6 replies to this topic

[garych]

Hi,

I would like to confirm my understanding regarding the hydraulic calculations.

 

Assume that its a closed loop and I would like to carry out they hydraulic calculations for the maximum flow rate case and minimum flow rate case in order to calculate the control valve pressure drop.

 

1. Maximum Flowrate Case

    - The pump runs at the rated capacity = 30m3/hr, with a pump head of 50m.

    - Pump Head = 50m

 

2. Minimum Flowrate Case

     - As the flowrate decreases, we only need 10m3/hr.

 

So, what i am curious is:

For the min. flowrate case, do i use the same pump head (50m) to carry out the hydraulic calculations?

Or should i refer to the selected pump performance curve to find the pump head @ 10m3/hr?

 

Because as i understand, based on the pump performance curve, the pump head will increase with a decreasing of flowrate. 

 

What is the normal practice when calculating control valve pressure drop for max. and min. case?

Kindly advice me and it would nice if example can be provided.

 

 

 

 

[Zauberberg]

You are correct. You should look at the pump curve for head developed at the relevant flow condition.

 

As for the sizing of control valve with regards to pump and hydraulics of the loop, see the discussion and attachment in my post at: http://www.cheresour...c-calculations/

[garych]

But then there's one thing that has been quite confusing.

If we design the pump rated capacity as 30m3/hr, do the pump always run @ 30m/hr eventhough the users downstream required only 10m3/hr?

For instance, the control valve downstream of the pump will throttle to limit 10m3/hr of flow, while the remaining 20m3/hr will be recirculated back through the pump minimum line? So, in this case, we can say that the pump runs at the rated capacity even during the minimum case. Do this sound correct?

 

And another thing is: let's say the minimum required flow provided by the vendor is 15m3/hr, can the pump runs @8m3/hr, considering that control valve at the pump minimum flow line will recirculate the pump back to the suction?

[fallah]

garych,

 

Min and max flow will specify the required Cv at min and max flow conditions and required flow will be set at a value in between. If downstream of the pump will limit the flow at 10 m3/hr; then control valve will be in wide open condition but due to restriction at downstream only 10 m3/hr will be passed through it.

 

About the minimum flow in any case, either the recirculating line back to the suction vessel or suction line, the flow passing the pump shall not be lower than the value has been specified by vendor...

[MTumack]

It also depends if you plan on operating your Motor with a Variable Frequency Drive Motor.

 

In many cases you can find points on your VFD where you can have near identical Head at all required flowrates.

[garych]

The pump doesnt hv a VFD.
To make it clear,let's say the pump rated capacity=30m3/hr with 50m head and when carrying out the hydraulic calculation for the min. flowrate case,there are options as below.

1. Flow through pump : 10m3/hr
Pump head : Head@10m3/hr based on performance curve.

2. Flow through pump : 10m3/hr
Pump head : 50m @30m3/hr

From above discussion,i pressume that option 1 will be the right choice,correct?

[Teknas]

Hi,

 

Lets us look at the latest options you have mentioned, and derive some more.

 

1. Assume minimum safe flowrate for the pump to be 40% of the rated flow, which means that your pump should never run below 30*0.4 = 12 m3/hr.

 

2. Assuming you are having a control valve in the recycle line, If the delivery point requires 10 m3/hr, then control valve in minimum flow line will have to pass 2 m3/hr because 12m3/hr is the

    minimum flow which your pump can achieve. Look for the developed head at 12 m3/hr flowrate.

 

3. Assuming you have a RO in the recycle line, then developed head will be for 30 m3/hr which is 50 m, and 20 m3/hr will flow back to the source vessel. Same will apply in case you have a

    manual globe valve on recycle line.

 

4. Assuming you are having no minimum recycle line and a globe valve at destination, the increase in head will correspond to the reduced flow. Check for overheating within the pump.

 

Regards