Hi!
I just ran into this website. I am calculating the NPSHa for a pumping system, and the only data i´ve got is that the fluid is petroleum 44ºAPI @ 130ºF and that de RVP is 5,6 psi. How can I estimate the vapor pressure at operation temperature?
API for oils is normally taken as the density of the oil at 60F compared to water at 60F though I guess you can do it at any temperature. Just haven't seen it done that way.
Do you know someone with a copy of the GPSA Engineering data book? I believe it has curves for converting TVP to RVP for crude oils.
You could be able to locate your crude at 5.6 RVP (which is at 100F) and then estimate the TVP at your operating temperature. Caution, this is not a precise number. This is not the time to start pushing NPHSA versus NPSHR numbers, especially when one considers that crude oils can change with time to say nothing of blends.
Sorry, the GPSA doesn't have the correlation for crude oils. They do have a correlation for gasolines but for crude oil, they direct you to the API data books, figure 5B1.2.
A 5.6 RVP oil using this nomograph has a TVP of 6.5 psia at 100F. 8.2 psia at 115F, 10.5 psia at 130F and 12 psia at 140F. You should be able to curve fit this using Excel and that the ln of the vapor pressure is proportional to 1/T (absolute).
