boiler chemistry!

[bigdog50]

IS there a time frame or formula that would indicate when it's okay to test the water chemistry. Standard I have used is 2 to 4 hours after a start up of a HRSG operating 1300 to 2400 psig hp steam.

FLORIDA,
BIGDOG50

 

[davefitz]

I think you are in big trouble.

The condensate feed to the HRSG should have an online pH and conductivity monitor to provide immediate alarm indication of a condenser tube failure or contaminatin of feed , to avoid ruining the eavporators . If you wait 2 hours after contaimination, there will have been a lot of hydrogen damage to the evaporator tubes.

The time period of most feedwater O2 upsets is during startups, so the feedwater out of the LP economizer/preheater should be sampled and the feed rate of scavenger shuld be based on maintaining the O2 at that location at 2 ppb. If excess O2 is measured , it should be countered by feeding scavenger at a rate proportional to ( feedwater flow * excess O2 in feed) at a mole ratio not exceeding 3:1. If you wait until 2 hrs after startup yuou have missed all the action and probably had overfed scavenger and can expect flow accelerated corrosion FAC of the LP evaporator.

If you are using nonvolatile additives to the LP circuit or coordinated phosphate to HP circuits, there can be "hideout" issues in which caustics that had accumulated on the tube ID at prior high load service will go back into solution and thus you will have excess casutics presetn during startups. If you wait 2 hrs after initial fires, you have missed all the exciting reactions that occur when the chemist realizes he has 10 time the right amount of caustics in teh boiler water.

 

[metengr]

I would agree with davefitz. There is too much that can go wrong during a unit start that after 2 to 4 hours could result in underdeposit corrosion damage and FAC damage to evaporator tubing.

If you supply steam to an HP turbine, chlorides or other contaminant carry-over could result in a serious problem.

 

[bigdog50]

I do agree, Hot starts we are Testing chemistry of the boilers at 50% load of the 501F or 95 MW. I also Agree on the O2 Level. We Have on line monitoring of this and Automatic process for Limanox and Amine Injection based on Flow rates and 02 and PH levels. This Is also proceded with Pegging to the Vac DA which we set on start up a 5 Deg F. above saturation temp of condensate water temp. FEC is a problem at start up I due agree. Also are other limintation is silica and Conductivty! Are Steam Turbine will not release until Silica and conductivity are with in Spec. it is fully automatic thanks to the Germans! But I quess what I ment to ask was COLD starts? Taking into Condsideration of ramp rates a 25 Deg. F a minute on HP which equals to about 72 minutes. to get to 1800 psi. So how long would you gents say on a cold start 2 hours for good test results Phosphates, silicas, Iron, ect? Thanks for your Positive CRIT.

FLORIDA,
BIGDOG50

 

[davefitz]

to reduce the delay and somewhat get around the steam turbine permisssives, the steam conductivity device should be a degassed conductivity, to remove eros caused by CO2 liberated from hi feed periods of carbohydazide or organic amines during startups.