Oil & Gas Separation Book 2 Flashcards
What 2 major controls do separators have in order to maintain optimum separation and this production? (2)
- Liquid level control
- Pressure control
In any continuous process such as oil and gas separation there are a number of factors which must be kept within certain limits.
(a) What are these factors referred to as? (1)
(b) What are the 4 factors that must be kept within certain limits?
(a)
Process Variables
(b)
- Liquid Level
- Pressure
- Temperature
- Fluid Flow
What does an oil and gas separation system rely on having built into it to achieve the desired control for optimum separation? (1)
- Control Loop
Describe:
(a) Closed Control Loop
(b) Open Control Loop
(a) An AUTOMATIC system which controls the separation parameters for optimum separation.
(b) An open control loop has a MANUAL input to control a system variable.
What are the four main elements in a closed control loop? (4)
- The Process Variable (Liquid, Pressure, Temperature, Flow)
- The Measuring Unit (Displacer Mechanism, LI, PI, PT)
- The Controller (Pneumatic)
- The Correcting Unit (Valve)
In a closed control loop we have 4 main elements listed below. Describe what each element is:
(a) - The Process Variable (1)
(b) - The Measuring Unit (2)
(c) - The Controller (2)
(d) - The Correcting Unit (3)
(a) - The Process Variable:
- Is the part of the process which has to be controlled within certain limits (i.e. level, pressure etc).
(b) - The Measuring Unit:
- DEVICE (such as a pressure transmitter, or level indicator) which MEASURES THE ACTUAL VALUE of the variable such as pressure or flow)
- Obtains the MEASURED VALUE
(c) - The Controller:
- The MEASURED VALUE of the process variable is COMPARED against the DESIRED VALUE.
- If it senses a deviation between the two, it sends a SIGNAL to the CORRECTING UNIT
(d) - The Correcting Unit:
- This part of the control loop is usually a VALVE
- RECEIVES SIGNAL from correcting unit
- MEASURED VALUE returns to DESIRED VALUE
Separator liquid level control is required for:? (5)
- To prevent liquids being carried out with the gas (known as carry-
over) - To prevent gas from leaving the separator through the liquid outlet. This is known as blow by or blow round
- To help maintain the pressure on the vessel (Fluctuating levels affect the pressure)
- In a three phase separator, to prevent oil from leaving through the water outlet or vice versa
- To ensure optimum retention time
What does ‘Blow By’ or Blow round mean? (1)
- When gas leaves a separator through the liquid outlet due to the levels not being maintained to the desired value.
What could fluctuating levels of liquid alter in side a separator? (1)
- Fluctuating liquid levels affect the pressure inside the vessel)
Draw a diagram showing a simple Oil Level Control System for a two phase horizontal separator (5)
Showing:
- Separator with corrects inlets/outlets (3)
- Displacer Mechanism (Measuring Unit)
- Level Controller
- Level Control Valve
- Process Flow
Shows:
- Separator with corrects inlets/outlets (3)
- Displacer Mechanism (Measuring Unit)
- Level Controller
- Level Control Valve
- Process Flow
What is
a) Displacer Mechanism? (2
(b) How does it operate? (5)
(a)
- Piece of equipment which measures liquid level by a method which is:
- Based on the principle of Archimedes
(b)
- Usually a cylindrical weight partially submerged in a liquid
- If the levels rise and fall, more or less of the cylinder will be submerged. Its apparent weight will therefore vary.
- The weight can then be measured to give an indication of the level of the liquid.
- The loss or gain of apparent weight is transmitted to the controller as a signal which is proportional to the increase or decrease in liquid level.
- Signal transmission is achieved by means of a torque tube assembly
What are:
(a) The basic components of a torque tube assembly? (6)
(b) Describe what each component is/does (6)
(c)
(i) What is the rotation of the torque tube transmitted to?
(ii) How is the
(a):
- Float Rod
- Knife Edged Bearing
- Torque Tube
- Torque Tube Plate
- Outer Flange
- Torque Tube Rod
(b) :
- Float Rod: - What the displacer is attached to. As the liquid level inside the vessel goes down the weight on the float rod will increase, the increased weight hanging on the torque tube will twist the torque rod which is connected to the level controller.
- Knife Edged Bearing: - The component which the Float rod is resting on, its in the shape of a triangle allowing the rod to rock up or down on the knife edge of the bearing.
- Torque Tube: - A hollow rod which the torque tube rod fits inside.
- Torque Tube Rod: - Fits inside the torque tube and is welded at the torque tube plate
- Torque Tube Plate: - The component which the displacer is attached to and the torque tube rod is welded to
- Outer Flange: - The housing in which all the components fit inside.
(c)
(i) To the next element in the control loop - The Controller
(ii) Rotation transmitted via linkage
What is the job of The Controller (2)
- To compare two values, the measured value signal and the desired value signal
- If a deviation exists, it sends a correcting signal to the control valve
In a Pneumatic Controller, what are the four separate but interconnected units?
- The Differential Mechanism
- The Flapper/Nozzle Assembly
- The Feedback Unit
- The Pilot Relay
Draw a block diagram showing the four units of a pneumatic level controller (10)
Shows:
- Four Units of Controller (4)
- Measured Value Input (1)
- Desired Value Input (1)
- Route to Correcting Element (valve) (1)
- Air in and correct pressure (2)
- Controller Limits (1)
What two types of Differential Mechanism exist? (2)
- The Motion Balance Mechanism
- The Force Balance Mechanism
Inside a level controller What is the job of the Differential Mechanism? (2)
- Compare the desired value against the measured value
- If a deviation exists between the two, the Differential Mechanism feeds the information to the next unit in the controller: The Flapper/Nozzle Assembly
How does a Motion Balanced Differential Mechanism Operate? (3)
- Uses 2 mechanical linkages to compare the MV & DV.
- One end of The MV Link DV Link are attached to the Differential Arm which is connected to the Deviation Link
- The Deviation Link transmits any motion of the Differential Arm to the Flapper/Nozzle Assembly.
Draw a diagram of a Motion Balanced Differential Mechanism (1)
Shows:
- MV Link
- DV Link
- Differential Arm
- Deviation Link
- Movement Direction & Interaction
(a) - How does a Force Balance Differential Mechanism Operate? (5)
(b) - Describe the action of this mechanism (3)
(a)
- Uses pressure applied to bellows to compare desired and measured values
- Output from the measuring element fed as a pressure to the Measured Value Bellows
- Movement of bellows is opposed by a second set of bellows
- The Desired Value Bellows are pressurised by a signal which relates to the desired value of the process variable.
- Between the two bellows is one end of a bar known as a Force Bar. The bar is pivoted using a fulcrum. The other end from the bellows is free to move.
(b)
- If the measuring element output gave an increased signal, the MV bellows would expand against the DV bellows
- This movement would cause the free end of the force bar to move
- Movement then fed to the flapper/nozzle assembly as a deviation
Which type of Mechanism is most commonly used in pneumatic controllers? (1)
- Force Balance Mechanism
What are the main components of a flapper assembly? (3)
- The Flapper
- The Nozzle
- The Restrictor
Draw a diagram of a flapper assembly (6)
Shows:
- 20 PSI Air Supply
- Restrictor
- Nozzle
- Flapper
- Back Pressure Outlet
- Movement of link from differential unit
What is the job of the flapper assembly (1)
- Send correcting pressure signal from the controller to the final element in the control loop - The Control Valve
Describe how a flapper assembly works (
- Air supply fed to line upstream of restrictor, typically at a pressure of 1.3 bar.
- The air can pass through the restrictor to the nozzle outlet and also through the variable back pressure outlet
- When flapper is positioned away from nozzle air can pass through the restrictor and out through the nozzle.
- Because the diameter of the restrictor is small compared to that of the nozzle there will be very little pressure build up in the space between the restrictor and the nozzle meaning there will be no pressure build up in the back pressure line.
- If the flapper is moved towards the nozzle, the area of nozzle through which the air can pass is reduced. This means that the nozzle back pressure will increase.
- If the nozzle is completely covered then the back pressure would build up to the supply pressure of 1.3 bar.
Draw a graph showing the back pressure against position of flapper relative to the nozzle in a Flapper Nozzle Assembly
Draw from Memory
What typical pressure range will pneumatic instruments record, transmit and control in? (1)
- 0.2 to 1 Bar
If the movement from a differential unit is linked to the flapper what will occur? (2)
- A varying back pressure signal will be obtained from the nozzle
- This signal will be proportional to the deviation between measured and desired values of the process variable
What is a feedback unit? (1)
- Feedback simply means feeding back the output signal of the controller to the input of the same unit
Describe how a feedback unit works? (5)
- Pivot point of flapper attached to movable end of set of bellows. The movement of bellows is opposed by a spring.
- Nozzle back pressure fed to these bellows in addition to being the controller output signal to the correcting unit.
- When a deviation occurs, the flapper moves towards the nozzle and causes an increase in back pressure.
- The increased bellows pressure will move the pivot point of the flapper against the spring until the spring and bellows forces are balanced - when this happens the flapper moves away from the nozzle until a steady state is reached
- When steady state is reached, flapper will have moved very slightly neared to the nozzle — just enough to increase the output to the correcting unit.
