2.1.3 Gate Valve with Pneumatic Membrane Actuator
Pneumatic gate valve, also known as air control valve operates by converting the energy from compressed air into mechanical motion. The motion can be either rotatory or linear, depending on the type of the actuator. There are some type of the pneumatic actuators such as tie rod cylinders, rotatory actuators, grippers, vacuum generators and others. It is one of an array component that used in control the pressure, rate and volume of air that move through he pneumatic system. There are various type of pneumatic valves can be found in the heart of the device according to a given type of the pneumatic used in the pneumatic system. For example, functional control valve that used to regulate the direction of flow or inhibit the flow is a class of pneumatic valves that house multiple variants. It is the fundamental components of any pneumatic system. Therefore, it is important to select the best pneumatic control valve will help to optimise the regulation of system pressure, flowrate and flow direction.
2.2 Pressure Relief valve
Pressure relief valve act as a safety valve that used in control the pressure in a process system, the pressure may be produced when there is a process, upset, equipment failure or fire. Therefore, the pressure will be released by allow pressurised fluid low from auxiliary outlet out of the system. The purpose of the relief valve is to protect the pressure vessels and other equipment from suffered condition of pressure that may exceed their design limit. When the pressure limit is exceeded, the relief valve will be forced to open and part of the fluid will passes out through the auxiliary path. The diverted fluid will be will be directed to flare header, where it will burn and produced combustion gases and release to atmosphere. Pressure relief valve control the pressure at the inlet port of an equipment by releasing the pressure to the atmosphere. In other cases, a bypass valve may also act as a relief valve by being used to return the discharged fluid by pump or gas compressor. This is to protect the pump or gas compressor.
2.3 Petrochemical Industry
Petrochemical industry that making products from chemicals that mainly derived from petroleum or natural gas. Crude oil or petroleum fresh out from the ground. Hundreds years ago, Native Americans used crude oil for fuel and medicine. However, the oil industry is actually start since 1859. In that year, Edwin L. Drake drilled a well near Titusville, Pennsylvania. By 1860s, wooden derricks covered the hills of western Pennsylvania. In 1865, the first successful oil pipeline is built from oil field near Titusville. This industry is concerned with the production of the petrochemicals. It is a direct linked industry with oil and gas industry, where it is known as the downstream sector of the oil and gas industry. Large scale of production of methanol, benzene, ethylene, toluene and propylene. Ethylene and propylene also known as olefins, which is a class of unsaturated aliphatic hydrocarbons with formula CnH2n. Benzene and toluene commonly known as aromatics, where they are unsaturated cyclic hydrocarbons with more than one benzene rings. The petrochemical industry will giving the globalization and integration impacts to the world economy.
2.4 Benzene
Benzene was first isolated in 1825 by Michael Faraday from an oil film that deposited from the gas that used for lightning. Several experiments have been done by Faraday and identified that there are same number of carbons and hydrogens in that compounds, therefore, it is named as “carbureted hydrogen”. Another chemist, Laurent, due to the ability of the compound to illuminating gas, it should be named as pheno. Nine years later, France and England have name it benzene instead of benzin, which come from Mitcherlish, another chemist that produced the same compound by heating the chemical that isolated from gum benzoin. Benzene is the simplest organic aromatic compounds and parent compounds for other aromatic compounds. The structure of benzene is hexagonal with C-C bonds of same distance and its stability is contributed by the P-orbitals that overlap to form delocalised molecular orbital that extended around the ring, decreased its reactivity as shown in Figure below. Benzene is a colourless liquid and odour. It has a boiling point of 80.1°C and melting point of 5.5°C. In addition. It will dissolves readily in organic solvent, but only slightly soluble in water. It is also carcinogenic as when it exposed to human body, it may cause leukemia. Therefore, benzene emissions has been strictly controlled.
2.5 Benzene Distillation
Benzene-Toluene Fractionation is a separation technique that used to produce pure benzene via benzene distillation. In past, multivariable controllers have been applied extensively in benzene production plants. These controllers have increased process efficiency and plant capacity. Traditionally, benzene distillation process model is derived by open-loop perturbance of the process. This is a step test and it is one of the part in the control project implementation. Furthermore, due to the cost cutting in the process industry, there are major reduction number of engineering staffs, causing in-house step-testing is no longer available. Benzene-Toluene fractionator is the major equipment in aromatic extraction unit. The benzene will be separated from distillation column as it is the light component compare to heavy component as toluene. The relative volatility between them are approximately 2.5, which means that a moderate feed to reflux ratio is needed. However, due to the high cost of control techniques, the control project has not been applied widely in the benzene distillation industry. In the process, temperature and the flowrate of the reboiler steam always need to be maintained at their setpoint in order to achieved desired benzene purity and recovery.
CHAPTER 3
ANALYSIS AND DISCUSSION
3.1 Final Control Element used in Petrochemical Industry
Final control elements are the equipment that used in regulate and control the flow of the process. Therefore, valves are among the most commonly used final control elements in petrochemical industry.
3.2 Historical Usage and Advanced Technologies of Valves
During the Greek and Roman time, the development of valve start with uses of plug cock valve and the design remain unchanged until 19th century. During the 19th century, some eminent engineers has focused on design of valves, notably Timothy Hackworth, who introduced the adjustable springs for replacement of weights to the steam safety valve. Hence, the valve is now well preserved in Science Museum of London. In the past 60 years, many type of valves are invented to cope the new and hazardous process developed over the years. The market nowadays showing that nearly 85% of the control valves applied in the industry were developed in the 1960s.
Figure : Plug cock valve.
Hence, the traditional valve such gate valve and globe valve have nowadays improved to new design of valve such as lubricated taper plug, ball and butterfly valves. On of these is Control-Disk is manufactured by Fisher Controls by adding of control profiles with on the basic of metal-seated and double-eccentric butterfly valve, which giving the valve equal percentage of inherent flow properties. Another invention happens focused around rubber-lined valves where it is pressure resistant and not particularly sensitive to impurities that may remained between slide and sealing surface.
Figure : Rubber lined gate valve.
3.3 Type of Gate Valves
Gate valves open and close the flow of process in the pipeline, hence, they are known as the family of the on/off valves. Gate valve is designed to be used as an isolation valve. When they are under operation, they are usually in a fully closed or fully open position. Therefore, when it is opened, it allows the fluid to flow in a straight line through at the least resistance. Instead, if it is fully closed, it will not allow any fluid to pass through. However, gate valve cannot be being used to adjust or regulate the flow due to its fully closed or fully opened limitation. In addition, if the gate valve is partially opened, this may result in corrosion to the valve discs and seating surface. Besides, the vibration that occurred due to the high fluid flowrate on the discs may cause the discs to be chattered. Therefore, a special design gate valve is used for low velocity throttling. For example, guillotine gate valve is used for pulp stock process. In benzene distillation, there are gate valves with electrical, manual and pneumatic actuator involved, including of other type of valves such as pressure relief valve.
For the operating principle of gate valve, gate valve is usually used gas pipelines in control the fluid flow. When the handle wheel is rotated in clockwise direction, the stem and gate will move downward and start to move across the gas flow and hold tightly between two seats. Thus, the gate will help in prevent the flow of gas and seal the pipeline from any gas leakage through the vales as the valve is completely closed. So, when the valve is closed, the gate surface will seal by only depend on the medium pressure to pressure the sealing surface of the gate plate to the other side of the valve seat to ensure sealing surface, known as self-sealing. However, in most cases, the valve will be forced to close by external forces to press the gate to its seat to ensure sealing surface.
Figure : Schematic diagram when closing of a gate valve.
However, when the wheel is rotated in anti-clockwise direction, the stem and gate will move in upward direction across the gas flow to open the valve and hence, allow the gas to flow through the valve. Hence, when the valve is fully opened, there are no resistance of gas flow so the gas can flow freely across the valve.
The main advantages of gate valve include the gas resistance in the gate valve is typically low compared to other type of valve as the valve body of the gate valve is a straight type and causing the gas flow not to change direction. This is because when there is blockage at the pathway of gas flow, it will cause to reduce of gas flowrate and hence increase the gas pressure. By maintain the gas to flow in straight and smooth direction can minimise the resistance of gas flow. Besides, gate valve is also a valve that has dual flow directions. Since it is not subject to the flow directions of the medium, it is suitable for use in pipeline where the medium may change the flow direction. In addition, it also has wide range of application as beside of its application in oil and steam media, it also available in medium that contain granular solid and with a large viscosity as it can either operate at fully open or fully closed condition, hence the condition of the medium will not cause harm to the discs of the valve.
3.3.1 Gate Valves with Electric Actuator
the electric actuator in gate valve uses an electric motor to provide force and torque to operate the gate valve. It converts the electrical energy into mechanical torque. It is quiet, non-toxic and energy efficient as it does not require any energy to operate the valve. But, there must be an electric source available in order for the actuator to function. If not, it must be also operated by batteries. Also, there is a break applied in the valve actuator to prevent the media from opening the valve. This is because when there is no brake installed in the actuator, the actuator will uncover the opened valve and rotate it back to closed position. If this continue to happen, the motor and actuator will eventually damage. However, it is also known as one of the most readily available forms of actuator because it does not involve oil or other fossil fuels.
In benzene distillation, gate valve with electric actuator is commonly used as it has shortest response time and greater stiffness which cannot being done by other valve actuator. It has also being proved that can increase the revenue of petrochemical company as it able to prevent eliminatation the unnecessary benzene emission to surrounding. By eliminate the requirement of vent natural gas (benzene) to the environment, this make the electric actuator valve a very efficient FCE in benzene distillation process and environmental friendly. This valve enhances its performance by using the servo motor and roller screw technology to achieve its fast and accurate performance.
Figure : Gate valve with electric actuator and its symbol.
The advantages of gate valves with electrical actuator is that it offers the highest precision of control positioning. An example of range of accuracy is ±0.000315in. and repeatability of about 0.0000394in. Also, the electric actuators can be network and programmed easily and they offer immediate feedback of diagnostics and maintenance. At the mean-time, the also provide complete control on the movement of flow of gas include encoders to control velocity, position, torque, and applied force. In addition, as compared with pneumatic, it is quieter as it involves only electrical movement and no any flowing of fluid that may produce noise. Besides, it will also being avoid of fluid leaking problem as it only involved electric current flow. Therefore, environmental hazard of pollution may be prevented.
3.3.2 Gate Valves with Manual Actuator
Manual actuation is the basically operated by human driving force. A manual actuator is operated by using wheels, gears or levers to move the valve stem. It can be made directly by devices such as handwheel on the axis of the valves to provides greater torque on more turns. Other than that, if the valve is located at the place that difficult to be operate, it can be operate in a distance with a chain wheels.
Figure :Gate valve with manual actuator and its symbol.
Generally, in benzene distillation, the manual actuator serves as a motion device that operators use as override devices for the gate valve that actuated with electric powered actuator. As the manual actuated valve need always to be operated under non-hazard location, it must be located at the place that can easily operated by operator. Thus, the valve is usually installed at the pipeline between the distillation column and heat exchanger that least chances to adjust it as it must be always open to allowed maximum flow from the distillation column output to the heat exchanger to cool down the purified benzene. Furthermore, the valve with manual actuator is also located at the output pipeline of the solvent recovery tank to allow the desired fuel gas in the solvent to be removed from the plant after the solvent has been used for bezene-toulene separation. Thereby, the recovered solvent will be transferred back to be reused.
Figure : Diagram of operating principle of manual actuator valve.
The advantages of manual actuator valve is that it is inexpensive, typically reliable and easy to operate as it does not require any power source. It is usually self- contained as well as since it used the same motion to open and close the valve, it is easy to trace an error occur during the control process or any cause of discrepancy. Due to its simple operation, it has been use widely in industry that does not cause human health problem or any harm to the operator at the location where the manual actuator valve located.
3.3.3 Gate Valves with Pneumatic Actuator
Basically, pneumatic actuator valve is operated by gas such as air as it power source. The two common types of manual actuator valve are linear or quarter-turn valves. The air pressure will act on a piston or bellow the diaphragm to produce a linear force to the valve. However, a quarter-turn actuator will create torque that provide rotatory motion to operate the valve. Valve with pneumatic actuator can either designed in spring-opened or spring-closed condition and the air will provide the force to overcome the spring and cause movement to operate the valve. The clean, dry and compressed air supply is provided by central air compression system.
Figure : Gate valves with pneumatic actuator and its symbol.
As shown in P&I Diagram of benzene distillation, the pneumatic valve act as a flow control valve, level control, pressure control and temperature control. where it can control the velocity of the flow more easily compared to hydraulic actuated valve. This is because the pressure differential between the work port of cylinder play a larger part. It basically located after every controller to receive the signal from controller to carry out its function to open or closed the valve. For example, in benzene distillation plant, the pneumatic flow control gate valve will control the flowrate of the recovered solvent from solvent recovery column back to distillation column to be further reused. Also, the pneumatic valve also regulates the flowrate of the cooled stream that contain majority of toluene from reboiler back to distillation column for further separate. Pneumatic valve also used as a level control valve to control the flow of purified benzene to ensure the level of benzene in the storage tank to prevent overflow.
Advantages of pneumatic actuator valve in petrochemical industry is the probability of contamination is minimised as process in petrochemical industry is strictly prohibited from contamination as the quality of petrochemical products will be affected and the products may also can be altered by contaminant on the way of production. Also, the gate valve with pneumatic valve can be designed in smaller size, lighter and highly responsive on the signal received from controller. This is because air compressibility has little or no resistance which make it to be easily compressed. When the stroking power is needed, the cylinder of air compression can be enlarged to suit the requirement. Furthermore, the gate valve with pneumatic actuator is usually used for purpose of quick opening and closing, beside of its wide range of operational temperature from -40° F to +250° F. This advantage is mostly needed by control valve in petrochemical industry as it able to control the petrochemical flow at the shortest response time. This is because pneumatic actuator can react immediately to open or close the gate valve even under high temperature condition which is mostly happen petrochemical industry.
3.4 Pressure Relief Valves
The purpose of installation of pressure relief valves (PRV) is to release the excess pressure trapped in an equipment such as boilers or pressure vessels. If not, the pressure may build up in the vessels and causing the vessels to failure or fire. Thus, the valve will open automatically when pressure is exceeded the setpoint. The released gas will pass through gas flare by combustion before released to the atmosphere. PRVs are mostly made from carbon steels is past, however change to stainless steel in recent years.
Figure : Pressure relief valve and its symbol.
As we know, pressure is one of the most important safety valve in petrochemical industry. In normal operating condition, the valve will usually in closed condition and no fluid will flow through the valve. However, when the pressure in pipeline exceed the maximum pressure limit, the valve will open to relieve the pressure. The PRV usually open with throttles to release reduce pressure or closed to maintain constant pressure in the pipeline. The valve is opened by spring and the pressure in the pipeline is sensed at the outlet port and feed to the end of spring-loaded spool as shown in Figure below. When the pressure rises, the valve will close against spring pressure. Flow of pressurised gas will pass through the small bleed hole to the spring chamber in preventing build up of pressure in the pipeline. This valve is operated to protect the expensive machinery such as motors, pumps and actuators from damaged by high pressure condition. PRV are available to operated with flowrate up to about 45 l/m and maximum pressure of 210 bar.
Figure : Operation of pressure relief valve.
In petrochemical industry, PRVs used either as safety valves or relief valves depends on their applications. If it is functioned as pressure relief, it will release overpressures in vessels by opening the valve with proportional to the level of pressure. However, safety valves is used on mechanical devices that contains pressurised fluid where it will opens suddenly when pressure is exceed the available limit.
In benzene distillation, PRV is located at the heat exchanger of E-104 and E-106 to release the pressure in the heat exchanger that exceed the setpoint. This is to ensure that the pressure in the heat exchangers is always maintain at the desired level for heat exchangers to function optimally. The excess of hot gas will be released from the heat exchanger in order to reduce the pressure back to setpoint level. Therefore, PRVs in benzene distillation plant functioned as gas releasing valves to remove unwanted gas and to ensure that the process of the plants carried out properly.
For instance, overhead pressure controller in benzene distillation influences the overhead pressure of the extractive distillation column. Input signal to this controller is the overhead pressure signal, which changes the flow rate of the overhead vapor exiting the column in order to maintain the pressure at set point. Therefore, the valve, which is pressure relief valve will receive signal from the controller and opened in this case, allowing the vapours to leave the column, reaching the pressure determined by the set point. If the pressure decreases, the valve would become close and the vapours accumulate in the column and causing increase in pressure.
Meanwhile, for bottom level controller, the liquid level in the bottom section of the column is maintained. The PRV on the liquid output will be closed when the liquid level is low in order to maintain the liquid inside the column until it reaches the desired level.
As we know, PRV is important in benzene distillation process. This is because distillation will produce a high pressure gas flow that may be exceed the maximum limit of pressure that can be withstand by the equipment and pipelines. This is because excessive pressure that stay in the equipment or pipeline may causing damage to the equipment and subsequently harm the operators nearby the equipment if explosion occur.
On the other hand, PRV also needed in maintain the pressure in the setpoint which require for the optimum condition of the process. When the pressure in the equipment reached the maximum availability and the excess pressure is not releasing to surrounding, this will cause process cannot running under optimum condition and hence less efficient of the process will occurred. By releasing the pressure from the equipment, the process in equipment can happen at its optimum pressure condition.
Lastly, the role of PRV in maintain the pressure condition of the process at the optimum level also help in minimise the operating cost of the plant as the pressure will be always maintained at the desired level tom optimise the process in the equipment, thus less operating will be required per yield.
CHAPTER 4
CONCLUSIONS
As we know, FCE play important roles in control the flow of the process plant in petrochemical industr. Final control elements are basically operated after received the signal from controller and regulate the flow of fluid in the plant. Gate valves being one of the major FCE found in benzene distillation plant, allow or prevent the flow of fluid involved to help in optimise the process of separation between benzene and toluene. Since, the separation process is usually carried out under high temperature condition, it is necessary that gate valves is applied as it ability of withstand high temperature besides of its cheap in price and low maintenance costs advantages. Gate valves with different actuator is implemented in the plant with different purposes as electric actuated valve help in control flow of high velocity fluid in the plant in addition of avoid elimination of benzene to environment. Gate valve with manual actuator is used in control the flow column output t heat exchanger and removal of fuel gas from solvent. Meanwhile, pneumatic actuator valve acted as temperature, pressure, flowrate and level control valve by receives signal from each controller. Also, PRV is installed mainly with a purpose of releasing the excess air from the major equipment to maintain the pressure inside. Therefore, final control elements are exist in every oil and gas plant in industry with the role of control and optimise the process condition in the plant.
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APPENDIXES
Figure 1: P&I Diagram of benzene distillation.
Figure 2; Process Flow Diagram of benzene distillation.