Section 12 — Instruments and Controls

Control Centers

IPE Engineering Practice IPE-EP-12-6-1

Document number: IPE-EP-12-6-1 · Section: 12 — Instruments and Controls

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SCOPE

A control center is a building from which the control of a plant or plant's units are coordinated. The primary function of a control center is to accommodate the necessary personnel and equipment to provide safe, continuous operation of the plant or plant's units.

In some cases, the control equipment which interfaces to the process input/output (I/O) signals and which processes the raw signals may be located in a building separate from the operator. This document addresses both types of buildings.

The basis for the Plant Practice is the installation of a distributed control system. Many of the principles embodied within this document are equally applicable to single–loop instrumentation, but no specific treatment of this instrument type will be given.

Any deviation from this Practice shall be approved by the procedure described in EP 1–1–3.

REFERENCES

The latest edition of the following standards and publications are referred to herein, and shall be used with this Practice:

STANDARDS AND PUBLICATIONS

DEFINITIONS

Owner - Inflection Point Engineering, LLC.

Owner’s Engineer - A Inflection Point Engineering, LLC. appointed engineer.

Purchaser - The party placing a direct purchase order. The purchaser is the Owner’s designated representative.

GENERAL LAYOUT AND SIZE

• The control center shall provide the process control operator or operators a comfortable and secure environment in which control equipment will display operating data in a manner to permit unambiguous interpretation of the condition of the process.
• The Owner shall ensure that this data is displayed such that current values, alarm conditions, and historical values are readily available. In addition, the Owner shall ensure that the control equipment provides facilities for the operator to manipulate the operation of the process by a combination of controls. These may include manual operation of the final control elements, auto/ manual switching, controller setpoint changes, and/or the ability to remotely shutdown certain equipment.
• With the advent of modem digital control systems, it is practical to locate centers remote from the process plant. This has allowed consolidation of control of an entire plant into a single building to become practical. Centralized control has several advantages:
• Improved coordination between different process units.
• Rationalization of control operator duties.
• More effective operation supervision.
• Reduced installation costs through shared facilities.
• Some of the factors which shall be considered in the design of these buildings include:
• The type of control equipment to be housed.
• The number of process units to be controlled and the degree of integration.
• The location of the building.
• The environment in which the building is located.
• The desired environment within the control center.
• Requirements for future space.
• Office requirements.
• Adequate space to perform maintenance on the equipment.
• Space for the storage of critical spare parts.
• Proper kitchen and sanitary facilities for personnel.
• Adequate space for storage of safety equipment.
• The general layout of a control building is a single story rectangular structure with a flat roof. No equipment shall be located on the roof of the control buildings. No windows are permitted in new design or additions to existing buildings. The operating area which contains the DCS console(s) shall be located so as to not be in the normal pedestrian traffic flow of the building.
• Control center size shall be determined by the number of process units to be controlled, the number of operators required to control the units, a minimum complement of supervisory/technical personnel, and the amount of equipment within the building. This equipment may include:
• Field termination or marshalling cabinets.
• Communication systems, including but not limited to public address (PA) or paging systems, VHF/UHF radio systems, telephone equipment, general purpose communications associated with Ethernet or similar communication networks, etc.

• Uninterruptible Power Supply (UPS) equipment, batteries, associated switchgear, breaker panels, transformers.
• DCS equipment, including consoles, I/O racks, communication racks, support equipment required by technical or maintenance personnel.
• Programmable Logic Controller (PLC) equipment including I/O communication, and processor equipment.
• Digital interfaces to chromatography equipment.
• HVAC and air purification equipment.
• Utility equipment such as hot water heaters.
• Computers used directly for control or management of the process.
• Unless otherwise specified by the Owner, consideration shall be given to providing kitchen and eating facilities, separate toilet facilities, and shared offices for the shift staff. Day–only office space, locker rooms, and significant conference rooms shall not be located within control centers.
• The single most common trait of control buildings is that generally they are too small, requiring expensive additions or in the case of blast resistant designs, compromising the original design assumptions. Incremental floor space is of trivial cost compared to building additions and gracious allowances should be made in rack rooms, power rooms, and operator console areas.

SAFETY

• Safety of personnel is the prime consideration in the design of control centers or associated facilities. Any new design shall include safety of personnel as the first objective, and revamps to existing control centers or associated buildings shall include review of the protection provided to personnel.
• Safety includes not only protection from the most significant hazard such as blast effects but also fire, exposure to vapors or dusts, falls, lack of proper storage for safety equipment, an environment which promotes fatigue, noise, and the full gamut of related issues.
• For a new central control room design, the recommended location is on the periphery of the process units, located with good access to roads and upwind of any units likely to release hydrocarbon vapors which could travel along the ground. They are generally alkylation, light end facilities, spheres and bullets. Propane, Butane, and other hydrocarbons of approximately the same molecular weight are the prime candidates to form vapor clouds. Similarly, H2S and HF releases can travel as vapor clouds or fog. Distance from the potential release source, ignition source (if appropriate), and proper location with respect to prevailing winds are appropriate and economical defenses for new control room designs.
• As described above in Section 4.4 of this Practice, no personnel other than those necessary to man the control center should be located within it. Since the governing premise for the design is personnel protection, minimizing personnel within the risk area is appropriate.
• When the required proximity of a new control center to the process is such that a hazard exists, the appropriate solution is a standard building design optimized for blast resistance or a complete blast resistant design. Refer to EP 4–1–2 .

• There is no single blast resistant design appropriate to all situations. If a hazard scenario is developed which leads to an extraordinary (greater than 25 PSI overpressure) design basis, the location of the proposed structure shall be re–evaluated rather than simply proceeding with the design. Such a result implies that no reasonable structure can protect personnel, so an alternative may be to remove the building from the threat area.

CONTROL CENTER INTERIOR DESIGN

• The control center is the focal point of the plant. Consideration in its design must accommodate visitors both from within and outside the company. The visitor should be able to view the operations area without becoming a nuisance to the operators. A small meeting or conference room immediately adjacent to the control room, with appropriate glass paneling and blinds can serve a dual purpose as a training room and a showroom for visitors.
• In no case shall the actual control room (where operators are seated) be a part of the general traffic flow within the building.
• In the layout of the control center, the following shall be accounted for:
• A minimum of three (3) exits. For blast resistant designs, there is generally no exit on the side towards the greatest blast hazard.
• All exits should be provided with “air lock” configurations, generally with the “air lock” enclosed within the building.
• Any equipment installed within the control center should be removable.
• Consistent with the above, corridors and hallways should be a minimum of six feet wide, with double doors to accommodate equipment removal. In blast resistant designs, double doors shall be located on a building face opposite the blast risk. This is because double–door configurations are unlikely to survive a significant blast if directly exposed to it.
• Kitchen facilities and eating areas appropriate to the shift complement.
• Separate toilet facilities for each sex.

CONTROL ROOM

• The control room is the operational area of the control center. The control operator(s) is located in this room with all equipment necessary to perform his job.
• Design of the control room and its equipment is of overwhelming importance to the safe operation of the plant.

CONTROL CONSOLES

• With the advent of DCS systems the use of control consoles has gained prominence. A control console for an operator is generally made up of three or more CRT’s along with associated keyboards, and other input devices (trackball, mouse, touch screen). Consoles are usually modular and may include space for alarm and logging printers. The design of the console generally permits an operator to accomplish his duties while remaining seated.
• onsoles may house magnetic loading devices (disk, tape), dedicated alarm annunciators, hardwired switches, and communication equipment such as telephone, PA’s or radios.

• Entry of cabling to the console shall be from below utilizing the suspended flooring system. If the console weight or access requirements justify, a steel support system may be used to support the console independent of the suspended flooring system.
• A minimum clearance of four (4) feet shall be provided around a console for easy access during operations and maintenance.
• Operators seated at control consoles should generally not use portable radios during their operation of the console due to the possibility of interference to the DCS. Instead, remotely– mounted radios with controls located on the operator console shall be provided.

AUXILIARY EQUIPMENT AND UTILITIES

Space shall be provided for auxiliary equipment and other control center facilities. The following list covers the most common requirements:

• Termination cabinets shall be located within the I/O equipment rack room adjacent to the field cable(s) entry point into the rack room unless otherwise approved by the Owner’s Engineer. Termination cabinets may be segregated by type (digital, analog) depending on local practice.
• Computer equipment should be located in a room adjacent to, but separate from the DCS equipment.
• A separate electrical utilities room should be provided for the UPS and other electrical equipment required to support the control systems, computers, HVAC, lighting, and emergency circuits.
• Batteries for UPS equipment which can evolve Hydrogen shall be located in a separate battery room. This room shall be vented only to the outside. A doorway into the battery room from outside shall be provided. Coatings appropriate to resist acid spills, eyewash stations and emergency showers shall be provided as appropriate.
• Motor control centers and large scale power switching/handling equipment shall not be apart of a new control center design.
• Installation of computer flooring throughout the control room, rack room, utilities room, and offices shall be required. Segregation of wiring shall be enforced to minimize undesirable interactions between cabling systems. Installation of AC power cabling in conduit is highly recommended.
• Shared office space for shift and operating supervisors shall be provided.
• A mechanical equipment room to house general purpose HVAC and utility equipment. HVAC equipment shall not be mounted on the roof of any building due to poor access and lack of protection in blast resistant designs.
• Rack rooms shall use stand alone redundant HVAC systems. Stand alone systems generally include heating, air conditioning, and humidity control in a single unit. Two units, each sized for 100% of the room requirements are the minimum acceptable configuration.
• Positive pressure shall be maintained inside of all control buildings. All makeup air shall be scrubbed of trace acid gases. The air inlet to the scrubber shall draw air from a clean source unlikely to contain dusts or hydrocarbons.

• The location of laboratory or testing facilities within the control center is prohibited. Any samples of material brought to the control center for examination shall be removed as soon as the examination is complete.

LIGHTING

• Lighting is an extremely important factor in determining the efficiency, comfort, and general effectiveness of the control room operator. With the proliferation of new lighting fixtures and a growing appreciation of the interaction between lighting, interior design, and CRT based systems professional consultation is recommended.
• The CRT based operator interface of DCS’s require special lighting provisions. Lighting fixtures shall be arranged and the surrounding environment structured to eliminate glare. Because current CRT screens are a low level light source, a low level of ambient lighting is generally required to minimize operator eye fatigue. Since individual operators will vary in their interpretation of the appropriate lighting level, provisions shall be made to provide a low level of background (ambient) lighting with local task lighting adjustable by each operator for his console area.
• For non–control room areas, proper lighting is equally important. Almost every office space has a CRT device of some kind, and provisions shall be made to provide, as a minimum, step– adjustable lighting in these areas. These lighting fixtures generally take the form of three–tube fluorescent fixtures with parabolic diffusers. The center bulb in each fixture has switching independent of that provided for the outer bulbs. More sophisticated lighting requirements may be applied to office lighting as appropriate.
• Lighting in rack room and general purpose areas has no unusual demands, but a degree of adjustability may be desirable to minimize night vision adaptation problems. In no case shall lighting from general purpose areas be permitted to compromise the control room environment.
• During failures of AC utility power, enough lighting shall be provided for the control room operation to continue and as provided by statutory regulation for personnel safety. Some amount of lighting shall be provided by battery powered emergency fixtures, however, when allowed by local codes, powering a minimum of lighting from the UPS provides a more normal environment.

CEILING

• The minimum recommended ceiling to floor height is ten (10) feet to accommodate equipment and provide good appearance. A non–dusting acoustic ceiling tile with an exposed grid suspension system is recommended throughout the structure. A minimum unobstructed distance of three (3) feet between the ceiling and the roof support members shall be provided to allow for cabling, ductwork, and lighting fixtures.
• The layout of lighting fixtures in the finished ceiling shall not proceed until the console layout and lighting arrangements are approved by the Owner.

FLOOR DESIGN

• A raised (computer–type) flooring system with steel jacks shall be installed throughout the control center, except in areas such as toilet facilities, kitchens, and hallways. For purposes of segregating HVAC, the rack room area shall be segregated from the remainder of the building with only interconnecting underfloor conduit stubs between areas. The entire underfloor area shall be connected either directly or by provision of multiple (minimum of 3, equally distributed) large diameter (greater than one (1) square foot inside area) connections through a non–raised flooring area (such as a hallway which separates the control room from office space).
• A minimum floor height of 18 inches from the top of the concrete subfloor to the bottom of the raised flooring system shall be provided. Careful planning of the underfloor cable system is required to allow for future changes and provide required electrical isolation between various cabling systems. Installation of underfloor cabling is not to proceed until the cabling plan is approved by the Owner.
• Termination, DCS, PLC, and computer equipment cabinets exceeding 50 pounds in weight shall not be installed directly on raised flooring systems. Instead, structural steel supports designed to support both the cabinet and the floor tiling system shall be installed. The steel support assembly shall be rigidly bolted to the floor slab and the equipment shall be bolted to the steel support assembly. The design of the assembly shall be sufficient to fully support the cabinets while maintaining the maximum of open space below the cabinet for easy installation of cabling.
• To simplify design of the flooring system, most DCS manufacturers will provide empty cabinets which can be used to hold auxiliary equipment such as chromatograph programmers, miscellaneous communication equipment, and other small rack mounted equipment.
• The floor covering material of the control room should be a plastic laminate or vinyl material. Office areas may be carpet for noise reduction purposes. Other areas may be vinyl, plastic laminate, ceramic or quarry tile. In all areas with sensitive electronic equipment, the antistatic properties of the flooring material shall be approved by the Owner.
• The exposed concrete under the raised flooring system shall be sealed then painted with an appropriate white paint.

PAINTING AND WALL COVERING

• The treatment of interior walls shall consider the effect of surroundings on worker efficiency. Professional consultation may be appropriate during the design phase.
• Wall coverings may also effect ambient noise levels and the effect of wall coverings or acoustical wall treatments shall be considered. Generally, the equipment rack rooms are the noisiest environment encountered, but any and all rooms shall be considered, especially those adjacent to public areas.

INTERIOR FURNISHINGS

• The interior furnishings of the building shall be selected before detailed design commences. In addition, the furnishings shall be selected for a specific purpose. For example, if a supervisory office contains a CRT device, the furniture supporting it shall make appropriate allowances for keyboards, cabling, required reference manuals, antistatic chair designs, etc.

• Similarly, if an office is shared by individuals covering a shift position, sufficient allowance shall be made for both shared and personal equipment.
• If a conference or meeting room is included in the design, the location of audio/visual aids, projection screens, provisions for remote control of audio/visual equipment and lighting shall be considered.

HEATING VENTILATING AND AIR CONDITIONING (HVAC)

• When designing an HVAC system for a control building, the following load factors shall be considered:
• Inside design conditions.
• Outside design conditions.
• Size and physical characteristics of the control building.
• Average number of occupants.
• Heat load from the equipment housed in the control building, including provisions for future expansion.
• Makeup/pressurization flow rate required to maintain positive pressure on the building.
• The HVAC system design shall consider worst case summer and winter conditions. The use of steam injection for humidity control shall be strictly prohibited. Recording devices for temperature and humidity shall be installed in all rack rooms and control rooms.
• The basic HVAC design is as follows:
• A central HVAC system that provides heating, cooling, filtering, and humidity control shall be installed in the utility area of the building. It may use the suspended flooring system as a plenum or as space for ductwork as appropriate. The central HVAC system shall service all areas except the rack room(s).
• In rack room(s), stand alone redundant HVAC units provided by Liebert Corporation (Columbus, OH) or equivalent shall be installed. Each unit shall be sized for 100% of the anticipated load. Loss of one or both units shall alert the operators by audible and visual means other than through a DCS console. The space below the raised floor shall serve as a plenum supplying cool air, with the warm air return to the top of the units. The number of raised flooring tiles with air diffuser grates shall be balanced with the need to provide good circulation through all equipment cabinets.

AIR PURIFICATION/POSITIVE PRESSURIZATION

• In addition to providing for human comfort, air purification and positive pressurization work together to protect the equipment in the control building(s) from corrosion, abrasive or conductive particles, and potentially hazardous fire or explosion conditions.
• Air purification attempts to eliminate these hazards by chemical and/or physical treatment of the fresh and recirculating air. Positive pressurization of the control building reduces the likelihood that contaminants can enter the building except in the makeup air stream, which is constantly being treated.
• Air purification includes:
• Filtering suspended particles using fiber, electrostatic, or other filtering devices.

• Minimizing the likelihood of undesirable vapors entering the building by locating the air inlet duct appropriately.
• Removing corrosion species (Hydrogen Sulfide, Sulfur Dioxide, Ammonia). The make–up air stream shall comply with ISA–S71.04 condition G1.
• All makeup air provided by the positive pressurization system shall pass through the air purification system.
• Positive pressurization systems shall include equipment to detect combustibles and/or other potential gases H 2 S, HF) in the makeup air stream. If detected, the positive pressurization system shall automatically shut down (HVAC now on recirculation only) and the cause of the shutdown shall be clearly indicated both audibly and visually to the operator(s) through a system other than the DCS console.

FIRE PROTECTION

• The ultimate requirements for fire protection are established by a wide variety of regulatory agencies and national standards. However, some basic guidelines are generally applicable.
• In general, sprinkler systems are not installed in control buildings. The risk of inadvertent water damage to DCS equipment results in a much larger risk to human life than would be protected by a sprinkler system. In general purpose areas such as kitchens such systems may be applicable.
• Smoke detection is required under raised flooring systems. Under floor water detection shall be included in all installations. Floor tiles located above detection equipment shall be marked to improve maintenance and testing of the detectors.
• Thermal and smoke detection is appropriate in all areas.
• All detectors shall be connected to a central fire alarm panel for the building. Remote annunciation of alarms from the panel shall be made if appropriate.
• Carbon dioxide extinguishers shall be provided for all electrical and electronic equipment areas. The use of Halon “flood” systems in continuously manned buildings is prohibited. For general purpose areas the extinguishing agents shall be selected as appropriate to the threat.

GROUNDING

• Proper grounding of DCS and computer equipment is critical to stable operation. The recommended practice is to install a single point grounding system for all equipment located within a building. Some DCS or computer equipment vendors may insist on separate grounds, but the risk of different ground potential is severe. EP 13–2–1 and EP 13–15–1 governs the design of grounding systems.
• Connections between DCS or computer equipment with similar equipment in other buildings should use fiber optic media without metallic shields or pull supports to electrically isolate the building and system grounds.
• Final design of the grounding system shall only proceed after consultation with the DCS equipment manufacturer.

• Designers should be alert to the introduction of “sneak” circuit grounds, especially between computer equipment.

ELECTROMAGNETIC INTERFERENCE (EMI)

• Modem DCS and computer equipment both cause and are susceptible to (EMI). VHF and/or UHF radios are the most significant threat to DCS and computer equipment. EMI from DCS and computer rack rooms can be reduced by including aluminum–faced fiber board under the finish wall surface. The aluminum–faced fiber board should extend below the top of the raised floor and above the suspended ceiling.
• Some equipment manufacturers prohibit the use of portable transmitters near their equipment or under some specified conditions such as when cabinet doors are open or removed.

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