SMART check: how boiler maintaining wet tubes

Study Case: water tube steam boiler (once through type)

Symptom: boiler tripped (by safety interlock) due to overheated tube

uSer blackbox view: boiler continuously heats the flowing water inside tube

Modular Analysis: firing system fires the gas fuel to furnace & water system pump feedwater inside tube passes in the furnace ... enough water is needed to ensure acceptable safe tube temperature

Suspects: Heat Flow not Uniformly-Maximized Absorbed: creating hot spot; may be due to either A.Fuel-Fluegas, or B.Water-Steam problems:

A. Fuel-Fluegas:

A.1. gas fuel fired too much;

B. Water-Steam:

B.1. heat surface altered (blocked or thinned) or B.2. water pumped less.

Each of suspects checked as following:

A.1. Flue gas temperature still within the acceptable ranges: so suspect # A.1. is unlikely the cause

B.1. Difficult to ascertain tubes blockage/thickness by gage or dismantle

B.2. The level control works fine & feedpump pressure is enough: unlikely the cause, BUT still water is the coolant for tubes, then ...

B.3. What other water-steam flow problem potentially cause non-uniform heat absorption?

gRounded Remedy: on suspect # B.3.: to check PLC control of the water system:?

1. the pump SPEED (VFD setting) increased?more: corelated with steam pressure increase (pump speed output responds to steam pressure) ... in other word: Control Output range adjusted to allow more flow

2. min-max FEEDwater flow value range set more: GENEROUSLY to flood more feedwater (feedwater flow allowed tobe more FAR above steam demand flow value) ... in other word: Control Output range adjusted toward high but lower range to allow more flow speedy response

Test: the system run in automode without trip consistently.

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ps: Relevant THEORY how boiler control/maintaining wet tubes (author's view):

A. once through - water tube boiler?(relatively most difficult)

too much firing will not immediately rise steam pressure but may lead to tube overheating when less water flow relative to firing heat flux, (similar to superheater case) operator needs to ensure tube getting enough water flow to absorb heat to keep tube temperature down

B. Drum - water tube boiler (medium difficulty)

steam pressure is correlated with more firing: balanced gravity water recirculation between downcomers/bottom headers & riser tubes - which absorbing heat more - ensure full water inside to keep tubes temperature down

C. Fire tube boiler (relatively easiest)

steam pressure is easily correlated with more firing: heat transfer to water body is easer: tubes fully immersed in the water inside shell so that it always has enough water to absorb the heat (keeping tube temperature down)