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  • Fuel Pump Check Valves

    I think I am going to need check valves on my dual MSD fuel pump installation.
    Why ?
    What kind ?
    What does Rotax use on their 900 series engines ?
    What pressure should I have at the rails on my Subaru 2.5 ?

    Thank you for any input !
    Happy Flying, Chris S.

  • #2
    Google " Aluminum Fuel Check Valve ", and you will find hundreds of examples for ~$2 a piece.

    If your pumps are in series, you will need the valves to bypass the pump that is not pumping.
    Without the valves, the fuel will just go in circles back to the input of the pump

    The valves should produce less than 1psi of restriction.
    You should be able to blow through them in one direction with only slight difficulty with only lung pressure.
    The other direction should produce a total blockage.

    Expect to see 40-45psi before starting the engine.
    The gauge pressure will vary with the intake manifold pressure on a running engine.
    Click image for larger version  Name:	FuelPumpValve.jpg Views:	1 Size:	9.8 KB ID:	1118842

    Last edited by Uncle Willie; 04-10-2017, 10:22 AM.

    Comment


    • #3
      I think most aftermarket fuel pumps have built in check valves,I use walbro pumps and they have the check valves in place.

      my fuel rails carry about 30-35 psi at idle.
      Best Regards,
      Eddie Sigman,Polvadera,nm
      (575) 835-4921

      Comment


      • #4
        Fuel pumps will have "Backflow Check valves" just by their design.
        Low-pressure pumps (<10psi) as used with carburetors are typically Diaphragm Pumps, may allow fuel to flow through them when the pump is not powered.

        High-pressure pumps (>20psi) as used in fuel injected engines are positive displacement Gear Pumps, capable of destroying themselves if the output of the pump is blocked.
        The pressure could theoretically reach into the 100's of psi if not relieved.
        This is why fuel injection systems have external pressure regulators that dump the excess fuel back to the fuel tanks.
        These pumps will typically have "Internal Relief Valves" to limit the pressure to under 100psi.
        These pumps will also not typically possess the ability to pass fuel through them when they are not powered.
        These pumps are typically designed with the automotive industry in mind.
        Most automobiles do not have dual redundant fuel pumps plumbed in series.

        The pumps supplied with the Rotax 912iS are internally relieved at 90psi each, 180psi as a pair.
        The fuel lines and fitting must be designed to handle at least that pressure in a worst case scenario.

        Therefore there is a need for external bypass valves when plumbing fuel pumps in series.
        The bypass valves will not be required if the pumps are plumbed in parallel.

        If you are plumbed in series and can not blow through the pumps via lung power, you are going to need to install bypass valves.
        Otherwise, if either pump stops, so will the fuel flow.
        Simulate each pump failing before the first flight attempt.


        Comment


        • #5





          First......thanks for the replies, I really appreciate them ! Here are some pics of my fuel pumps. The one on the left side will be run by the ECM (hopefully) The right one is the aux powered by a switch on the dash. My 2 seat tanks are teed together & go to the filter (1/4").I then leave the filter with 3/8" ID clear tygothane tubing. Then teed off to the 2 pumps. I leave the pumps with 5/16" fuel injection hose. The pump outputs then tee back together & then to the motor.

          I primed & ran the aux pump & caught the return fuel. Not sure where I am going to return it to yet. (2 tanks ) The fuel appeared to be going back through the other pump. I hope to only run the main pump most of the time.


          Attached Files
          Last edited by CLS447; 04-30-2017, 02:26 AM.
          Happy Flying, Chris S.

          Comment


          • #6
            Originally posted by CLS447 View Post
            ... My 2 seat tanks are teed together & go to the filter (1/4").I [I] then leave the filter with 3/8" ID clear tygothane tubing. Then teed off to the 2 pumps. I leave the pumps with 5/16" fuel injection hose. The pump outputs then tee back together & then to the motor.
            That means you are running the pumps in parallel.
            Each pump typically pumps 30 gallons per hour.

            In parallel, with both pumps ON, you will pump twice the amount of fuel (60gph) at the same pressure.
            This could overwhelm the capacity of the fuel regulator, And flood the return tank faster than the crossover flow through your Tee can balance the tanks.

            In Series. with both pumps ON, you will pump the same amount (30gph) with the capability of twice the pressure. (The Prefered Plumbing.)
            The actual pressure will be determined by the Pressure regulator.

            You can return the fuel to either tank as long as it can cross flow to the other tank easily.
            A 1/2 to 1 inch crossover hose would be advised

            AutoGyro installs two tanks.
            Only the main tank has a filler neck, Fuel Level Sender, and fuel pickup.
            The second tank has a 3/4" crossover hose at the bottom and a 3/8 crossover vent at the top.
            The fuel is drawn from, and returns to, only the main tank.
            The crossover hose only needs to deliver HALF the fuel used by the engine per hour to the main tank in order to balance the tanks.
            Both tanks are ~10 gallons each.
            If you burn 5 gallons per hour, that gives the second tank 4 hours to crossflow its 10 gallons. An easy requirement!
            In the 4 hours it takes to empty the tanks, 120 gallons of fuel will have circulated through the engine and back to the main tank.
            The last 5 gallons in the tank makes the round trip every 10 minutes.

            When you shut down one fuel pump, does the fuel pressure change?

            Add a relay in parallel with the power to the first pump and in series with the switch controlling the second pump.
            The ECM will disable the main fuel pump in the event the engine stops.
            You want the second pump to shut down automatically in the event of an accident where the engine stops.
            Having the second pump continue to blindly pump fuel into a fire is not a good design.

            Comment


            • #7
              Besides the power shutoff, what would you change ?

              I did not check the pressure yet. http://www.jegs.com/i/MSD/121/2225/1...R_JBoC9eHw_wcB
              Last edited by CLS447; 04-30-2017, 02:27 AM.
              Happy Flying, Chris S.

              Comment


              • #8
                Here are some expensive ones & have AN connections.....http://www.summitracing.com/search/p...illet-aluminum

                Could you possibly show me some good quality $2 checks....?
                Happy Flying, Chris S.

                Comment


                • #9
                  The pumps you selected have a 43 gal per hour rating.
                  Operating two in parallel will pump over 85 gallons an hour.
                  The pressure regulator may have a problem dumping over a gallon a minute.

                  See the attached PDF for plumbing pumps in series back to two tanks.
                  DualFuelSystem.PDF

                  - - - - - - -

                  I have found that these Valves are as good as any.
                  http://www.ebay.com/itm/252592584958

                  You could buy the same thing in the US for $100 each if it would make you feel better.
                  http://www.ebay.com/itm/252520845340

                  Comment


                  • #10
                    Uncle Willie......You are the MAN ! Thank You so much. I'll post more later !
                    Happy Flying, Chris S.

                    Comment


                    • #11
                      Originally posted by Uncle Willie View Post

                      That means you are running the pumps in parallel

                      Add a relay in parallel with the power to the first pump and in series with the switch controlling the second pump.
                      The ECM will disable the main fuel pump in the event the engine stops.
                      You want the second pump to shut down automatically in the event of an accident where the engine stops.
                      Having the second pump continue to blindly pump fuel into a fire is not a good design.
                      Willie , question for you.....The ECM shuts off the power to the main pump if the engine shuts off ? What if it was a pump failure ?

                      Turn on aux pump & turn key. If no restart, turn off key & land her !

                      How did the RAF & Sparrow hawk have their fuel pumps installed ? FI 2.2 & FI 2.5 ?

                      What engine do you fly ? When do you use the aux pump ?



                      Happy Flying, Chris S.

                      Comment


                      • #12
                        Can high pressure fuel pumps handle alot of dirt in the fuel ? The petcocks on my tanks have screen filters....I always use a Mr. Funnel when fueling the tanks.

                        I then filter the fuel again before the pumps. Why would I need to filter the fuel after the pumps ?

                        Maybe to catch pieces of the self destructing fuel pumps ?

                        If I run one pump at 35 psi & it not only goes to the fuel rails, but back through the other pump....That means I will get 35lbs to my low pressure lines & a reverse flow situation.

                        You could say it is just going in a circle to be picked up again at the tee ?

                        As far as the return goes....I have the sight tubes up the sides of my seat tanks, These are also interconnected. The petcocks on the tanks are also connected. (1/4" )

                        I am going to try putting the return line into a tee in the sight glass / crossover tube. Hopefully the fuel will follow the path of least resistance.....which will be a perfect 50/50 split to the two tanks !!!! Who knows ...I might get lucky.

                        Do you feel a fuel pressure gauge is a must ?

                        Eddie, tell me more about your system.....please ?

                        Happy Flying, Chris S.

                        Comment


                        • #13
                          So Many Questions !!!

                          Your automotive ECM will run the fuel pump continuously.
                          There was a Crash Switch in the trunk of the car to disable the pump in a crash.
                          You have likely not installed this important safety feature.

                          I have a Rotax 912iS that this information is based on.
                          This engine has dual independent alternators. One for the engine/ignition, one for the plane.
                          The pumps in the 912iS are activated when START Power is applied.
                          After engine start, START Power is removed and the ignition and the pumps are powered by the independent "A" Alternator.
                          If the engine stops for any reason, so does the alternator, and the pumps lose power and stop also.

                          If your main pump fails, activate the auxiliary pump.
                          The Aux pump is normally ON when below pattern altitude (1000 ft) while on approach and departure, just in case.
                          Also ON whenever not High enough to have enough time to perform a restart.
                          - - - - - - - - -

                          The fuel pickups have 1000u (micron) screen filters that block anything large enough to plug up the hoses.
                          The Coarse Filter (100u) blocks anything visible to the human eye that would bother the pumps.
                          The Fine Filter (10u) is immediately before the fuel rail and stops microscopic particles that may flake off the fuel hose from blocking the injectors.
                          The injector nozzles are in the 50-200u range and will not tolerate much debris.

                          Get out of the carburetor mindset! Fuel injected engines move a lot of fuel continuously.
                          1/4"fuel line is a too small to be moving >40 gph. AN6 (3/8) hose is recommended for fuel injection systems.
                          40gph in 1/4 hose is well over 4 ft per second. 3/8 hose will reduce that to under 2 fps.

                          When running two pumps the fuel passes through both pumps serially.
                          When running on One pump the fuel does NOT go through the other Pump.
                          It bypasses the other pump through the bypass valve.
                          The other bypass valve prevents backflow around the working pump.
                          These valves are NOT optional!

                          After the pumps, and before the Fine Filter, there is a small Orifice (Leak) to the return lines.
                          If the pumps ever try to pump air/vapor, they can only generate ~3psi.
                          This is not enough pressure to open the pressure regulator and you will be vapor locked. Bad things follow!
                          The only escape for this vapor will be cranking the engine long enough for the injectors to vent the air. (20-60+ seconds?)
                          The orifice will relieve the pressure and dump the air/vapor back to the tanks allowing the pumps to restore their prime.
                          The orifice also relieves the pressure in the lines after engine shutdown.
                          It bypasses a minor amount of fuel. We are talking about a pinhole here. (~0.015")

                          The return line from the engine also contains significate vapor due to the pressure drop as it comes out of the regulator.
                          Is must return to the tanks and never directly to the pumps.
                          The tanks need to be cross vented otherwise fuel sloshing through the lower Cross Flow will pump all this vapor to the atmosphere.
                          The vapors will recondense in the tanks as it cools.
                          Otherwise, you will be distilling your gas into kerosene and the fuel gets worse the farther you go.

                          Hopefully, the fuel will follow the path of least resistance.....which will be a perfect 50/50 split to the two tanks !!!! Who knows ...I might get lucky.
                          Hard Love follows! ... It is this kind of attitude that gets people killed and gyros a bad name.
                          Design the system so the fuel absolutely takes the path you intended.

                          The engine return will always have > 35gph of flow in it. your sight tubes are going to be overflowing with bubbly gas.
                          If this cross tank tube is also the same line that supplies fuel to the pumps, you are going to be recycling all that vapor. Very BAD!
                          The CrossTank connection and the fuel pick-up must be separated.
                          - - - - - - - -

                          The fuel pressure gauge is optional but very useful.
                          It is after the pumps but before the Fine Filter.
                          The Engine pressure regulator maintains ~45psi ABOVE Manifold AIr Pressure.
                          A Non-Differential Pressure gauge will indicate pressure above ambient.
                          It will indicate Gauge pressure Plus fuel filter pressure drop.
                          Expect to see ~45.5 psi before engine start.
                          ~35psi at idle. And 30-45 psi in flight depending on altitude and throttle.
                          If it ever goes above 50psi, The fine filter needs to be changed. (Assuming NO Turbocharger.)
                          If it goes below 25psi The coarse filter or the pumps need servicing.
                          It is not required, but is worth its weight in piece-of-mind.
                          - - - - - - - - -

                          If you are using Two Seat Tanks, is one seat higher than the other?

                          Bill.

                          Comment


                          • #14
                            MY RAF seat tank has two separate lines one on each side of the main frame rail,they connect to two gascolators one on each side of the main frame rail they are also at the lowest

                            point of the fuel system and have a cross connection fuel line between them,this allows the separate portions of the seat tank to equalize the fuel load. Each gascolator has a

                            fuel line to one of the two fuel pumps mounted side by side on either side of the frame rail, on the fuel out side of the pumps there is a fuel line connecting the two pumps to one fuel line up

                            to the fuel injector rail. with the gascolators connected with cross feed line either pump will draw from both sides of the seat tank at the same time.the gascolators are also my in line filters.

                            all of my fuel line are AN-6 with the outer steel braided ,the return line go's into the top of my fuel tank with fittings to seal out the smell of gas. I have used the fuel spout as a return point

                            but it had a tendency to leak around the rubber fittings and smelled like fuel inside. Sorry that there not any pictures to show you. If you buy walbro pumps or any good pump they probably

                            have built in one way valves. WALBRO 255LPH pumps/GSL392,$130 buck apiece..I have more than 700 hours on my pumps,I altnerate use between them.



                            Best Regards,
                            Eddie Sigman,Polvadera,nm
                            (575) 835-4921

                            Comment


                            • #15
                              Eddie,

                              Other than having your pumps in parallel, you have a reasonable system.
                              255 lph is over 65gph.
                              With both pumps running the fuel system must handle over 125gph.
                              That's over 2 gallons a minute.
                              Pumps in parallel do not need the check valves.
                              The valves are NOT to prevent fuel flowing backward through the pumps.
                              They are to allow fuel to flow around (Bypass) the standby pump.

                              You have a single tank with a hump down the middle.
                              This is easy to deal with.

                              Chris is dealing with two separate seat tanks located a distance from each other.
                              This complicates the situation.
                              Getting the return flow to equalize the tanks will be trickier.
                              The same amount of fuel that was drawn, needs to be returned to each tank.

                              There are ways to treat the two tanks as one if you design it properly.


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