9 replies to this topic

[raisie]

Hi guys.

 

I have a situation I would appreciate some help on.

 

There are two gasoline storage tanks (Tank 1 and Tank 2), two pumps (Pump 1 and Pump 2). Currently, Tank 1 and Tank 2 are connected to Pump 1 through a 4 inch pipeline. For future operation purposes, an additional connection will be made from Tank 1 and Tank 2 to Pump 2. Thus, in the future operation mode, fuel will be supplied to from either Tank 1 or Tank 2 to both pumps simultaneously. This means that, in order to deliver the nominal flow to both pumps, the flow through the 4 inch pipeline will double. My colleagues seem to think that the 4 inch pipeline will have to be replaced with a 6 inch pipeline in order to optimally deliver fuel to both pumps and are trying to justify this thought with calculations. With “optimally” I mean that both pumps receive a flow rate equal to their nominal flow rate continuously. The question is, which calculations do you perform to check this??? (Please see attached sketch)

 

I think that the size of the pipeline has nothing to do with whether or not both pumps can be provided with their nominal flow, because once the pipelines are filled with fuel and the tank has enough fuel, the pumps will just keep receiving the needed flow through suction. However, I think that due to the doubled amount of required fuel flowing through the same size pipeline, the speed of the flow will be larger and THAT is why the size of the pipeline must be increased. So basically, now I’m looking for a code/standard which specifies recommended speeds of fuels through pipelines.

 

Am I right on this so far??

 

 

 Drawing.png   5.05KB   5 downloads

[fallah]

raisie,

 

I don't think while both pumps are in operation, the flow through 4" line will be doubled. Why do you think so?

[raisie]

@ Fallah,

 

Because then two pumps will require flow

[fallah]

raisie,

 

If pump2 was installed at the same side of pump1 and in parallel with it, you might be right; but it is installed in oposite side of pump1 and each pump will take flow mostly from closer tank, i.e pump1 from tank1 and pump2 from tank2; then the flow in 4" line can't be doubled...

[Bobby Strain]

Did you consider doing some actual calculations?

Bobby

[shan]

Check your NPSHAs for both pumps.

[raisie]

Thanks everyone!

I have performed the cslculations and they seem okay. However, I have on more concern.
NPSHa is calculated as follows:
NPSHa = Ha +/- Hs - Hvp - Hf
where: 
Ha= pressure exterted on the fluid surface
Hs = Suction head, this is positive in my case because the liquid level is above the pump suction)
Hvp = Vapor pressure head
Hf = Head loss (sum of major losses and minor losses)

The tanks in my case are 10m high.
Assuming that the tank is full when operation starts, Hs will be somewhat less than 10 m (I based my calculations on Hs = 9m).
For this case, the NPSH requirements are fulfilled for all pumps, so no problem there.
However, the tank level drops during operation right, so that means that Hs is continuouslly decreasing.
According to my calculations, when the tank has a liquid level of 7 meters, one of the pumps' NPSHa becomes less than NPSHr.
This means that the tank is still more than half full when one of the pumps starts cavitating. This also means that the tank always needs to be full for operation.
This does not seem right to me......
Any ideas?

[shan]

Increase you suction piping size to reduce Hf.

[MTumack]

There are a ton of options to remedy the situation;

 

a) Raise your vessel.

 

Add a booster pump.

 

c) Increase pipe sizing.

 

d) buy a new pump that requires less NPSH

 

e) Put level protections / alarms on your vessel.

 

f) Move pump closer to vessels to reduce frictional losses.

 

etc etc

[Linda1978]

Based on the picture that you have provided, if you have proper valving on your system, you don't need to change the size.

 

It is not clear for me both tanks are feeding pumps or one is feeding both pumps?

 

Here are my recommendations:

 

1- check the pressure drop based on actual system (considering valves, actual working system and operating manual) and compare the result with maximum allowable pressure drop (see your local standard and/or applied standard in your plant). You can use free online pressure drop calculators (here) to calculate pressure drop and compare the result with standard or use free desktop software (here) to re-size the pipe for the new case.

 

2- Check available Net Positive Suction Head (NPSH) with your pump requirements. Make sure your pump won't face cavitation.

 

Good Luck

Edited by Linda1978, 04 June 2015 - 12:58 AM.