Instrumentation basic

• Type and application of instrumentation devices , selection and finalization of equipment depends largely on the process , system and plant.

• Process refers to manufacturing / refining / distribution / storage etc

• Plant size, location and availability of logistics form a major part in decision making regarding finalisation and selection of field and control instruments .

SELECTION OF INSTRUMENTATION SYSTEM

• Philosophy of operation and Size of plant contribute directly to development of design and engineering.

• Owner’s range of selection are :

• Pneumatic controls

• Electric controls

• Electronic systems

• Electro-pneumatic controls

• Electro-hydraulic controls

• Automation• Warning and trip devices



Owner’s range of selection are :• Fail-safe control devices

• Prime movers

• Valves

• Smart devices

• Emergency shut down systems

• Supervisory functions

• Data acquisition and remote control functions

• Local ( station ) and Master control centres


• Location of various processes in a plant allows the Owner to have a range of selection as below:

• Centralised

• Distributed

• Hot and cold stand by

• Local

• Upon finalisation of above, communication links are developed such as processor to processor, peer communication and various interface protocols

Utility of various processes in a plant allows the Owner to have a range of facilities as below:

• Diagnostic messages

• Remote calibration and maintenance checks

• MIS reports

• Data storage and retrieval facilities


• With these guidelines and requirements we will be in a position to step into the exact instrumentation device and control equipment.

• For a typical pipeline and pump station application , we need the following:

• Fail-safe , reliable instrument devices

• Integrated auto control system

• SCADA systems

• ESD system


• Before we go to the selection of instruments ( both field and control ) we need to understand the prevailing industry safety guidelines and standards for solid state control .

• All field equipment are to necessarily comply with certification for use in hazardous areas. These compliances are discussed in latter part of the session.


FIELD INSTRUMENTATION :-

- Types

- Characteristics


FIELD INSTRUMENTATION :-

There are three types .

1) Monitoring : These are used to monitor the health of equipment and operation as well. Basically they are alarm devices such as level / pressure / flow / temperature transmitters or switches.

2) Accounting : ( Measuring / Indicating ) : Basically to record values eg. Pressure / temperature indicator / gauges, level indicator, flow meter

3) Protective : are used to achieve fail safe operation and are used as shut-down or trip devices such as level / flow / vibration switches. These can be called field control type also.


FIELD INSTRUMENTS :-

- Turbine meter

- Pressure transmitters

- Temperature transmitters

- Density meters

- Relief valves

- Control valves


CONTROL FIELD INSTRUMENTATION :-

- These are generally discrete devices which generate contact for

signal interlocks or alarm or tripping of equipment.

- Analog control of equipment through control systems is also

possible from various field transmitters such as pressure and

temperature transmitters.

- Control equipment include pressure or flow control valves.

These are configured in auto control loop of a process.


Apart from measuring / protective field instrument, we have calibrating type field devices / equipment such as Meter prover. These are used to calibrate turbine flow meters . Construction and design of these provers are as per API codes and standards. This is one critical equipment used in pipeline industry


Besides analog / digital field instruments, there are certain instruments which are located in control room. These are generally as follows:

- Data loggers

- Flow computer

- Vibration monitor

- Recorders

- ESD devices

- Alarm annunciators


Objective of a Pipeline Control system:

The system is designed to facilitate an effective , safe and smooth operation of pumping as well as delivery / storage system. The system takes care of opening and closing of motor operated valves ( MOVs ), starting and stopping of mainline pump, booster pumps, monitoring sump tank levels, provide audio and visual alarms for various parameters and status . It interfaces with analog control system for throttling of control valves.


Various other functions of SCC and logic control system in a pipeline application:

• Starting and stopping of tank stirrers

• Starting and stopping of auxiliary equipment such as air compressors, centrifuges, control valves, pig launch / receive system

• Auto control of mainline pumping units

• Continuous monitoring of process parameters, alarming and tripping . This includes batching, monitoring of tank levels.

• Maintaining line balance and line integrity through application such as pressure profile, flow monitoring etc.


Station control centre

Pipeline system ( consisting of tank farm, pump station and terminal facilities ) is operated by established procedures so as to have a smooth and fail-safe operation. To ensure these tasks at each site ( originating tank farm cum pump station, intermediate station or terminal ) a station control centre is installed.

Objective of a Pipeline station control centre :

The logic system also ensures that the station is operated safely or the station is left in safe state in the event of failure .

It is the nerve of the station control centre which in turn communicates with the Master control centre.

Application software, hardware to be used depends on the application, I/O capacity and system time responses desired. Scan cycle and other updation times will depend upon the owner’s design with a vision for future developments / replacements / addition / augmentation etc.


Main features of Station /Master control centre worth consideration during design and selection:

• Dual redundancy at logic controller processor as well as work station / operator station processor , memory, power supply and communication level so as to have minimum downtime

• High speed communication bus, WAN and interfaces with logic control system

• Real time tagging of data

• System design for open connectivity

• Serial interfacing on MODBUS protocol


Main features of Station /Master control centre worth consideration during design and selection:

• Operating system – Windows NT

• Computer architecture – 64 bit for server and 32 bit for operator station

• Programming terminal to handle station control in case of exigencies

• Portability and remote downloading of software

• SCADA and control system commands shall have SEL-CHK-EXE features. This is select-checkback-execute instruction on all digital commands and analog set point controls .


Main features of Station /Master control centre worth consideration during design and selection :

• Dual redundant PLC or intelligent RTU in hot stand by mode for logic control

• Open architecture and distributed intelligence type

• Bumpless switch over between processors

• High speed data transfer rate ( state of the art available )

• Reports shall be generated on MS Excel

• MMI updation 2 seconds or better

• Highest ingres protection rating



• On-line servers shall hold alarms, events and trend, history for minimum 30 days.

• Networking as per IEEE standards

• Continuous on-line diagnostics

• Easy archival and retrieval systems



• Colour RAL 7032

• Temperature withstand ( specified by owner )

• Electro-magnetic interference suppression

• Surge protection, transient protection, RFI fiters


Design considerations of PLC / SLC based control system : • Design for the dual redundant field interrogation power supply shall be

such that one power supply can take full load at worst loading condition. Regulation shall be .05% or better for both line and load.

• All outputs shall be individually fused

• Standard 19 inch modules

• Spare 20% capacity in I/Os and other installed items like relays etc

• Switch-on protection

• IEC 65 design procedures


Design considerations of PLC / SLC based control system : • Flow algorithm compliant to API

• Minimum I/O handling capacity 4096 any mix

• Floating point processing

• On-line editing without effecting outputs

• On-line self-diagnostics

• Processor fail, redundancy failure, rack failure , I/O module failure and communication failure signaling to operator MMI

• Individual power supply for each I/O rack


Design considerations of PLC / SLC based control system : • Time of scan/K word length of application shall be better than 0.5 ms/K

• Very high noise immunity

• On-line replacement of module possibility

• Isolation level between logic and I/O min. 1 kv dc

• Reverse polarity protection in each I/O

• Contact de-bouncing

• Lightning protection


Design considerations of PLC / SLC based control system : • Digital filtering

• Oscillation suppression

• Conversions from BCD to Binary and vice-versa


Design considerations of field instruments • Accuracy of pressure gauges 0.5 % or better

• Accuracy of pressure switches 0.5 % or better

• Accuracy of Pressure transmitters 0.1 % or better

• Linearity and accuracy of turbine flow meters 0.15 % with repeatability also at 0.15 %


Design considerations of field transmitters • Intrinsic safety interface must be met.


Design considerations of field transmitters • A certified IS ( intrinsically safe ) interface passes signals to and

from the process, but limits the voltage and current that can reach the hazardous area under fault conditions.

Usually the interface is made up of a number of discrete devices such as shunt-diode safety barriers.


Design considerations of field transmitters • Shut diode barriers must be suitably earthed to the

equipotential bonding system for the plant via an insulated conductor whose resistance measures more than one ohm.


Design considerations of pressure switches

• Hermetically sealed micro-switch for clean environment

• Corrosion resistant material design

• Integral sensor design : Diaphragm sensors with welded construction eliminates O-rings


Other Design considerations

• Life cycle of operations, switching time cycles, surges, pulsations, over-pressures, process temperatures can reduce sensor life.

• Dead band analysis


Design consideration for selection of flow meter:

- Type of device ( capacity and accuracy related )

- Length and diameter of upstream and downstream piping

- 3D and 2 D concept

- No . Of pulses / cubic meter

- Floating rotor principle

- Output signal type and level . Distance criteria

- Flow counting method using flow computers


Design consideration for selection of flow computer - Repeatable analog outputs

- Input handling ( pulse atleast two streams and other process parameters)

- Accuracy

- Isolation

- Displays

- Diagnostics

- Meter proving function


Design consideration for selection of Meter Prover:

Definitions


Design consideration for selection of Compact Meter Prover:

{ as per API }

• Pulse generation of minimum 10000 pulses from meter under test

• Usage of double chronometry


Double chronometry


D D

Count - A

Count - B

No. of Integral Flow meter pulses Count - C

Total Flowmeter Pulses = A * C / B

Low wear and tear of piston and seals


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Instrumentation basic