Instrument Power System Philosophy

Instrument Power System Philosophy

1. Pneumatic Supply

For pneumatic instruments, dry instrument gas / air supply shall be as follows:

- 5.5 Kg/cm2 (min.)

- 7.5 Kg/cm2 (nor.)

- 10.5 Kg/cm2 (max.)

2. Electric Power Supply

Components of power supply system shall be of highest available quality for

reliability and long service life.

Power supplies for all transmitters, controllers, signal converters, electric

system and components in shutdown system shall be supplied from

uninterruptible power supplies.

Power distribution to each consumer shall be through proper, independent

switch and fuse. Protective fuses shall be of indicating cartridge type mounted

in fuse holders.

In general, the following Power Supplies shall be used for instrumentation and

Control:

- For Process Platforms: 110V AC + 5%, 50HZ + 1% (UPS) for all

instruments control. However, all components / instruments / system shall

be suitable for 110 V + 10% AC, 50 Hz + 3%

- For Process & Well Platforms: 24V DC + 5% Battery Negative earthed for

Platform interlock system, solenoid valves, Fire and Gas system and

status lamp.

Instrument Earthing System Philosophy

Three separate earthing systems shall be provided:

a. Electrical Safety Earth – Bonded to the site structure and utilized for electrical

safety of metal enclosures and chassis on all instruments and electrical

components.

b. Instrument Clean Earth – Insulated from the site structure and other metal

work, utilized for instrument cable screens and bonded to the main electrical

earthing system at a single point.

c. Intrinsically Safe Earth – Insulated from the site structure and other metal

work, utilized for termination of IS zener barrier earth connections, and

bonded to the main electrical earthing system at a single point.

Equipment Protection Philosophy

1. Environmental Protection

All instruments / equipment and installation material shall be suitable for the

overall climatic conditions, the position within the installation and the local

environment, with particular attention to site ambient conditions. The

conditions will include exposure to Hydrocarbons, H2S (in case the process

fluid is sour), moist salt laden atmosphere, sea spray, sunlight, monsoon

rainfall, temperature, humidity, wind, fungal growth, vibration and shock, EMI

and RFI. All equipment shall also be able to withstand these conditions during

shipment, storage and installation prior to commissioning. Instrumentation

shall withstand not only the quoted environmental conditions, but also the

periodic testing of the Deluge or Fire Hose System.

As all of the Company’s sites are subject to seismic activity as indicated in the

General Basic Bid Work and General Design Criteria, all instrument /

electrical frames, panel and racks shall be fixed in position suitably secured.

In view of the highly corrosive ambient conditions, all internal and external

parts which are not inherently corrosion resistant by choice of material shall

be prepared and finished by plating or paint finish in accordance with the

General Specification for protective coating. Seals and purges shall be used

as necessary, to ensure reliable instrument performance.

All field instruments shall be provided with necessary weathering and

anticorrosion protection. All field instruments shall be provided with plastic

bags (min. 1.5 mm thick) to protect them during handling, installation and

commissioning. The bags shall be kept in place at all times except during

work on the devices. Drying agent (desiccant) with humidity indicator shall be

put inside the bag and it shall be replaced when color of the indicator changes

from blue to pink Additional protection by other means such as canvas shall

be provided to prevent damage caused by welding during construction work

at nearby location. Labels and tags that may be exposed to paint spray, shall

be temporarily masked with a transparent material during construction

activities, which shall be removed at the time of hand over of the work. Plastic

plugs shall be fitted to all instrument tubing and air, process and cable entry

ports until final connections are made.

2. Ingress Protection

All field instruments shall have ingress protection to IP 65 or better.

Pneumatic field instruments used for control applications shall have ingress

protection to IP 55 or better. All instruments installed inside pressurized

equipment / control rooms shall have ingress protection to IP 42 as a

minimum.

3. Tropicalization

All electrical components shall be tropicalized to protect against humidity,

moisture and fungal growth by means of hermetically sealed units, protective

coating on circuit boards, gold plated edge connectors, etc.

4. Hermetic Sealing

All relays and switches shall be hermetically sealed, and those utilized in 24 V

DC control logic circuits shall have gold plated contacts rated 0.5 Amp at 24 V

DC. Those interfacing with field equipment shall be rated 2 Amp 24 V DC.

5. Hazardous Area Instrumentation

The Contractor shall classify hazardous areas in accordance with API 500

and specify various equipments accordingly. All instruments which are

mounted outside of normally pressurized control / equipment rooms shall be

certified by bodies such as FM / UL / BASEEFA / CSA / DGMS / CMRS for

use in Class I, Division I, Group D, T3 hazardous area, even if the

instrument’s location is classified as a normally non-hazardous area. Intrinsic

safety approval shall be based on entity concept and necessary compatibility

checks shall be carried out by Contractor before selecting any equipment.

Intrinsically safe protection using external barriers shall be provided for all

process transmitter loops (closed as well as open). Isolating barriers shall be

of the plug-in type, mounted on modular back plane termination units. Each

input and output in a loop shall have a separate barrier. No barrier shall be

shared between two loops in input / output. All other instrument loops shall be

provided with explosion proof / flame proof protection. Solenoid valves,

electric hand switches, signaling lamps and Intercom / Paging system shall be

Explosion proof / flame proof to Ex d or NEMA 7.

If specialist instrumentation cannot be provided with the above methods of

protection, then alternative methods suitable for the classified area and

certified by an acceptable Authority may be proposed. The Contractor shall

submit a technical report justifying the instrument selection for the Company’s

consideration.

6. R F Interference

All equipment shall remain unaffected by radio transmissions (Levels of

permissible RFI shall be as per IEC 801). Band-pass and / or band stop filers

shall be fitted, as necessary.

7. Sealing

Seal systems shall be used to isolate instrument from the process fluid

encountered in the following services:

a. Wet gas, which may condense in the instrument lines.

b. Process fluids that vaporize, condense or solidify under operating

pressure and ambient temperature.

c. Process fluids that will subject the element to high temperature.

d. Process fluids that will subject the element to high temperature.

e. Viscous liquids.

Sealing may be accomplished with diaphragm seals.

All venting instrument and pilot valves shall have bug screens fitted to

atmospheric vents.