BN-SP-K02 Specification for The Accessibility of Instruments and Instrument Connections

Table of Contents

1. General Design Basis
2. Accessibility
3. Specific Requirements
4. Plant Model
5. Responsibilities

1. General Design Basis

1.1 This specification covers the general design requirements for the accessibility and readability of instrument components and is of direct concern for the piping and vessel design.

1.2 In the installation of an instrument, the various components must be accessible for efficient maintenance and certain elements must be readable for good operation.

Successful instrumentation depends upon a workable arrangement of pipe routing, platforms and instrument hook-up installation.

1.3 Usually the most satisfactory installation as well as the most economical installation is achived by coupling the instruments as close to the process connection as practicable, consistent with required accessibility and visibility.

1.4 Careful piping arrangement shall satisfy accessibility requirements by routing the process line to a convenient height above grade or near existing platforms or other permanent means of access.

Where this would result in an impractical piping layout additional platforms or permanent ladder shall be considered whenever feasible and justified.

The necessity to use portable ladders shall be limited as much as possible.

1.5 Consideration is to be given to the location, accessibility and possible readability of instrument connections, in line instruments, transmitters and gauges/indicators.

1.6 The orientation, location and size of instrument connections shall be in agreement with the engineering flow diagrams and the engineering and design specification for instrumentation BN-SP-K1

1.7 For plant personnel safety, it is required that instrument installations do not obstruct walkways.

1.8 Mounting of instruments on hand rails is not allowed.

1.9 Mounting of instruments on fire-proofed equipment is not allowed. If there is no alternative location, clamping may be considered.

2. Accessibility

2.1 Direct Accessibility (Grade or platform)

Direct accessibility to instrument components is provided when maintenance can be performed from grade, permanent platform or fixed walkway.

Instrument components will be installed not more than 0.6 m horizontally away from or not more than 2 m vertically above grade or permanent platform.

Heavy items, such as in line metering equipment, control valves etc. which cannot be reached by movable hoisting equipment shall be provided with adequate provisions for dismantling and/or transport.

2.2 Permanent Accessibility (Permanent ladder)

An instrument component is pemanently accessible when its location permits cleaning, rodding-out and calibration inistu, without the use of tenporary facilities.

Instrument components shall not be installed more than 0.6 m horizontally away from permanent ladders.

2.3 Limited Accesibility (Portable ladder)

When access from grade, permanent platform or ladder is impractical, temporary access facilities can be used to provide infrequent access. Limited accessibility shall be restricted to occasional inspection, cleaning and dismantling of easy to handle parts.

To be accessible by temporary facilities instrument components shall not be installed more than 4.5 m vertically above grade.

Portable ladders may only be supported from grade. Arolling platform may be employed where free access is available to the space below the instrument.

Temporary access facilities shall be consistent with local safety rules.

2.4 Visibility

Instruments providing an indicating function shall be readable from grade, operating platform or related equipment location with a maximum line of sight of 2.5 m.

It is advisable to locate indicating instruments at an elevation of 1.0÷2.0 m above grade or platform.

Gauges must be installed at locations where readability is ensured.

3. Specific Requirements

3.1 Instrument Connections and Take-off Points

3.1.1 Take-off points for instruments and analyzer sample points shall not be located in the lower quarter of horizontal lines. Connections at the side are preffered for liquids and steam.

Connections for gases should be at the top of the line but may be mounted at the side when a connection at the top would result in bad accessibility, especially for large bore piping.

Pressure gauges on horizontal lines may have their take-off point located at the top of the line.

3.1.2 The first block valve and instrument connections (i.e.thermowell) shall preferably be permanently accesible. However, when the addition of extra steelwork is not economically justified or would be impractical, or obstruct access ways, limited accessibility may be considered.

3.1.3 A clearance of approximately 0.7 m above or below the first block valve or instrument connection is to be observed to install the instrument.

3.1.4 Withdrawal space must be allowed for thermowells. Unsealed thermowells must be self draining.

3.1.5 Analyser pre-conditioning racks must have direct access.

3.2 In line Meters

3.2.1 In line meters like positive displacement flowmeters turbine flowmeters, rotameters, probe type instruments etc. must be directly accessible from grade or permanent platform.

3.2.2 Limited accessibility is allowed for:

Vortex meters, swirl meters etc.; temperature elements; instrument air take-off valves.

3.3 Orifice Flanges, Venturi’s, Dahl Tubes, etc.

3.3.1 For the accesibility reference is made to the requirements for instrument connections.

3.3.2 Where close coupling is impractical or uneconomic (eg. orifice meters in elevated pipe racks) remote mounting may be considered provided limited access is available for process block valves, direct access is available for the instrument, and adequate supports are available for impulse lines between them.

3.3.3 For orifice flow meter installations in saturated steam service vertical downwards flow is preffered.

3.4 Vessel Mounted Instrument Equipment

3.4.1 Instrument equipment mounted directly to vessels, like external displacers, ball float switches, gauge glasses etc. should be permanently accessible.

3.4.2 When installing nozzles close to permanent ladders the clearance for and the position of external displacer type transmitter heads, lightining fixures for level gauges, etc. shall be observed.

3.4.3 Permanent accessibility is preferred for block valve connections on level control and alarm applications.

3.5 Control Valves

3.5.1 Control valves and on/off valves should be directly accessible and are generaly located at grade unless pressure head or other design conditions make such an arrangement impractical.

For noise abatement it might be more economical to leave the control valve in a straight pipe run and build a special platform for access.

3.5.2 Where there is a choice of location, it is desirable to have the control valve installed near the process equipment which must be observed while on manual control. The distance between the control sensing point and the control valve shall preferably be as short as practical, consistent with straight length requirements (flow) and mixing characteristics (temperature, desuperheaters and analytical control loops).

3.6 Gauges

3.6.1 It shall be observed that level gauges have a tendercy to foul-up necessitating frequent cleaning and reading from short distance.

3.6.2 In applications where a process variable should be manually controlled at a control valve station, as indicated on the EFD, the local indication as obtained from the level, pressure or temperature instrument shall be visible from the location.

3.6.3 Pressure gauges installed to provide local indication at the safety relief valve shall be clearly visible from the relief valve location.

3.6.4 Connections for local dial thermometers may have limited access, with the cenerline of the well horizontal or with a maximum 45° from horizontal, in such a way that readability is ensured from grade or platform.

3.6.5 Remote reading dial thermometers feature a flexible capillary of 1.5 m unless otherwise specified. Readability should be obtained within the capillary length.

3.6.6 Gauges shall preferably be located with the scale pointing in an upright position. (Pressure gauges with diaphragm seals shall be installed with the diaphragm horizontal).

3.7 Transmitters

3.7.1 Transmitter installation shold be permanently accessible. Only in rare cases, as determinedin consultation with the instrument engineer, may the transmitter installation have limited accessibility.

3.7.2 Accessibility requirements, vibration or high ambient temperature may preclude close coupling and dictate remote transmitter installation.

Impulse lines, however, should be as short as possible.

3.8 Instrument Boxes and Local Panels

3.8.1 Boxes shall be located with centerline 1.60 m above grade or platform.

3.8.2 Free access to be maintained to instrument boxes, cubicles and local panels. Piping shall not obstruct openings, panel fronts and access doors.

4. Plant Model

4.1 When a model is prepared, as a focal design tool, the model shall demonstrate the accessibility by the modelling of vessel mounted instruments, in line instruments, boxes, local panels, take-off points and main cable trenches.

It may become necessary to relocate instruments/connections after model has progressed sufficiently to indicate better grouping and access.

Transmitter locations in general will not be shown in the model.

5. Responsibilities

5.1 Accessibility and location of instrument connections, vessel mounted instruments and in line instruments is primarily the responsibility of the Piping Department. Deviations from this specification, however, are to be discussed with the Instrument Engineer.

5.2 The location and accessibility of instrument boxes, transmitters and panels will be determined by the Instrument Design Department in consultation with the Instrument Engineer and Piping Design Section.

5.3 Instrument locations will be communicated to the model maker by the Piping and Instrument Design Departments.

5.4 The Instrument Design Department will evaluate the model on instrument accessibility and location.

The Instrument/Process/Project Engineer will review the model on engineering and operability aspects.

This review and evaluation shall be conducted during the development stages of the model.

Summary of Minimum Accessibility Requirements

(X) = Permanent accesibility is preferred but additional steelwork will, in general, not be provided.

Remarks

1. Following block valves to be permanently accessible:

- Level applications for control and alarm

- Take-off points with rodding facilities.

2. Readable from short distance, over whole range.

3. Locate close to manway.

4. Allow for calibration access.

5. Check position of displacer head.

6. Check for clearance of lighting fixture.

7. Allow for occasional access for test measurements with dial thermometer.

8. Inform Instrument Department when horizontal well or readability is not possible.

9. A platform might be more economical, allow for handwheel and positioning access.

10. Consult Instrument Department for limited accessibility.

11. Purge rotameters must be permanently accessible.