Blower Control with Recycle

[dadams]

Need help on controlling a blower for vapor removal from a surge drum.

Drum operatres at 2 psig. It is purged with natural gas from municipality (98% CH4) via a pressure control valve. If the valve fails (fail open) the flow of gas would be about 150 scfm. I need to get this gas to a another compressor knockout drum that operates at 5 psig. Looking at a small blower.

The blower has to operate continually so it is available in case of a valve failure. Obviously, more than 90% of the time it will be on recycle. My question is how to control the blower on full recycle?

I am guessing operating just past surge point with the gas taking a pressure drop across a recycle valve or orifice. I am wondering if I have to put in an exchanger to remove the heat from the recycled gas?

In the case of an upset, the suction pressure would rise slightly and I would open a discharge valve to the other compressor knockout drum.

Can a blower be operated on minimum flow similar to a centrifugal pump?

Also, there are upset cases where the gas would be butane at approximately 100 scfm.

Surge drum that provides blower suction operates at 2 psig and about 100 deg F.

The gas will also contain small (say 0.5%) concentration of H2S.

 

[djack77494]

Sir,
I very much dislike your approach to install a continuously operating blower to provide essentially pressure control. However, I suspect there's more to the situation than what we now know. Where does the vent gas from this surge drum normally go? You say there's a flow of natural gas into this drum, so tell me where it goes today. Typically, you'd have a low pressure destination for vent gases. You might have a second pressure control valve in a line to this system that would control the drum's backpressure. Properly sized, both valves would have to fail before you could overpressure the drum.

The system you describe sounds dangerous to me. Failure of the blower you describe is much more likely than a valve failure. If that were to happen, you would have a higher destination (5 psig) connected to your surge drum. Could this cause the drum to fail? Really, you'd need to provide quite a bit more detailed information on how your system is designed and operated before anyone on this forum could provide meaningful assistance.
Good luck,
Doug