1. Natural Gas Production Chapter 8 Instrumentation
PTRT 2323
Natural Gas Production
Chapter 8
Instrumentation

2. Instrumentation Related to the control of a gas or liquid
Controlled medium
Maintain specific values of these control variables
Temperature
Pressure
Flow rate
Liquid level
Final control achieved by adjusting orifice size through which the controlled medium (or some related flow) is passing

3. Measurement
Value of variable to be controlled must be accurately measured
Measurement must be compared to an optimum value (set point)
Corrective action to correct offset
Manual
Automatic
Mechanical
electrical

4. Pressure Measurement
Manometer most commonly used device for pressure from zero to a few psi

5. Manometer U-tube One end connected to tank
Other end open to atmosphere
Positive gauge pressure in tank pushes fluid down on the leg next to tank and up on the leg open to atmosphere
Negative gauge pressure in the tank does the reverse
Difference in height is the gauge pressure measured in inches (Hg or H2O)
Used for low pressures where visual indication is required

6. Diaphragm Element Also used for measuring low pressures (a few psi)
Very sensitive to small pressure changes
Can provide an indicated reading or movement to operate an automated control device
Components
Heavy metal base
Pressure connection
Corrugated diaphragm (thin sheet metal or elastomer for very low pressures)
Barometer is this device with pressure connection sealed

8. Bellows Element
Similar to diaphragm except corrugations expand in series instead of outwards
Movement of bellows is opposed by a spring (extends the range of pressures tolerated)
Ranges up to 100 psi or so available

10. Bourdon Tube
C-shaped tube, sealed at one end connected to pressure at the other
Outer circumference is greater than the inner so pressure tends to straighten the tube
Linkage used to amplify the movement
Multiple Bourdon tubes connected together make a helical tube

13. Temperature Measurements
Thermometer – common indicating instrument
Contains mercury, pentane, toluene, alcohol, etc
Volume of fluid in bulb is about 1000 times more than volume in the stem bore.
Temperature increase produces a volume increase which moves the fluid column up or down in the stem bore (Vacuum above)
Thermometer well protects thermometer from fluid flow

14. Well should be filled with thermal transfer fluid such as a light oil
Well provides seal against pressure or leaks

15. Filled System Thermometer
Sealed unit
Increased T produces increased P measured with Bourdon tube
Thermal expansion of the fluid in the device is not accounted for in this design
Temperature compensation requires a separate Bourdon Loop

16. Temperature Compensation
Second Bourdon tube aligned opposite to sensing element
Expands or contracts the same but in opposite direction thereby correcting for the expansion of the fluid inside the tube

17. Bimetallic Temperature Element
Bimetal strip – two strips physically bonded together
Bends in response to temperature changes
Often seen in Thermostats in the home
Helical winding saves space

18. Thermocouple Element Two metals electrically bonded together
Temperature change produces a voltage
Sensitive voltmeter can detect the changes and display or transmit temperature

20. Liquid Level Measurements
Most common is the sight glass
Transparent
Armored
Reflex
Level in the glass tube matches the level in the tank
On pressurized vessels the bottom and top of the sight glass must be connected to the tank (top above the liquid)

23. Float-type Control Bulb or ball floats on liquid
Linkage transmits motion through a packing gland(seal) to move an indicator or trip a valve
Generally used for on/off control

24. Pressure Devices
Depth of fluid generates a hydrostatic pressure at the bottom of the fluid
Pressure device and convert this pressure to a level measurement
Vented to atmosphere

25. Pressurized Tank
Must compensate for vapor pressure in order to determine liquid level
Two bellows oppose one another
Pressure DIFFERENCE is measured

26. Flow Controllers
Manual control can be achieved in most circumstances by well-informed operator
All control functions can be performed automatically:
Better control
More safety
More accurately
Lower cost

27. Flow Controllers Automatic system must contain:
Measuring device (detect variation)
Controlling element (use variation to produce a control signal)
Final control element (use the control signal to produce the control; i.e. valve, heater, etc)
Connected in a continuous closed loop
Setting the control limits is the only manual step required
Measuring and controlling usually in a common instrument that operates the final control element

28. Flow Regulators All flow regulators contain three essential elements
Restricting device (limit flow)
Measuring device (measure flow)
Loading device (control flow)
Self-contained regulator contains all three elements in a single device

29. Spring-loaded Regulator
Loading device
Restricting device
Measuring device

30. Spring-loaded Regulators
Up to 1000 psi inlet pressure
Up to 400 psi outlet pressure
Outlet pressure applied to diaphragm (limiting factor)
Spring stretches as it’s opened farther causing outlet pressure to drop off as regulator is opened farther

31. Temperature Control Regulators
Diaphragm replaced by bellows
Bellows connected to filled system thermometer
Temperature changes create pressure changes inside the bellows
Orifice opens or closes against the seat
Unit ONLY control flow rate pressure is irrelevant.
Pressure regulation must be provided separately

32. Temperature Controller
Measuring device
Loading device
Restricting device

33. Pilot-operated regulator
Similar to temperature regulator
Gas pressure replaces the spring
Small spring used to help seat the orifice against the seat when closed
Constant load independent of the position of the diaphragm (unlike regulator)
Eliminates the pressure drop
Used to regulate burner fuel supply

34. Motor Valves Diaphragm Loading spring Valve stem and body
Inner valve and seat
Normally open (spring tries to hold the valve completely open)
Normally closed (spring tries to hold the valve completely closed)
Operating pressure (control signal) overcomes the resistance of the spring
Typical values for operating pressure 3 psi (start travel) and 15 psi (full travel) – some valves 2x these values
In failed condition spring returns the valve to it’s normal position

35. Normally OPEN
Normally CLOSED

36. Hydraulic Piston Motor Valve
Diaphragm attached to a hydraulic piston instead of metal stem
Hydraulic pressure squeezes on a rubber bladder expanding it against the inside wall of a seat and stopping the flow
Streamlined flow since there is little change in direction during operation
Can accept a 1000 psi pressure drop and still close (even around trash)
Control pressure can usually be must less than the 3 to 15 psi for motor valve

37. Hydraulic Piston Motor Valve

38. Types of Inner Valves
Double ported (also called balanced valve) (shown previously)
Requires lower opening forces (pressures)
Used when process requires extended throttling at less than 10% capacity
Difficult to fully close
Single ported
Requires higher opening forces
Used typically for on/off situations

39. Single-ported Valve

40. Valve Forms Selected for specific flow characteristics
Linear (20% travel yields 20% flow, etc.)
V-port (40% travel yields only small % flow)
Excellent throttling valve
Good when upsets in flow can occur
Quick opening (small travel yields large flow)
Used when control variable is difficult to adjust
Good when drastic flow changes occur
Motor valves operate equally either normally open or normally closed

41. Valve Plugs
Linear
Throttling
Equal Percentage
Quick Opening
V-port

43. Final Comment on Valves
Choice depends on application
Long transmission lines or distribution lines
Never completely shut off
Not required to throttle for extended periods
Ideal for double-ported with linear plug
Flow control
Double-ported with throttling plug
Controlled variable that is reduced two or three times in a relatively short section of pipe
Control variable can be changed easily
Dump valves or liquid level control
Single ported with quick opening plug is adequate
Also can be good in corrosive environments

44. Pneumatic Controllers
Controls operation of motor valve
Must be supplied with clean, dry, non-corrosive gas
Field gas
Compressed air
Typical supply pressure about 20 psi
Performs the following functions
Method of setting the value for the controlled variable (set point)
Measure the actual value
Detect deviation from set value
Transmit control signal to provide correction
Select controller matched to system (i.e. don’t use a 600 psi element to control 45 psi system)

45. Principles of Operation
Error detection (on output side)
Nozzle and flapper system
Acts like orifice and seat
Bourdon tube
Reacts to pressure changes
Pivots flapper over orifice of nozzle
Back pressure adjusts position of motor control valve or other pneumatic device

47. Proportional Control
Simple ON/OFF (Bang-Bang or Snap Action) often not sufficient
Hydraulic hammer
System sensitivity
Erratic control response
Proportional control means that a small error produces a small response while a larger error produces a larger response

48. Proportional Control
Generally given in percentage of total control range required to fully open the valve
Example: controller with 100-lb full range and a 10% proportional control would require 10-lb error before starting to open the valve.
If the control setting is at 50-lb then the pressure would have to drop to 40-lb before the valve would respond to the error
Moving the pivot point is one way to accomplish this action

49. Setting Proportional Control
Gauge piping, backpressure, nozzle size, etc define a maximum speed at which the controller can respond and a resulting lag in the response
Always set the proportional control starting from a high setting – this allows for the existence of a large error without causing a response leaving the controlled variable in a stable condition
Make small reductions in the proportional setting – the response of the valve will reach a level where the controller is trying to respond faster than the system can tolerate
Increase the proportional control slightly to eliminate the instability – this yields the maximum response rate

50. More elaborate system
Setup such that any pressure in bourdon tube would close the flapper against the nozzle
Spring-opposed bellows extends in response to pressure and opens the flapper
Position of the pivot point controls the amount of proportional action

51. Reset Action Used when deviation from control setting is not tolerable
Spring is replaced by a second bellows
Response of the second bellows is delayed by a needle valve (repeat per minute) 1
Diaphragm

52. Reset Action
System response is set to 3 – 15 lb to stroke the valve and 10% proportional setting
System is stable at 14 lb
Response if system drops off by 1 lb is as follows:
Pressure decrease is sensed at the Bourdon tube causing the nozzle to open and the diaphragm to vent
This change is sensed immediately at the proportional bellows but delayed by the needle valve to the reset bellows
Reset acts like a spring resisting the change but it will slowly vent through the needle valve and allow the change to occur
Without reset the system pressure would drop to 12.8 lb by proportional action alone
Without reset the system would maintain this value
With reset the system returns slowly to the original values
Read the text for more details

53. Rate of Response
Reset action can only occur at a specific maximum speed defined by tubing and the leak rate of the needle valve
During the time the reset action takes to occur the product being produced in the process can be ruined
Rate of Response controls can partially eliminate this problem by allowing the controller to operate temporally at 0% proportional action
Another needle valve is added to the inlet line for the proportional control bellows
This needle must never be closed more than the reset needle or the controller will always function at 0% proportional control

54. Proportional, Reset and Rate Summary
Proportional – sets the range over which the controller will take action – large proportional can generate an offset – small proportional will introduce instability
Reset – allows the controller to act with large proportional band in response to a disturbance and then with a small proportional band as control is established eliminating both the offset and the instability
Rate – allows the controller to respond to how quickly the error develops – not usually needed for many control applications – most important when large rapid upsets are expected

55. Proportional Only
Proportional plus Reset
Upper control limit
100%
10%
Lower control limit
Proportional plus Reset plus Rate

56. No-Bleed Relay
Controller of the previous types are best suited for controlling processes change slowly
Air relays are used when faster response and better control are required
The back pressure from the nozzle is applied to an air relay instead of the diaphragm of the motor valve
The air relay amplifies the response of the back pressure from the controller using a pair of diaphragms that have a 12:1 area ratio
This means that only a 1 lb back pressure signal from the controller is sufficient to provide 12 lb to stroke the motor valve

57. (20 lb inlet)

58. Valve Positioner
Pressure from controller may not provide enough force to overcome friction at the motor valve stem
Signal from controller is passed to a bellows mechanically connected to a second orifice/nozzle and an air relay
Position of valve stem is not the controlling input for passing supply air to the motor valve diaphragm
Typically used on large motor valves

59. Electric Controllers Electric Valve Controller (ON/OFF)
Proportional Controller

60. Control Methods Controlled variables
Flow rates
Liquid levels
Pressures
Temperatures
In almost all situations control is effected by the use of regulating valves

61. Control of Process Variables
Measure the variable
Compare the measurement to the set point
Use the error to adjust the position of the regulating valve
Provide feedback to the controller

62. Flow rates
Control generally accomplished by changing the output of a pump of compressor
Control installed on the output side
Three examples:
Centrifugal pump controlled with throttling valve
PD pump controlled using throttling valve and a bypass loop
Plunger pump (steam drive) controlled using throttling valve to control the supply of steam to the pump

63. Centrifugal Pump
Closing throttling valve (control valve) will change the flow from A to B
Back pressure between the pump and valve will rise along the performance curve to point B
No bypass is required because the pump can spin on closed line without damage
Flow Recorder Controller

64. Positive-displacement Pumps
Pump output is constant for each stroke
Control is still effected by throttling the output from the pump but the unused flow must bypass back to the suction side of the pump to avoid damage due to excess pressure
VFD can also be used to reduce the pump cycling rate

65. Plunger Pump
Control is established by controlling the flow of steam to the pump piston
The same system can be used with a variety of air-operated pumps

66. Level Control
Level control – float or diaphragm element regulating flow
Into the tank
Out of the tank

67. Pressure Control Regulating the flow of vapor leaving a tank
Separate from a level controller in a closed system

68. Process Temperature
Controlled by regulating the amount of sensible heat added to a system
Note once again the flow control is on the output side of the flow

69. Process Control Efficiency
Influenced by system response – time required for process variables to adjust to the set point after a disturbance
Time delay caused by
Signal path – detect the error, adjust the flow, interpret process feedback
Process time – heat created or lost by reactions, volume changes, fluid flow limitations

70. Process Control Effect of Proportional Band
100% = more stable process but larger variation
40% = tighter control but less stable process
Proportional only will also typically introduce an offset from the set point
Offset can be minimized by using reset in addition to proportional control
Some controllers (especially temperature control) use a rate function to increase the response to large disturbances

71. Cascade Control
Two measured variables are interlocked into one control loop
Objective is to provide precision by overcoming time lag in control response
Uses a primary control loop to adjust the set point of a secondary control loop
Adjustment of set points should always be made gradually when manual changes are made

72. Control System Examples
Absorber tower
Flow indicating controller (FIC) – regulate flow of lean oil into absorber tower
Level controller (LC) on the output side regulates rich oil leaving system
Low level alarm (LLA) installed in the tower as back up in case of failure of the level controller or its control valve
Scrubber
Level controller along with a high-level alarm (HLA) indicating that the level controller is activating
High Level shutdown (HLSV) is a simple float-type switch
Absorber pressure (in fact entire system pressure) is maintained with a pressure control (PC) on the residue gas outlet
The tower has a pressure safety valve (PSV) as a back up to this primary device
Feedback loop in some plants links the lean oil flow to the inlet gas flow using a ratio relay (RR)
Freezing problems can be detected using a differential pressure indicator (DPI)

74. Fractionator
Temperature and pressure control were covered in Chapter 3 (keep in mind for final exam)
Pressure control using a hot-gas bypass
PC – A controls the tower pressure
Pressure in the accumulator varies with the temperature of the condenser outlet temperature

75. Fractionator
Without intervention the pressure can become so low that the reflux pump could not pump reflux back to the tower
Another pressure control loop is used to send hot gas bypass to the output side of the condenser
The hot gas maintains the accumulator pressure

77. Control when output is vapor
Pressure control regulating the amount of vapor leaving the accumulator

78. Flooded-condenser Pressure Control
Flow control on the product output line
Control valve closes until condenser tubes are flooded with liquid product
This reduces the heat transfer surface and raises the temperature
Problems can occur is some of the product does not condense to liquid – can be corrected with the addition of a level controller and three-way valves to prevent the accumulator from running dry