Hi,

There is a flow transmitter that its impulse lines are connected
to a vertical line (between them there is an orrifice).
The flow comes from down to up and the impulse line is connected correct to the transmitter (H is down from orrifice L is up), but there is a difference between them about 8 cm. the range of transmitter is 0 to 4000 mmh2o.

so, in order to have an accurate measurement need to do a zero trim (when there is no flow) or it is correct as it is?

Sorry for my bad English!

 

For upward flow in a vertical pipe, the total pressure drop is the sum of the pressure loss due to elevation and the pressure drop due to the flowmeter element.

If you were to shut off the flow upstream of the orifice plate, with the pipe full of liquid, the reading at zero flow would be 8cm water column * specific gravity of the fluid.

We'll assume water at SG = 1.00 for sake of argument.

The elevation distance between the high and low ports is 8cm. 8cm = 80mm, so the water in the pipe exerts a static pressure of 80 mm w.c. between the high side and the low side.

80mm/4000mm = 0.02 or 2% of max DP (at max flow rate)
The square root of 2% of DP = 14.14% flow rate. sq rt 0.02 = 0.141 = 14.1%

So at no flow, the meter reads 14.1% flow rate.

At the high end of flow, with the 80mm of offset at zero flow due to elevation error included at an indicate max flow at 4000mm DP, the real flow rate is 4000mm - 80mm = 3920mm

3920mm/4000mm = 98% of max DP.

sq rt of 0.98 = 0.9899 or 99% of max flow rate, so the error is only 1% at the high end.

 

> If you were to shut off the flow upstream of the orifice plate, with the pipe full of
> liquid, the reading at zero flow would be 8cm water column * specific gravity of the fluid.

That is incorrect.

There may be a greater height of liquid in the low pressure sense line, but that is compensated for in the high pressure sense line by the liquid in the process pipe. Both sides of the element are subjected to the same amount of pressure.

The only instance where there will be "8cm' of differential pressure at no flow condition is if the process level is near or below the orifice plate (effects vary with liquid level and gaseous pressure, or lack thereof, above the liquid).

Calibration for flow at high end should include the calculated offset for the '8cm' differential.

 

I stand by my assessment.

> Both sides of the element are subjected to the same amount of pressure.

The two ports do not see the same head pressure.

The pipe is vertical. The DP high side is the tap at the lower elevation, the DP low side is the tap at the upper elevation.

The DP low side 'sees' 8cm less liquid/water column pressure than the DP high side because it is elevated 8cm higher than the high side.

In fact, in the static no-flow state, the vertical installation is a classic DP density measurement with the taps at a fixed elevation distance apart from one another, albeit an 8cm elevation difference is too little, it is impractical.

 

I understand why you are confused, so I'll attempt to explain. If the following doesn't work, I can draw a diagram.

The DP element, being a liquid system, is mounted below both process ports with the chambers (H & L) horizontal. There is equal head above both chambers. On the L side, 8cm more in the sense line, but the H side sees the same 8cm of head in the the process pipe.

 

Assuming the same fluid is in the impulse lines as in the pip the zero flow DP is zero.

It doesn't matter if the transmitter is above, below or beside the tapping points zero flow results in zero DP

Draw the installation out with rough dimensions then to make it easy assume the pipe is full to a point above the highest tap or transmitter then add and subtract all the dimensions you will find out the net result is the same for both sides (zero DP)

You can do a zero in the normal way closing one side and opening the zero valve.

 

Why you believe that should include the calculated offset for the '8cm' differential?

My opinion is that does not need, because when flow exist The pressure in the line is bigger than the hydrostatic pressure in impulse lines. so transmitter will not take account the hydrostatic pressure but the real pressure in the line.

 

Here's a couple of liquid vertical flow DP installation diagrams.

http://i61.tinypic.com/oac6br.jpg

http://i58.tinypic.com/302yxpz.jpg

The low side tap is elevated above the high side tap in each, which has to reflect a head pressure difference.

A zero flow, with liquid in the pipe, a DP will show the elevation difference between the upper tap and the lower tap.

If DP does not 'see' a head pressure between the two taps (with the pipe full, but at no flow), then DP level or DP density measurements would not be possible.

> Why you believe that should include the calculated offset for the '8cm' differential?

The total pressure drop is the sum of the pressure loss due to elevation and the pressure drop due to the flowmeter element.

 

>"so transmitter will not take account the hydrostatic pressure but the real pressure in the line."

On the space station, yes, I would agree, because of near-zero gravity. However, on the planet earth, the acceleration of gravity makes the liquid mass in the pipe weigh something which is measured as a head pressure by the DP transmitter.

Apparently the Starship EnterPrize generates its own gravity, and even more remarkably, somehow eliminates the crushing effects of accelerating to warp speed, which would really throw off a DP measurement. But until your technologies can manage gravity or anti-acceleration then you're stuck with today's physics.

 

Maybe my English is bad but I think you have an understanding problem! As I wrote "doesn't take account" (because I spoke for dp transmitter and the gravity is equal on both sides with the pipe full) not " doesn't exist"!

>>"so transmitter will not take account the hydrostatic pressure but the real pressure in the line."

> On the space station, yes, I would agree, because of near-zero gravity. However, on
> the planet earth, the acceleration of gravity makes the liquid mass in the pipe
> weigh something which is measured as a head pressure by the DP transmitter.

 

Yes there is a difference in pressure at the process ports. However, given the same liquid fills the impulse lines, the vertical distance from process port to element chambers add (or subtract) to (from) the port pressure.

In both images to which you linked, the element is below the process ports, impulse lines are filled with process liquid, and the difference in vertical height of the low pressure impulse line to the high pressure impulse line is equal to the difference in elevation of the process ports. This difference in height of the impulse lines negates the difference in pressure at the process ports. Both sides of the DP element realize the same hydrostatic head.

 

To Comment - I expect we are saying the same thing really.
In my first post I said "Assuming the same fluid is in the impulse lines as in the pipe the zero flow DP is zero".

I will use the first example you posted.

>>"so transmitter will not take account the hydrostatic pressure but the real pressure in the line."

>On the space station, yes, I would agree, because of
>near-zero gravity. However, on the planet earth, the
>acceleration of gravity makes the liquid mass in the pipe
>weigh something which is measured as a head pressure by the
>DP transmitter.

The pressure in the lower orifice tap at zero flow will be 8cm higher than the upper tap but don't forget the upper tap to the transmitter is 8 cm higher in elevation, by the time the pressure reaches the diaphragm of the transmitter it cancels out to zero DP

Yes a similar installation can be used to measure density in the line if the impulse lines are flushed with some reference fluid e.g. water.

If we were trying to measure a slurry flow and using water as a flush fluid it would be a whole different calculation.

>Apparently the Starship EnterPrize generates its own
>gravity, and even more remarkably, somehow eliminates the
>crushing effects of accelerating to warp speed, which would
>really throw off a DP measurement. But until your
>technologies can manage gravity or anti-acceleration then
>you're stuck with today's physics.

 

This discussion does not seem to be resolving the issue with words, which seems pretty much clear to me. However, seems like Bernoulli's Equation applies to this as it is the basis for orifice and other head measurements. You can find the answer in any good book on orifices that derives the orifice equation or even in any beginning fluid mechanics book. I believe that it will be found that Newton was correct in his deductions underneath the apple tree. Since the pressure is assumed to be static (e.g. the S.G. is constant), the pressure difference at zero flow is normally zeroed out at the transmitter with the pipe full.

Please see:

1. Richard Miller's - Flow Measurement Engineering Handbook or

2. L.K. Spinks - Principles and Practice of Flow Meter Engineering, 9th Ed.

Or just google: Bernoulli's equation + orifice. You can start with Wikipedia.

William (Bill) L. Mostia, Jr. PE
ISA Fellow, SIS-TECH Fellow,
FS Eng. (TUV Rheinland)
SIS-TECH Solutions, LP

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