Section 12 — Instruments and Controls

Control Systems for Steam Generators

IPE Engineering Practice IPE-EP-12-4-1

Document number: IPE-EP-12-4-1 · Section: 12 — Instruments and Controls

SCOPE

• The purpose of this Practice is to present requirements for the design of a steam generating unit instrument and control system.
• Any deviation from this Practice must be approved by the procedure described in EP 1–1–3.
• A revision bar indicates all changes made to this Revision.

REFERENCES

• All equipment and materials shall be of such construction, and shall be installed in such manner as to be in accordance with the latest edition of these and other applicable local, state, or national codes and standards.

STANDARDS AND PUBLICATIONS

• In case of conflict between this document and these Codes, Standards, and Specifications, the more stringent requirements shall govern unless specifically noted otherwise.

DEFINITIONS

• Contractor - Company or business that agrees to furnish materials or perform specified services at a specified price and/or rate to the Owner.
• Inspection - An activity to check that design, materials, fabrication, and testing comply with the requirements of the specification.
• Manufacturer - The recipient of a direct or indirect purchase order for materials and/or equipment. In this context, a direct order is one issued to a manufacturer by a contractor or the Owner. An indirect order is one issued to a manufacturer by a vendor (recipient of a direct order) for materials, fabricated components, or subassemblies.
• 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.

QUALITY ASSURANCE

• The contractor shall operate a quality management system to ensure that the requirements of this specification are achieved. Each contractor shall state, as part of the quotation the quality assurance program and procedures used. IPE may require demonstration of the quality system.
• The implementation of such a quality system shall be subject to monitoring by IPE and in addition may be audited following an agreed period of notice. IPE personnel shall have the right to inspect at all reasonable times and at all locations during the engineering, configuration, assembly, and testing of the steam generator controls, systems and cabinets.
• The system vendor and contractor shall ensure that technical and quality requirements specified in this document are applied to all materials, equipment and services provided by subcontractors and to free issue materials.

INSTRUMENTS AND CONTROLS

• General
• IPE steam generators shall be arranged for fully automatic operation, of the full modulation type, and shall be provided with all safety devices required by the National Fire Protection Association (NFPA). Approval of the National Fire Protection Association must be obtained by the vendor.
• Detailed requirements for control equipment shall be as specified by IPE in accordance with the applicable portions of EP 12–1–1 and EP 12–2–1.

• The steam generator supplier shall be responsible for the satisfactory design and operating capability of the instruments/controls and safety equipment and associated with the steam generator. The supplier shall submit details to IPE for approval before purchasing the specified equipment.
• Electrical equipment design and installation shall conform to EP 13–1–1 and EP 13–2–1.
• Control Valves
• Control Valves shall be specifically selected for the full dynamic turndown of the system, i.e. for start–up and over the full firing range. For high turndown where the required control is greater than the normal range, two valves in parallel may be used.
• Control valve selection shall conform to EP 5–3–17.
• Fuel Supply to Burners

Where automatic valves are proposed forthe “on” and “off” control of the fuels and steam to individual burners, separate manually operated valves shall also be provided at the steam generator front. All these valves, both automatic and manual, shall be specifically selected to give reliable operation, tight shut–off, and no external leakage over the full operation period between steam generator overhauls, which should be not less than 26 months. Valves shall be of the ball valve type subject to the operating temperature and pressure being within the rating of the valve seat, etc. Overtravel on automatic valves shall be sufficient to operate limit switches satisfactorily.

• Combustion Control
• The combustion control system shall be of the type described in API 550 and must conform to NFPA 85 series. It shall be cross limiting, lead–lag type and maintain a predetermined fuel/air ratio so that the air rate leads the fuel rate on load increases and lags the fuel rate on load reductions. In addition:
• The CO2 content of the flue gas throughout the entire normal firing range shall remain within

” 1% CO2 of that attainable by expert hand operation under similar conditions.

• The steam pressure shall remain within ” 2% of the desired value at any point within the entire operating range of the steam generator.
• Combustion controls shall be fully integrated with the cross limiting control system and the flame safeguard system to assure low–fire light–off, proper furnace purging controls locked in low–fire position until after the main flame has been proven.
• The vendor shall provide a master pressure controller for steam pressure which shall obtain its process signal from the main steam header via an electronic transmitter.
• The master pressure controller shall make adjustments to air flow and fuel flow according to steam pressure changes.
• The control system shall allow manual adjustment of fuel/air ratio.
• A steam pressure gauge, with siphon and shut–off valve shall be provided.
• The control system shall allow selection of automatic/manual operation and manual positioning of valves and dampers.

• A reliable O2 analyzer shall be supplied. The air flow shall be constantly trimmed within adjustable limits dependent on load, to maintain the specified excess O2 in the flue gases. Analyzers shall meet the requirements of EP 12–7–1.
• In steam generators having balanced draft, the ID fan inlet guide vane setting or fan speed shall be varied to maintain a constant negative pressure in the furnace of minus 0.25 inch to minus

0.5 inch water gauge.

• Feed Water Control
• A three (3) element feed water control system shall be provided to control steam generator drum water level unless otherwise approved by IPE.
• The feedwater control system shall include a steam flow meter and orifice plate, drum level control system, feed water control valve, flow meter, and orifice plate. Shutoff and drain valves shall be included for feed water control elements.
• The control system shall function to maintain the water flow input equal to the steam flow output from the steam generator, with readjustment as necessary from drum level deviation from set point.
• The capability shall be provided to permit manual operation of the system.
• The feedwater control valve shall be a pneumatically actuated spring opening design. Valve body material shall be suitable for pressure and temperature process conditions. Valve internal trim shall be hardened stainless steel. THE VALVE SHALL BE EQUIPPED WITH A DECLUTCHABLE SIDE MOUNTED HANDWHEEL and a cam actuated positioner. Valve shall be sized for maximum steam generator capacity at 80% travel, with at least a 25 psi pressure drop.
• The three–element control system shall maintain the drum level within 7% of the set point for which it is adjusted, (allowable for shrink and swell) provided the rate of change of steam load does not exceed 20% per minute, and provided there is adequate feed water supply pressure.
• A double block and bypass with bleed valve configuration around the control valve to permit continuous steam generator operation during periods of valve servicing shall be furnished. The bypass piping around feedwater control valves shall be of the same size as the main feed piping. The bypass valve shall be a globe valve.
• Installation of two (2) direct–reading water level gauges shall conform to ASME Section I, PG–

60. Each gauge shall be capable of being blown–down or isolated for removal and repair without taking the steam generator off load.

• The level gauging system shall be provided with separate high and low level alarm and a “low– low” level fuel cut off. The extra low level switch shall cause the fuel to the steam generator to be cut off and issue an emergency alarm signal, both visual and audible, in the steam generator control room. The point at which this switch operates shall be at a water level high enough to protect all pressure parts from overheating and still be visible in the gauge glasses. It shall not be so near to the low water alarm level that there would be insufficient time for an operator to make adjustments for the first condition before the second arises for most failure modes, causing the shut down of the steam generator.

• Where the drum elevation above operating floor level prevents the operator from viewing the direct water level gauges, a remote gauge of a proven type shall be provided in addition to the minimum gauge arrangement required by ASME code. It shall be located at operating floor level and positioned so as to be easily seen by the operator standing at the feedwater regulating and bypass valves. Bicolor gauges equipped with mirrors to reflect a view of the drum gauges down to operating floor level shall not be used, although the bicolor gauge itself is acceptable to improve identification of the water column.
• Steam Superheat Control
• Where attemperators are proposed to control the degree of steam superheat, the temperature of the steam at the steam generator stop valve shall be the control criterion. The steam temperature leaving the attemperator shall also be used in the control loop to ensure maximum response rate and accuracy.
• Control of surface–type attemperators shall be by regulating the amount of steam passed to the cooling surfaces. The spraywater type shall be controlled by regulating the amount of spray water injected into the steam.
• The final temperature of the steam shall not vary more than 1% from the specified set point at any load, for steady state operation of the steam generator.
• Burner Management and Control Logic Programming System
• A complete burner management system with programming facilities, “Fireye” or approved equal, shall be furnished and shall be fully integrated with the combustion control and limit control system for maximum safety. It shall be installed local to the burners.
• The system shall jointly monitor the burner and steam generator to ensure safe start–up and shut–down of burners and steam generators.
• Unless otherwise required by IPE, the system shall, on the pressing of push–buttons, arrange for the entire sequence of burner light–up or shut–down to be automatically carried out with a high degree of safety and reliability. It shall also automatically shut down burners on identification of a fault condition serious enough to warrant such action, or raise alarms to indicate faults of a less serious nature.
• Separate buttons shall be provided to initiate purge and individual burner start–up, and also for individual burner and steam generator shut–down.
• Reset facilities shall be provided for both steam generator and individual burner trips.
• The burner management system shall utilize two individual flame sensing devices for detection and supervision of flame behavior on each burner, with any one detector signal arranged to initiate an alarm and the two signals together to cause lockout of the fuels to the burner.
• The flame detector shall monitor the pilot flame during light–off and shall monitor the main burner flame during normal operation.
• The detector shall readily detect flame under all fire conditions of both pilot and main flame but shall not be actuated by hot refractory or other hot bodies.

• The burner management system shall shut off the fuel supply to the burner and sound an audible alarm in case of flame failure. A shut–off valve and a locally mounted alarm shall be furnished by the Vendor. Contacts shall be provided for connecting to a remote alarm.
• The burner management logic and operation shall conform to NFPA 85 series. It shall provide all the functions necessary for safe operation. The logic shall consist of:
• Purge Cycle
• Precombustion purge timer to assure operation of the draft fan for a period of time sufficient to provide four (4) complete air changes within the fire box before the pilot flame is ignited.
• Postcombustion purge timer to continue operation of the draft fan, after the fuel supply valve has been shutoff, until four (4) complete air changes within the fire box have been made.
• Ignition with air damper and fuel control valves at the minimum firing position.
• Pilot flame proving.
• Main flame proving.
• The burner management and electrical control logic programming system shall be designed to operate with the electrical control voltage specified by IPE.
• Key–operated override switches shall be provided for all shut–down functions. These switches shall also override those start “permissives” which are also shut–down functions. The override switches shall normally be located on the front of the main control panel. If located on the rear of the panel, then indication of the override condition shall be given on the panel face and in the main control room.
• All shut–down systems shall be capable of full function testing from primary sensor up to final actuation device while the plant is on line. Key–operated override switches shall be provided for this function. These shall override the minimum number of function components. Indicators shall be provided to show when the trip circuit is being overridden for test. Final element trip testing on a “single fuel” basis should be provided where more than one fuel is used. Where the loss of an individual burner can be tolerated in the steam system, the test facility shall include tripping individual burners. IPE will specify those trip functions where it is necessary to provide duplication for trips or a two–of–three voting system to give increased reliability.
• All override test facilities shall be mechanically protected and accessible only to personnel authorized to perform testing.
• All systems shall be energized during normal operation, but if systems that are de–energized during normal operation are used, they shall be provided with power supply and trip circuit monitoring.
• Safety Controls

The steam generator safety controls shall conform to NFPA 85 series. The Vendor shall furnish the following safety controls, all factory installed on the steam generator insofar as possible.

• High and low water level alarms and “low–low” water level shut–off of fuel.
• Airflow measuring element and draft fan interlock to shut off the draft fan and fuel supply in case of loss of combustion air.

• High and low fuel pressure shut–offs.
• Low atomizing steam or air pressure shut–off (for fuel oil only).
• Low fuel oil temperature shut–off (for heavy fuel only).
• All safety shutdown devices shall be connected to an audible alarm, with individual alarm indicators to isolate the source of the alarm.
• Operation of any safety device shall require a manual restart after the cause of the trouble has been corrected.
• Low flue gas oxygen alarm.
• High deviation alarm between steam load and feedwater supply.
• Control System
• IPE will specify whether controls are to be pneumatic or electronic. All control system vendors shall be approved by IPE.
• Control Panel
• Vendor shall supply a control panel, completely shop assembled, wired, and piped.
• The panel shall contain all automatic control instruments and alarms.
• Reference shall be made to EP 12–1–1 for general requirements of panel design.
• Distributed Control System: The steam generator control system shall conform to EP 12–3–1.
• Unless otherwise specified, automatic control of the following functions shall be provided:
• Fuel supply to burners.
• Combustion conditions.
• Steam superheat.
• Steam drum water level.
• Feed water supply.
• The following indications and shall be included unless specified otherwise:
• Pressures at:
• FD fan outlet.
• Air heater outlet (air side).
• Burner windbox.
• Furnace at burner level (three individual measurements per NFPA 85G).
• After superheater.
• Steam generator outlet.
• Economizer outlet.
• Air heater outlet (flue gas side).
• ID fan outlet.
• Feed water supply.
• Drum and superheater outlet (also local indicator).
• Instrument air.
• Fuel supply (also local indicator).
• Fuel pressure after main control valve (also local indicator).

• Atomizing steam (also local indicator).
• Air and flue gas temperature at:
• Air heater inlet (flue gas).
• Air heater outlets (air and flue gas).
• Flue gas after secondary stage of superheater.
• Flue gas after primary stage of superheater.
• Flue gas at steam generator outlet.
• Flue gas O2 analyzer/controller/trend (also local indicator).
• Flue gas opacity indicator with audible alarm.
• Fan speed.
• Fuel temperatures at burners for oil firing (also local indicator).
• Burner and pilot ON/OFF indication.
• Feed water temperature at inlet to economizer.
• Superheater outlet temperture.

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