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Diesel Vapor Lock - Vapor Pressure?

Diesel Vapor Lock - Vapor Pressure?

Diesel Vapor Lock - Vapor Pressure?

I am working on a fuel oil transfer piping system that will draw No. 2 fuel oil from a remote fuel storage tank to a steam boiler. The pump is a Positive Displacement type pump located in the boiler house. I have performed pressure drop calculation on the suction leg of the pump which shows the pump inlet pressure to be around 9 PSIA, which is acceptable per the pump vendor data. However, I am concerned about vapor lock of the pump on a hot day. Portions of the suction leg are exposed to the boiler house temperatures, which I estimate could hit 120 F. Does anyone know what the vaporization pressure is for No. 2 Fuel oil (diesel) at 120 F. I have not been able to find it in any of my references. If you have the data please let me know where you got it so I can reference it in my calc.



RE: Diesel Vapor Lock - Vapor Pressure?


First, Use GOOGLE and search "vapor pressure" fuel oil.... your first hit is


Go here and read.....

This superb website gives you some do's and don't's for fuel oil systems... please note that you are making a classic mistake by locating fuel oil pumps far from the tank!! I worked on a similar system in 1993 where a utility abandoned an underground fuel oil tank and installed a remote aboveground unit. Western PA had 100-year coldest weather the next year and the two Viking pumps failed to deliver required pressure at the burners.... it seems that the long run of piping coupled with the cold temperature caused "waxing" of the fuel oil.

Oh yeah,..... the MBA manager of the utility "downsized" the suction pipe size specified in order to save money on the installed cost....The installed pipe size was smaller than the pump suction nozzle.....Another "mortal sin" in pump system design!!!

When the fuel oil pumps would'nt work, the MBA was off managing a different project, screaming at different people.... the operation of the new pumps was "not of his concern"  (his words)

I believe your problem could be lack of NPSHa due to equipment location. Additionally, contact your fuel oil supplier and ask about additives if cold weather operations are to be considered.

Good Luck ...................................MJC

RE: Diesel Vapor Lock - Vapor Pressure?


Thanks for the advice and the web site link. I had been to the pump school site earlier and did find it very informative, unfortunately I did not find the vapor pressure data for No 2 fuel oil at 120 F. Do you know anywhere else I might look?

Your comment about low temperature waxing of the diesel causing problems on your project brought up a few more questions. At my site the tank and lines are underground (at least 2' of cover), or inside the boiler plant. I assumed that the fuel temperature would stay at or above 40 F throughout the winter. I was able to find viscosity data for No. 2 fuel oil at 40 F and used it in my pressure drop calculation (15.5 centipose). I assume your problem had to do with the tank being above ground and subject to colder temps. Do you know of any problems with waxing occuring above 40 F with No. 2 oil?

You are not the first person to tell me I am going to run into trouble with the long suction leg for the PD pump, but so far no one has been able to explain to me why it will be a problem in this instance. I agree that ideally the tanks should be as close to the pump inlet as possible when using PD pumps, to reduce pressure loss in the line and increase the pressure at the pump inlet. The reason I would like to use the remote tank location is financial - the tanks were already in place prior to the project and are not being used. Additionally, the remote tanks are at a higher elevation (+3 feet) than the available tank site local to the pumps. I have completed a detailed calc on the expected pressure drop in the suction leg including line loss, fitting loss, elevation change, etc.. and I come out with an acceptable inlet pressure at the pump (8.81 PSIA, and the pump vendor says keep it above 8 PSIA). I know that is close, but I figured I could add a small booster pump at the tanks to bump up the line pressure if it doesn't work after installation. The suction run is about 320', with a lift of 8.33'. I have sized the suction line at 2" ID, which is the same size as the pump inlet. The pump will flow about 17.25 gal/min. I included 21 elbows, 5 tees, a foot valve, 2 gate valves, and a strainer in my fitting losses. Can you think of anything I left out of the calc that would cause a higher pressure drop?

I want to make sure this system will work once it goes in, but I can not go to the client and suggest installing new tanks unless I have a legitimate reason why the existing remote tanks won't work. Any advice on things I failed to account for would be greatly appreciated.


RE: Diesel Vapor Lock - Vapor Pressure?

Can you just re-locate the pump close to the storage tanks?

We suffered "waxing" during that same cold winter, but it occured at the suction strainer of the pump. We "solved" it by pulling the strainer basket for the cold snap. There wasn't much else to do, except shut down. The oil from the above ground storage tank was cold enough that the oil lines inside the plant had frost on them.

RE: Diesel Vapor Lock - Vapor Pressure?


Good ! Your pipng and tanks are underground...."waxing" does not become a problem unless the fuel oil is nearer 0-10 degrees F.

I still feel that location of the pumps, in a little pre-fab house near the tank is your best bet. Are you routing a fuel oil return line back to the tanks ? The long suction line plus the long FO return line may be an expensive installation. Have you contacted VIKING (www.vikingpump.com) pumps and asked for help from a sales rep ? Fuel oil transfer is a "plain vanilla" application for those guys and they have a lot of practical experience

2" NPS pipe at ~17 gpm gives a velocity under 2 feet/sec - OK !! Be aware that a suction system will be operating at a partial vacuum, so any flanges/threaded pipe/pipe unions may permit air entrance into you system. Consider an all welded system construction...

The places where I have found the vapor pressure of fuel oil at various temperature are MSDS datasheets and a book called "Cameron Hydraulic Data". I believe that the info you want is contained there..

Good Luck ......MJC

RE: Diesel Vapor Lock - Vapor Pressure?

I don't know the exact number, but, I can tell you that the vaper pressure of #2 fuel oil is extremely, extremely low!  I agree completely with MJCronin.  Your problem is likely due to friction losses as this is a very viscous material.


RE: Diesel Vapor Lock - Vapor Pressure?

From the MSDS sheets:

Sun Oil                1.6mmHg @ 70F
Amerada-Hess     0.5mmHg @ 70F
Fisher Scientific   2.6mmHg @ 122F (50C)

#2 fuel oil is a petroleum distillate ranging from 26 to 34 degrees API specific gravity.  The vapor pressure will be a range of numbers, not a nice constant value.

Call your Viking Pump salesman.

RE: Diesel Vapor Lock - Vapor Pressure?

There are a number of factors to consider when looking at suction line design. One problem found on long suction lines is the entrapped air/vapour which needs to be cleared by the pump before liquid can enter the pump inlet. 320 feet of 2" pipe represents a large volume of air/vapour and as the pump operates on this mix, the air heats up and re expands thus increasing the volume. Another point to consider is that the pump needs to push the air through the discharge line and will not be able to clear air if the discharge pressure is too high. I normally advise customers that a pump may be able to "wet" prime a system, and to include a manual or hand pump to prime the system when maintenance is carried out or when the suction line drains back to tank etc.

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