3 replies to this topic

Regards,

 

I am designing a positive displacement pump, screw type for a refinery revamp project. They handle a flow rate of 18,000 bbl/h of Fuel Oil @ 130 °F and 570 cP with a design discharge pressure of 600 psig.

 

Art has commented on various forums the importance of relief systems of positive displacement pumps (relief valves and piping). It is clear for me that the relief pipe of this type of pumps "should not go back" to the suction line because all the mechanical and piping flexibility implications this would entail. Art suggests a return to the suction vessel. In fact, I'm very agree with him... http://www.cheresour...no-minium-flow/

 

But my doubt arises regarding the specific design features on which I’m working. The length of the suction line of the pump is around 400 m away. Yes, it’s a very big dike (four fuel oil tanks) because a Venezuelan law (from 1973) requires a ridiculous separation between tanks (very absurd in my opinion and experience).

 

The suction is performed from atmospheric Fuel Oil tanks of 433,000 (nominal capacity). According to the mentioned criteria, the relief lines from pumps to tanks must travel a long distance. The relief system (valves and pipes) were design to handle the design flow of the pump (18,000 bbl/h), so, I have a very long line of a considerable diameter back to the tanks.

 

My questions are, is it strictly necessary to do this? Could I return the relief flow to the suction line respecting a considerable distance from the pumps? Will this impact the fuel tank considering its working pressure?... I don’t think so, It’s a big tank (diameter: 220ft) and It is true that the operation is not continuous and I have a ESD system for the pumps, but, would not be appropriate, given all these features (economic impact) connect the relief pipe to the suction line instead the tank considering the long distances?

 

In the attached file I found the following information:

 

“The safety relief valve outlet connection should ideally be piped back to the suction supply vessel. Piping back to the suction pipe can cause discontinuities in the suction pipe flow that can result in poor pump operation and damage. Should it become necessary to pipe the safety relief valve back to the suction piping, the connection into the suction pipe should be a minimum of 10 suction pipe diameters in length back toward the suction supply vessel away from the pump suction connection”.

 

Suction line diameter: 34”.

 

 

I would like to know your opinión.

 

 

Thanks.

Attached Files

•  PDRP FUNDAMENTALS.pdf   6.86MB   72 downloads

[Sathya R]

The tutorial is about reciprocating pumps, which may not be appilicable for gear type (rotary type pumps) which you are intended. The requirment for reciprocating pump as stated in the tutorial may be due to reduce pulsation in the suction piping.

Yes, of course. I know that is about reciprocating pumps but like all those types of positive displacement pumps there is not a shut-off pressure and considering the high flow rate and the high discharge pressure, so, actually what I meant is this... If there is such a criterion for this type of pumps, more delicate and complicated in terms of operation and maintenance, Could not be applicate to a PDP, gear type? I think yes, sure. Just want to know your opinion.

[Art Montemayor]

 

 

Idavidqs:

 

Instead of 300 meters, I come up with an estimated 247 meters for your suction line.  Nevertheless, I agree with you that this is a considerably long suction line and not the normal set of circumstances.  However, if this is the scope of work from the client, then that is what you have to work around.

 

Please refer to the attached workbook.  I was expecting you to generate at least a layout of the equipment showing all dimensions, but since you didn’t generate this required piece of basic data, I went ahead and did it.  Please correct and indicate any errors or corrections that you spot or identify on the workbook and submit it as Rev1.  Note that regardless of what problems you might have with routing a discharge line from the pressure relief valve, you still have to furnish a “pump-around” line for startup and shutdown service.  Since this line has to be sized for the maximum flow, then it coincides with PSV capacity and it would seem that you could then pipe both together as shown on my sketch.  Don’t forget that you will need to compensate for the hydrostatic head of fuel oil that fills the vertical run of the return line.

 

Your size of 34 inches for the suction line seems small to me for the capacity of 12,600 gpm.  Even a 36 inch size seems a little small – although I haven’t checked the estimated pressure drop at the calculated velocity.  You should be very cautious about accepting a velocity of 5 ft/sec - especially with a potentially high fluid viscosity.  Any sudden increase in pressure drop or a sudden Venezuelan rain shower could lower the temperature and lead to pump NPSHA problems.

 

I believe you are on the right trail in resolving potential problems in this exceptional fuel oil pumping application.  I would insist to the client that in keeping with proper engineering design criteria and good judgment, a 36+ inch suction line seems to be the correct one (after running rigorous pressure drop and NPSHA calculations.  Additionally, I would point out to the client that a top entry with a dip pipe would seem to be the best solution in avoiding any potential spills within the diked area.  The top entry of the return line maintains the containment principle of the tank: a minimum of bottom nozzles means a minimization of potential nozzle ruptures or leaks and allows the tank to act a true containment vessel within the dike.  A return line back to the tank is required due to the need to employ a positive displacement pump.  This allows the pump to be started and shut down without any undue load on the motor drive – and that is the correct way to start and stop all positive displacement devices.

   Ingemar Fuel Oil Tanks.xlsx   24.98KB   56 downloads