Mag Meter 3

[PUMPDESIGNER]

I spoke with Siemens today about their Magmeters they want us to use on our equipment.
He stated flat out that they care not at all about flow runs or pipe and fittings before or after their flow meter.
The 2" model I looked at was flanged on either side, about 10" long.

This seems about right to me because this is a mass meter, but is that agreed upon by peers or the engineering community?

PUMPDESIGNER

 

[bmsg]

I have used magnetic flowmeters of Krohne Marshall and Yokogawa makes. As I remember, they recommend a straight pipe run of about 5D upstream and 3D downstream of the meter - with pipe size the same as the meter size.
(Often, for a given flow, you will find that it is more economical or even necessary to use a flowmeter one size smaller than the process pipe size, depending on the flow velocity through the meter).

Within this 5D / 3D distance, no bends, fittings, etc. are allowed.

Further, the angle for reducers / expanders in the vicinity of the flowmeter has to be maximum 4 degree.

I think, it would be prudent for you to follow the above guideline - since your line size is only 2", this might not pose any problems.

 

[PUMPDESIGNER]

Thank you bmsg.
We will be using 2" - 6" most of the time, so that pipe run is important, and we always put that flow run in. But this guy claims it is not necessary for the Siemens magmeter. He was a factory guy, not some local rep.
It is tempting to consider what they say because those flow runs are hard to get, would release us from that design parameter, lower cost too.
Here is one thought, if turbulence reduces accuracy in a predictable way and not too much reduction in accuracy, we could work with that, "suitable for purpose". We are not involved in custody transfer, or sales per volume, we just want a general idea of flow rate with accuracy of 3% and high repeatability and reliability. The Magmeters are attractive because they have no moving parts, and potentially could work in tougher piping situations with less up and downstream requirements.

PUMPDESIGNER

 

[khan101]

I would suggest that you put the onus on the manufacturer and let them know that the meter will be purchased only subject to satisfactory testing in actual condition.

 

[jsummerfield]

Go with the manufacturers recommendations where longer runs are not proactical. However, if located The upstream and downstream diameters related to swirl, etc. appear less significant for a mag meter than a head-type element. If longer length exists, then use it.

Magnetic flow meters are only suited for the conductive fluids.

John

 

[bmsg]

I think that if you need an accuracy of only about +/-3%, you can ignore all straight length requirements in a magnetic flowmeter. Only, ensure that the liquid in the pipeline has a conductivity of atleast 20micro siemens.

 

[quark]

If the supplier is asking for a premium for such a meter or if he claims it as an advantage when compared to others then I feel that is a gimmick. What you should ensure is full pipe flow(better to have full flow detection sensor in the mag). 3% accuracy is not a big deal with mags. You should ask for testing the accuracy of meters at site, as suggested by Khan, with a master meter that will be marginally effected by the absence of straight lengths.

Last point in your first post,

This seems about right to me because this is a mass meter

is not clear to me.

Regards,

 

[CARF]

For sure a magnetic inductive flowmeter is NOT a mass meter. It measures volumetric flow [m3/sec].

 

[PUMPDESIGNER]

CARF - Thank you, you are correct, Mag Meters are not mass.
I think quark would have gotten to me also on this.

Thank you guys, I am weak on flow meters so I have some study to do on the subject.

PUMPDESIGNER

 

[PUMPDESIGNER]

kahn101 - Good idea about letting them prove themselves in the field.

bmsg - We are dealing primarily with groundwater on this application, but sometimes we do potable water. Do such fluids easily accomplish 20 microsiemens by your experience?

Can I put a 110 line into the water and tell with my tongue?

PUMPDESIGNER

 

[quark]

Actually, volumetric flowmeters that measure fluids which have no significant effect in density with respect to changes in temperature and pressure can be considered as massflow meters. If the density gets effected then you have to have density compensation.

But there are true massflow meters(for ex. corriolis, thermal etc.) that don't require density compensation. Straight length requirement is insignificant in this case.

There is no problem with conductivity if the water is potable(you will have problems with purified water and better quality waters apart from other pure liquids). As a thumb rule you can calculate conductivity as TDS x 0.6.

I once again caution you to check for full pipe flow as the volumetric flow calculations consider full pipe diameters. BMSG's suggestion about the expander and reducers should not be overlooked.

Good luck,

 

[PUMPDESIGNER]

Interesting comment about temperature density relationships.
Our water temp is 60-90 F., very little density variation there.

We never have less than full pipe, high pressure systems only (High pressure meaning 60-120 psi), always full pipe.

So quark, if our pipes are always full, water temperature is always 60-90, is a magmeter's accuracy much affected by fluid flow conditions as affected by upstream/downstream conditions?

PUMPDESIGNER

 

[quark]

With those conditions, I feel any magmenter should work as faithfully as your pet dog.

Generally magmeters can be selected upto 8m/s velocity, to reduce first cost, if the permanent pressure drop is insignificant. The expander and reducers may create a chance for formation of vena contracta. Except that everything is perfect for a magmeter to be in your application.

If a 2" magmeter has a length of 10" and if the sensor is at the midlength then you already have 2.5D straight length on either side. Seems to be ok.

Regards,

 

[hydrae]

Pumpdesigner

I had an application of using 2 mag meters, I was limited in space so the configuation was, 10" mag meter, 12" diaphram operated control globe valve, 12" tee (run-run), 24" of 12" pipe, second 10" mag meter, On each side of each mag meter was a 12" x 10" reducer, we were not looking for high accuracy in the second mag meter, just to find out about how much water was flow in or out of the tee, The first mag meter was within the accurancy of a turbine meter 2 miles up stream, so it was OK, the second mag meter read up to 10% low or high (depending upon the position of the control valve) when it was known there was no change in flow due to the tee. When the valve was closed and flow from the tee (bull) to the second mag meter accuracy was better.
Results: 5D is vital when a globe valve is upstream, anything downstream 1d maybe

As for conductivity, a means of grounding the meter to the water is required, you can use ground rings installed in the flanges or a corporation on the downstream side, lack of a good ground will give fluctating readings at steady flow.

A Mag meter works by inducing a magnetic field and measuring the deformation of the field by the fluid moving through the pipe, so small eddies should cancel out hence the reducers had no effect except to increase velocity for higher accurancies.

Hydrae

 

[PUMPDESIGNER]

Hydrae,
Thank you, Tribal Knowledge good stuff, send you fire water if you want some.

PUMPDESIGNER