixture such as a mixture of crude oil and natural gasoline liquids may actually be a changing value since the vapor pressure of the so called lighter ends of the mixture will be lower at a given temperature than the heavier or more dense hydrocarbon materials. Accordingly, it has been deemed desirable to provide an apparatus which can readily measure at least the cavitation pressure of a complex liquid mixture so that pumping operations and the like may be controlled to prevent cavitation and leakage of vapors from the transport network. On the other hand, knowledge of the true vapour pressure is important for certain processing or separating operations. Apparatus is known from EP-A-0 216 706 for making measurements of vapour pressure having a housing including a chamber conduit means for conducting liquid to and from the chamber, chamber volume adjustment means movable relatively to said chamber to a first position to permit flow of liquid to and from the chamber by way of the conduit means and to a second position to reduce the volume of the chamber to a predetermined minimum and an actuator for moving the volume adjusting means from the first to the second position and back towards the first position. This known apparatus is not suited for making vapour pressure measurements of fluid flowing in a pipe line and it is accordingly one object of the present invention to provide such apparatus which enables vapour pressure measurement in those circumstances. A further object is to provide apparatus of the kind set forth which facilitates vapour pressure measurements at the temperature of the liquid flowing to the apparatus. The present invention consists in apparatus for making measurements of vapour pressure of a liquid composition having a housing including a chamber, conduit means for conducting said liquid composition to and from the chamber, chamber volume adjustment means movable relatively to said chamber to a first position to permit flow of said liquid composition to and from the chamber such that the liquid composition flows through the chamber by way of the conduit means and to a second position to reduce the volume of the chamber to a predetermined minimum and thereby to entrap a sample of the flowing liquid composition in the chamber and an actuator for moving the volume adjusting means from the first to the second position and back towards the first position, characterised in that said chamber comprises a cylinder bore in said housing, the volume adjusting means comprises a piston in the bore movable by said actuator from the first to the second position and back towards the first position and the conduit means comprise inlet conduit means for placing the bore in communication with a source of said liquid composition and outlet conduit means in communication with the bore for discharging liquid composition from said chamber, and there are provided valve means in said outlet conduit means to permit flow of liquid composition out of and prevent flow of liquid composition into said bore, pressure sensing means for measuring the pressure in said bore during movement of the piston to expand the volume of the bore with a sample of liquid composition in the bore and a passage which is open over part of the piston stroke from the second position towards the first position thereof to circulate liquid composition from the inlet conduit means (19), around the bore and out of the housing (34) to stabilise the temperature of the apparatus at a temperature related to the temperature of the liquid composition flowing to the apparatus. Advantageously, control means are provided for sensing the pressure in the chamber and for controlling the movement of said piston to increase the volume of the chamber until the pressure of fluid in said chamber is reduced to a constant value with increasing volume of the chamber. The invention further consists in the method of operating the apparatus of the invention to determine the vapour pressure of a liquid composition which comprises supplying said liquid composition to the chamber, moving the piston from the first to the second position thereof thereby to entrap a sample of liquid composition in the chamber, and moving the piston from the second towards the first position while monitoring the pressure in the chamber to determine when the pressure remains substantially constant with increasing volume of the chamber and recording the substantially constant value of pressure, characterised by the step of circulating liquid composition through the apparatus to maintain the temperature of the chamber at a substantially constant value during measurement of pressure in the chamber. Preferably, this method includes the step of replacing the liquid composition sample in the chamber with a second liquid composition sample and expanding the chamber to a volume different from the volume during the first expansion, measuring the pressure in the chamber at a predetermined time after arresting the second expansion of the chamber, and comparing the final pressure in the chamber after a predetermined time during the first expansion and the second expansion and extrapolating a curve defining the final pressure as a function of the size of the chamber to determine a value of vapour pressure of the liquid composition. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a combined side elevation and schematic diagram of the vapour pressure measurement apparatus and system of the present invention; FIGURE 2 is a central section view of the vapour pressure measurement apparatus in somewhat schematic form; FIGURE 3 is a diagram illustrating the total pressure versus time of a fluid sample in the chamber of the measurement apparatus during a cavitation pressure and vapour pressure measurement cycle; and FIGURE 4 is a diagram illustrating the expected value of true vapour pressure based on measuring a selected sample at various chamber volumes. DESCRIPTION OF A PREFERRED EMBODIMENT In the description which follows like parts are marked throughout the specification and drawing with the same reference numerals, respectively. The drawing figures are not necessarily to scale and certain features of the invention may be shown in somewhat schematic form in the interest of clarity and conciseness. Referring to Figure 1, there is illustrated a system in accordance with the present invention for measuring the vapor pressure of liquid compositions and mixtures. The system includes apparatus, generally designated by the numeral 10, which is particularly adapted for application to measure vapor pressures in petroleum pipelines, chemical process systems, and the like. For example, there is illustrated a crude oil pipeline 12 which may be a branch line off of a main pipeline system or comprise a main transmission line itself having suitable fittings 14 and 16 for the connection of conduits 18 and 20 leading to and from the apparatus 10. The apparatus 10 is comprises a closeable housing 22 having a transverse upper flange 24 and a removable cover 26. The housing 22 is suitably mounted on a skid 28 for movement to selected test sites, as needed. A support bracket 30 is provided for supporting an enclosure 32 which is adapted to house control circuitry for the apparatus 10 and which is shown in somewhat schematic form in Figure 1. The apparatus 10 includes a cylinder housing 34 disposed within the housing 22 and including means defining a bore to be described in further detail herein in which a piston is reciprocal and is connected to a piston rod 36 extending from the cylinder housing. The piston rod 36 is connected to a linear actuator 38 driven by a reversible motor 40 for moving the abovementioned piston to increase or decrease the size of the chamber volume as will also be described in further detail herein. The control system for the apparatus 10 includes a signal conditioner unit 50 for receiving an electrical signal from a pressure sensing transducer, not shown in Figure 1, and a measurement and control A/D converter 52 for receiving a conditioned signal from the unit 50 and also for receiving plural electrical signals from a temperature transducer 54 in the conduit 18, a transducer, not shown in Figure 1, located within the housing 22 and to be described herein and a suitable transducer, not shown, located within the housing 22. A digital computer 56 is adapted to receive signals from the A/D converter 52 for controlling the operation of the apparatus 10, and an operational program disk drive 58 and a data acquisition disk drive 60 are operably connected to the computer 56. The computer 56 and disk drives 58 and 60 are adapted for use in modifying operating characteristics of the system and could be replaced by a suitable microprocessor, now shown. The A/D converter 52 is also operably connected to a digital signal conversion unit 62 which is adapted to effect operation of a control unit 64 for the motor 40. A D/A converter 66 receives signals from the converter 52 and provides suitable pressure and temperature displays through display units 72, 74, 76, and 78. A 120v a.c. power source, not shown, is connected to and converted to dc power through a suitable power supply unit 79 for operation of the components described above. The housing 22 also includes a suitable electrical heater 82 disposed therein and controlled by a control unit 84 for maintaining a predetermined temperature of the interior of the housing 22. The signal conductors extending between the housing 22 and the enclosure 32 are preferably routed through a separable multiple pin connector or the like, generally designated by the numeral 90 and having a socket portion 92, Figure 2, extending through the bulkhead formed by the cover 26. The connector 90 may be of a type commercially available. Commercially available devices corresponding to certain ones of the elements described hereinabove are as follows: signal conditioner unit 50 Model 4428, Endevco Corp. A/D converter 52 Micromac 4000™, Analog Devices, Inc., Norwood, MA. conversion unit 62 Micromac 4020™, Analog Devices, Inc., Norwood, MA. converter 66 Micromac 4030™, Analog Devices, Inc., Norwood, MA. actuator 38 Model 15D electric cylinder Industrial Devices, Novato, CA controller 64 Model AC2001 el Industrial Devices controller 84 Model 808, Eurotherm Corp. Reston, VA. Referring now to Figure 2, certain structural features of the cylinder housing 34 are illustrated in detail. The cylinder housing 34 is suitably supported within the interior space 23 of the housing 22 on a depending support member 25 which is secured to the cover 26 so that the assembly of the cylinder housing 34 and the actuator 38 may be removed from the housing 22. The housing 22 is provided as a protective covering for the apparatus 10 and to minimize the chance of escape of volatile vapors into the atmosphere when the apparatus 10 is utilized to measure the vapor pressure of potentially hazardous substances. The cylinder housing 34 preferably comprises a removable insert part 102 which is mounted within the housing 34 and disposed on a transverse shoulder 104. The insert part 102 includes a cylindrical bore 106 in which a piston 108 is slidably disposed and defines a variable volume chamber 110. The insert part 102 includes a transverse head surface 112 on which a head part 114 is removably bolted in sealed relationship thereto. A portion of the chamber volume 110 may be formed by a cavity 116 in the head part 114. Locating pins 118 are provided for locating the head part 114 on the insert part 102 so that side walls defining the chamber 110 do not have any discontinuities which would provide an area for trapping a vapor bubble. An outlet port 122 is formed in the head part 114 and opens into a chamber 124 in which a spring biased check valve 126 is disposed. A removable cover part 128 retains the check valve 126 and a biasing spring 180 within the chamber 124. A conduit 130 is connected to the cover part 128 and is connected to a further conduit 132 in communication with the return conduit 20 extending to the cover 26. The head part 114 includes suitable cavity for receiving and retaining a pressure transducer 134 of a type commercially available such as a model 8530-50 manufactured by Endevco Corp. of San Juan Capistrano, CA. A temperature sensor 136 is also disposed on the head part 114 for sensing the temperature in the chamber 110. The temperature sensor 136 may also be of a type commercially available such as a type RTD Model PR12 manufactured by omega Engineering of Stamford, CT. Referring further to Figure 2, the cylinder housing 34 and the insert part 102 define therebetween an annular passage 138 which is in communication with the bore 106 by way of suitable passages 140 and 142 formed in the insert part. An annular groove 144 is formed in the piston 108 between the piston head 111 and an integral collar 145 on the rod 36. The conduit 18, for conducting fluid from the pipeline 12, is in communication with the passage 138 by way of a passage 146 formed in the cylinder housing 34 and by way of an intermediate conduit 19. A bypass chamber 150 is formed in the cylinder housing 34 between the shoulder 104 and a removable lower head member 152. Return conduits 154 and 156 are in communication with the passage and chambers 138 and 150, respectively, and are connected to the conduits 130, 132 by way of a minimum pressure or pressure relief valve 160. The pressure relief valve 160 is set to allow pressure fluid to flow therethrough but at a pressure nominally greater than the pressure at which the valve 126 will allow fluid to flow through the chamber 110 to the return conduit 130. Referring still further to Figure 2, the piston rod 36 is suitably coupled to an actuating rod 170 of the actuator 38 for linear reciprocation of the piston 108 within the bore 106. Position limit switches 172 and 174 are adapted to be operably engaged by a cam 176 on the rod 170 for providing control signals to operate the motor 40 to extend and retract the piston 108 within the bore 106. For example, upon engaging the switch 174 the motor 40 is typically operated to stop extension of the rod 170 and, upon a suitable signal from the control unit 64, the rod 170 may be retracted to move the piston 108 to increase the volume of the chamber 110. A lower limit position of the rod 170 is achieved when the cam 176 engages switch 172. The apparatus 10 may be operated to determine the cavitation pressure and the vapor pressure of liquid flowing through the pipeline 12, for example, at will, and such vapor pressure measurements are typically carried out at selected time intervals to continuously monitor such pressures of the fluid flowing through the pipeline. A small pressure boost pump, not shown, may be required to be interposed in one of the conduits 18 or 20 to provide for flow of fluid through the apparatus 10. The apparatus 10 is operable to determine the minimum or so called cavitation pressure at which vapor will form as well as what may be considered the true vapor pressure which, for a complex liquid mixture, is normally somewhat greater than the cavitation pressure. Referring to Figure 2, in the operation of the apparatus 10 to make a vapor pressure measurement, the piston 108 is normally in a retracted position so that the piston head 111 is disposed below the passages 140 and 142. With fluid being supplied to the conduit 19, it will flow continuously through the chamber 110, across the valve 126 and through the conduit 130, 132 to return to the pipeline 12. By continuously flowing fluid through the chamber 110, the temperature of the apparatus 10, including the cylinder housing 34 and associated parts, is stabilized at the temperature of the fluid from the pipeline 12. When a sample vapor pressure measurement is to be taken, the piston 108 is moved to reduce the volume of the chamber 110 and to momentarily cut off flow of liquid through the conduit 19 as liquid is pushed out of the chamber 110 through the valve 126. When the piston 108 reaches its maximum extended position, corresponding to a minimum volume of chamber 110, groove 144 is placed in communication with the passages 140 and 142 and flow through the chamber formed by the groove 144 provides for the apparatus to maintain a continuous flow of fluid, to minimize the formation of vapor bubbles within the apparatus 10 and to stabilize the temperature of the apparatus. Leakage flow past the piston portion extending between the passages 140, 142 and the chamber 150 maintains this chamber flushed and full of liquid and which flow exits the housing 34 through the conduit 156. This flow also assists in stabilizing the temperature of the components of the apparatus. The piston 108 is retracted to increase the volume of the chamber 110, without uncovering passages 140 and 142, while pressure measurements are taken as the volume of the chamber 110 increases until the chamber pressure decreases to a relatively constant value with increasing chamber volume, thereby indicating the minimum so called vapor pressure or cavitation pressure of the liquid being analyzed. At this point, the volume of the chamber 110 is held constant for, a predetermined period of time and the pressure in the chamber is monitored by the control system to record the characteristic change in pressure with time. The piston 108, in the maximum retracted position during a vapor pressure measurement, must not, of course, retract far enough for the face or head surface 111 to place the passages 140 or 142 in communication with the chamber 110. When a suitable pressure characteristic has been recorded, the piston 108 is then retracted to open the passages 140 and 142 into the chamber 110 to flush the vapor out of the chamber and resume the continuous circulation of liquid through the chamber and back to the pipeline 12. The piston 110 may be maintained in the maximum retracted position until a new vapor pressure measurement is to be taken. Thanks to the configuration of the piston 108 and the bore 106, there is little likelihood of formation of unwanted vapor bubbles in the chamber 110 except through expansion of the chamber volume to create a bubble due to lowering the pressure to the cavitation and/or true vapor pressure of the liquid mixture being measured. A seal is maintained between the piston 108 and the cylinder wall defining the bore 106 by a very close tolerance fit between the piston and the insert part 102, preferably by utilizing diesel engine fuel injection pump manufacturing techniques and materials. Moreover, the configuration of the chamber 110, which does not have any discontinuous surfaces or grooves, also reduces the likelihood of formation of unwanted vapor bubbles. During the measurement of cavitation and/or vapor pressure, the temperature in the chamber 110 is continuously monitored by the temperature sensor 136 and this temperature is recorded for each vapor pressure measurement. The controller 84 is operable to cause the heater 82 to maintain either a preset temperature in the space 23 or to maintain the temperature sensed by the sensor 54, whichever is desired. Referring now to Figure 3, there is illustrated a diagram of the pressure sensed in the chamber 110 during a vapor pressure test of a sample of crude oil in which various natural gasoline liquids have been blended. One pressure-time trace 189 includes a steadystate portion 190 which is the pressure sensed in the chamber 110 when the piston 108 is retracted to permit flow of fluid continuously through the chamber from the inlet conduit 19 through the valve 126 to the conduit 130, 132. During this mode of operation, substantially all of the fluid flowing through the cylinder housing 34 will pass through the chamber 110 since the pressure relief setting of the valve 160 is higher than that of the valve 126. When a vapor pressure test is commenced, the piston 108 is extended to decrease the volume of chamber 110 and cut off the flow of fluid into the chamber from passage 140. When the piston 108 reaches a position corresponding to a predetermined minimum volume of chamber 110 the piston is retracted to increase the volume of the chamber 110 until, with increasing volume, there is essentially no change in sensed pressure in the chamber. This condition is represented by the segment 192 of the pressure-time trace 189. After a predetermined time at which there is essentially no change in the pressure sensed in the chamber 110, the computer 56, through the signal converters 60 and 62 causes the controller 64 to effect operation of motor 40 to arrest movement of the piston 108 and the chamber volume is maintained at a constant value. As indicated by the segment 193 of the pressure-time trace, with no change in volume of the chamber 110 there will be a gradual rise in pressure, particularly for liquids which are complex mixtures of more than one liquid or have dissolved gases formed therein. The pressure indicated by the trace segment 193 may be read at a predetermined time as the vapor pressure, which pressure will rise until an equilibrium condition is reached. The pressure-time trace 194 illustrated in Figure 3 represents a further test utilizing the apparatus 10 wherein a fluid sample is tested in the apparatus by expanding the volume of the chamber 110 until the cavitation pressure is reached as indicated by the trace segment 196 at which time the volume of the chamber is maintained constant while the pressure increase in the chamber 110 is observed as indicated by the trace segment 198. The chamber 110 is then flushed and a new sample of the same fluid is trapped and the chamber volume expanded as indicated by the trace segment 200 until the cavitation pressure is reached at point 202. Under this second test, however, the volume of the volume chamber 110 is not expanded as much and the pressure rise indicated by the trace segment 204 is to a higher final pressure at point 205. Referring to Figure 4, there is indicated a comparison of the final equilibrium pressure for a given time period versus the chamber or bubble size. With larger chamber volume for chamber 110 corresponding to a larger vapor bubble volume, the final equilibrium pressure is indicated to be lower for a particular fluid. Accordingly, by plotting a series of final pressures at different chamber or vapor bubble sizes, a trace or curve 210 may be developed which may be extrapolated to indicate the so-called true vapor pressure of a particular liquid mixture at zero bubble size such as indicated at point 212. Moreover, it has been observed that with complex liquids such as crude petroleum, when a liquid sample expanded in a closed chamber until the cavitation pressure is reached and then the sample is allowed to reach an equilibrium condition that it may require a substantial period of time before a final equilibrium or vapor pressure is reached. However, with the present process two or three liquid samples, which may be taken over a period of a few minutes according to the process described herein, and wherein the equilibrium pressures at various chamber volumes are recorded, may be utilized to extrapolate substantially true vapor pressure. In any case, the minimum pressure which can be tolerated in a system, as indicated by the cavitation or so called bubble point pressure, is readily determined with the method and apparatus of the present invention.

Claims: An apparatus (10) for making measurements of vapour pressure of a liquid composition having a housing (34) including a chamber (110), conduit means (19,130,140,146) for conducting said liquid composition to and from the chamber, chamber volume adjustment means (108,36) movable relatively to said chamber to a first position to permit flow of said liquid composition to and from the chamber such that the liquid composition flows through the chamber by way of the conduit means and to a second position to reduce the volume of the chamber to a predetermined minimum and thereby to entrap a sample of the flowing liquid composition in the chamber and an actuator (38) for moving the volume adjusting means from the first to the second position and back towards the first position, characterised in that said chamber comprises a cylinder bore (110) in said housing, the volume adjusting means comprises a piston (108) in the bore movable by said actuator (38) from the first to the second position and back towards the first position and the conduit means comprise inlet conduit means (19) for placing the bore in communication with a source of said liquid composition and outlet conduit means (130) in communication with the bore for discharging liquid composition from said chamber, and there are provided valve means (126) in said outlet conduit means to permit flow of liquid composition out of and prevent flow of liquid composition into said bore, pressure sensing means (134) for measuring the pressure in said bore during movement of the piston to expand the volume of the bore with a sample of liquid composition in the bore and a passage (138,146) which is open over part of the piston stroke from the second position towards the first position thereof to circulate liquid composition from the inlet conduit means (19) around the bore and out of the housing (34) to stabilise the temperature of the apparatus at a temperature related to the temperature of the liquid composition flowing to the apparatus. The apparatus set forth in Claim 1, characterised in that the passage (138,146) is formed in the housing. The apparatus set forth in Claim 2, characterised in that there is provided a further outlet conduit (154) which connects with said passage for conducting liquid composition out of the housing when said passage (138,146) is open. The apparatus set forth in Claim 1, characterised in that the inlet conduit means (19) connect with a further passage (140) in the housing which opens into the bore at a location such that in the movement of the piston from the first to the second position thereof flow of liquid compositoin to the bore at the side of the piston in the direction of movement thereof is cut off. The apparatus set forth in any preceding claim, characterised in that control means (134,40,172,174,176,178) are provided for sensing the pressure in the chamber and for controlling the movement of said piston to increase the volume of the chamber until the pressure in said bore is reduced to a constant value with increasing volume of the bore. The method of operating apparatus according to Claim 1 to determine the vapour pressure of a liquid composition which comprises supplying said liquid composition to the chamber (110), of moving the piston from the first to the second position thereof thereby to entrap a sample of liquid composition in the chamber, and moving the piston from the second towards the first position while monitoring the pressure in the chamber to determine when the pressure remains substantially constant with increasing volume of the chamber and recording the substantially constant value of pressure, characterised by the step of circulating liquid composition through the apparatus to maintain the temperature of the chamber at a substantially constant value during measurement of pressure in the chamber. The method set forth in Claim 6, characterised by replacing the liquid composition sample in the chamber with a second liquid composition sample and expanding the chamber to a volume different from the volume during the first expansion, measuring the pressure in the chamber at a predetermined time after arresting the second expansion of the chamber, and comparing the final pressure in the chamber after a predetermined time during the first expansion and the second expansion and extrapolating a curve defining the final pressure as a function of the size of the chamber to determine a value of vapour pressure of the liquid composition.

Inventors: REED, DONALD B.
Applicants: ATLANTIC RICHFIELD COMPANY
Application year: 1987
Main classification: G01N7
IPC codes: G01N7