10 replies to this topic

[sargon 1979]

Dear all
I have to size hot oil pump, the oil data used for the pump sizing is the data of normal operatipn in which the oil temperature is 240 ℃ and oil viscosity is 5 Cp. At startup condition, oil temperature is 45℃ and oil viscosity is 100 Cp. What case should govern the sizing of the pump? The pump capacity is 135 cubic meters/hr and the pump differential pressure is 13 barg at 240℃. Thank you

[Bobby Strain]

I would ask potential suppliers. For a cold startup it seems you have only to pump the minimum flow required by the pump. So, we can't give you any answer.

 

Bobby

[rdcrags]

Normal operation plus the usual extra GPM ("design GPM").

[breizh]

Hi,

Can you provide a sketch for a better understanding of your operation ?

Breizh

[fallah]

Dear all
I have to size hot oil pump, the oil data used for the pump sizing is the data of normal operatipn in which the oil temperature is 240 ℃ and oil viscosity is 5 Cp. At startup condition, oil temperature is 45℃ and oil viscosity is 100 Cp. What case should govern the sizing of the pump? The pump capacity is 135 cubic meters/hr and the pump differential pressure is 13 barg at 240℃. Thank you

 

sargon,

 

One point you should consider the govern case for motor sizing is the viscosity; hence you should size the motor for start up and normal conditions and select the one with higher required absorbed power...

[sargon 1979]

Dear Naser;

if the pump is sized according to the cold case then, the pump will be oversized according to the hot case also the piping system is designed for a hot oil velocity of 4.5 m/s which will be exceeded if the pump is sized according to the cold case. again, the NPSHA is limited to 45 m only.

 

thank you 

[ankur2061]

Hi,

 

I have faced a practical problem on hot oil pumps during the start of my career. The pump was not designed for cold start-up and when we started up the plant in winter conditions with near about freezing ambient conditions the pump kept on tripping whenever we opened the discharge valve. The key is the density, which governs the absorbed power with other parameters remaining constant. Refer the link below and specifically the post#2 to understand what happens if the liquid is very dense (high density) and the design does not consider the cold start-up condition.

 

https://www.cheresou...n-with-density/

 

Regards,

Ankur

[fallah]

 

The pump was not designed for cold start-up and when we started up the plant in winter conditions with near about freezing ambient conditions the pump kept on tripping whenever we opened the discharge valve. The key is the density, which governs the absorbed power with other parameters remaining constant. 

 

I think in your case for pumping a specified liquid fluid at a wide range of temperature from freezing ambient condition to higher and normal condition the more important key affects pump's absorbed power is viscosity than to be the density; because the viscosity variation would be so high relative to the density change which is limited to a few percent...

[ankur2061]

Naser,

 

If you have read my post and followed the link in my post, you will notice that the absorbed power equation has a density term in it and not viscosity. Pump was tripping because it was drawing more power than designed during cold start-up and hence tripping. The electrical maintenance team would not allow us to change the motor overload relay setting (and rightly so) in order to protect the motor.

 

Density differences for some common hot oils (such as Dowtherm A ) are significant for large temperature differences. Dowtherm A has a density of 1065 kg/m3 at 12 deg C and a density of 812 kg/m3 at operating temperatures of 295 deg C.

 

Hence the pump trip can only be attributed to high density difference and while providing engineering data related to pump absorbed power, cold start-up density needs to be carefully evaluated for the impact on absorbed power.

 

Regards,

Ankur.

[fallah]

 

If you have read my post and followed the link in my post, you will notice that the absorbed power equation has a density term in it and not viscosity. Pump was tripping because it was drawing more power than designed during cold start-up and hence tripping. The electrical maintenance team would not allow us to change the motor overload relay setting (and rightly so) in order to protect the motor.

 

Density differences for some common hot oils (such as Dowtherm A ) are significant for large temperature differences. Dowtherm A has a density of 1065 kg/m3 at 12 deg C and a density of 812 kg/m3 at operating temperatures of 295 deg C.

 

Hence the pump trip can only be attributed to high density difference and while providing engineering data related to pump absorbed power, cold start-up density needs to be carefully evaluated for the impact on absorbed power.

 

 

Ankur,

 

I know the effect of the fluid density on the required absorbed power of a centrifugal pump; but you certainly well aware about the effect of the fluid viscosity also.

 

In most cases the variation in the fluid density due to temperature change (not due to fluid itself change) can be handled by the over design in pump's rated power being created in motor selection stage; means if a pump's motor being designed for the fluid density in lower ambient temperatures can mostly handle the increased power due to minimum operating temperature but the same pump might not handle the simultaneous effects of high density and high viscosity due to minimum ambient temperature because of reduced efficiency due to high viscosity. I think the pump trip, if any due to not already being considered the effect of high viscosity, in this situation can mostly be attributed to high viscosity difference rather than density difference.

Edited by fallah, 03 August 2017 - 10:39 AM.

[breizh]

Hi ,

In a previous project we had hot oil stored in an underground tank , to overcome the issue with temperature( viscosity/density) we had a HX installed on a dedicated  recirculation line to warm the product ( start up)  back to the tank . The heating medium was steam , pressure was about 40 bars .

 

Hope this is going to help you .

 

Breizh