Faculty of EngineeringDepartment of Chemical EngineeringCape Town Campus

PROCESS CONTROL III (PRC300S)Assignment 1Name & SurnameDirk Myburgh

Student Number212282247

Lecturer: M AzizI certify that this is my own unaided work, except for the assistance received from the teaching staff. I undertake not to pass this assignment onto any other student.

Signature (student):. Date:15/08/2014Assessment Criteria%Mark

Introduction (Summary & Problem Statement)10

Report Structure and Presentation15

Theory & Literature Review

15

Problem solving and Practicality40

Conclusion and Recommendation10

Referencing10

TOTAL100

IntroductionProblem StatementTemperature is a very important and commonly measured parameter used to monitor and control processes, the accuracy of those measurements are of great importance. Within the Oil and Gas industry, temperature is used as a means of calculating the exact amount of liquid in a storage tank. When calculating the net standard volumes of these liquids in a tank, the temperature measurements needs to be very accurate since a temperature change of 1C can cause a change in volume of 0.1%, therefore accurate temperature measurement is vital.Theory & Literature ReviewSince the volume of petroleum or other hydrocarbons vary with fluctuating temperatures it is necessary to know the temperature of the liquid when petroleum or other hydrocarbons are transferred between tanks. Primary measurement data is obtained at storage conditions, and therefore differ depending on the specific process conditions. This poses the problem of knowing exactly how much petroleum is being transferred. An international standard for calculating the net standard volume has been developed in order to solve this problem, and requires temperature and pressure to be at standard conditions, which are 15C and 101.325 kPa, respectively. The primary or process conditions data is corrected by using correction factors, therefore precise temperature measurements are needed for calculations. The formula used to calculate the net standard volume of petroleum is as follows: , where GSV is the Gross Standard Volume and CSW is the correction for sediment and water (Hyne, 1991).The Gross Standard Volume is calculated using the indicated volume of the tank, corrected for the effect of temperature on the liquid density (CTL), as well as the indicated pressure, corrected for the effect of the compressibility on the liquid density (Hyne, 1991).The Correction for Sediment and Water (CSW) corrects the volume for non-merchantable components such as sediment and water. It is expressed as follows: (Hyne, 1991).

Different types of temperature measurement equipment.There are many temperature measurement devices for many different applications, for the measurement of liquid temperature the following are important. Thermometers (not considered) Glass tube Thermometers Bi-metal Thermometers Non-Contact Devices (not considered) Single reading devices Camera field devices Probes(considered for problem solution) Thermopiles Thermocouples Resistance element types Thermistors Resistance Temperature Detectors(Eckersdorf et al. 2001)The two measuring devices considered for the solution are Thermocouples and RTDsThermocouples

Thermocouples work on the basis that two dissimilar metals joined together will generate a predictable voltage that relates to a temperature difference between the junction end and the tail end. When these ends are maintained at different temperatures a thermoelectric electro-magnetic-field (EMF) is generated within the thermocouple. The magnitude of the EMF is proportional and is related to the temperature difference between the two ends. The generated EMF is measured by a millivolt meter or a potentiometer that is built into the circuit to determine the temperature (Eckersdorf et al. 2001).Resistance Temperature DevicesThe RTD is a temperature sensor that works on the principle that a materials electrical resistance changes with a change in temperature. This relationship between electrical resistance and temperature is predictable, therefore allowing for accurate and consistent temperature measurement. The TRD is supplied with a constant current and the resulting voltage drop, caused by a change in temperature, can be measured. Once the voltage drop is known the corresponding temperature can be determined. Materials used in RTDs include platinum, nickel and copper, platinum being the preferred material (Eckersdorf et al. 2001). There are various wiring constructions available for RTDs depending on the system requirements. The constructions include 2-wire, 3-wire and a 4-wire configuration. The 2- wire configuration: The measured resistance Rt = R1 + R2 + Rb. This configuration is least accurate since there is no way to eliminate the resistance of the lead wire used.

The 3-wire configuration is the most commonly used since it provides a means of removing the resistance of the lead wire used. The resistance is calculated as follows: (R1 + R2) - (R2 + Rb) = Rb

These devices have resistance coefficients which is the ratio of the resistance change per 1C change in temperature over a range of 0-100C. The ratio is dependant of the type and purity of the material of construction. A positive temperature ratio indicates that the resistance of the device will increase with an increase in temperature. It is this increase in resistance that is measured and correlated to a specific temperature (Liptak, 2006). Resistance Temperature Devices Characteristics Commonly used in accuracy and repetitive applications. An inexpensive wire can be used to connect the RTD to the measurement and control equipment. RTDs can cost more than thermocouples in the same application, and are more sensitive to shock and vibrations and are only effective to a lower temperature than that of a thermocouple (Liptak, 2006).Thermocouple Characteristics Thermocouples can be used up to very high temperatures, they are less expensive that RTDs and less susceptible to shock and vibrations. They are more responsive to immediate temperature changes. The thermocouple needs to be connected to the temperature measuring and control equipment with a wire of similar composition (Liptak, 2006).Both these temperature measuring devices can be connected to a transmitter instead of extension wires. This option reduces installation and maintenance costs, and increases the setups robustness. Problem SolutionAn averaging RTD can be used to measure the temperature average temperature of a liquid in a storage tank. This ensures the most accurate temperature measurement of the liquid as a whole. The measuring sensor is distributed along the entire length of the probe and therefore can give and accurate average temperature measurement. In order to ensure accuracy, multiple probes with varying sensor lengths are fixed in the tank, the sensor that is most submerged is then selected in order to negate the temperature of the empty/vapour space above the liquid level. Diagram depicting the proposed setup of an averaging Resistance Temperature Device in Storage Tank.

ConclusionAlthough there are many different types of temperature measurement devices and setups available to engineers, the chosen setups has been selected on the bases of the following: the setup is easy, less expensive and robust in total compared to that of a thermocouple setup. The proposed setup also allows for easier maintenance of the equipment and gives an accurate and reliable temperature measurement. The solution that is proposed is not limited to a stagnant body of liquid. With slight modifications the setup can be implemented in a continuous stirred storage tank, which will allow for a uniform temperature profile as well as prevent stratification.

References 1. Bentley, R.E. 1998. Resistance and Liquid-in-Glass Thermometry. Singapore: Springer-Verlag Singapore.

2. Eckersdorf, K., Kucharski, J., Mcghee, J. & Michalski, L. 2001. Temperature Measurement Second Edition. England: Wiley.

3. Hyne,J. 1991. Dictionary of Petroleum Exploration, Drilling & Production. USA: PennWell.

4. Liptah, B.G., 2006. Instrument Engineers Handbook Process Control and Optimization 4th edition. USA: CRC Press.

0-100deg

mV10-50