9 replies to this topic

[Prasanjit]

Dear Sir,

 

Need to know this in regards to throttling of the differential head.

 

I have a Centrifugal pump handling 4 liquids with different density and constant flow.

 

The flow rate is 2200 m3/hr and the differential head varies from 98.67 m  to 110.13 m.

 

In this case, how the differntial head can be throttled when the variation in head comes.

 

We can throttle the flow but differential head....how ???

 

Please note that NO VFD is envisaged due to high cost. In this case, what should be my design case.

 

How the pump Vendor shall take into account the rated parameters.

 

 

[shan]

In contrast what you mentioned, flow can not be throttled (valve inlet flow = valve outlet flow) but head or pressure can be throttled (valve inlet pressure > valve outlet pressure).  Therefore, you may obtain the desired head by adjusting pump discharge throttle valve.

[fallah]

In contrast what you mentioned, flow can not be throttled (valve inlet flow = valve outlet flow) but head or pressure can be throttled (valve inlet pressure > valve outlet pressure).  Therefore, you may obtain the desired head by adjusting pump discharge throttle valve.

 

A throttle valve doesn't necessarily change either flow or pressure and depend on the application it can control only pressure, say in closed loop refrigeration cycle with almost constant flow, or can control only flow, say along a line between two pressure vessels with different but constant pressure. In all cases inbetween, say in centrifugal pumps, a throttle valve by restricting the flow can simultaneously change the pressure drop across the valve; then i think always the application has to be taken into consideration to evaluate if a throttle valve changes /controls the flow or the pressure in that application...

[rychurek]

What do you want to throttle the differential head for?

[Padmakar Katre]

Dear Sir,

 

Need to know this in regards to throttling of the differential head.

 

I have a Centrifugal pump handling 4 liquids with different density and constant flow.

 

The flow rate is 2200 m3/hr and the differential head varies from 98.67 m  to 110.13 m.

 

In this case, how the differntial head can be throttled when the variation in head comes.

 

We can throttle the flow but differential head....how ???

 

Please note that NO VFD is envisaged due to high cost. In this case, what should be my design case.

 

How the pump Vendor shall take into account the rated parameters.

 

Hi,

It's confusing, a single centrifugal pump handles 4 liquids? what does this mean? and why the head changes at constant flow which is 2200 m3/h? Can you please explain the issue further?

[shan]

The pump discharge head varies with the pumped liquid density from 96.67 m to 110.13 m at 2200 m3/h along the individual pump curve of corresponding liquid density .  You should design the discharge head to satisfy the highest density liquid at 96.67 m at 2200 m3/h and control the head by reducing 13.46 m=110.13-96.67 for the lowest density liquid with the throttle valve and so on.  Therefore, you will have all the pumped liquid discharge at identical head and flow rate 96.67 m, 2200 m3/h.

[Chemitofreak]

If you have time in your hands, I would suggest you carry out hydraulics for all the 4 cases, by doing so you will analyse the impact of the following

1. High density case (for design pressure of the system)

2. Low density case (for high head)

3. High viscosity case etc (for high friction i.e. high head)

 

Since, flow in all the cases is constant, you will arrive at different heads in all the 4 cases. The highest head case shall be the governing case. The rest of the cases can be achieved by simple control valve throttling.

 

As for the vendor, specify the high flow-high head case as the governing case, rest of the cases can be attached as check cases (for high viscosity etc)

 

Revert in case of any query.

[S.AHMAD]

1. Dear Prasnjit, my BIG question is why you want to control the pump head?

2. I believe that your concern is more related to pump discharge pressure which is related to pump head.

3. For the sake of education to other  members, please take not that the reason why we plot pump head in m or ft vs flow rate in kl/h or gpm?

When we express the pump head in m or ft, the pump curve can be used for any liquid. It does not depends on density. The one that is change with density is pressure.

4. Pump head in m or ft is only a function of flow rate! nothing else. The only means to control the pump head is by controlling the flow rate that is why we normally install flow control valve downstream of a centrifugal pump.

4. If discharge pressure is the one that restricting the oeration, one possibvle solution is to trimmed pump impeller or change the smallest pump impeller. From the pump curve normally it includes the biggest and smallest impeller size.

 

Hope the simple explanation helps.

 

S. Ahmad

[S.AHMAD]

Dear members, another important point is that we must understand that pump discharge pressure is the requirement of the system piping size and configuration. The job of the pump is to deliver the required flow rate at the required system pressure. Please enhance your understanding of Bernoulli equation- go back to basic. That is  why we must select the pump size according to the requirement of the system.

[S.AHMAD]

Sorry guys, I forgot to mention that viscosity affect centrifugal pump performance curve. There is a chart that can be used for correction to the flow rate and head.

 

In real life normally the viscous fluid is heated up  to the right operating temperature to lower the viscosity so that the effect is minimal.