3 replies to this topic

[popay73]

Hi,

I have a dilema about flow calculation.
If I have reciprocating pump, more precisely sulphur dosing pump, and if I have formula involving piston size, "travel time"-TT (don't know the word in english, so sorry, but I hope u know what I mean; value is say 10 mm) and max flow for calculation of desired flow :

Desired flow (volumetric) = (TT * max volumetric flow) / piston diameter

My question is: are these flows for water ? I guess pumps are calibrated with water.

If it's for water how do I get volumetric flow for sulphur ?

I actually need mass flow, so can I use calculated volumetric flow for water and multiply it by sulphur density ( at required T) to get mass flow of Sulphur ?

I hope my question is clear enough, thx in advance !

[Art Montemayor]

Popay:

In order to put the query into perspective, let us state that a reciprocating pump is a type of piston pump and, consequently, a POSITVE DISPLACEMENT device. What this implies is that the piston displacement is essentially the volume of the fluid pumped. If we know the RATE at which this displacement is taking place, then we also know the fluid’s flow rate through the pump – i.e., the pumping rate. Basically, the pump doesn’t know (nor does it care) what fluid is flowing through its valves and its pumping chamber. It could be water just as well as it could be sulfur – or any other liquid.

The volume rate displaced is the cross-sectional piston area times the stroke of the piston rate (assuming the piston is single-acting):

Volume flow rate = (pi/4) (D^2) (Stroke) (rpm)

The mass flow rate is the volumetric flow rate times the fluid’s density:

Mass flow rate = (pi/4) (D^2) (Stroke) (rpm) (density)

There is no “travel time involved. I have never heard of this term.

Pumps are not necessarily “calibrated”. They are tested. Positive displacement pumps pump pretty close to what is mathematically calculated. There is some “slippage” of fluid – but this depends on the style of pump, the precision of the machining, the type of valves, the clearance given, the fluid involved, and the empirical data collected from actual tests. All this information is known only to the manufacturer, so you as a student are in the dark as to what “factor” to attach to the calculations to arrive at the true, actual diameter and rpm of the pump. Therefore, just allow for a contingency by either increasing the diameter, the stroke, or the rpm by say, 10 to 20%. You require extra capacity anyway since there is no practical manner to exactly calculate and machine out a pump to meet your specific needs and specifications. The extra capacity is controlled with either recycle or speed adjustments on the pump.

This is probably one of the easiest pump calculations around; you are, in my opinion, making it more complex than it really is.

[popay73]

Thx for quick answer, and U are right about pump being positive displacement type.

Let me explain it more precisely. Our faculty arranged a visit to a plant. They have sulphur dosing pump, positive disp. They showed us the pump and at the back of the pump there is a "button" for changing stroke lenght (that's what I ment by TT). It's usually set on 25 mm. As I understand stroke lenght is distance that piston travels from one end to another. So, the longer the path, the more volume flow we get. Am I correct ?

One of our question was how they could tell what the actuall flow is ? Then they gave us that formula which involves stroke lenght, piston diameter and maximum volumetric flow. From that formula they get volumetric flow for desired stroke lenght. Then they multiply it by density to get mass flow.

Your formula is :
Volume flow rate = (pi/4) (D^2) (Stroke) (rpm)

Can U explain what is meant by stroke ? I'm not from english speaking country, so I have some difficulties in translation :-)

And one more question. U mention rpm. How do we chagne speed (rpm) on this kind of pump ?

Thx one more time.

[Art Montemayor]

Stroke is defined as the length that a piston (or a plunger) travels. We are talking about the same thing. The industry calls the travel that a piston does as the "stroke".

Speed - or the revolutions per minute (rpms) that a crank shaft turns - is varied by either a VSD (Variable Speed Drive, which varies the electrical frequency of the power feed) or by a variable diameter pulley - sometimes called a "Reeves Drive". You can also use a variable speed motor or a motor connected through a gear box which can also change the speed through a gear change.

Look carefully at my equation. It is the same equation that you are alluding to.