Ch5 A-12


Objective 1: Describe Blowoff, Blowoff Equipment, and Blowoff Procedures

  • The focus of this chapter includes understanding blowoff processes, related equipment, and procedures essential for boilers.

Bottom Blowoff

Importance of Bottom Blowoff

  • Bottom blowoff is crucial as condensate may be polluted with other fluids like glycol and oil. Condensate that is lost through humidification or blowdown (B/D) needs to be replaced with make-up (M/U) water, which may contain varying amounts of impurities, including suspended and dissolved solids or gases.

  • Make-up water is typically softened and filtered to reduce the concentration of impurities. Over time, suspended solids will settle at the bottom of the shell as sludge, which must be periodically removed using bottom blowdown.

Frequency of Bottom Blowoff

  • The frequency of blowoff operations is influenced by:

    • The rate of steam production.

    • The type and concentration of impurities present in the make-up water.

    • The percentage of make-up water added.

  • Typically, bottom blowoff occurs at least once per shift, but this can vary based on operational conditions.

Blowoff Piping System Design

Design Considerations

  • The blowoff piping and system are subject to extreme temperature fluctuations requiring special designs to handle these shock conditions.

  • Key design features include:

    • Strong blowoff valves that resist cavitation and control flow.

    • Compliance with standards like ASME B31.1 for pipe specifications regarding diameter and materials based on maximum allowable working pressure (MAWP).

Bottom Blowoff System Components

Specifications for Valves

  • Different types of blowoff valves are specified based on their construction and size:

    • Single Blowoff Line: Minimum piping diameter of 20 mm and maximum of 50 mm.

    • Multiple Blowoff Lines: Requires an additional valve on each individual blowoff line for safety and reliability.

    • Small capacity boilers can use blowoff connections as small as 15 mm, but the design pressure must not be less than 690 kPa.

Hard-Seated Slow Opening Blowoff Valves

Design and Functionality

  • Hard-seated valves utilize a globe-type mechanism with a seat and plug made from hard, wear-resistant metal alloy, suitable for throttling.

  • Y-Type Globe Valve: Known for reducing pressure drops due to its angled stem design, which reduces the effects of flashing and cavitation. Sediment and water can freely discharge through the valve, without being obstructed by 90* turns in flow path of regular globe valve. Because the flow through this valve is fairly straight, sediment and debris cannot build up under the valve seat to eventually interfere with the closing valve.

  • Slow-opening valves provide better thermal shock management, requiring multiple turns to transition from closed to open. Requires at least fie 360 turns

Blowoff Valves

  • Bronze body Y-type

  • Rising stem open and closed

Seatless Slow Opening Valves

  • No plug or seat

  • Open when ports in lower plunger align with inlet ports, allowing fluid to flow freely into the system, and close when the plunger moves out of alignment, effectively blocking the inlet to prevent backflow.

Quick Opening Blowoff Valves

  • POLISHED 2 PIECE SLIDING DISC THAT MATES WITH SEAT

  • SPRING B/N DISCS FORCES THEM OUT CREATING SEAL

  • IF VALVE LEAKS RETRY BUT EXTENDED LEAKING CAN CUT METAL

Quick Opening Blowoff Valves

  • Tuff to tell whether open or closed so usually stamped onto forging

Blowoff Piping

  • RAPID FLUCTUATIONS AND EXPANSION/CONTRACTION

  • PIPING MUST BE ANCHORED SO AS TO NOT STRESS MUD DRUM CONNECTION

  • PROVISIONS MUST BE MADE FOR LEAKAGE INSPECTION

Blowoff Vessel

  • HI TEMP HI PRESSURE WATER WOULD DAMAGESEWER SYSTEM SO A BLOWOFF TANK IS USED TO: depressurizes the blowoff water by venting steam to the atmosphere and it cools the remaining blowoff water to a temperature suitable for sewer system

  • INLET LINE DISCHARGES BELOW LIQUID LEVELMIXING WITH COOLER WATER AND AGITATINGSLUDGE FORCING WATER TO SEWER

  • NOT REQUIRED ON HEATING BOILERS OR BOILERSUSING A POND

  • COLD WATER CONNECTIONS HELP TO PREVENT D/CTEMPS ABOVE 65C, ensuring that the blowoff water remains at a manageable temperature before it enters the sewer system, thereby protecting the infrastructure from thermal shock and potential damage.

Bottom Blowoff Procedures

  • ONLY DONE TO CONTROL SEDIMENT ACCUMULATION

  • EXCESSIVE BLOWOFF IS WASTEFUL

    Large steam generating units use very high purity make-up water. For this reason, utility boilers do not accumulate much waterside sediment. Therefore, these boilers are blown off infrequently, and only under light loads or when not in operation

  • Waterwall header blowoff is never performed when these boilers are in operation. To do so might cause temporary loss of water circulation, and expose the tubes to failures from overheating

  • For Large steam generating units, bottom blowoff valves are mostly operated during boiler startup and for draining

  • Blow off piping is connected to the bottom of the boiler for 2 reasons:

    • Although small amounts of suspended matter are dispersed throughout the boiler, the greatest point of deposition and accumulation occurs at the bottom of the water space

    • Bottom blowoff valves are also used to drain the boiler waterside for maintenance and inspection

  • Also used for level control, especially S/U to control swell and ensure circulation

Key Considerations During Blowoff

Safety and Operational Guidelines

  1. In plants with multiple boilers, clear identification of valves is essential to prevent cross-contamination.

  2. When performing a blowoff, ensure related valves are securely locked or tagged if boilers are undergoing maintenance.

  3. An assistant should monitor gauge glass levels during blowoff to prevent low water conditions.

  4. Always verify that quenching water is on before starting the blowoff procedure, and never leave valves unattended.

  5. Open and close blowoff valves in the proper sequence to avoid thermal shock and water hammer effects.

  6. Limit blowoff operations to one boiler at a time during off-peak loads to minimize the risk of circulation disruptions.

  7. Open the blowoff valves slowly, and in the proper sequence. This will permit the blowoff piping to warm up and expand gradually. As well, this will prevent water hammer.

  8. Close blowoff valves slowly, and in proper sequence

  9. Only one boiler should be blown down at a time

  10. Perform bottom blowoff when boiler load is low or moderate. During blowoff, boiler water circulation is disrupted. There is less circulation disruption at low or moderate firing rates.


Blowoff Valve Operation

  • QUICK SLOW SLOW QUICK

  • SLOW OPENING REDUCES CHANCE OF WATER HAMMER

  • SAME PROCEDURE FOR 2 HARD SEATED SLOW OPENING VALVES

UPSTREAM VALVE AT BOILER ISCALLED THE SEAL VALVE ORGUARD VALVE

SEAL VALVE SUFFERS LESSWEAR AND TEAR

Two seatless slow opening valves

  • PLUNGER WOULD COMPRESS THE WATER B/N VALVES AND CAUSE DAMAGE

  • THEREFOR DOWNSTREAM VALVE IS OPENED 1ST AND CLOSED LAST

  • MAY USE A 3RD GUARD VALVE AT THE BOILER

Surface Blowoff System

  • SOME CONTAMINANTS GATHER AT THE WATER SURFACE AND MAY CAUSEFOAMING OR PRIMING

  • CONTROLLED WITH PERFORATED PIPE IN THE SHELL JUST UNDER THE NWL

  • CONNECTS TO THE BLOWOFF TANK

  • OFTEN USES A GRADUATED METERING NEEDLE VALVE

  • MAY DO AN INTERMITTENT SURFACE BLOWOFF?

Objective 2

PURPOSE OF CONTINUOUS Blow down

  • IN ADDITION TO SOLIDS AND PRECIPITATE BOILER WATER ALSO HASDISSOLVED SOLIDS SUCH AS CARBONATES, BICARBONATES, CHLORIDES,PHOSPHATES, SULFITES, CHELANTS, AMINES, SILICA ETC

  • These compounds may be

    • Impurities that entered the boiler with the make0up water

    • Impurities that returned to the boiler dissolved in condensate

    • Compounds produced by external feedwater treatment

    • Compounds formed in boiler drum by chemical reactions between impurities and treatment chemicals

    • Chemicals fed to the boiler drum to control corrosion and scale deposits


  • DISSOLVED SOLIDS CONCENTRATION INCREASES AND WILL CARRY OVERWITH WET STEAM DEPOSITING ON S/H SURFACES AND CAN CAUSE FOAMINGAND CARRYOVER

  • TDS is controlled by:

    • Producing high quality feedwater. Feedwater is made of returns and make-up water. If returns are hard, or contain suspended matter, they can be filtered and softened with condensate polishing equipment. Ultra-pure make0up water can be produced with filtration, coagulation, precipitation softening, ion exchange, and reverse osmosis equipment

    • Draining the appropriate amount of concentrated water from boiler. This is done by measuring the solids concentration of the boiler water, and operating metering valve to maintain the solid concentration at the prescribed level. This process is called continuous blowdown, which helps to remove accumulated solids and maintain optimal water quality within the boiler system.

Determining the Continuous blowdown Rate

  • CBD RATE DEPENDS ON THE TDS OF THE WATER. TDS DEPENDS ON:

    • Rate of steam production

    • Type and concentration of impurities in the make-up water

    • Percentage of make up water added

  • IF F/W FLOW IS 5000KG/HR AND B/D IS 250KG/HR THEN250KG/HR/5000KG/HR=.05 OR 5% BLOWDOWN RATE

CBD Piping Systems and Components

  • SHOCK SERVICE

  • CBD NOT EXPOSED TO EXTREME SHOCK AS IBD SO PIPING ONLY HAS TO BEDESIGNED TO MAWP OF BOILER

  • PIPING TERMINATES AT THE B/D TANK

ASME b31.1 (122.1.4) states that boilers operating at 690 kPa or less can use non-ferrous blowdown piping material. For pressures higher than 690 kPa, all blowdown piping must be steel and at least schedule 80

Internal CBD Collection Pipe

  • PERFORATED COLLECTION PIPE EXTENDS THE LENGTH OF THE DRUMABOUT 15CM BELOW NOL WHERE TDS IS MOST CONCENTRATED

Valves

  • ASME requires single isolation (usually gate) to isolate when boiler is out of service (Upstream of Needle)

  • CBD valves are usually needle or V-Notch plug valves

  • May have multiple valves for HP service

V-Notch CBD valve

  • NOTCHES ASSIST WITH FINE FLOW CONTROL AND GIVE BETTER FLOWCHARACTERISTICS

  • TDS IS CONTROLLED BY SMALL ADJUSTMENTS TO CBD

  • SOME HP BOILERS WITH PURE FW MAY OPERATE WITH CBD CLOSED

  • CBD SHOULD BE KEPT TO MINIMUM (HEAT AND CHEMICALS)

Reducing Losses From CBD

  • AUTOMATED CBD SYSTEMS REDUCE B/D AND SWINGS IN TDS

  • OPERATE BY COMPARING TDS TO SP AND CONTROLLING CBD BY USINGCONTROL VALVES, TIMERS, OR SELF POWERED VALVES

  • USUALLY HAVE SOLENOID THAT SHUTS OFF CBD WHEN BOILER DOWN

  • HEAT IN CBD CAN PREHEAT M/U WATER



Blowoff System maintenance

  • HAZARDOUS WITH THERMAL SHOCK AND WEAR AND TEAR (CORROSIONCRACKING ETC)

  • SYSTEMS SHOULD BE INSPECTED FOR:

    • Boiler blowoff vessel: Corrosion at the waterline, erosion of internal wear plate, leaks, scale deposits, and pressure gauge operation

    • Blowoff valves: Cracks, packing leaks, wire drawing, corrosion. and erosion

    • Blowoff piping: Leaking joints, leaking elbows, damaged or missing pipe supports, wrong materials, uncertified welds, and lack of room for expansion

Piping System

IF REPAIR IS NECESSARY:

  • Galvanized piping must never be used in a blowoff or blowdown system

  • For boilers operating above 690kPa,blowoff piping must be atleast Schedule 80 steel

  • Piping bends should use 45degree elbows, if possible. Avoid using 90degree elbows

  • Threaded elbows and connectors on blowoff lines must be strong enough for schock service. Malleable iron fittings of Class 150 and Class 300 are prohibited in blowoff use. So are cast iron fittings of Class 125 and Class 250. For schedule 80 pipe, only Class 2000#(or stronger) fittings can be used

  • Welded pipe installations and weld repairs must be approved by the jurisdictional inspector prior to commencing the work. Welding must be performed by certified pressure welder.

  • VALVES MAY BE REBUILT IF PASSING OR MAY REQUIRE REPLACEMENT

  • ENSURE VALVE IS RATED FOR SERVICE BEING USED – CHECK CODE OR ASKINSPECTOR