Q & A > Question Details
In our Delayed Coker Unit (DCU), we have a Feed Preheater which is a Balanced Draft Fired Heater. The Draft of heater is continuously hunting whenever the stack damper is fully closed so we are operating the same with stack damper 15% open.
Also the Excess Oxygen & Draft are on the higher side in this condition.
What can be possible cause for the same & possible solutions?
27/06/2019 A: keith bowers, B and B Consulting, kebowers47@gmail.com
The reported pressure oscillations that the draft/pressure control system is 'resonate.' All mechanical systems are subject to resonance. The pressure measurement and control system must be 'tuned' to avoid exciting the natural resonance frequency of the system. Partially opening the stack damper shifts the physical resonance frequency away from the control system resonate frequency.
It is better to 'slow down' the draft damper actuator than utilizing the in-efficient 'pressure leak.''

There is no practical need to have a rapid response of the stack damper as furnace firing rates typically move slowly.
28/05/2019 A: Satyalal Chakravorty, Sr Consultant, satya1354@yahoo.co.in
Possibility cause is plugged/choked APH as mentioned by Mr Mukherjee.
22/05/2019 A: Lindsay McRae, Pall Corporation, Lindsay_McRae@pall.com
Is this heater fuel gas or heavy fuel oil fired? If this heater is fuel gas fired you should check burner fouling as this can result in unstable burner operation.
Also what type of burners does this fired heater have? Conventional, low NOX or ultra low NOX? Should your burners be fouled you may need to upgrade fuel gas treatment to high efficiency LG coalescer to remove particulate and liquid aerosol contamination from the fuel gas. KO drum / mesh pad is widely used for historical reasons but is not best practice to protect fired heaters.
21/05/2019 A: RUPAM MUKHERJEE, ENGINEERS INDIA LIMITED, rupammukherjee86@gmail.com
Since you stated that it's a balanced draft heater, malperformance in the ID Fan circuit is causing the draft to worry. It can be due to plugged or choked APH which is causing more than desired pressure drop on flue side. Even it can be because of a worn out I D fan which has lost it's mechanical integrity over years of use. Or, it can also be because of intermixing between the flue gas and air due to any leakage in the circuit, most commonly the broken glass APH tubes..
21/05/2019 A: Bruce Carr, Suncor Inc, bcarr@suncor.com
First off... a stack damper should never be fully closed, you are asking for trouble... In my opinion there should be a mechanical hard stop to prevent it..... Never trust an excess oxygen analyzer to determine how any heater is controlled, it is dangerous. The only real way to monitor how a heater is operating is by having operators that have been properly trained, that know what a good flame pattern looks like.