4 replies to this topic

[Mech.Engineer]

Hi everyone.

Please take a look on attached image...

Later on...

I would like to know if i need to close partially the gate valve (ø =10'') in the tank discharge nozzle to obtaining 110 gpm needed on the suction of a screw pump (double screw) wich is designed for operating with that Gallons per minute? , or if pump is gonna take only the 110 gpm that it requires...?.

Thank you in advance...!.

[Mech.Engineer]

....SORRY...! Here is the image..!

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[breizh]

You shoud investigate about the technology of the pump and how is delivered the flow rate !

http://en.wikipedia....ive_cavity_pump

Hope this helps

Breizh

Edited by breizh, 15 December 2011 - 07:02 PM.

[Art Montemayor]

Mechanical:

As an engineer, you are exhibiting a lack of practicing the basic understandings of the equipment you are operating (or trying to understand).

A screw pump is a rotary type of pump that is a typical POSITIVE DISPLACEMENT (PD) device. It will displace the volumetric flow rate that it was designed for until something happens to counter that action – such as:

A resulting discharge high pressure activating a pressure relief device – or a mechanical failure (rupture) of the discharge piping, the pump itself, or the pump’s driver (the best option). A high discharge pressure in this type of pump is a result of a discharge resistance to flow – such as your proposal to throttle the discharge.

This is what I suspect Breizh is trying to alert you to by recommending you study the features, characteristics, and basic design of such a pump.

It is very difficult to control the flow rate capacity of a PD pump without having to resort to relative expensive controls. Speed variation is one method. Recycle control is another. Throttling the discharge of such a pump is never an option. This type of pump will displace what it was designed to displace. In other words, it will take a constant quantity of suction fluid and deposit it on the discharge side – regardless of whether the discharge side is prepared to take the constant flow or not. It is imperative that a properly designed pressure relief device be installed on the discharge side of such a pump.

If you require LESS than the designed 110 gpm from the pump, you can recycle the excess flow back to the suction side (using flow control) or you can reduce the speed of the pump (by varying the frequency of an electric motor drive).

One curious item: a 10-inch pipe is an exaggerated size for transporting 110 gallons per minute of fluid. Why such a large size?

[Mech.Engineer]

Mechanical:

As an engineer, you are exhibiting a lack of practicing the basic understandings of the equipment you are operating (or trying to understand).

A screw pump is a rotary type of pump that is a typical POSITIVE DISPLACEMENT (PD) device. It will displace the volumetric flow rate that it was designed for until something happens to counter that action – such as:

A resulting discharge high pressure activating a pressure relief device – or a mechanical failure (rupture) of the discharge piping, the pump itself, or the pump’s driver (the best option). A high discharge pressure in this type of pump is a result of a discharge resistance to flow – such as your proposal to throttle the discharge.

This is what I suspect Breizh is trying to alert you to by recommending you study the features, characteristics, and basic design of such a pump.

It is very difficult to control the flow rate capacity of a PD pump without having to resort to relative expensive controls. Speed variation is one method. Recycle control is another. Throttling the discharge of such a pump is never an option. This type of pump will displace what it was designed to displace. In other words, it will take a constant quantity of suction fluid and deposit it on the discharge side – regardless of whether the discharge side is prepared to take the constant flow or not. It is imperative that a properly designed pressure relief device be installed on the discharge side of such a pump.

If you require LESS than the designed 110 gpm from the pump, you can recycle the excess flow back to the suction side (using flow control) or you can reduce the speed of the pump (by varying the frequency of an electric motor drive).

One curious item: a 10-inch pipe is an exaggerated size for transporting 110 gallons per minute of fluid. Why such a large size?

Hi Art..

Thanks for your answer...

I see you respond to questions with excellent professionalism...

In this case, you`re right... i`m trying to understand some points in this little system...maybe this is of basic undertsanding but really important in this case & I don`t forbear to express my concern as you can see.

Well, Why such a large size?. That`s a good question with an only answer: bad engineering...!. I didn`t do this so-called engineering, but in this moment is not possible to correct this mistake because pipes are installed.

In fact...Pump suction is 4'' diameter; i mean , there`s a 10'' x 4'' reducer at pump suction (pump supplier`s mistake), originally pump suction flange had to be 6'' diameter.
Doing the calculations pump can operates safely.

My question arises because simulation of system was run assuming 110 gpm of the fluid through the suction line. What do you think about this?.

Fluid through suction line from the tank to reducer in the pump suction is not gonna be 110 gpm at least that gate valve in the tank outlet remain partially closed and that ain`t good.

All suction line had to be 6 inches...that was enough...

Best;