Lift Pump Problems - Has Anyone Tried This?

[RustyJC]

The following is from a post that's buried on another thread - I throw it out here for discussion.



OK, how's this for thinking outside the box?



As a rule, large industrial (stationary) diesel engine installations are designed to conform to Diesel Engine Manufacturers Association (DEMA) standards. DEMA specifies that you will have a transfer pump to move fuel from the outside storage tank to the "day tank". The day tank is a small fuel tank normally located in the rafters of a power plant building which then gravity feeds the diesel fuel system on the engine - a flooded suction design. Fuel level in the day tank is controlled by either a float-type system that returns excess fuel to the storage tank (reduces transfer pump cycling) or a level controller that turns the transfer pump on and off to maintain day tank level.



What if one were to rig a small tank in the engine compartment that is fed by a pusher pump back near the main fuel tank. The outlet of this small "day tank" would then be piped to the inlet of the stock lift pump. This way, the lift pump has a much easier life - it doesn't have to draw fuel all the way from the fuel tank in the rear of the truck, and it will operate at a slight positive pressure at its inlet (assuming the tank is mounted higher than the pump inlet. ) At the same time, the transfer (pusher) pump is only having to push the fuel against gravity since the day tank is not pressurized, so it should last longer as well.



Hey, just a thought... ... .



Rusty

 

[cold Diesel]

In all of our diesel applications ( I run a power plant) the 'day tank' is also typically set from 15 to 25 feet above the ground in order to meet the NPSH requirements of the injector pumps. The lift pump in our trucks is there to perform the same function ie meet the NPSH requirements of the IP. Although I am new to these trucks I believe that the lift pump must require almost no NPSH or it would not work. The pusher pump is probably easier also than another tank as well and would increase the NPSH far more than another tank due to height limitations under the hood. Very creative thought though. Let me know if you try it!

 

[RustyJC]

The reason I made the suggestion is that one reason the stock lift pumps seem to fail is because they have to pull fuel from the stock tank at the rear of the truck all the way to the engine compartment. By design, this pump is really intended to push, not pull. The day tank arrangement would eliminate this problem by (as you say) providing a slight NPSH at the lift pump inlet - the lift pump can now push the fuel at the desired pressure to the VP-44 without having to drag it all the way from the main fuel tank, so the power required and work performed by the lift pump would be reduced.



At the same time, the pusher pump would no longer have to pressurize the fuel line to the lift pump's suction - most of the guys who are using pusher pumps are reporting pump life of only a year or so in this service. With a day tank configuration, the pusher pump would only be working against gravity instead of having to produce 5-6 PSI continuously to feed the stock lift pump inlet (or, if the stock pump has been eliminated, 12-13 PSI to feed the VP-44 inlet. ) Therefore, I would expect to see its lifespan extended as well.



The day tank in the engine compartment wouldn't have to be that large. A small aluminum tank of 1/2 to 1 gallon or so should work fine.



Rusty

 

[Steve St.Laurent]

The concern I would have would be fuel foaming in that small of a tank. You'd have some air and some fuel in there - I'm thinking that you'd have the feed line to the tank pouring fuel in through the top of the tank (as opposed to pushing it into the bottom of the tank where the weight of the fuel in the tank would be pushing back against the pump). I would think that would cause the fuel to foam. I'm guessing that the "day" tanks that you are talking about are of a large enough capacity that it wouldn't be a problem. Just thinking out loud here - nothing to back that up.

 

[RustyJC]

Steve,



Yep, finding the right fuel entry point and tank volume would take some experimentation. You might want the fuel to impinge tangentially on the tank wall rather than spray into the "air space" to prevent foaming. You also wouldn't want too much excess capacity (GPM) on the transfer (pusher) pump - the less fuel you pump into the day tank, the less foaming would occur. Part of the engineering is to ensure that "velocity" of the fuel through the tank would always be low enough to permit deaeration - a function of residence time. With the right feed rate and tank volume (maybe deaeration baffles as well), if you're gravity feeding the lift pump from the bottom of the tank, you would hope to have solid (non-foaming) fuel down there even with some minor foaming on the surface of the fuel.



Another thought would be to vent the day tank back to the main fuel tank filler pipe to provide some slight pressurization. This should help prevent foaming and also eliminate fumes and the possibility of fuel spillage in the engine compartment.



I was just trying to figure out how to integrate this time-proven diesel fuel system design into our application. Heck, we've tried about everything else, right?



Rusty

 

[DiTrani]

Lately there seems to be a number of reported lift pump failures for which the cause was a broken plastic coupler between the motor and the pump head. It'd be nice to know if this is the primary failure mode for these pumps, because if it is, and if there are no other significant failure modes, then the whole notion of push vs. pull and pump location could be largely moot.

 

[JTBMC]

I agree with three lift pumps of four dead opened up and checked out it was the plastic connector that failed. The erradic needle movements on my gauge prompted me to check the pump preasure with another gauge to verify the failure.

I would like to have a machineist look in to building the plastic connector out of brass or stainless or what ever is compatable with diesel fuel . Maybe then we will have a lift pump that will last longer than 10,000 miles or so as mine have.

TJ

 

[Steve St.Laurent]

Keep in mind that this is just a guess - but that plastic coupler may be there to act as a fuse. If the problem was as simple as a plastic coupler I'm sure that after 12-13 revisions they would have figured that out! What could be happening is that the rotor is getting to an interference fit and it is breaking the plastic coupler. If that coupler was strengthened then the rotor may start making metal shavings that would go into the filter and possibly onto the injection pump killing it. Just thinking out loud here - there must be a reason for a weak link like a plastic coupler (especially after 12-13 revisions).

 

[tgbol]

The way my pump acts, I think that the actual failure in the plastic drive coupling is caused by a binding in the vanes. I think that my pump the vanes are sticking in the recessed position and then bypassing. if I jump on the loud pedal when the fuel pressure is low it jumps back up. I think that when the injection pump lowers the line pressure the lift pump speeds up and fling the vanes outward and it starts to pump again. I have noticed that when my pressure drops you can hear the lift pump speed up.

 

[JTBMC]

The plastic coupler could be a planned weak link as you stated to save the IP or these pumps could be designed for gasoline and the heavy diesel fuel is killing them.

TJ