Fans & Heaters technical seminar Variable Pitch Axial Fans Henrik Bagger

Variable Pitch Axial Fans Seal air Stalling 2 Stall warning 3 Stall probes Corrosion protection 4

Variable Pitch Axial Fans Seal air fans All flue gas fans are fitted with seal air units to prevent the flue gas from entering the inner tube of the diffuser and intermediate piece. The sealing air units can be supplied as stand-alone units or with back-up. Forskellen mellem standardløsningen før 2011 og efter 2011 er, at den nye løsning er billigere, og at indsugningen (air in-take) er gået fra at være rund til at være firkantet. Standard seal air, 2011 onwards

Variable Pitch Axial Fans Seal air fans Trykket fra spærreluft skal være 200 Pa højere end i ventilatoren Spærreluft i diffusor og mellemstykke er standard Om der skal spærreluft i sugekasse er afhængig af beregningerne

Variable Pitch Axial Fans Stalling The fan can be operated safely inside the performance envelope Above the stall line the fan cannot operate for a long time

Variable Pitch Axial Fans Stalling Description of stall cells One or more stall cells from B to D Stall cells rotate at a lower frequency From B to D blades are in and out of cells At D stall cells cover the full circumference From D to E the layer of cells gets thicker Stall cell

Variable Pitch Axial Fans Stalling Stresses in blades Stress amplitude 1 10 103 106 104 102 105 107 108 Number of cycles SN Curve σg Rm Infinite lifetime Stress levels in blades Normal operation – shaded area Stall situation – non-shaded area Example: Natural frequency of blade: 35 Hz No. of cycles for 15 min.: 15x60x35 = 31,500

Variable Pitch Axial Fans Stall warning A stall situation mostly occurs when a system is operated incorrectly creating a situation where the resistance in the system is too large in relation to the volume flow the fan must deliver. Howden offers two solutions at which the risk of stall can be minimized or eliminated. Stall Probe (Green line) The stall detector is connected to a manometer which registers the pressure difference in the airflow. The stall detector gives a signal when the fan works in an unstable area before the fan stalls. Stall Elimination system (Red line) The anti-stall ring is placed on the suction side of the fan where it absorbs and turns the recirculated air coming from the blades when a fan is working in a stall area. The anti-stall ring is increasing the stable working area of the fan Howden Variax has a patent on this solution. Stall Avoidance system (Blue lines) The actual operating fan load is calculated by means of a differential pressure measured across the side tappings of the inlet box and inlet cone installed on the fan from the inlet of the inlet box to the inlet cone (volume flow) and across the fan from inlet box to diffuser end (pressure rise) respectively. The actual operating point is specified in a graph characterised by a combination of pressure and volume. An alarm line and a trip line are drawn on the fan graph. If the operating point (combination of volume flow and pressure) is within the stall alarm area, an alarm is released. If the operating point (combination of volume flow and pressure) moves outside the stall alarm area and into the trip area, the blade angle is automatically regulated and the operating point moves outside the trip area. Thus the risk of stall is avoided. Red lines: Stall Warning System Green line: Stall Probe

Variable Pitch Axial Fans Stall warning Fan equipped with stall warning measurement rings and pressure transmitters All tubes are designed to have gravity drainage – First generation difficult to purge and clean

Variable Pitch Axial Fans Stall warning Calculation of fan operating point Performance curve with stall warning line

Variable Pitch Axial Fans New constant purge air system

Variable Pitch Axial Fans New constant purge air system Design rules for new constant purge air system: Single ports only – no rings connecting several ports Two identical measuring systems are applied – averaged in the DCS Static ports are still designed for gravity drainage ½” OD stainless steel pipes are used Calibration of purge air system: Fan is not running Purge air is switched on Absolute transmitter adjusted to show barometric pressure The two differential transmitters adjusted to show zero

Variable Pitch Axial Fans Stall probes Pictures of plugged ports from site Stall probe is plugged Stall probe is plugged and cleaned with wire brush

Variable Pitch Axial Fans Stall probes Two different sizes of stall probes Stall probe detects stalling Mounted upstream next to first stage Standard probe has 5mm holes Large probe has 10mm holes Probes are calibrated at site

Variable Pitch Axial Fans Stall probe signal for axial fans Fan characteristic  = Angle of attack

Variable Pitch Axial Fans Stall probes Location and design of stall probe Stall cell

Variable Pitch Axial Fans Constant purge air system Stall probe from European site with high dust load

Variable Pitch Axial Fans Calibration of purge air system Stall probe transmitters Fan is not running Purge air is switched on Adjust so differential pressure across probe is zero To ensure correct functionality, the calibration procedure for stall probe transmitters should be made annually Note: The calibration of the stall probe itself (turning) must be made when the fan is running and in accordance with the manual

Variable Pitch Axial Fans Corrosion protection Bearing Housing : Normal Intermediate casing : Corrosion on the guide vane root Inlet box : Ash and inner tube corrosion Diffuser : Normal Hub surface (DE): Corrosion Hub (NDE): Surface normal Blade (NDE): Normal Blade root(DE): Corrosion Hub side plate (DE/NDE): Corrosion 3

Variable Pitch Axial Fans Corrosion protection Corrosion on inlet box inner tube surface Enhanced condensation of sulfuric acid (H2SO4) on the surface of inner tube caused by cooling effect of sealing air entering from the atmosphere and heated to 70~80℃ (160- 175 deg.F) and a high acid dew point temperature 160 C (320 F) 70~80 C (~170 F) Heater [ Countermeasure ] - Blocking and reducing air entering from outside 4

Variable Pitch Axial Fans Corrosion protection Corrosion of blade root disc and hub caused by cooling effect of sealing air which passes through the hub side seal and blade root gap from inner tube of inlet box Blade root thinning Sealing air : 70~80 ℃ (170 F) Corrosion around blade root 1st side (DE) Hub [ Countermeasure ] - Apply special seal for seal air blocking - Block and reduce air entering from outside Corrosion on hub surface 5

Variable Pitch Axial Fans Corrosion protection Condensation of sulfuric acid (H2SO4 ) on the surface of hub caused by cooling effect of sealing air and high acid dew point temperature Enhanced corrosion of drive/non-drive hub side plate caused by condensation of sulfuric acid (H2SO4) from flue gas which unexpectedly penetrated through seal air exit Corrodsion hole Sealing Air Flue gas Hub side plate : Corrosion < Hub side plate > Labyrinth Seal 2nd side (NDE) Hub 6

Variable Pitch Axial Fans Corrosion protection Countermeasure - Improvement of sealing air system (reduced seal air) Minimizing seal air flow for main bearing assembly and inlet box inner tube which prevents condensation by minimizing the cooling effect of relatively cold seal air (30 ~ 40℃ ) (85-105degF) Contact type Labyrinth seal Suction Box Heater Damper and heater off Add valve on seal air pipe Blocking the intake air to inner tube 13

Variable Pitch Axial Fans Corrosion protection Countermeasure - Improvement of sealing air system (additional seals) Minimizing cooling effect by reducing the amount of seal air flow by an additional labyrinth seal Minimizes the temperature difference between hub surface and flue gas Prevents condensation of sulfuric acid on the hub surface Preventing unexpected penetration of flue gas to hub side plate by labyrinth seal and shield plate Ensure no penetration of flue gas – no condensation on side plate of hub Shield plate Hub Surface Special Special Seal Labyrinth Seal (Contact type) Hub Side Plate Blade root hole of the hub surface Hub side plate & casing tube 14

Variable Pitch Axial Fans Corrosion protection Countermeasure – Applying special coating on hub surface and blade When non-metallic coating frequently is in contact with sulfuric acid, it degrades chemically (blistering and delamination). Because of potential risk to accelerate local corrosion beneath blister, it cannot be a permanent solution w/o seal air improvements After improving sealing air system condensation of sulfuric acid is reduced and non- metallic coating can suppress corrosion rate Mild environment (no immersion ,limited H2SO4 condensation) Severe environment (frequently Immersed in H2SO4) Non-met. Non-met. Blistering Before corrosion test and after 6 cycles Acid dew point corrosion simulation cycle 1h Immersion 20% H2SO4 15 min Air drying 16-17h Oven 150C/300F 5-6 h Cooling The test for C-276 showed a similar result – no protection

Coated blade with PFTE after 6 month operation

Coated Blade with PTFE (NEW)

Thank You Any Questions?