Nitrgen gas flow correction to standard conditions 2

[rustyhooks]

We have a situation where we were given the following set of conditions regarding nitrogen gas flow through a pressure reducing valve:

Flow = 360 lb/hr
Pressure at Inlet = 34.7 psia
Temperature at Inlet = 306 deg F

Because these are actual conditions, I would like to convert them to standard conditions (assume 14.7 psi, and 70F)so I can compare that flow to our catalog flow which is based on standard conditions. We have gone through numerous calcs using the ideal gas law but we differ on how to correct to standard conditions. Can anyone offer advise?

Thanks in advance.

 

[25362]

Rustyhooks, since you have the mass flow rate you don't need the actual conditions, because the mass doesn't change when changing these conditions.

If you are after the volume rate at 70 deg F and 14.7 psia, look for the tabulated density of nitrogen at these conditions. I found 13.804 CF/lb which multiplied by 360 lb/h would give you the volumetric flow rate at those selected standard conditions.

 

[Montemayor]

Rusty:

What 25362 has advised you is probably the fastest and most direct way to get to the answer. There is another way – one where you don’t have to know or acquire the specific volume for the Nitrogen at the stated standard conditions. What you do is:

1. Convert the mass flow rate of nitrogen to molal flow rate by dividing the mass flow rate by the molecular weight of nitrogen. This gives you (360 / 28.0134 = 12.851 mols/hr)

2. Multiply the molal flow rate by the specific volume of a mol of ideal gas at 60 ^oF and 14.7 psia (379.49 scf). Then correct for the 70 ^oF by multiplying the product by (530/520). The result is:

(12.851) (379.49) (530/520) = 4970.6 scfh at 70 ^oF and 14.7 psia

Which is pretty close to the 4969.44 that 25362 obtains. I believe 25362’s method is more accurate but you must know the specific volume at the stated standard conditions. This you can obtain at:

http://webbook.nist.gov/cgi/fluid.c...nit=kJ/mol&WUnit=m/s&VisUnit=uPa*s&STUnit=N/m

 

[mbeychok]

The earlier response from 25362 is correct. You could also use the ideal gas law of PV=nRT, where P is the absolute pressure in psia, n is the number of lb-mols, R is the universal gas law constant of 10.73, and T is the absolute temperature in degrees Rankine:

Since the molecular weight of nitrogen gas is 28.02, then
n = 360/28.02 = 12.85 lb-mols/hr

T = (70 + 460) = 530 degrees Rankine

Thus, V = (12.85)(10.73)(530)/(14.7) = 4,970 ft³/hr

Using the density of 13.804 ft³/lb found by 25362:
V = (360)(13.804)= 4,969 ft³/hr ... which is essentially the same answer as obtained above by using the ideal gas law.

You should understand that there is no universally accepted set of "standard conditions". I would urge you to read the Wikipedia article below to see how very many different sets of "standard conditions" are in use:

http://en.wikipedia.org/wiki/Standard_conditions_for_temperature_and_pressure

Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[rustyhooks]

Thanks for taking time to answer guys. We are good.

 

[dcasto]

Standard conditions are either set by contract or by state laws.

Oh, N2 has a zfactor of .999847 at 14.696 psia and 60 F so you need to make this adjustment to the above..

Never mind its close enough.

 

[israelkk]

I am puzzled. The flow at pressure inlet of 34.7 psig (psia) is a choked flow where the flow at 14.7 psig is unchoked flow. How can you convert them the ways offered. As I understand the problem the mass flow rate is a function of the inlet pressure and a function of the SQRT of the absolute temperature. Using the compressible mass flow formulas and the actual data in the first post I will calculate the valve orifice equivalent drag coefficient (Cd) or the orifice equivalent LOHMs according to the Lee company methods. Then using the same formula with the different inlet temperature and pressure I will calculate the mass flow rate.

 

[Montemayor]

israelkk:

I am also puzzled. How is it that you can predict sonic flow? We have been given only one pressure condition:

Pressure at Inlet = 34.7 psia

The OP has not mentioned what his pressure drop across the pressure reducing valve is. For all anyone knows, it could be 20 or 25 psia - which would take it out of the possibility of sonic flow. The OP asked: "I would like to convert them (the actual flow conditions) to standard conditions" - and that is what we all have done.

 

[israelkk]

Montemayor

I accept your remark about the oulet pressure without more data then given. However, I believe the correct way to receive the mass flow rate at other then the given conditions is as I suggested by using the mass flow formula for compressible fluid.

 

[mbeychok]

israelkk:

I am confused about your "34.7 psig (psia)". I don't understand that unit "psig (psia)". What is it?

The original poster clearly stated 34.7 psia as the inlet pressure. And his stated standard conditions of "14.7 psi and 70 F", although not explicitly stated, was obviously meant to be "14.7 psia and 70" since atmospheric pressure is often stated as 14.7 psia (in reality, 14.696 psia). So I also don't understand why you changed it to 14.7 psig.

I agree with MOntemayor that the above responses by 25362, Montemayor and myself are all correct answers to his clearly stated question.


Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[mbeychok]

israelkk:

I would also point out that the Lee Company to which you referred is a company that manufactures miniature micro-hydraulic components and electro-fluidic systems. They also seem to have developed what they call "Lohms liquid laws" and "Lohms gas laws", neither of which set of laws have I ever seen in any fluid flow textbook. I would caution you to use what you call "orifice equivalent LOHMS" with a bit of healthy skepticism.

Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[israelkk]

mbeychok

In the last 30 years of R&D of pneumatic servo actuators, pressure regulators, high speed electro-pnematic ON-OFF valves, pressure vessels, etc. This included full mechanic, dynamic and thermodynamic simulaion of charging and discharging constant volumes and dynamically changed volumes such as a piston moving against forces, torques and inertias for aerospace use.

I have good experience with the LOHM system although for the full blown simulations we use our own formulations developed from scratch using the incompressible fluid flow theory combined with heat transfer through the volume walls during the charge and discharge of the volumes. The LOHM system is a "cooked" system where some of the basic parameters are hidden and we use it mainly for quick and dirty calculations or to compare with other calculation methods.