Dear Community,
I am seeking advice on suitable type(s) of flowmeter, if any, for measuring extremely variable flow. By that I mean flow that fluctuates from zero to full scale continuously with an unsteady period of 5-10 seconds.
I am a mechanical engineer working on a wave energy project and only have a superficial understanding of and limited practical experience with flowmeters. I have used vortex and PD flowmeters in the past on similar applications, but I was not involved in their selection and the measurements were poor. A Google search located the article “Optimizing Flowmeter Selection” by Brian Fretschel and Amy Johnson, which I found very informative, but did not deal with the subject of non-steady flow.
I have contacted a range of suppliers who have variously quoted DP, magflow, turbine and vortex flowmeters. However, when I ask them about the dynamic responses to unsteady flow, they have no information to offer or they say sorry, too hard. I’m sure there’s people out there with the capacity to assess our requirements properly, but it’s clear I’m going to have to do some detailed investigation myself first.
So my question is: does this requirement to measure unsteady flow exclude certain types of flowmeter? If so, what types should I consider?
Thanks,
David.
Hi David,
The question you pose is a good one. In looking at the standards they say things like "The standard is limited to single-phase Newtonian fluid flow in which the flow can be considered sufficiently free from pulsation effects (ASME MFC-3M, general DP Flow Standard)." Or, "it applies only to differential pressure devices in which the flow remains subsonic throught the measurement section, flow is steady or varies only slowly with time, and the fluid is considered single-phase (ASME MFC-14M, small bore orifice standard)."
Unfortunately, as good as some of the standards are, they do not do a great job of explaining what "slowly with time" means exactly which is the question of your application. I have some colleagues that have performed some extensive testing which was reported at the 5th International Flow Sympoisum and ISA in 2002. Many of their tests show that, for your application (0.1- 0.2 Hz), there will not be much of an issue in accurately measuring flow with a DP device. This is because most of the models accurately fit a linear model that could easily be corrected for in the calculation or the transmitter.
It would be of interest to know how large of flow changes we are talking about (zero flow to max flow, reverse direction, etc.) as that will be better handled by some meters than others, as well as the metering section as that can have impact on flow profile development. Long story short, I would love to hear more about your particular application and can definitely get you connected to some of the data testing we have done over the years to model this type of application. I encourage you to reach out to your sales representative and get a hold of me and we can get the problem addressed in more detail.
Regards,
Brian Fretschel
In reply to Chuck Gray:
Hi Brian & Chuck,
Thanks very much for replying to my question. The basic requirements are in the table below (sorry this image isn't the best):
In summary, we need a subsea flowmeter that can handle up to 12,000 L/min (200 L/s) at 80 bar abs and another that can handle up to 15,000 L/min (250 L/s) at 160 bar abs.
The process fluid is a water-based hydraulic fluid (in 100 % liquid phase) similar to a 5 % solution of this wellhead control fluid: <http://www.houghton.no/files/stack-magic_eco-f_pds-e_2006.pdf>. The available data on kinematic viscosity for this solution is: 1.033 cSt @ 20 °C and 0.717 cSt @ 40 °C. In other words, in absolute terms, there will not be significant changes in viscosity between 10-40 °C.
Some indicative simulated time traces of the flow profiles we expect under 2 significantly different process conditions are below. These are caused by different sea states acting on our wave energy converters. The first shows the expected flows under the most common sea state, while the second shows the expected flows under a rare, but powerful sea state. The flows are in cubic metres per second, multiply by 1000 to get to litres per second. Note that “Outlet Flow” refers to the high pressure (HP) flow. “Inlet Flow” refers to the low pressure (LP) flow. The charts don't hit the flow limits in the requirements I've been given and might seem counter-intuitive without a detailed knowledge of our hydraulic systems, so just interpret them as an indication of the flow variability we expect.
We would be satisfied if we could meter these flow with +/- 5 % relative accuracy, but would prefer +/- 2 %.
In reply to PorridgeMan:
Here is a brouchure of the istrument, also flow measurement:
www2.emersonprocess.com/.../Liq_Brochure_00803-0101-6144.pdf
Here you can download the toolkit for specifying the meters:
www2.emersonprocess.com/.../index.aspx
Niklas Flykt
Klinkmann Oy
Key Account Manager safety products
nikfly@gmail.com
In reply to Niklas Flykt:
I am looking into this application with a few of my colleagues and the information you have provided is perfect. The only other detail we will need to further analyze this would be the line size and pipe schedule if possible.
In reply to Brian Fretschel:
Hi Brian,
The nominal line size is 150 mm (~ 6"). The pipework hasn't actually been designed yet, but the intention is to specify ASME B31.3 schedule 80 for the high pressure (160 bar) piping and schedule 20 for the low pressure (80 bar). The preferred mechanical interface between the pipework and flowmeter is a flange to ASME B16.5, class 900 for the HP flowmeter and 600 for the LP flowmeter.
Whilst I really appreciate the advice so far and am very keen to see what is recommended based on the requirements I have provided, I still want to understand in a broader sense why one technology is more suitable than another. Maybe there's no simple answer, but note that already in this discussion Chuck has suggested Micro Motion coriolis mass flow meters, whereas Niklas has suggested 8705 magnetic flowmeters. Why, for example, would these be any better than a turbine flowmeter?
"whereas Niklas has suggested 8705 magnetic flowmeters." I just played with the program to see how accurate the meters are with your kind of flow. I also tried to use some "peasant wit" to try to get the pressure drop as low as possible. If you can avoid? sticking something inside the pipe, the flow will be better. Was it so that the meters also will be under water?
A link to some tips for DP measurement:
community.emerson.com/.../2457.aspx
A masters thesis on flow: http://www.idc-online.com/technical_references/pdfs/instrumentation/industrial_flow_measurement.pdf
"Another radical innovation, introduced by Emerson, is the Conditioning Orifice Plate[16]. In place of the conventional concentric round hole (orifice) through which theliquid flows the Conditioning Orifice Plate makes use of four equally spaced holesthat are arranged in such a fashion as to leave a metal section of the plate in the centreof the pipe. This causes the flow to condition5 itself as it is forced through the fourholes. The arrangement reduces swirl and irregular flow profiles and removes therequirement for a flow conditioner to the extent that only a total of 4 pipe diameters(2D up-stream and 2D down-stream) is required. Furthermore, the dischargecoefficient uncertainty (UCd), a major factor in determining accuracy, is in many casesreduced to ±0.5% from a typical value of ±1.0%."
Hi Niklas,
I had a look at the literature you cited, but could not find an answer to my basic question about dynamic response. I haven't heard back from Brian or Chuck, so will try contacting a local representative for more help.