Good Afternoon,

We have orifice plates in 10" line pipe for air flow measurement in air blower suction line. DP transmitter is connected where we get air flow. Now it is showing less air flow as compared to blower discharge flow. We checked all impulse lines and transmitter configuration also. But there were no changes. Due to negative pressure in line we could not justify the leakage. What will be probable cause? We want to install a new parallel flow meter, what type will be good for it?

 

What is the percentage difference between the two flow transmitters? How are the orifice plates installed? How much straight run do you have? When did you last calibrate the transmitter?

Walt Boyes
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Something to check: Where are you doing the square root extraction?

I remember once checking and re-checking a gas flow calculation looking for a <10% error. Eventually I found that the transmitter was doing the square root as well as the input module. I won't be caught by that again.

Regards,

Roy

 

It is in 20% difference, orifice plates are installed correctly. Earlier it had shown accurate reading. It is in the suction line of air/methanol blower line. For cross checking now we are using capacity of blower. There are three blower. We want to install a new meter. Which one will be best?

 

If you are planning to install a secondary flow meter, try Emerson annubar series (multivariable or standard transmitter). I had recently a very good experience with this type of device and a DN 450 pipe for the combustion air of a furnace.

 

If you need to measure low pressure air flow in a short pipe with no straight run I have a very cheap method.

Roy

 

Okay Roy, I'll bite. I know of several ways to do it, none of which are cheap. I'd like to learn something new.

Walt Boyes
Editor in Chief
Control and Controlglobal.com
www.controlglobal.com
Mailto:[email protected]

 

Several years ago I was faced with the challenge of controlling air flow to a flotation cell. There was only about 3 ft of straight run after an elbow followed by a 10" damper valve. I tried to measure with a pitot tube but the flow profile was unworkable (thermal dispersion was also tried) and the low pressure available ruled out anything like an orifice.

I ended up using a large saucer fan (cabinet ventilation type) that just fitted inside the pipe. The fan gave a nice sine wave output which I put into a F/mA converter.

In front of the fan I placed a flow straightner made from cutting up a length of EMT conduit. I don't know that the straightner was required because all the air had to flow through the fan.

The actual flow was not important so I just scaled it 0-100% but I did think about mounting it in a tube on top of the car so I could relate frequency to velocity.

The flow meter was still working like a charm 2 years later. I figured at it's reduced speed and without heat from powered windings it should have lasted for many years.

The total cost of materials was less than $100 (including a home built F/mA converter.

Hope someone finds this useful
Roy

 

We have orifice plates in natural gas line, process natural gas line and methanol line. It has been running for last 20 years. There are about 35 Kg/cm2 pressure and 410 o C temp. All flow elements are orifice plates? Is there any chances of wrong reading from orifice plates since it has servicing last 20 years?

 

As earlier, the transmitters were showing the correct readings, the possibility of square root corrections at the 2 places shall be very remote. There has to be some leakage or blockage in impulse lines or blockage in orifice plates.

In a similar situation long back I found a block of wood obstructing the orifice plate. The line was boxed-up 10 years back and never opened. Transmitter started giving flawed reading after 10 years of service. After so many days of different exercises with the transmitter and associated impulse tubes the 12" line was opened and a wooden block was found which had traveled slowly in so many years to block the orifice.

Again if the transmitters have shown correct readings for 20 years, what is the need to install a different type of flow meter? Except if it is for the academic reasons. It shall be a better thought to find out the source of the erroneous reading at this stage after so many years of good service.

 

Yes, after 20 years your orifice plate may have worn, but that would show up on examination. A worthwhile exercize might be to have someone re-calculate the orifice plate to verify the original calcs were correct.

Roy

 

With time, Orifice Meters do have problems, it could be because of wearing or tearing which shifts the flow co-efficient of the orifice plate, hence if it is not re-calibrated the inaccuracy will increase. To get good accurate metering you should recalibrate your orifice to find the new flow co-efficient & recalculate DP based on the new constant value.

To avoid this issues you can look at ANNUBAR or VERABAR which are nothing but Averaging Pitot Tube. There are 1000s of locations where this units replaced orifice plate to give trouble free performance & great accuracy for decades together without any problem. Moreover less pressure drop hence more energy efficient devices.

Alternative solutions could be Thermal Mass (Direct Mass Metering, if required), Vortex (Though not very common on this application), etc.

 

Thank u NG. We think there may be leakage some where. But line has -ve pressure line. For that we cant locate where leakage is present.

 

Sanjib,

For checking the leakage in vacuum systems, I have seen sometimes operators using Agarbati (to see the direction of smoke).