PD Pumps to Meter Flow?

[EITJeff]

I was wondering if anyone has any experience with using screw pump (speed to meter flow rate. In theory it should work (i.e. volume per rotation x RPM = volumetric flow), but there has to be something that I am missing otherwise EVERYONE would be doing it. Can this be done? If so, what kind of accuracy/repeatable am I looking at?

 

[itdepends]

Not sure exactly what you mean by screw pump- but most PD pumps suffer from some kind of inefficiency- e.g. bypass around lobes on lobe pumps, worn stators on mono pumps (perhaps this is what you mean by screw pump?) In most cases this bypass changes as the unit wears- so your initial calibration becomes inaccurate. Yes you can do it (not use a flowmeter) but it depends on how critical the flow control is to you- and the type of pump you're going to use and how prone it is to bypass (e.g. peristaltics are relatively good, low bypass until the hose ruptures).

 

[waross]

You may have better luck at a better price if you start with a Positive Displacement Flowmeter.
respectfully

 

[EITJeff]

To clarify, the pump I am installing is a 3-screw pump from Imo Industries.

 

[Ashereng]

I think you have it right - the problem is accuracy.

There are many PD metering pumps (additive injection comes to mind) that are used to meter the dosages without a flow meter. Hence, in practice in addition to theory, it also works.

You didn't give much information, such as size of pump, flow range, process fluid, accuracy required, etc. More information would help the fora to give a better informed answer.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[twr]

Th big problem I run into with the PD pump is the "slip". A low viscosity product such as water will pass around the pump impellers/screws aand not give you as accurate a reading. At the same time, with a thick high viscosity product you can have excellent control until the rotors wear out, as itdepends mentioned.

 

[EITJeff]

Ashereng suggested that I might get more detailed responses with process conditions so here goes...

Flowrate = 150 m3/hr
Suction P = tank head
Discharge P = 700 kPa
Temp. = 15 C
SG = 0.93
Viscosity = 350 cSt

Thanks to everyone for their answers.

 

[Ashereng]

Jeff,

Your application is not what I was thinking - 150 m3/hr is substantially more than additive injection.

What accuracy are you looking for? Is it a "at least" case, or "within x% case"? Depending on the accuracy you need, looks like you will need a flow meter.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[f1636]

I agree with others who have already said this could work for the parameters you have provided but how well it will work for your situation will depend on the accuracy you are looking for.

You can determine the pump's volume versus speed factor from the manufacturer's data and periodically determine a correction factor by pumping at a constant speed and taking tank guage readings (assuming you can stop flow into the tank while doing this and that you have a tank gauge table). This deals with changes in pump performance as it wears or as product characteristics change and also enables you to determine correction factors at various speeds, pump pressures and tank heads for additional accuracy.

Alternatively, if you have access to a mobile meter prover and provide typical meter proving connections downstream of the pump, you should be able to determine the above-noted correction factors (more quickly, accurately and with less operational disruption) by feeding a frequency signal from a variable speed drive into the prover counter (or a constant frequency signal if you don't have a variable speed drive).

I think it will be difficult to predict with confidence the accuracy you will achieve; you'd better treat this as an experiment and leave a spool in the piping for a meter in case you need to add it later.

 

[Ashereng]

Clamp-on ultrasonic meters are good for checking flow rates, and easy to set-up.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[SandeepRaheja]

Strictly speaking inorder to qualify as a "Metering" pump, the pump is expected to deliver an accuracy level of +/-1%. It should also demonstrate this accuracy over a reasonable turndown range (usually 10:1) and must also demonstrate a reasonable degree of repeatability in delivering the same flow for the same adjustment/settings.

Designs that have met these challenges more easily are the diaphragm type, plunger type (both recips) and gear type. These designs can be manufactured to tighter tolerances and with lower clearances which is must for this application.
Screw pump designs probably fall short here.

By virtue of being positive displacement pumps some degree of accuracy would definitely be realised. It is difficult to quantify that with these designs.

It is advised that u get hold of the vendor. They can provide you with performance curves for the pump. The performance curves of these pumps would not only give you flow rate with respect to speed but also flow rate at different viscosities and differential pressures. You can also inquire from their design staff about the test tolerances on capacities. You may like to visit the IMO site:

http://www.imo-pump.com/tech.htm

for some of the models the performance curves given are quite detailed. You may not need all the experimentation to calibrate the pump.

good luck

 

[blackwed]

Just a bit of anecdotal evidence for you. I was once at a bulk facility that used a 400 gpm PD pump in a tank truck offload bay for jet fuel. They consistently offloaded 8000 gallon trailers in 20 min +/- a few seconds. Obviously not the accuracy needed for some processes, but if you only want to know if the answer is 8000 or 7900, it might be good enough.

DB

 

[EITJeff]

Thanks to all those who responded! Plenty of info for me to proceed!