Deaerator level control
Deaerator level control
(OP)
Hi all,
I am Bala Krishna Godavarthi, i am an DCS operator in QAFCO. We have a strange problem with newly commissioned Deaerator,
i.e level control at higher loads is becoming difficult, controller is not responding properly, even though it was tried to
tune several times. I suspect a problem with feed water temperature as feed water is beng supplied at 45 deg c, as there are
no preheaters at the feed inlet and steam consumption for Deaeration is higher, i suspect swelling or shrinking in the level
when fresh water is fed at low temperature, but this is not the case at lower loads, but this comes into picture while
increasing loads and at higher loads. What can be done with level measurement? is the level can be biased with pressure just
like the case in boilers??
I am Bala Krishna Godavarthi, i am an DCS operator in QAFCO. We have a strange problem with newly commissioned Deaerator,
i.e level control at higher loads is becoming difficult, controller is not responding properly, even though it was tried to
tune several times. I suspect a problem with feed water temperature as feed water is beng supplied at 45 deg c, as there are
no preheaters at the feed inlet and steam consumption for Deaeration is higher, i suspect swelling or shrinking in the level
when fresh water is fed at low temperature, but this is not the case at lower loads, but this comes into picture while
increasing loads and at higher loads. What can be done with level measurement? is the level can be biased with pressure just
like the case in boilers??





RE: Deaerator level control
Have you checked that the valve is responding correctly in the field to a changing set point? Is there a local level indication, is it showing the same level as your instruments when this is happening?
RE: Deaerator level control
RE: Deaerator level control
Dearator pressure control at all loads is critical and you may need to preheat your makeup water.
RE: Deaerator level control
Thanks for the replies, may be the problem was not quite elaborated.Let me try to explain,i previously worked in a process plant where deaerator is operating at 125 deg C and feed is entering at 110 deg C, so steam required for mechanical deaeration was quite less, so pressure change in deaerator during process upsets was not much significant. But in this case
as water is entering at 45deg C and operating temperature was at 125 deg C steam consumption is higher,for example to treat
200t/hr of BFW we are injecting around 35t.hr of steam to keep the deaerator pressure constant, the Deaerator operating pressure is at 160 kpa to keep the outlet DO <5 ppb. So with change in water flow during process upset this pressure is varying which makes me think that it causes either swelling or shrinking in the level.we have a master controller on the level cascaded with a flow control, master control is level and slave is on flow control.I think level indication is working correctly as all the LT's including trip transmitters are varying in the same manner.
RE: Deaerator level control
Deaerator pressure and level
Flow rate of water to the deaerator; the control valve, SP, PV, OP
Flow rate of steam to the deaerator; the control valve, SP, PV, OP (I assume the steam is directly sparged to the deaerator)
I believe the problem lies in the fact the feed water is much colder than the temperature in the deaerator. When there is an upset in the level, say the level drops, the control valve dumps more cold water in, lowering the temperature and pressure in the deaerator and causing a reduction in boiling but increasing the level. The lowered pressure causes the control valve to increase the steam to the deaerator to increase the pressure. The lag time between the the cold water getting boiled to saturation causes the steam valve to over compensate causing the water to boil more and raising the pressure. The control valve controlling the feed water encounters more pressure in the deaerator and therefore for the same OP, less water goes in the deaerator. This causes the level to drop again. This cycle starts to continue and depending on the tuning of the two valves maybe keep on going and a dynamically coupled loop. Analysing the trends can quite easily confirm this.
RE: Deaerator level control
RE: Deaerator level control
Though not close to boiler Operations, I have never heard of above problem, probably because useful volume of each deaerator surge drum (just below spray-tray tower) has 20 min hold up time at maximum outlet rate of water (102 oC). This may offer adequate inertia.
RE: Deaerator level control
RE: Deaerator level control