Centrifugal pump sizing for recirculating and transfer line

[chemks2012]

Hi all,

I need your help/input please.
Please see below link where I have attached a simple drawing of the system.


As you can see, pump P1 (7m3/HR) pumps hydrocarbons to dryer (D1) via heat exchanger (HX1). In the dryer, liquid is sprayed and pressure is maintained at atmospheric pressure and water traces are removed at 90 to 100 degC. Dried liquid is then either recirculated to the dryer via centrifugal pump, P2 or transferred to the further process via high pressure positive displacement pump, P3 (70barg, 7m3/hr).

My queries are:

1) How would I design pump P2? For what flowrate and head? Here recirculation and transfer line to P3 both are having manual operation and no flow control valve on any either recirculation or transfer line.

The head required (i.e. NPSH required for P3) for the transfer line (from P2 to P3, blue line) at the flowrate of 7m3/hr is 60m.

http://files.engineering.com/getfil...e-6ef9-4247-a379-d4dc48fb2db7&file=image.jpeg

Thanks in advance
KS

 

[georgeverghese]

The recirc pump flow may most likely have something to do with the turndown of the spray drier. At some low flow, the atomiser nozzles on the drier may not be able to create the right degree of atomisation ( droplet size depends on flow to the nozzles) ?

So when you have low feed flow at P1, P2 flow makes up for the loss in flow to keep drier eff high.

The operation here may be such that turndown ratio on this drier is poor, hence there is no flow control on the recirc branch from P2.

You may need to ask someone for what may be the permissible turndown for the nozzles on this drier ? That may clear the air on the basis for this recirc scheme. The rest of the info required should fall into place after that.

 

[chemks2012]

Hi georgevergese

Thanks for your reply.

Ok. I will ask details on dryer. Now let's say if we assume recirculation flow rate from pump P2 is 10m3/hr and at that flow rate, the differential head (total head) required for recir line (red line) is 30m while the differential head required for transfer line (blue line) is 60m at 7m3/hr.

What duty point should I consider for pump P2?

1) Is that 17m3/hr (10m3/hr + 7m3/hr) at 90m?

Or

2) Is that 17m3/hr (10m3/hr + 7m3/hr) at 60m? If your answer is yes for this option, would pump not pump only to the least resistance path I.e. Recirculation line?

Thanks in advance

 

[georgeverghese]

So for this case, the pump P2 flow range would be from 10-17m3/hr.

If the required suction head to P3 is 60m, then the duty for P2 would be 17m3/hr at 60m.

However, as you suspect, to enable this, a backpressure control valve should be installed on the recirc branch to maintain a constant suction head of 60m to P3. Without this, the suction head to P3 would collapse to about 30m.

 

[FMJalink]

Chemks2012 aren't you better off using separate pumps for the recirculation and boostering? The operation points appear to be quite far apart for a centrifugal pump, even with speed control and back pressure valve.

 

[georgeverghese]

Agree its a waste on pumping power to burn off 30m of developed head across a backpressure PCV in this current scheme. Depending on what the cost of electric power is at this location, it may pay off in the long run to split out the pumping duties between recirc loop and the exit route.