Operating fan against closed suction

[Alwynb]

Hi there,

I have an application where I am thinking of installing a fan in a vent stack. This stack is 32m high and contains gas with a SG of 3.5. The vent line is a closed system except for a number of relief valves that will vent into it in a emergency. Thus the problem is that the fan will run most of is lifetime with a "closed" suction damper. Can fans operate like this?

Regards
Alwyn

 

[CRG]

Hello,

Can you add pressure switches upstream of the relief valves set to a pressure just under the relief valve setting? Then the fan will only run when pressure approaches one of the relief valve settings. Are the relief valves used to prevent an upset in a process or protect equipment? Do you need an alarm signal to verify proper operation of the fan? What will happen if the fan is not running and a relief valve opens? Does this require the fan be installed with an uninterruptible power supply?

CRG

 

[ChasBean1]

I agree w/CRG. And also what fluid will be relieved and how often, at what volume and with what backpressure, and you should accommodate how many simultaneous lifts in the design. There could also be a material compatibility issue with the fan... If you decide the fan is needed, a low pressure, high volume type fan (such as a standard propeller fan like you would see in your house) would probably do better than one that was built to overcome system friction like you might see in an air handler (e.g., a centrifugal, vane-axial, plug type fan, etc.).

 

[Alwynb]

Thanks for the replies CRG and ChasBean1.
I will investigate your issues.
Regards

 

[drdust]

A fan operating with zero flow is not hurt. However, when the gas is released into the fan inlet there could be vibrations and surging of pressures. Also, can the inlet plenum stand the negative pressure the fan will produce. Given teh s.g. do you need much static pressure to send the gases out the stack?
The fan selection should be a non-overloading wheel design (backeard inclined) so it can operate with and without airflow without overloading the motor.

 

[lilliput1]

The fan will overheat with no airflow. Also it will not be safe to depend on a fan that could fail to vent the relief. You should provide adequately sized vents dedicated to each equipment protected by the relief valve so as to avoid the danger of a relief valve popping causing another equipment relief valve to also pop, rendering the equip not operational.

 

[gnordo]

Several issues to consider:

1. The fan motor, may or may not overload, depending on the type of fan blading- backward curve centrifugal, forward curve centrifugal, propeller blade, radial, etc.

. Look at the fan curves for CFM/HP/SP, etc. for the answer.

For sure, a backward curve never will over load on no flow.
On the other hand, a propeller blade will overload the motor as static pressure is increased, such as when the intake gets blocked.


Another issue is the blocked intake static pressure heat that is generated [and motor heat too, if it is located in the air stream.] The fan/motor may require a minimum flow to dissipate generated heat. The higher the fan static pressure rating, the more likely this will demand attention.

Gnordo