Fuel pump question

[santelmo]

I have a project in mind to modify my fuel delivery system. I want to arrange the plumbing so that in the event of a secondary filter blockage I can quickly introduce a separate, filtered supply, pumped by a 12v fuel pump, either form an emergency supply of clean diesel or taking a tee from just downstream of the primary filter, thus bypassing the secondary altogether. I propose to use 1/4 turn ball valves. My question is would there be any damage to the mechanical fuel lift pump since it would be pumping against a blocked filter and beyond that a closed valve ? I am thinking here of ruptured diaphram.

 

[pvb]

I have a project in mind to modify my fuel delivery system. I want to arrange the plumbing so that in the event of a secondary filter blockage I can quickly introduce a separate, filtered supply, pumped by a 12v fuel pump, either form an emergency supply of clean diesel or taking a tee from just downstream of the primary filter, thus bypassing the secondary altogether. I propose to use 1/4 turn ball valves. My question is would there be any damage to the mechanical fuel lift pump since it would be pumping against a blocked filter and beyond that a closed valve ? I am thinking here of ruptured diaphram.

I've had primary filters block, but never a secondary filter. Are you looking for a solution to a problem which is unlikely to happen?

 

[santelmo]

The primary is a 30 micron racor and the last of the secondaries is a 2 micron racor/cav adaption. This one blocked on me last summer off the east coast of Sicily and I had to change it in a hurry before I was blown onto a fish farm. Not a happy experience.

 

[pvb]

Why not put a finer filter in the primary? The ubiquitous CAV filter, widely used as a primary, has a 6-9 micron filter.

On my last boat, I had 2 CAV primary filters installed in parallel, with valves to use one or the other (or indeed both).

If you run the engine driven pump without flow, you'll damage it.

 

[andrew-b]

Why not put a finer filter in the primary? The ubiquitous CAV filter, widely used as a primary, has a 6-9 micron filter.

On my last boat, I had 2 CAV primary filters installed in parallel, with valves to use one or the other (or indeed both).

If you run the engine driven pump without flow, you'll damage it.

Agreed. Running the lift pump block ended will break either itself or a joint in the fuel pipe (less likely). I'm also not sure why the extra 12V pump is required. A lot less effort to use the existing pumps and add a second filter in parallel with valves. Unless there is more to the story i.e. you've had the lift pump give up on you before and are trying to build in both pump and filter redundancy?

I would also add (and this can be argued both ways...), that by adding things like this to reduce the chance of breakdown, you are simultaneously increasing the number of joints (therefore air/fuel leak capability) - and chance of you dropping a ball with valve positions. Personally I'm on the simplicity side of the fence but I know and understand plenty who are on the other.

 

[VicS]

. My question is would there be any damage to the mechanical fuel lift pump since it would be pumping against a blocked filter and beyond that a closed valve ? I am thinking here of ruptured diaphram.

If you run the engine driven pump without flow, you'll damage it.

Agreed. Running the lift pump block ended will break either itself or a joint in the fuel pipe (less likely).
.

If the lift pump is the usual type of diaphragm pump:

.... running it with the outlet blocked will do no damage. The fuel pressure will simply hold the diaphragm down against the return spring.

Closing off the inlet would not be advisable!


.

 

[rogerthebodger]

If the lift pump is the usual type of diaphragm pump:

.... running it with the outlet blocked will do do damage. The fuel pressure will simply hold the diaphragm down against the return spring.

Closing off the inlet would not be advisable!


.

Sorry Guys I must totally disagree with you.

The action of the typical lift pump either mechanical of electrical is that the power stroke is the inflow (suction side) and the out flow is operated by the spring which will limit the pressure in the output line to only a low pressure maybe 10psi max.

An electional version will anyway have times of no flow when the power is on without the engine running.

VicS is the word in red an error or not.

 

[santelmo]

Thanks for the replies. Given that there is some doubt about the lift pump I am now inclined to radically rethink the idea or maybe drop it altogether and just continue changing filters regularly.

 

[VicS]

Sorry Guys I must totally disagree with you.

The action of the typical lift pump either mechanical of electrical is that the power stroke is the inflow (suction side) and the out flow is operated by the spring which will limit the pressure in the output line to only a low pressure maybe 10psi max.

An electional version will anyway have times of no flow when the power is on without the engine running.

VicS is the word in red an error or not.

whoops yes . Should have read "will do no damage"

Will correct the original

Thanks

 

[kolster]

My apologies if this is going off topic a bit, but should the diaphragm ever be changed / inspected or just left until it eventually wears out?

 

[VicS]

My apologies if this is going off topic a bit, but should the diaphragm ever be changed / inspected or just left until it eventually wears out?

Ideally you need to replace it before it fails or you end up with diesel oil in your lub oil sump and air in the fuel.

All the petrol engined cars I have owned in the past 50+ years, except probably the latest one, have had this type of diaphragm pump. I have never replaced a diaphragm and most have run for over 100K miles

 

[rogerthebodger]

My apologies if this is going off topic a bit, but should the diaphragm ever be changed / inspected or just left until it eventually wears out?

Last month I was looking for a service kit for the fuel pump on my engine but I could buy a complete replacement pump cheaper than a service kit both from the UK different suppliers. So worth checking costs.

 

[neil1967]

I don't think you have to radically change your plans. As others have said, why not fit two filters in parallel, either with separate feeds from the diesel tank (as I am about to do) or with a single feed from the tank 'T'd to both filters (valve on each inlet). The outlet from each filter should then be fed via a valve for each filter into another T piece and then go to the lift pump. I do have a separate electric pump fitted, in series with the mechanical lift pump, to ease bleeding the engine, but it could be used if the mechanical lift pump fails. In normal operation the electric 'cube' pump just sits there and does nothing. I've just ordered the parts from ASAP to do this- it might be belt and braces, but I have had a filter clog on me before (my fault - should have checked the filter when I bought the boat), and I don't want to have it happen again.

Neil

Edit: sorry just re-read the question. Clearly what I propose would not address a secondary filter problem, only a primary filter blockage. I would be surprised if a secondary filter got blocked if it was replaced routinely - I do mine annually and they never have much in them - even when I had a fuel bug problem, it was all caught by the primary filter.

 

[rogerthebodger]

One of the things i Didn't state in my previous post is that I do have twin secondary filters with changeover valves between the lift pump and the injection pump so in my setup it is possible to close off the outlet from the lift pump and no damage would happen to the lift pump as stated in my previous post.

I have 3 primary filters/water separators one for each of my 3 fuel tanks. between the primary filters and the lift pump is a 2 bank 3 position valve to select the tank to draw from and return to.