Deaerator operating pressure at 40 bar? 1

[steamandpower]

I am involved with the design of a new steam generating plant consisting of two 225 tph boilers with 102 barg and 320degC outlet steam condition. The main return condensate flow [Stream 1] from the associated process plant can be up to 240 degC, making up 85% of the boiler feedwater flow. The make-up water and other return condensate streams [Stream 2] combine to a temperature of about 100 degC. Stream 2 is deaerated, whilst Stream 1 is not (excellent quality with no chance of contamination). After deaeration the two streams are combined in a pressured condensate vessel pegged at deaeartor operating pressure. The deaerator pressure needs to be just higher than the saturated steam pressure of Stream 1 [about 250degC], otherwise flashing will occur in the condensate vessel.

My question is: Is there likely to be any problems with deaerating Stream 2 from 100degC up to 250degC ?? I'm used to a differential temperature of about 40 degC, but 150 degC is way higher than I'm comfortable with.

 

[dberner]

Wouldn't a DA sized for 250C (482F), which has a saturation pressure of (570 psia), be extremely expensive? With not knowing much about your system - running the DA at 250C means BFW is at 250C which hurts plant efficiency since flue gas exit won't be below 550F and will cause steam to be produced in the economizer.

 

[steamandpower]

A Deaerator operating at 570 psia would certainly be expensive, but operating at this pressure allows the process plant to remove several 80 barg pressure vessels from their system, so the overall cost impact is balanced to some extent. A small economiser could still be installed to bring the feedwater temperature up to around 280 degC, but removing a small part of the flue gas heat. To get boiler efficiency up would require large air heater surfaces and a subsequent higher combustion air temperature, and larger combustion air ducts after the air heater. We'd certainly have enough heat in the primary air to dry and convey the PF coal from the mills to the burners. I haven't done a heat balance yet, and will need to do so in conjunction with the boiler manufacturer.

The alternative is to operate the Deaerator at 15.4 bara, with a resultant feedwater temperature of 180 degC (the final temp of un-deaerated return condensate (Stream 1) mixed with deaerated condensate and make-up water (stream 2). However the process plant engineers believe there are some process concerns with their systems if this alternative is adopted (an alternative they have been working on for some time). I sure don't want them solvig thier problems at the expense of the boiler plant design.

 

[steamandpower]

Some boiler feed pumps are rated to operate at 400 degC water temperature, implying that they are fed from deaerator systems operating at pressures substantially higher than the 570 psia mentioned in dberner's response. Anyone have any comments about their experiences with deaerator operating pressures?