Measuring air flow with differential pressure instrument

doomsword

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Aug 2016
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On site we have Yokogawa EJA110E differential pressure transmitter connected to process air line. I'm not sure if this can be used to measure air flow... I already told client it can't, now I see it's possible to generate flow from delta pressure, so I've possibly made a fool of myself today. :p


First I received overrange from instrument (30000+ in my input word in Siemens PLC), then I went down to check what's wrong. Now it's not sending anything, could be because nothing is inside the pipes yet (I mean, there's no air flow).

I presume I could read delta p and use Bernoulli to do some math or is not possible at all?
Air is supposed to flow in only one direction.
 
To measure the flow, the transmitter must measure the loss of pressure through an orifice plate, but for a correct reading a calibrated orifice must be used that should be inserted in a straight section of pipe, according to the specifications of the orifice manufacturer.

The flow rate will be a function of the square root of the measured differential pressure. Some transmitters can do the square root by itself.

Without flow the mesured pressure must be 0, if you are measuring overrange then something is wrong.
 
To measure the flow, the transmitter must measure the loss of pressure through an orifice plate
I wondered, too, if you have a primary flow element in the line. If you want measurements that mean anything, then there has to be a primary flow element.

An orifice plate is a primary flow element.

All primary flow elements create a permanent pressure loss in the line, which is undesirable because it costs money to pump air and losing line pressure in a measurement is counter productive.

An averaging pitot tube is also a primary flow element with a fraction of the permanent pressure loss of an orifice plate. The trade-off is that the measurable signal from an averaging pitot tube is a fraction of the measurable signal (at the same flow rates) as an orifice plate. So the DP transmitter has to be able to measure a lower range of pressures when used with an averaging pitot tube.
 
On site we have Yokogawa EJA110E differential pressure transmitter connected to process air line.

What kind of air line? Is it a large duct? A small pipe?

How is the transmitter connected to the air line?

Is there an orifice plate installed?


I'm not sure if this can be used to measure air flow... I already told client it can't, now I see it's possible to generate flow from delta pressure, so I've possibly made a fool of myself today.

Just tell your client that you thought about it, did some research, and found that it will indeed be possible, as other posters have already explained.


First I received overrange from instrument (30000+ in my input word in Siemens PLC), then I went down to check what's wrong.

What did you find? If you were receiving an over-range signal then there must have either been positive pressure applied to the instrument, a wiring error, a defective instrument or power supply, or a module configuration error.


Now it's not sending anything...

What do you mean by that? Is the signal now at zero, or is there no signal?

What is the milliamp reading from the transmitter (if it's a current signal) or voltage reading (if it's a voltage signal)?


could be because nothing is inside the pipes yet (I mean, there's no air flow).

No, That wouldn't explain it. No air flow should equal 0 pressure, if the transmitter is plumbed properly. Does the transmitter have a local display? What do you see there?
 
]Okay, so signal is 4-20 mA, now I'm reading 0 (4 mA). Two days ago it was in around 30000 (20 mA is 27648 in Siemens), and instrument was showing 110%. Now it shows 0%. I did not make any changes on instrument. Electrically all is okay, we checked this.

Regarding sensing element P&ID says it should be pitot tube (instrument list says multihole element with 5 valves manifold). Process air is coming by a large pipe 300 mm in diameter and incoming air should be at 6 bar.

Range for this instrument should be 0-20000 Nm^3/h (according to instrument list I have). On nameplate it's written Max Working Pressure is 16 Mpa and calibration range is 0 to 1059.

I've seen AL.10 and AL.30 on this transmitter, according to manual Al.10 means input pressure is out of range, AL.30 means output is set to high/low limit value.
 
You need to decode the model number on the transmitter's ID tag to see what pressure range the transmitter is designed for. The pressure range is one of the alphanumeric characters in the long model number. YOu can find a spec/datasheeton the manufacturer's web site that decodes the model number.

To configure the transmitter range, you need to know what the design maximum flow rate is and the DP at the flow rate.

The averaging pitot tube came with a 'sizing sheet' that tells what the DP is at the 'design' maximum flow rate.

For example: Max design flow rate: 0-2000 Nm^3h

DP at max flow rate: xxxx kPa

Averaging pitot are typically down in the range of 10.59 kPa (frequently less than that)

You provide no engineering units for the 1059 value. So it isn't clear what the 10.59 means.

It looks like that transmitter might have a display on it. If it has a display, it might have keys/buttons to navigate a setup menu. If no buttons/keypad, then you need a HART communicator (handheld or PC/software/HARTmodem to figure out what the engineering units are and whether the transmitter is setup correctly, whether its setting match the setting on the sizing sheet.

Zero flow can be caused when
- The root valves are closed
- the process connection valves on the manifold are closed
- the equalizing valve on the manifold is open (should be open for purposed of calibration.

You need to check the condition of all the valves on the manifold.

Many DP transmitters have a means of setting the LRV and URV by applying pressure and use a key or button on the transmitter. It might be that someone has re-ranged the transmitter using that kind of function. That is not a practical using actual flow, that method is intended for tank level or for flow when doing bench configuration using a pressure calibrator.
 
Dear doomsword,

please have a look at this. You can even download a trial version to test.
The basic thing is to know a lot more about actual primary flow element. You said it is pitot tube, but do you have a mechanical drawing?

To correctly measure the air flow there must be a calculation based on geometry of primary flow element and properties of the flow medium (air in your case).
There are really a number of actual mechanical setups.
For example, have a look at the attached picture.
In my humble opinion, you must first know how primary flow element is built and then to have its calculation.

Air flow measurement - example.jpg
 
You need to decode the model number on the transmitter's ID tag to see what pressure range the transmitter is designed for. The pressure range is one of the alphanumeric characters in the long model number. YOu can find a spec/datasheeton the manufacturer's web site that decodes the model number.

To configure the transmitter range, you need to know what the design maximum flow rate is and the DP at the flow rate.

The averaging pitot tube came with a 'sizing sheet' that tells what the DP is at the 'design' maximum flow rate.

For example: Max design flow rate: 0-2000 Nm^3h

DP at max flow rate: xxxx kPa

Averaging pitot are typically down in the range of 10.59 kPa (frequently less than that)

You provide no engineering units for the 1059 value. So it isn't clear what the 10.59 means.

It looks like that transmitter might have a display on it. If it has a display, it might have keys/buttons to navigate a setup menu. If no buttons/keypad, then you need a HART communicator (handheld or PC/software/HARTmodem to figure out what the engineering units are and whether the transmitter is setup correctly, whether its setting match the setting on the sizing sheet.

Zero flow can be caused when
- The root valves are closed
- the process connection valves on the manifold are closed
- the equalizing valve on the manifold is open (should be open for purposed of calibration.

You need to check the condition of all the valves on the manifold.

Many DP transmitters have a means of setting the LRV and URV by applying pressure and use a key or button on the transmitter. It might be that someone has re-ranged the transmitter using that kind of function. That is not a practical using actual flow, that method is intended for tank level or for flow when doing bench configuration using a pressure calibrator.


It's written 1059 Pa. Anyways client told me they'll try to adjust it correctly, so far I'm either receiving 0 or some random values.


Dear doomsword,

please have a look at this. You can even download a trial version to test.
The basic thing is to know a lot more about actual primary flow element. You said it is pitot tube, but do you have a mechanical drawing?

To correctly measure the air flow there must be a calculation based on geometry of primary flow element and properties of the flow medium (air in your case).
There are really a number of actual mechanical setups.
For example, have a look at the attached picture.
In my humble opinion, you must first know how primary flow element is built and then to have its calculation.


Okay, I'll check this with client on site tomorrow.
 

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