6 replies to this topic

[villivord]

Dear all,

 

When you need to design a restriction orifice in a minimum recirculation line for a centrifugal pump, I'm wondering if I'm doing a good approach. There is a lot of information available about minimum flow for centrifugal pumps but I'm missing some information to understand it properly. 

 

During preliminary pump sizing, I calculate my system head losses at the desired flow rate and determine the required differential head. Because a minimum recirculation line will be installed, I have to add a certain unknown minimum flow (I don't know what the minimum flow requirement of the pump is as I'm just in the initial designing fase) to the desired flow to account for the continuous minimum flow through this line. Can I do this by taking 20% of the desired flow and add this to this flow rate? So in my pump datasheet (which I'll send to the pump vendor), I state the total flow rate (desired flow rate + minimum flow rate) and calculated differential head.

 

Hereafter, the vendor proposes a suitable pump with pump performance curve and specifies the required minimum flow rate. I think this is the moment when I can start designing the restriction orifice to ensure a minimum recirculation flow. I would do this by determining the pump's differential head at minimum flow rate from the pump curve. The pressure drop across the restriction orifice + friction losses in the minimum recirculation line + static head - suction pressure at minimum flow shall be equal to the differential head at minimum flow. 

 

As I know the differential head, I can calculate the orifice's bore diameter. Is this a correct approach? 

[Mahdi1980]

Hi,

 

In case of considering the minimum flow, it depends on the vendor's recommendation and the pump he offered fro your case. However, for the initial calculation you can normally take into account a flow about one third of your maximum rated flow (30% of Max.)

 

Regarding the orifice sizing, exactly you are right. The pressure drop is the discharge pressure at the minimum flow curve of the pump ( proposed by vendor) minus the suction pressure of the pump and in most cases the friction losses are ignored unless the connecting pipelines are long and friction is high.

 

regards

[villivord]

Hi,

 

In case of considering the minimum flow, it depends on the vendor's recommendation and the pump he offered fro your case. However, for the initial calculation you can normally take into account a flow about one third of your maximum rated flow (30% of Max.)

 

Regarding the orifice sizing, exactly you are right. The pressure drop is the discharge pressure at the minimum flow curve of the pump ( proposed by vendor) minus the suction pressure of the pump and in most cases the friction losses are ignored unless the connecting pipelines are long and friction is high.

 

regards

Thanks for the confirmation Mahdi1980

 

Kind regards,

Villivord

[GANESHD]

thank you for the informative discussion , I am also onto a similar exercise in our plant. I am sizing RO for pump which has a kick back line back to tank.(tank is at atm pressure).
From vendor H vs Q curve I see 1m3/hr is the required minimum flow and Rated flow is 12 m3/hr . So I assume that in worst case scenario if operator close the discharge valve completley still pump has minimum flow 1m3/hr going through kick back line to tank.

My question ,
1)should the pressure drop through the orifice plate be equal to
pressure drop of kickback pipeline + pressure developed at minimum flow.

2) If I follow this approach as question 1 I get very high pressure drop and my beta ratio (orifice dia/ pipe dia) comes to 0.15 . ( that is a very small orifice bore) is it advisable to go below 0.2 beta ratio ?

3) please suggest some refernces for low beta ratio RO sizing

thank you

[fallah]

thank you for the informative discussion , I am also onto a similar exercise in our plant. I am sizing RO for pump which has a kick back line back to tank.(tank is at atm pressure).
From vendor H vs Q curve I see 1m3/hr is the required minimum flow and Rated flow is 12 m3/hr . So I assume that in worst case scenario if operator close the discharge valve completley still pump has minimum flow 1m3/hr going through kick back line to tank.

My question ,
1)should the pressure drop through the orifice plate be equal to
pressure drop of kickback pipeline + pressure developed at minimum flow.

2) If I follow this approach as question 1 I get very high pressure drop and my beta ratio (orifice dia/ pipe dia) comes to 0.15 . ( that is a very small orifice bore) is it advisable to go below 0.2 beta ratio ?

3) please suggest some refernces for low beta ratio RO sizing
 

 

GANESHD,

 

1) Pressure drop across the RO for sizing: (Pressure developed by the pump at 1 m3/hr minus pressure drop along the min flow line upstream of the RO) - (Pressure of the suction drum as destination of the min flow line minus pressure drop along the min flow line downstream of the RO

 

2) Where the beta ratio will lead to an orifice bore size less than 1/8" API doesn't allow using such orifice.

 

3) For the orifice bore size less than 1/8" you can use multiple RO's
 

[GANESHD]

Fallah, Thanks again.

Regarding 1.
For calculating pressure drop for pipeline piece upstream and downstream of RO , which flow has to be considered 1m3/hr or 12 m3/hr.

I came across a code on Small Bore Liquid Orifice sizing (ASME MFC-14M-2001) equation from my internet search. This code is actually for orifice flow meter. Can I use this for RO plate sizing.

[fallah]

Regarding 1.
For calculating pressure drop for pipeline piece upstream and downstream of RO , which flow has to be considered 1m3/hr or 12 m3/hr.

I came across a code on Small Bore Liquid Orifice sizing (ASME MFC-14M-2001) equation from my internet search. This code is actually for orifice flow meter. Can I use this for RO plate sizing.

 

GANESHD,

 

The flow should be considered 1m3/hr as min flow.

 

You can try to use and examine such equation but with caution and cross checking with other sizing methods. Anyway if the bore size is pretty small it's better using multiple, say two, RO's.