I have a riddle for you

[Buzzen]

I have a flow problem..here is the riddle. We use positive displacement pumps for chemical feeders. The flow is really low on some applications in the area of 3-4 gallons per hour, with about 5 psi. I am using a flow meter that will respond fairly accurately when the tank is full. As the tank empties the psi is around 1-2 which will throw off flowmeter. I am thinking of using a small booster pump to improve the flow hopefully improving the conditions for the flow meter. What do you think?

 

[GlennA]

Is the flow meter at the pump inlet or outlet ?

 

[Buzzen]

The flowmeter is on the inlet side along with a inlet stabilizer installed after the flowmeter. This gave me some stabalization, but the velocity changes when the tank is nearly empty.

 

[GlennA]

So the obvious stupid question is can you move the flowmeter to the output side of the pump ? What type of flowmeter is it - is it built like a hydraulic gear pump ?

 

[Buzzen]

No not a stupid question. I could move it to the outlet side, but I would need to buy another flow meter. On the outlet side we have water and chemical. I was trying to monitor the chemical only. I would have to measure the water then subtract it from the chemical. OPerators can only handle small numbers.

 

[GlennA]

5 psi is about 11 feet of water column. Is your flowmeter at the same height as the bottom of the feed tank ? How big is the feed tank ? Can you raise the feed tank or pressurize it ?

 

[Buzzen]

The tank is about 11' tall and the flowmeter is slightly under the tank. The tank is on a pedestal. It's about 8' diameter. I could give you more accurate meas. if needed. raising it would not be possible. I was thinking of using a booster pump that would always be at 5psi, with a constant pressure switch/valve. But I am not an engineer. Would the velocity still raise and lower, Giving me a good flow for the meter?

 

[Tom Jenkins]

If it is a positive displacement pump and you know the pump speed and displacement you really don't need the flow meter at all. I'd calculate the flow from pump speed and use the flow meter for alarming only.

 

[TConnolly]

If it is a positive displacement pump and you know the pump speed and displacement you really don't need the flow meter at all. I'd calculate the flow from pump speed and use the flow meter for alarming only.

This.

If you can count pump rotations (or stokes if a diaphragm type) then you know how much fluid was pumped.

 

[jstolaruk]

I was thinking of using a booster pump

This is what I've always used with positive displacement pumps - they just don't want to work right when starved.

 

[Buzzen]

If it is a positive displacement pump and you know the pump speed and displacement you really don't need the flow meter at all. I'd calculate the flow from pump speed and use the flow meter for alarming only.

This may sound like a ridiculous question, but how do I calculate the pump speed? The pumps are Milton Roy and they do not sell decoders or anything to monitor stroke of the piston, is there a way to do this?

 

[Buzzen]

This is what I've always used with positive displacement pumps - they just don't want to work right when starved.

jstolaruk, how do you figure out what kind of booster pump to use? There are a million different types. The great thing here is that it would never be starved. We are a Water district. so the water never stops and neither does the chemcials.

 

[jstolaruk]

jstolaruk, how do you figure out what kind of booster pump to use? There are a million different types. The great thing here is that it would never be starved. We are a Water district. so the water never stops and neither does the chemcials.

My projects used a positive displacement pump to create 5,000 to 10,000psi at 50gpm. We knew, from experience, that delivering at least 50gpm at no less than 10% of the outlet pressure would keep the circuit from cavitating. Gravity feeding or relying on the pump to draw from the tank wasn't enough. YMMV.

 

[Buzzen]

Okay Thanks Tom

 

[Tom Jenkins]

Chemical metering pumps come in many flavors, such as peristaltic, piston pumps (fixed and variable stroke), gear pumps, and so on. Most are "positive displacement" types - that is, for every rotation of the pump a specific volume of liquid is moved from the inlet to the discharge. For most pumps this volume remains fixed, and at a constant speed the flow rate is fairly constant across a wide range of of inlet and discharge pressures. The volume moved on each revolution is the pump displacement, and is usually expressed in liters per revolution, or cubic inches per revolution, or such. If you know the pump speed you essentially know the flow rate:

Q = d x rpm
Q is flow rate
d is displacement

If you have one of these pumps you don't need a flow meter - multiply the speed by the displacement and you have the flow rate.

Now some pumps have variable displacement - they can adjust the effective stroke of the piston by moving a swash plate or by some other mechanism:

http://www.miltonroy.com/corp/details/0,10294,CLI1_DIV39_ETI3654,00.html


In this case, at 50% adjustment the displacement is 50% of maximum, and again you can calculate the flow from the pump speed. The pump manual should explain all of this, and give you the accuracy of the adjustment and so on. The stroke adjustment can be manual, or respond to a 4-20 mA signal or whatever.