3 replies to this topic

[chemks2012]

Hi

We are planning to use a Coriolis mass flowmeter at the double diaphragm pump discharge (2" discharge line) to measure the flowrate of viscous liquid (viscosity of 800 cP) which contains some engrained gas (no idea how much).
The pump seems working at it's maximum capacity at present. The 2" flowmeter has pressure drop of 1.5bar while 3" flowmeter has pressure drop of 0.15bar.
As the pump is working st it's max capacity (5barg discharge pressure, 9000kg/hr), I think we need to have a flowmeter with mini pressure drop otherwise it will reduce the flowrate and therefore I suggested 3" flowmeter while supplier suggested 2" flowmeter which has higher DP. Supplier thinks that 2" flowmeter would be good for engrained gas (also viscosity is higher and gas bubble needs higher velocity??) due to high velocity. However I personally think that higher DP of 2" flowmeter will reduce the flowrate.

Am I missing anything?

Is there any other suitable flowmeter for the engrained gas which could give lower pressure drop?

Any help will be highly appreciated.

[breizh]

Hi ,
Let you consider these resources :

http://www.mainpress...osing Pumps.htm

Hope this helps

Breizh

[JMW]

I would think the coriolis meter a good option here with its entrained gas capability if you have a significant amount of gas i.e. in the 1-10% range.
But, be sure to check two things with the manufacturer:

• The effect of installing downstream of a double diaphragm pump. This type of pump may produce a pulsation in the flow. Be sure to check if this is OK for the coriolis as some of them are affected by pressure pulsations. It may require you modify the installation to accommodate this.
• At 800cP, any air/gas entering the fluid isn't going to be dispersed easily. If the gas is present as a dispersion of bubbles and the fluid is thus relatively homogeneous, then the coriolis meter should be fine.
  You'll find a chart on the Neftemer web site (in one of the presentations) which illustrates well flows with different amounts of gas.
  Bubble flows are fine.
  But be sure to confirm with the manufacturer if slug flow is expected.

Reading the article posted above they say that this can be a cause of damage to mechanical meters. Sure it can. But not to all mechanical meters. Piston and nutating disc meters, unbalanced rotor meters yes. Some gear meters should be fine.
But while slug flow will not damage the coriolis meter, I am not sure that it will measure the flow with slug flow and the article does not say it will. From the trials by BP in Singapore (with Invensys coriolis meters) they amply demonstrated the the capability of the meter to handle what is called the "cappuccino effect" - a dispersion of bubbles in fuel oil. But they were concerned with tank stripping which is when the tanks are empty and the pumps draw the last dregs through and also large amounts of air.

Fuel oils run at 150-380cst typically but some are 500cst and raw residual fuels can be higher so these are pretty much equivalent to your fluid.
The only question is the one you don't know the answer to, how much air you should expect.
It would help also to understand how the air/gas is introduced and if you should expect this to be as bubbles or pockets.

If it is just a small amount of air - less than 1% say, you might figure that coriolis is not the only choice. Conventional coriolis do not like air because the bubbles (VOS effects) perturb the measurement. The entrained gas capability overcomes this problem but are you paying extra for this?
Coriolis meters also have comparatively high pressure drops. This may mean that at low gas volume fractions you may consider a suitable PD meter a good choice. It will have lower pressure drop and will report the volume flow accurately so the only error is the error attributed to the proportion of the flow that is gas.
If you want good accuracy then you may need to think about this. The coriolis will give you around 0.5% for entrained gas flows. A 1% gas inclusion is going to cause a PD meter to under record liquid flow by upto 1% plus the meter accuracy.
You don't say what accuracy you need.
A couple of other options are possible but since your line size is 2", using PD with an Entrained gas density meter which will give you around 0.75% accuracy or better irrespective of the gas volume fraction,is a problem because the density meter is 1" and needs a bypass. However, the Sonartrac GVF measurement is available from 2" to 6" and will measure Gas Volume Fraction upto 20% with 5% accuracy.

[JMW]

PS a couple of added points.
At 800cP, if we are dealing with bubbles, they aren't going to go anywhere except where the liquid goes and at the same pace. At these viscosities it would take days for bubbles to migrate out of the liquid if allowed to stand.
And, depending on accuracy, Sonartrac also do clamp on flow measurement...... 2" to 60" lines.
No pressure drop (they are not ultrasonic, as they tell you on the web site)... but I would ask for references if you go this route. I have no experience of using them.
http://www.cidra.com...ucts/vf100.html