You don't need to use an affinity law to find pressure at a specific flow.

[ContractorDave]

Quoting my friend skdesigner from a closed thread (Pump Affinity Laws August 2012)
"You don't need to use an affinity law to find pressure at a specific flow."

I agree but then what do you use the affinity laws for?

NFPA 25 8.3.5.2.1:
Theoretical factors for correction to the rated speed shall be applied where determining the compliance of the pump per the test.

and from the handbook:
"NFPA 25 requires the use of affinity laws for correction to the rated pump speed. This correction is needed as the pump ratings were established under ideal factory conditions and most pump performance will vary from the factory ratings under field conditions. Uncorrected pump performance must be sufficient to supply the required system demand as required in 8.3.5.7, to assure that the active systems are properly supported with appropriate pressure and flow."

I'm not understanding the purpose of 8.3.5.2.1 as it applies to a technician doing the test and subsequent report given the last sentence in the paragraph above.
Please correct me if I'm misunderstanding this, but what I read from the handbook is this correction is needed to compare real world test results with perfect world manufacturers test conditions, but it doesn't matter because the real world result are the ones that count.



Regards
D

A thing moderately good is not so good as it ought to be
Thomas Paine

http://www.ykfireprevention.ca

 

[Sdpaddler50]

I agree, this is confusing. When a pump is tested, lets say a booster pump taking suction from a public maint, you get the results, and plot them. Bottom line - can the pump supply supply the sprinkler system, plus hose, in-racks, etc. IE, is the demand point below the pump curve?

Regarding the affinity laws, these are used for variable speed engines, ie, diesel drivers. The engine speed can vary over time due to governor issues, and just simply wear and tear. Lets say you test the pump and note the diesel rpms are on the low side for churn, 100%, etc. The affinity laws can be applied to see how the pump would have performed (Q, P) if the rpm was correct. You can then identify the issue as the driver, and not neccesarily the pump.

As you probably know, the pump can also be the problem, not the driver. Pumps loose performance due to wearing rings, volute/impeller issues, etc. IE, like us, they get old, and start wearing out over time.

The key is to find out which is the problem with a poor pump test result ( generally 10% below the mfg., or shop curve) is time to be concerned.

Constant speed drivers such as electric motors will have a more constant rpm. Frequency = PN/120 if remember correctly. IE, the motor speed is dependent on the number of poles in the motor, frquency (60 hz), so these are constant factors. Electrical guys, this is by heart, so correct me if i am wrong. The affinity laws are therefore more applicable to diesel drivers.

In addition to NFPA, if you want more stuff to read, check out FM Data Sheet 3-7N.