API 661 ISO 13706 clause 7.2.1.7. Fan Stall.

[RonJeremy]

API 661 had, some time ago, a clause to the effect that fans had to be designed such that, by increasing fan blade angle, a 10% increase in air flow can be achieved. Some of the major operators understood this to mean "and thus the motor must be sized for this increased airflow". Often, in specs that would be manifested as something like "you shall use one motor size bigger than you theoretically need".
Now ISO and API seem to have tried to clarify the clause, as follows (my italics)...


Fan selection at design conditions shall ensure that at rated speed the fan can provide, by an increase in blade angle, a 10% increase in air flow with a corresponding pressure increase. Since this requirement is to prevent stall and inefficient operation of the fan it is not necessary that the resulting increased power requirement govern the driver rating.

So, the motor need only be sized for the design point, and the margin is in the fan only. Then increasing fan pitch (to 110% x CFM) effectively moves your operating point upwards and to the right along the system curve - but if the motor may not be sized to drive that combination then I fail to see what is going to be achieved. Are we supposed to assume that motor factors can take up the slack? Also, I do not follow that this action necessarily prevents stall: in increasing the pitch, might you not actually drag the stall point over to the right, toward your operating point?

Any ideas? Thanks!

 

[vpl]

Didn't I just read this same question over in the mechanical engineering other topics forum? Hmmm ... yes I did. thread404-325138. Please don't double post! Choose one of your two threads and request that it be deleted by using the "inappropriate post, red flag" in the bottom middle of a post.



Patricia Lougheed

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[RonJeremy]

Thanks! That was a big help.

 

[vpl]

RonJeremy

Hope this is a little more helpful. My view is that ISO/API decided to clarify that it wasn't necessary for people to oversize motors. I haven't dragged out fan curves in years, but, yes it could move you closer to your operating point. However, that is part of designing the system -- you need to choose an operating point that allows for that 10%.

Patricia Lougheed

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[speco]

RonJeremy,

I believe the intent is to make sure the fan is sufficiently sized. Normally, the motor is somewhat oversized, since it usually needs to be sufficient for the specified minimum ambient temperature. Note that the fan is to be designed for an extra 10% air flow at the corresponding static pressure. This would result in about a 33% increasse in fan horsepower. I don't think it was ever the intent of the API spec to require the motor to handle this extra load, and the clarification was a good idea.

Regards,

Speco (

http://www.stoneprocess.com)