External Gear Pump

[MickMcC]

Hi everyone,

Let's say I have an external gear pump driven by the camshaft of a marine diesel engine. The nominal speed of the engine is 1800 rpm and the capacity of the pump at nominal speed is 8 L/min. Diesel fuel is supplied to the gear pump suction at 0.4 bar g pressure via a 15 mm pipe.

If everything remains the same on the engine side of the pump. What would happen in the fuel is supplied to the pump at 1.3 bar g at 11.5 L/min?

 

[LittleInch]

Is this the same question you already asked?

Which side is the engine? Discharge or inlet

What is the pressure of the discharge

Can you post a pump curve?

Not sure goitre type going to get 11.5 l/min in and only 8 out??

What pressure relief system is there? Inside or outside the pump

What does the pump vendor say?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MickMcC]

Yes, didn't get any joy so trying to ask in a different way. Engine is discharge side. Don't know what the discharge pressure is. Don't have a pump curve, I think there is a pressure relief system outside the pump. Haven't contacted the pump vendor. Than you for the reply.

 

[LittleInch]

Well data makes a massive difference.

If the outlet pressure is close to the inlet then all the gear seals etc are either not active or in reverse. Then you could get bypass of the main gears which are now not needed.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[TugboatEng]

A couple of things here, you say the fuel rate varies widely with electrical load. Changes in power factor can contribute to this.

Second, what is the injection duration of the engine during these different operating states? If you have an electronic governor you should one able to get the duration using your engine diagnostic tool. If you have a mechanical governor use the rack length. If you see decreased rack length or increase injection volume during the periods of high fuel rate then the engine is indeed burning more fuel.

 

[hydtools]

The pump will not accept more than 8lpm, not accounting for leakage.
There better be a relief valve between the source and the pump driven by the camshaft.
What is suppling the fuel to the pump intake?

Ted

 

[MickMcC]

Thanks for the replies. A fuel oil ring main is supplying the pump intake. The ring main is fed by a boost pump. I will try to find out the discharge pressure from the manufacturer.

 

[LittleInch]

When I look at the diagram in your other list I can't work out what this pump is doing. It seems to be there to take fuel from a tank and deliver it to the high pressure injection pumps.

Now you're feeding the engines fuel from a header at 1.3 barg then why doubt you just remove the pump and pipe through?

Your diagram also had reliefs set at 1.5 and 2.5 barg. Could flow also be going through the hand pump?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MickMcC]

Thanks for your help littleInch, that's an interesting observation regarding the hand pump. The fuel delivery system has a pressure reducing valve that is notorious for causing problems so it has been bypassed. Hence the 1.3 bar instead of 0.4. I'll keep researching and see if some of the flow is being diverted, which makes the most sense.

 

[TugboatEng]

Inch, this is a fairly common setup. Engine manufactures want to sell there engines in as complete if a package as possible. All pumps, heat exchangers, etc are attached to the engine frame. This isn't always suited to the application so the Band-Aids start. If the engines are located far from the fuel source or at too high elevation, booster pumps are often necessary. The on engine pump supplies fuel to the individual high pressure injection pumps. The pump is sized to exceed the full fuel rate of the engine. In the the case of unit injector engines the pump is sized to provide sufficient cooling of the injectors as well. There is sometimes an internal relief within the gear pump set well above normal operating pressures. There is usually a back pressure regulating valve on the exit of the system. It is a vented valve so it will pass fluid.

 

[MickMcC]

Thanks TugboatEng

 

[LittleInch]

I appreciate what it is doing. My point is that a pump doing nothing creates issues.

I suspect that if the pressure regulating valve is set very close to the supply pressure you now have there could easily be surges of fluid which either goes via some relief valve and isn't being measured or the meter can't cope with these sudden increases in flow.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.