Page 436 - Boiler_Operators_Handbook,_Second_Edition

Controls 421

to open it. Of course there were times when we forgot pumps in operation to determine when recirculation is
to open an isolating valve on that recirculating line too! necessary and open a solenoid valve in the recirculating
Why, I don’t know, because we should have left them line when necessary. I made it a point to have the sole-
open. noid valves supplied as normally open and the controls
Another feature of those recirculating lines that I energize it to close. That way it will fail to recirculating
used in later designs was combining all the recirculat- to prevent damage to the pump. You should still use
ing lines into one line returning to the deaerator with orifices but they can be a little larger.
another orifice (sized for all pumps) so some of the recir-
culating water would flow backwards through the other, Goofy Controls
idle, pumps to keep them warm (the recirc line was con- The advent of microprocessor controls allows us to
nected before the discharge check valve). Knowing what add more and more features to a control system. I like to
I now know about the effect of piping stress on pumps think of them as ways to help the boiler operator. There
(see piping flexibility) I think it also prevented damage are, however, some features added that make it more
to the pumps. difficult for the operator.
During work on a problem with some boiler feed I’ve encountered some fairly stupid concepts in re-
pumps in 1999 I also discovered that higher pressure cent years. Not because they were dangerous or wrong,
boilers require so much feed pump energy that the recir- they just made life difficult for the operator. One I ran
culating flow represented a significant amount of extra into involved a boiler control system where the designer
horsepower and, more importantly for that particular decided that any time any variable got out of range
customer, a significant reduction in the amount of water (more than 100% or lower than zero) all the controllers
that could be supplied to the boiler. should switch to manual. With that logic, on a balanced
Plants operating at pressures of 250 psig and high- draft boiler, every time the boiler was started and the
er have had a solution for this problem for many years, purge commenced all the controls switched to manual
it’s a self contained check and recirculation valve which because the boiler outlet dampers couldn’t close down
consists of a spring loaded check valve that checks the enough to prevent the furnace pressure dropping below
main fluid flow and an integral recirculating valve that the low end of the furnace pressure transmitter’s range
opens as the main flow decreases. The problem with so everything switched to manual. The boiler hung up
those valves is they are very expensive and most plant in manual until the operator switched everything back
owners scream at the cost, they can cost almost as much to automatic. Even though I explained what was hap-
as the pump! Regardless, they work and they pay for pening and that there was no need to switch controls
themselves in power savings. to manual simply because the measured variable was
There is another solution that I haven’t had an out of range the designer insisted that his system had
opportunity to try yet and I hope to compare to those to work that way. Hopefully it was finally resolved but
expensive recirculating valves. The cost of controls has I wouldn’t be surprised to visit that plant and watch the
dropped so much I believe you can justify installing two operators switching all the controls back to automatic
temperature sensors on the pump, one in the suction after every start-up.
piping and one that senses the liquid near the pump I also recently had a situation where a technician
discharge (without interfering with the flow patterns) insisted the drum level transmitter had to be set where
to detect a rise in the fluid temperature in the pump. It mid range was the center of the drum. There are very
would control a small pneumatic valve in the recirculat- few boilers out there which have a normal water level
ing line. As long as the temperature differential is low at the center of the drum, most are lower by two to four
enough there’s no need to recirculate water. It eliminates inches. I finally had to insist that the middle of the range
the pressure loss attributable to the spring on the auto- was at the center of the gage glass, that it had been that
matic recirculating valve check and, by using tempera- way for ages, and he had better set it there if he wanted
ture differential, is oblivious to any changes in deaerator to get paid.
pressure that would change the temperature at which On a recent excursion to California to look at a sys-
steam would start to form. tem that had nothing to do with a boiler other than use
Another system I am trying is using the integrated steam I got frustrated with the programmable controller
controllers of automation systems to create a feedwater logic. It prevented starting a pump while the water level
flow calculation based on the control signals to all the in a tank was high. It was high because the pump had
feedwater control valves in the plant and the number of been shut off! I had to drain the tank to get the pump