19 replies to this topic

[B-2 Spirit]

Dear All,

Please suggest how to convert liquid flow available in std m3/hr (i.e. obtained from flow meter readings in Plant) to actual m3/hr (at operating conditions).

regards,
Alok

[ankur2061]

Alok,

The difference is in the density at the standard conditions (you need to define the std conditions of temperature and pressure, e.g. 15 deg C and 1.01325 bara).

Let us take the example of hexane. The density of liquid hexane at 15 deg C and 1.01325 bara is 664 kg/m3 and at 52 deg C and 5 bara is 630 kg/m3. If your mass flow is 1000 kg/h then:

Std volume flow = 1000 / 664 = 1.506 m3/h

Actual volume flow (@52 deg C and 5 bara) = 1000 / 630 = 1.587 m3/h

Hope this provides some understanding regarding std and actual volume flow of liquid.

Regards,
Ankur.

[B-2 Spirit]

Thanks Ankur for your help.

Does it mean that there is no direct corelation between the two flow rates. The density at std condition and operating condition needs to be known for sure. What if we dont know them?

regards,
Alok

[ankur2061]

Alok,

Yes, the density of the liquid is required at both the standard and actual conditions to measure the volume flow at standard and actual conditions. Most modern flow meters (coriolis, vortex. thermal, Electromagnetic) can be configured to read actual volume flow since they can measure the density of the flowing liquid at the flowing temperature and pressure

Regards,
Ankur.

[B-2 Spirit]

Thanks Ankur,

In the same context one more question:
Pump datasheet states the specific gravity of the liquid at 323 deg C is 0.6.
Does this mean that the operating density is 600 kg/m3?
Please correct me if I am wrong.

[ankur2061]

Alok,

Yes, that is correct for your engineering calculations. Specific gravity of liquids is measured with reference to water which is considered as 1.0 for the purpose of engineering calculations. The actual definition of specific gravity is somewhat more complicated. Refer the link below for specific gravity definition:

http://www.cheresour...__fromsearch__1

Regards,
Ankur

Edited by ankur2061, 30 December 2011 - 11:02 AM.

[B-2 Spirit]

Thanks Ankur,

But I am not sure whether we should take the density of water as 1.

I am confused, as far as I know we always multiply the S.G by 1000 to get the density in kg/m3. You also stated same thing.

But if we go by the literal meaning of specific gravity, at the operating temperature i.e 323 °C, if we look from steam table the density of water is 659 kg/m3. If we use this the density of operating fluid for this case comes out to be 395.4 kg/m3 which I doubt is correct.

Can you please conclude this topic in view of above difference.

thanks

Alok

[ankur2061]

Alok,

You are confusing the issue. Your pump datasheet is mentioning the liquid specific gravity as 0.6, so for all practical purposes your liquid density is 600 kg/m3. Remember that while doing engineering calculations a lot of simplified assumptions and inputs are considered and in the case of a specific gravity of a liquid mentioned as a fraction, the density is just that fraction multiplied by 1000.

Regards,
Ankur.

[B-2 Spirit]

Thanks Ankur

[S.AHMAD]

SG = density at conditions/density of water at 60F and 1 atm.

[azhar_uk]

accoring to my standpoint i met with eng. Ankur about liquid from standard phase to actual, for liquid the variable is density .
according to GPA , standard condition is 15 C and 101.3250 kPa therefore you will measure density of your sample at this condition .but if you have LPG or ligher component you shall use general equation for gas .



Azhar
Refining Technology

Edited by azhar_uk, 10 February 2012 - 09:33 AM.

[Profe]

Excuse me Azhar
I disagree with the last part of your statement,
"but if you have LPG or ligher component you shall use general equation for gas."

We speak of liquids here, and liquids are not covered by the general gas law.
For LPG and for liquids lighter components applies the same corrections between standard and operation temperatures, but the VCF (Volume correction factor) calculation are diferent.

I think that this clarifies a little bit.

[Art Montemayor]

I agree with Profe's note.

It is important to know that LPG stands for "Liquefied Petroleum Gas" - and therefore, is essentially a liquid (and saturated at normal storage conditions). Therefore the gas law relationship does not hold while it exists in its normal state.

If you VAPORIZE the LPG (and it is no longer "LPG"), then you apply the gas laws.

[azhar_uk]

EXCUSE ME AGAIN !!!

acorrding to GPA, standard condition is 15 C and 101.325 kPa ,
what is your estimate for LPG is liquid phase or gas phase ?therefore lets me give you note about LPG ( C3 50%,C4 50%) or any mixing ratio ,at Normal condition 0 C and 101.325 kPa LPG IS GAS phase ,and you should check pressure vs temperature for propane and butane .
while if any one need to calculate total sulfur for LPG he shall be care about Temperature and Pressure and Generally gas measure at standard condition threrefore we see 343 Mg/SM3 (this value is equal to 140 PPMW) ,S=means standard condition
thank for all
Azhar
Refining Technology, Process and design engineer

Edited by azhar_uk, 23 February 2012 - 05:23 AM.

[Profe]

Hi Azhar.

I agree with you, when the conditions of pressure and temperature of hydrocarbon are vapor, we apply the general equation of gases, but if we talk of liquid hydrocarbon like LPG we apply VCF for SG of liquid.

For clarify that, consider the attached diagram of debutanizer overhed, the uncondensed vapor (mixed of C3s + C4s), use the general equation of gases.
For condensed of this stream (mixed of C3s + C4s), named LPG, we apply the VCF for this liquid.

In addition, also attached how you calculate the VCF for various streams and LPG as well.
My apologies for that are in Spanish.

I think this is useful for all.

Attached Files

•  Debutanizer overhead.jpg   43.14KB   83 downloads
•  FCV for hydrocarbon liquids.jpg   178.06KB   93 downloads

Edited by Profe, 23 February 2012 - 12:03 PM.

[azhar_uk]

HI Fausto
First of all LPG is vapor phase at standard and normal condition, but if we need to increase pressure on LPG the result is LPG Liquid therefore we see most of saturation gas plant (Open Art Design) and may be Unsaturation gas plant (depending on licensor) operating pressure or temperature for all is to liquefied feed to easy of purification and separation.
Secondly, I have some issues on your DWG , at overhead H.C vapor is LPG yes this is high temperature , you should take branch from drum for H.C vapor not from main process line to maintain or to keep operating pressure system for debutanizer and select one pump and high control system efficiency may be reach to SIL 2 or SIL 3.
thank for all
azhar
Refining technology-process and design engineer

[Profe]

Hi Azhar,

The diagram is for teaching purpose, therefore, not included control valves or other additional instrumentation. I think the purpose of this is to mainly LPG (Liquid mixture of C3s + C4s) and not the gas mixture C3s + C4s for fuel gas system, which is an operational practice when operational problems occur in that unit.

Good luck.

[JMW]

Ankur says: "Most modern flow meters (coriolis, vortex. thermal, Electromagnetic) can be configured to read actual volume flow since they can measure the density of the flowing liquid at the flowing temperature and pressure"
I think we need to be careful of this statement and understand the implications of different meter technologies.
Coriolis meters measure mass flow and if reporting volume flow depend on the measured density. They can today measure the density, not all with good accuracy but if with good accuracy this will be compensated for temperature and pressure conditions to give the density at process conditions as the primary density measurement.
This means coriolis meters can report either the actual volume flow (using the measured density) or the standard volume flow using the base density calculated from the measured density.

The other types, vortex, turbine PD etc measure volume flow.
They may be used with an inline densitometer giving you the equivalent functionality to the coriolis or they may depend on a mean density value or they may depend on peridoic sampling.
Periodic sampling may then be used to derive both the base density and the density at the process conditions by tables or calculation.

Be careful with volumetric flowmeters.
I say this anyway but particularly as your question assumes the plant meter to report standard volume and you want actual volume. It is more usual to encounter this as a problem to convert actual volumes determined by plant measurement to standard volumes for accounting purposes.
Possibly your meter does report standard volume in which case I have to assume you want the actual volume at some other set of conditions which might even be the conditions at which the meter operates.
Therefore with volume meters you need to consider:
Volume flow meters may report the actual volume flowing at process conditions and may incorporate a temperature and pressure correction to the meter factor (if measured or if operating at defined conditions) to allow for the effects of temperature and pressure on the meter.
They may alternatively be calibrated to report the volume flow at reference conditions if requested and if a mean density value and normal operating conditions were defined.
You cannot assume which is the case.
You need to check how the meter was specified and also check if the process conditions match those declared when the meter was specified.
You need to be careful if the meter was specified for conditions which do not now apply and may need to add a correction for meter factor error to compensate the measured volume.
You also need to determine if the meter is periodically proved in situ and investigate the proving procedure and what that was designed to do as this may have been used to compnaste the meter for changed consitions or changed expectations i.e. to shift from actual to standard volume flow.

The measured volume (actual) is then converted to the standard volume using the Volume Correction Factor which is a function of the density at measurement conditions to the density at standard or reference conditions.
This may be dynamic using inline density meters.
It may be based on periodic samples or it may be based on a mean value and the meter may already use these corrections to give standard flow.

Note Ankur gives an example of reference conditions.
These can vary according to where you are or industry standards.
You need to check.
If you know the density at some temperature (the observed density) you can calculate the base density and then, if the observed density is not the density at process conditions, the alternative density can be calculated.
The calculations will be defined in standards. e.g. the Manual of Petroleum Measurement Standards. This may also declare that the standard is now the calculation and not tables but which is used or mandated may depend on your industry or even site standards.
Note that the standard will recommend which tables or calculation constants are to be used for the hydrocabon being measured.

[Zubair Exclaim]

i completely agree with JMW. I personnaly had a very hard time commissioning a unit where performance guarantees required flow confirmation and that flow meter didn't had a actual conditions compensation factor with it. That thing delayed us for about 2 weeks. Volume measuring flow meters can be really tricky and hard to get noticed

[JMW]

Sorry, I forgot to mention that some volumetric sensors such as Vortex do give a mass flow signal but you need to understand how they get it. They monitor temperature and have use a reference density so all they do is calculate the density at the process temperature. Great if the density doesn't change except with temperature.
A similar approach is used with turbine meters that have a viscosity correction - they use temperature as a meas to "select" the appropriate linearisation based on a viscosity temperature calibration.
You really do need to be alert and assume nothing but check everything.
It is all too common for meters to be specified for a set of condiions that never actually are eperienced.