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# Vapor Pressure Of Crude Oil

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Posted 13 August 2012 - 07:40 AM

Dear All,

Can either RVP or TVP of crude oil be supply as the vapor pressure for crude oil for control valve sizing? I need to confirm because crude oil is a mixture of components.

### #2 Bobby Strain

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Posted 13 August 2012 - 03:52 PM

When in doubt, provide both.

Bobby

### #3 Dacs

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Posted 13 August 2012 - 07:54 PM

My understanding is RVP is taken at 100°F. Since vapor pressure is a function of temperature, I tend to think that you have to use TVP especially if you're working on temperatures far from 100°F.

I think RVP usage makes sense when comparing 2 different blends for their volatility.

My 2 cents

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Posted 14 August 2012 - 03:32 AM

Dacs,

What if the temperature is below 100 deg.F, can I still use the TVP as the crude oil vapour pressure?

From HYSYS simulation, I obtained the RVP and TVP at 38 deg.C as -10.55 psig and 0.1174 psig respectively at the pump discharge outlet on the HYSYS model. The inlet pressure of the oil into the LV is 526 psig @ 99.84 deg. F, and the outlet line from the LV is connected to a 1.5 psig surge tank. This shows that I would have a large pressure drop across the valve.

Can I go ahead and use the TVP values as the fluid vapour pressure?

Also, I am skeptical the usage of TVP. From the simulation model, the TVP obtained from crude oil outlet from the oil separator was 55.07 psig @ 38 deg.C. The vessel operates at 55 psig at 38 deg.C.

Edited by asade abiodun, 14 August 2012 - 04:05 AM.

### #5 kkala

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Posted 14 August 2012 - 11:02 AM

A. Please look into http://web.anl.gov/PCS/acsfuel/preprint%20archive/Files/48_1_New%20Orleans__03-03_0574.pdf , as well as http://en.wikipedia...._Vapor_Pressure - and "true vapor pressure", "vapor pressure" below the "See also" heading. Concerning petroleum liquids of 100 oF (37.8 oC), it is understood that: Reid VP < vapor pressure < TVP, the latter being a bit higher than (equilibrium) vapor pressure, since TVP also considers any dissolved gases. In a few cases vapor pressure = TVP (if difference quite small).
Since these are based on references, advice from practical experience is welcomed.
B. Consequently TVP of liquid crude oil at (max) operating temperature is slightly conservative and judged more suitable for control valve data sheet (liquid crude in, probably liquid+gases out).
Γ. Other remarks.
- RVP and TVP at 38 oC cannot be so different for same liquid, something may be missing, probably RVP=10.55 psig and TVP=11.74 psig? Is it psig or psia?
- I have limited knowledge on API separators only, not the case of the query. Nevertheless result of TVP=55 psig (temp=38 oC) seems too high for the "separated" liquid crude (being liquid, not a gas / liquid two phase mixture).
Note: Liquid crude is usually stored in floating roof tanks in refineries.

Edited by kkala, 14 August 2012 - 11:11 AM.

### #6 paulhorth

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Posted 14 August 2012 - 05:12 PM

What if the temperature is below 100 deg.F, can I still use the TVP as the crude oil vapour pressure?

you are missing the point.
The vapour pressure of any liquid, crude oil, water, mercury, whatever, depends on its temperature. The vapour pressure you should use for control valve sizing should be given for the operating temperature of that valve. If the operating temperature is say 300 C, or 10 C, then quoting the vapour pressure at 38 C is really no use.

TVP stands for True Vapour Pressure and is always quoted at a reference temperature, and doesn't have to be at 38 C. In contrast, Reid Vapour pressure is always at 100 F and is not a true vapour pressure measurement since it is measured on an oil sample in air.

So Bobby Strain's answer is also incorrect.

Paul

Edited by paulhorth, 14 August 2012 - 05:14 PM.