VFD

[slickstyles5]

Hi,

I am looking into installing a VFD (variable freq. drive) on an existing rotary pump (PD pump). This pump supplies several engines with fuel (3 branches). I want to figure a way to adjust the RPM of pump motor to vary the flow out of the pump all depending on what the engine really needs. Can I do this by monitoring the pressure I need at the engine? That way, as soon as the engine needs more fuel and the pressure starts dropping, the VFD would react and increase the pump speed, therefore increase the flow?

What do you think?

Any ideas?

Thanks!

 

[waross]

If these are diesel engines you may want to look into the use of excess fuel flow to cool the injectors.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[zdas04]

Notwithstanding potential benefits of excess flow, if you know the relationship that you want to maintain (e.g., always have 30 psia available regardless of consumption rate) then driving a VFD off of the 4-20 mA signal from a pressure transducere is a quite reasonable control approach.

David

 

[SNORGY]

How big is the pump and what is the volume in the piping? I am thinking that unless the system is pretty big, your rate of change in pressure will be faster than your rate of change in controller speed (VFD), so the system won't be smoothly controlled. I would use flow control with a PD pump.

Regards,

SNORGY.

 

[slickstyles5]

zdas04, yes we want to maintain a pressure of 30-40 psig approximately, while the consumption increases/decreases. my logic, is that as the consumption increases, the pressure will decrease, so in turn the VFD will make the pump turn faster and give more flow to bring the pressure back up. i am only concerned that it will not react quick enough.

snorgy, the piping is relatively big, between 1" and 2" pipe with aprox 15 gpm flowing at maximum conditions. the pressure will change when the engine starts consuming a little more, but i think the vfd can react quick enough, but i need to really look into it. the pressure we need at the engine is not that important and can vary, but the engine does need to be supplied in fuel.

 

[Pumpsonly]

By adding an accumulator in the line will help to smoothen the pressure / flow fluctuation.

 

[zdas04]

My experience has been that a pressure change at the discharge of a PD pump is reflected in the entire system almost immediately. Especially when you are talking about 10's of feet of piping (not thousands) and very small relative changes. An accumulator (or surge pot) is also often a good idea, but you have to be really careful that the bladder is compatible with the pumped fluids.

David

 

[BigInch]

Hey Slick, you almost got it right with your concern, but ..
my concern is not that the VFD will not react quick enough (Why should that control circuit be slow. Maybe you mean that the fuel pressure won't build fast enough???), but that your control logic will not work. As you seem to have said, then quickly forgotten, the logic is backwards. Isn't that your real problem.

I think that flow is not the problem, your control logic is the problem. Usually a diesel engine is controlled by the fuel flow rate; user gives it more fuel by manually increasing the rpm of the fuel pump, in this case, the VFD controlled fuel pump and the diesel engine, then the machine it is driving runs faster. You seem to want to turn it around somehow, driven machine wants to go faster, or slower and has to ask the VFD to give it the proper amount of fuel necessary to do that. How does the machine know how fast it wants to go and how does it tell the VFD to set the right freq to deliver the proper amount of fuel to reach the required speed. Your control logic is not fully explained. Trying to maintain a certain fuel pressure is destained to result in trying to hold the diesel to one speed. For example, a diesel driving a bus. Bus wants go go up hill. Speed slows, diesel turns less rpm, needs less fuel, and fuel line pressure increases. You try to hold the previous fuel pressure and the diesel gets less fuel and bus stalls.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[zdas04]

BigInch,
I was answering the question asked. I am assuming that he has a constant-speed governor on the engine. When load increases the engine tries to slow down, the governor calls for more fuel, fuel system pressure goes down, the VFD gets a declining pressure signal and speeds the PD pump up. It all makes sense to me, and it doesn't have to be instantaneous. If that governor piece is missing then the control logic is truly backwards.

David

 

[BigInch]

zdas I wasn't commenting on your response, but now that you mention it, the OP says, "I want to figure a way to adjust the RPM of pump motor to vary the flow", so we caN be pretty sure that it's not a constant speed application.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[Pumpsonly]

BingInch.
Your response of 24 May is not exactly correct for this application
The diesel engines it self can be a fixed speed operation with
varying load such as a generator and therefore varying fuel demand.
The proposed VFD controlled PD pump is just fuel booster or transfer pump from the storage tank to the engines.
The engine has its own fuel priming pump which would be sufficient to feed the diesel to the governor controlled injection pump if the fuel tank is near by. A few PSI drop in the fuel supply header line will not affect the engine operation.

 

[zdas04]

You see this kind of control with AC electric generators all the time. There is an engine rpm that results in the power being at the proper frequency, and any deviation can be devastating. But load is constatnly changing so fuel flow must constantly change. If the supply volume is constant then the ability to adapt to changing loads is compromised at the high end of capability. A VFD on the fuel pump would go a long way towards minimizing that problem.

David

 

[DLite30]

It would be much more simple to install a pressure regulator on the fuel line...such that it sends exceess fuel back to the tank in order to maintain a constant fuel header pressure.

 

[BigInch]

Guys you are right. I was thinking the engine was driving the pump, now I get that we are talking fuel pump. Sorry for running off the road there.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[zdas04]

BigInch,
I enjoy the occasional foray into the weeds. Usually I'm driving, but it is nice to tag along occasionally.

DLiteE30,
The fuel is delivered with a PD pump. PD pumps don't play well with pressure regulators.

David

 

[chief]

AS DLite30 suggests,simple is best, a pressure control or relief valve with spill back to the suction of the pump or fuel tank is the way to go.

Offshore Engineering&Design

 

[QualityTime]

I would say keep it simple and use the excess flow approach as sucggested earlier. You may get column separation in the fuel line if the VFD cannot deliver the fuel fast enough....too many extra peices of equipment to maintain and go wrong

 

[slickstyles5]

Yes that's what we will most likely be doing... Keeping it simple with a relief valve that sends the over pressure back to the fuel tank. That way there won't be any risk of missing fuel to the engine. I will keep note of this thread as there are some very interesting points that were brought up for my future knowledge!!