High Flow Through 2" Piping/Nozzle
High Flow Through 2" Piping/Nozzle
(OP)
Good afternoon everybody,
We are in the process of fixing the design error regarding the minimum flow of high-head, multistage centrifugal pumps. In brief, the existing single loop (both pumps have the minimum flow control through a single valve which simply does not work) is to be replaced by separate, individual loops for minimum flow control of each pump. The existing configuration simply uses the minimum flow line downstream of the air cooler at discharge of the pumps, while rest of the flow (flow-controlled) goes to the process.
However, this solution with individual recycle lines leaves us without the possibility to cool the recycle streams which may lead to progressive temperature rise across the pump(s) and subsequent damage. The questions arrisen are:
1. Is it worth considering re-designing the recycle loops without any cooling, and just recycling (partially) directly to the pump suction? Recycle flow varies from 0 up to 50% of total discharge flow of the pump. Personally I am against such solution as we don't know where we may end up.
2. Another option is to have the spillback header connected to the low-pressure column somewhere upstream. I have seen this type of solution successfuly implemented elsewhere but the problem here is that we have only a 2" line and 2" nozzle on the column, for the maximum recycle flow of 170,000 kg/hr. This section is 2 meters long only but according to pressure drop calculations this would create the loss/backpressure of around 10 bar. As the pump discharges at 80 barg and the column operates at 2 barg, having such high pressure drop does not represent a problem just by itself, but the velocity in 2" line and in the nozzle would be more than 20 m/sec which is extremely high.
Can you give your views on these two items? And perhaps come with additional suggestions that might enlighten us.
Regards,
We are in the process of fixing the design error regarding the minimum flow of high-head, multistage centrifugal pumps. In brief, the existing single loop (both pumps have the minimum flow control through a single valve which simply does not work) is to be replaced by separate, individual loops for minimum flow control of each pump. The existing configuration simply uses the minimum flow line downstream of the air cooler at discharge of the pumps, while rest of the flow (flow-controlled) goes to the process.
However, this solution with individual recycle lines leaves us without the possibility to cool the recycle streams which may lead to progressive temperature rise across the pump(s) and subsequent damage. The questions arrisen are:
1. Is it worth considering re-designing the recycle loops without any cooling, and just recycling (partially) directly to the pump suction? Recycle flow varies from 0 up to 50% of total discharge flow of the pump. Personally I am against such solution as we don't know where we may end up.
2. Another option is to have the spillback header connected to the low-pressure column somewhere upstream. I have seen this type of solution successfuly implemented elsewhere but the problem here is that we have only a 2" line and 2" nozzle on the column, for the maximum recycle flow of 170,000 kg/hr. This section is 2 meters long only but according to pressure drop calculations this would create the loss/backpressure of around 10 bar. As the pump discharges at 80 barg and the column operates at 2 barg, having such high pressure drop does not represent a problem just by itself, but the velocity in 2" line and in the nozzle would be more than 20 m/sec which is extremely high.
Can you give your views on these two items? And perhaps come with additional suggestions that might enlighten us.
Regards,
http://antwrp.gsfc.nasa.gov/apod/astropix.html





RE: High Flow Through 2" Piping/Nozzle
I wouldn't accept 20 m/sec in a line for a liquid.
RE: High Flow Through 2" Piping/Nozzle
RE: High Flow Through 2" Piping/Nozzle
Good luck,
Latexman
RE: High Flow Through 2" Piping/Nozzle
@TD2K: Wouldn't the suction and discharge temperature progressively rise if there is no cooling of the recycle stream? And yes, you are right, startup/upset are another issues which might make things even worse.
http://antwrp.gsfc.nasa.gov/apod/astropix.html
RE: High Flow Through 2" Piping/Nozzle
Let's say your pump has a 5F temperature rise across it (unlikely to be that high unless it's a very high head pump and low efficiency). Now consider 50% of the discharge flow is recycled back to the suction. The overall temperature rise across the pump will be 10F and the 50% forward flow is removing the same amount of energy from the system (the pump).
Why isn't a single recycle line working as it's currently designed? Do you routinely run both pumps? If you are, how come you need to recycle flow then?
RE: High Flow Through 2" Piping/Nozzle
I am not sure I understood your statement regarding the heat out-flow from the system. Just imagining the process in my mind. First run through the pump: 2 degC temperature increase, then we recycle 50% of this flow back to the suction. The suction flow coming from the process has the same temperature, so the combined suction temperature will increase due to warmer recycle stream. Now, we have higher discharge temperature than in the first run, and higher combined suction temperature, and so on... To me it looks like the temperature rise inside the system could continue without any control. Does that make sense?
http://antwrp.gsfc.nasa.gov/apod/astropix.html
RE: High Flow Through 2" Piping/Nozzle
Say the cold stream is at 20C. It goes through the pump and exits at 22C, 2C rise across the pump, and goes off to the process.
Now, you recycle 50% back to the suction. The cold fluid is still 20C but it's mixing with the recycle stream which is 24C now (will get to that number next). The combined suction to the pump is the average given a 50% recycle or 22C. You go through the pump and pick up 2C giving a discharge temperature of 24C, the figure I used at the start of the paragraph.
With no recycle, the energy added to the net stream as heat is Q*2 deg C*Cp (Q is the flow and Cp is the heat capacity, don't worry about the units for this example).
For the 50% recycle case, the net flow from the system is 1/2Q*4 deg C*Cp which is the same as with no recycle. It should be the same since the pump flow and its efficiency will not have changed.
RE: High Flow Through 2" Piping/Nozzle
Since this loop cannot work in 100% recycle due to pump overheating, is there any point to size the recycle line for 100% flow? If I use the design minimum flow of 200 t/hr I get 6" line size required to maintain velocities below 4.5 m/sec (15 ft/sec). For a 4" line the line velocity would go up to 6.5 m/sec. Is this acceptable as a short-term operation? Normally the 4" line will never see velocities higher than 4.0 m/sec.
I have attached the dynamic model, maybe it will be of use to other forum members.
Thanks again.
http://antwrp.gsfc.nasa.gov/apod/astropix.html
RE: High Flow Through 2" Piping/Nozzle
The recycle line should be designed to meet the pumps' minimum flow requirement which should be on your data sheet or is available from the vendor. Since you say that you operate two pumps at least part of the time, the minimum flow is 2x the individual flow. If you can't take credit for the minimum flow to the process then this needs to be the sizing basis for the recycle line. Then you can evaluate where it goes, directly to the pump suction, to another location where the stream can cool, etc.
RE: High Flow Through 2" Piping/Nozzle
The only true concern is if there are design scenarios when the pump is supposed to run in 100% recycle mode for a prolonged period of time, e.g. startup or upsets. Then recycling without cooling is meaningless as the pump would need to be shutdown after ~5 minutes due to danger of overheating, according to dynamic simulation.
http://antwrp.gsfc.nasa.gov/apod/astropix.html
RE: High Flow Through 2" Piping/Nozzle
http://antwrp.gsfc.nasa.gov/apod/astropix.html
RE: High Flow Through 2" Piping/Nozzle
that is a bit unusual given the 20 m/s flow condition isn't it?
RE: High Flow Through 2" Piping/Nozzle
Maybe I didn't understand your comment in a correct way?
http://antwrp.gsfc.nasa.gov/apod/astropix.html
RE: High Flow Through 2" Piping/Nozzle