Why is it the scale range of ROTAMETER is 10% of F.S. - F.S.?

I'd like to have good technical explanation, WHY?

ROTAMETER can't read below than 10% of F.S.?

 

Huh?

What system is this ROTAMETER located in? What is the application?

Have you used your preferred Internet search engine to research data sheets for ROTAMETERS to see what the manufacturer recommends for the application of their ROTAMETER? There is usually a lot of good application information contained in manufacturer's data sheets. In fact, you should have the data sheets in the documentation provided with the equipment the ROTAMETER is installed in.

 

How about a good technical educated guess:^)

This is not a positive displacement device, so some bypass occurs. It is negligible from 10 to 100% but significant, and worse, variable, at very low flow rates so you can't guarantee accuracy at those rates. There are also friction and phase of the moon effects close to zero speed. It probably takes a flow a small amount above zero just to get the thing rotating. So, it's a good indicator provided you don't rely on it at below 10% flows.

Regards
cww

 

Hi there,

There seems to be a bit of confusion as to what instrument you are talking about. This is a variable area flow meter or sometimes called a rotameter. Not a turbine meter as a previous reply seems to think.
The operating principle below should answer your question.

It is mainly used to measure the flowrate of liquids and gases. The rotameter consists of a tube and float. The float response to flowrate changes is linear, and a 10-to-1 flow range or turndown is standard. It has a linear scale, a relatively long measurement range, and low pressure drop.

It works on the variable area principle: fluid flow raises a float in a tapered tube, increasing the area for passage of the fluid. The greater the flow, the higher the float is raised. The height of the float is directly proportional to the flowrate. With liquids, the float is raised by a combination of the buoyancy of the liquid and the velocity head of the fluid. With gases, buoyancy is negligible, and the float responds to the velocity head alone.
The float moves up or down in the tube in proportion to the fluid flowrate and the annular area between the float and the tube wall. The float reaches a stable position in the tube when the upward force exerted by the flowing fluid equals the downward gravitational force exerted by the weight of the float. A change in flowrate upsets this balance of forces. The float then moves up or down, changing the annular area until it again reaches a position where the forces are in equilibrium. To satisfy the force equation, the rotameter float assumes a distinct position for every constant flowrate. However, it is important to note that because the float position is gravity dependent, rotameters must be vertically oriented and mounted.

 

Yes, the Rota prefix did throw me off. My apologies, but the reason is much the same. There is a certain amount of work to be done so there is a kink in the characteristic close to zero until that becomes an insignificant part.

Regards
cww

 

Dear Sam and cww,

Thanks a lot!

Let me clarify my understanding. As I understand, it requires some force (flow) to move the float which is design to be equal to 10% of the F.S., is it right?

If it is right, then it could not read below 10% of the F.S.?

It is important for me, because our minimum flow and low alarm set point is less than 10% of F.S., thus it could not read it, isn't it?

 

Dear Sam,

Thanks.
As I understand, it requires some amount of force (flow) to move the float which is design to have a weight of 10% of F.S., isn't it? Then if is the case, it could not read below than 10%?

Please clarify, as our minimum flow rate and low alarm setpoint are less than 10% of F.S.

Best regards,

 

Hi there,

If you have a alarm going off below 10%, how do you see this alarm? Where is it fed from, since this VA flow meter is a indication only, with no electrical connections. Do you just read the flow manually every time?

If so, and you need to see the flow below 10% of your current FS flow, install a smaller VA flow meter. (smaller range therefor a lighter float - FS might be smaller as well). If you need to keep the current VA flowmeter to see the FS flow, install a smaller one in series with it just to see the flow below 10%.

Depending on your situation and what is practically possible, install a orifice plate with a DP transmitter, in which case you will be able to send the signal to the supervisory system and set up the low flow alarm to go off in the CCR, and execute a action automatically. The DP transmitter will be able to let you see FS and will be able to measure this low flow as well.

More complex systems will always give you more result than the basics so if you need more than what the VA can do you need to install something more.

 

If you are interested in the basic principles of a rotameter see the following links:

http://en.wikipedia.org/wiki/Rotameter

http://www.fibu.yokogawa-usa.com/Rotameter - Variable Area/Overview_04.pdf

Also, the rotameter with some more flow meters and instrumentation.

utwired.engr.utexas.edu/che354/mom/flowme.ppt

William (Bill) L. Mostia, Jr. PE
Sr. Consultant
SIS-Tech Solutions

 

Hi there,

Actually its a rotameter integrated with the transmitter, thus we have wiring on the system.

It is a compressor package, i don't know with the HITACHI why they used rotameter and not DP since they want to measure the low flow alarm. Perhaps there a reason behind that. I got to ask the vendor in this case.

Thanks for sharing your knowledge guys.

 

> rotameter and not DP since they want to measure the low flow alarm. <

DP measurement is not particularly good below 25% full scale, accuracy falls off, the noise level increases the lower one descends on the flow scale.

The best low flow technology I'm aware of is thermal dispersion. There are switches or continuous flow meters. The design of the flow sensors minimizes permanent pressure drop. My experience is that thermal dispersion works better at low flow rates than at high flow rates. Look at something like FCI.

 

Hi there,

I think this is a bit of a wild statement to make and I have to disagree with it.

Surely if companies like BP and Adderley choose to select the orifice and DP flow meter as the most accurate gas flow meter for fiscal metering purposes there must be a good reason for that. I read a article once about the research they have done on the various flow meters and they gave their results as well on each flow meter. For gas they have concluded that the DP transmitter is still the most accurate and that is why they still use them for their final fiscal gas measurements today. For oil measurement they have chosen the turbine meter as can be seen on all fiscal metering skids world wide.
I also think there is a bit more involved than just to say the lower end of a DP Flow meter becomes inaccurate since the flow rate, normal dp, line pressure, viscosity and size will influence that statement quite a bit from meter to meter.

 

I always wondered why people like John D. Kopp didn't publish the inaccuracy curve envelope for DP flow measurement below 20% full scale (FS) flow rate, like shown here:

http://tinypic.com/r/j6pugk/6

published in D.W. Spitzer's "Flow Measurement" (1991) [pg 68].

Then I noticed that R.W. Miller in his "Flow Measurement Engineering Handbook" (1983) did extend the flow rate down as far as 10% full scale flow on a DP flow inaccuracy envelope curve, in his section on Wide Range Metering (pg 9-118), here:

http://tinypic.com/r/2n0u1q0/6

However, it should be noted that the conventional full range DP ceases at 20% FS flow (like Kopp's curve), but the wide metering range uses a 2nd DP transmitter (on the same OP) scaled at 10% of the full range DP (10% of DP, not 10% of flow). Even that 2nd DP transmitter has ±4% error at 10% FS flow.

The steam people, Spirax Sarco, bit the bullet and actually publish the inaccuracy curve down to 10% full scale flow rate on their web site, with this explanation:

"At very low flowrates, the value of the uncertainty accelerates. At between 20 and 25% of maximum flow, the rate of change of the slope accelerates rapidly, and by 10% of maximum flow, the range of uncertainty is between +18.3% and -22.5%."

http://www.spiraxsarco.com/resource...s/flowmetering/principles-of-flowmetering.asp
(take out any spaces that the control.com forum page inserts into the URL; graph is close to the bottom of a fairly long web page)

Can DP measure at 10% FS flow rate? Yes. Is it accurate. Depends. Can this compressor app tolerate the permanate pressure drop for a DP measurement? I don't know. Does a low flow alarm need a high level of accuracy? I don't know.

But I'm pleased to know that I stand in the company of men like Mr. Kopp and Mr. Miller in my wild assertions on low flow DP measurement conditions.

David