10 State Standads chem feed pump requirements

[chileheadcraig]

I was looking at permitting some chemical feed pumps and they technically do not meet the following 10 State Standsrds:
4.7.2, Fluoridation – Chemical feed equipment and controls
e. a fluoride solution shall be applied by a positive displacement pump having a stroke rate not less than 20 strokes per minute, and at a feed rate not less than 20 percent of the rated capacity of the feed pump,

5.0.3, Chemical Application – General Equipment Design
g. chemical feeders and pumps shall operate at no lower than 20 per cent of the feed range unless two fully independent adjustment mechanisms such as pump pulse rate and stroke length are fitted when the pump shall operate at no lower than 10 percent of the rated maximum,

Does anyone know the intent behind these requirements? The pumps we want to specify (Grundfos DME, cut sheet attached) do not technically meet this, but they are designed to be turned down farther than standard chemical feed pumps due to use of a stepper motor. I want to explain to the Department of Health that these pumps meet the intent even though they dont meet the explicit requirement. To do that, i want a better grasp on the intent. Thanks!

 

[JJPellin]

These standards feel a little bit out of date. In years past, I would have avoided pumps running at very low stroke rates because of the concern that they would stall and stop pumping. This was a problem particular to air actuated diaphragms and pistons. I would avoid pumps running below 10% of full capacity because of the inherent inaccuracy I had seen with pumps that drop capacity by reducing stroke length.

But with a microprocessor controlled solenoid driven pump or a stepper motor driven pump you probably have eliminated much of the risk of stall or flow inaccuracy. And, if you used the proper type of flow switch, you could probably detect either problems and alarm so that the failure does not go undetected.

Even with that, I would not feel comfortable with a 1:1000 downturn on a pump. The manufacturer has an incentive to claim the highest reliability and accuracy with the greatest down-turn to increase their market potential. It is just like a coupling manufacturer who claims that their coupling can run at full load with a certain degree of misalignment. Just because the coupling can take it does not make it a good idea to run that way. I would stay well away from the manufacturer’s maximum down-turn or minimum stroke length or rate.


Johnny Pellin

 

[micalbrch]

In general I agree with all Johnny wrote. The reason for the stroke rate limitation is (was) for sure the accuracy. But this Grundfos pump type is a metering pump, not an AOD or something similar. I'm sure the accuracy is still good below 10 % of its rated flow as metering pumps are designed for that. Having said that a 1:1000 turndown ratio is indeed something I would not try for a process pump.

 

[chileheadcraig]

Thanks for the feedback! We should be able to get these permitted then if the reviewer understands what we are proposing.

 

[bingopin]

Regarding metering accuracy...
While I have no experience with the specific pump mentioned, my experience with other metering pumps is that a consistent minimum back-pressure is required for metering accuracy. At least one manufacturer recommends a minimum of 25psig. This can be supplied by a simple back pressure valve on the pump discharge.