Lowering boiler pressure setpoint

[Sam654]

We have a 50,000 #/hr boiler with MAWP of 300 psi and operated since installation @ 200 psi. Over the last 10 years, the demand has decreased to between 10-25,000 #/hr (seasonal). Mechanical modifications have been made to the system and boiler to drop the pressure to 50 psig (none to the fuel train). The question that has come up is; "Does the boiler have to be re-tuned for the lower pressure?" I say no since tuning for air/fuel ratio doesn't take into consideration drum pressure but, a couple of other engineers have said yes without a logical reason why. Should I be eating my humble pie?

 

[Compositepro]

Air/fuel ratio is controlled by controlling the flow rate of air and the flow rate of fuel. Both of these will have changed significantly. so yes, retuning would be wise.

 

[davefitz]

The boiler mfr may indicate that operating at 1/6th design pressure at 50% design steam flow is equivalent to 3 times the design velocity thru the steam risers and thru the drum steam separators. This can advsersely affect boiler waterwall circulation and steam/water separation efficiency, leading to high steam conductivity. An alternate approach would be to operate at 150 psig ddrum pressure and reduce to 50 psig via a pressure reducing valve designed for low noise.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[crshears]

This isn't entirely within in my wheelhouse by any means, but unless I'm mistaken is it not the case that operating at a lower steam pressure will alter the delta T not only due to what davefitz said but but also because both the fire side and saturation temperatures will have changed considerably from design specs?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Sam654]

davefitz... The manufacturer has reviewed the setpoint change and said the internals do NOT have to be changed. His only recommendation was to change the non-return valve to a gate with a wafer check to prevent "slapping" during low loads. The downstream piping was increased to accommodate the increase in specific volume (velocity) of the steam.

compositepro... The fuel demand responds to the deviation from setpoint of the steam drum pressure. The point I'm making to my colleagues is the burner dictates the fuel/air required for proper combustion at high and low ends of the range, not the steam drum pressure.

 

[georgeverghese]

We get a better picture with @sam654's later posts. Air fuel ratio need not change if the fuel is the same, but a firing heat rate turndown of 5:1 and more is at the limit of conventional forced draft burners. Check if you can cycle out one or 2 burners at low load (and run them on / off with feedback reset from total fuel gas flow) if this is indeed the limit for your burners (talk to burner supplier), else if this is too problematic, replace with 8:1 heat rate turndown burners. Else the life of the burners may be affected due to flame lick with extended operation at low firing rates.