Often it is necessary to pump into a pipeline that may or may not be full of liquid. A user was unable to get the needed results because the discharge pipeline in her system was full under high pumping demands, and partially full under low pumping demands. This article describes how to use PIPE-FLO to determine if a pipeline is full, and how to handle a piping system in which a common discharge line is flowing partially full.

PIPE-FLO performs headloss calculations using the Darcy method, which is valid for fully charged pipe in either a pumped or gravity flow situation. The important thing to remember is that the pipeline must be full. If the pipeline is not full PIPE-FLO cannot give a valid result for the partially full pipeline.

A system similar to Figure 1 was submitted for our review. During periods of light pumping loads the discharge pipeline flows partially full, during periods of high pumping loads the discharge pipeline is full. This article describes how to set up a PIPE-FLO model to calculate the flow rate through the pumps in both conditions.



Figure 1.  Diagram of the example piping system showing pipelines and connections.

At the bottom of this article, a system entitled partial_full.pipe or partial_full_v2009.pipe can be downloaded so you can follow through with PIPE-FLO Professional or Flow of Fluids.  

Notice in the .pipe files that there are additional pipelines added to each Lateral line on the FLO-Sheet drawing, one called full pipe and the other called over flow pipe. Both pipelines are the same nominal size as the lateral pipelines, and both pipelines are short (1 ft) in length. These additional pipelines are placed in the system so we can control the direction of flow. The full pipelines connect the pumps to the discharge header and are open if the discharge pipe is calculated full. The over flow pipes also connected to the pump but are terminated at a boundary pressure. The pipeline is opened if the discharge pipe is calculated to be partially full - the pump then discharges into the partially full pipeline.


Figure 2. Modified diagram of the example piping system showing additional pipelines added to each Lateral line.

Here are the steps to determine if the discharge pipeline is completely or partially full.

• Perform the calculations with the full pipes open and the over flow pipes closed.
  
• If all of the calculated pressures at the nodes in the discharge header are above the vapor pressure of the fluid, the pipeline is full and PIPE-FLO displays the calculated results.
  
• If some of the calculated pressures at the nodes in the discharge header are below the vapor pressure of the fluid, the pipeline is not full, and the flow rates from the pump are not correct.  The pump is discharging into a partially full pipeline, which is similar to it pumping into the top of a tank.
  
• Close the "full pipes", open the "over flow pipes", and run the calculation again.  PIPE-FLO then does not take into account the pressure in the discharge line.
  

To demonstrate we will run through three examples:

First Example:  Discharge header partially full

1. Run the calculation assuming the discharge pipeline is full ("full pipe 1/2" open, and "overflow pipe 1/2" closed).  After performing the calculation the pressure at the main sump is about –28.9 psig, -17.4 psig at Node 1 and –10.5 psig at Node 2.  
   
2. Since some of the pressures at the nodes in the discharge pipelines are less than the vapor pressure of the fluid, the discharge pipeline is only partially full.   We will rerun the calculation with the overflow pipes open and the full pipes closed.  
   
3. Close "full pipe 1/2" and open "overflow pipe 1/2" then rerun the calculation. The flow rate through pump 1 is 1,429 US gpm and pump 2 is 1,143 US gpm.  These values are correct because the pumps are pumping to the overflow pressure boundaries that simulate dumping into a partially full unpressurized discharge header.

Second Example:  Discharge header full.

1. Choose the lineup called Second Example.  Notice the flow rate into the main sump is set to 4,000 US gpm
   
2. Run the calculation with "full pipe 1/2" open, and "overflow pipe 1/2" closed.
   
3. Notice the pressure at nodes 1 & 2 are above the vapor pressure of the fluid indicating the discharge pipe is completely full.  The flow rate through pump 1 is about 940 US gpm and pump 2 is about 1,143 US gpm.

In this lineup the program calculated node pressures above the fluid vapor pressure, indicating the pipeline is full.  As a result the pumps are now pumping into a full pipeline and pumping against a discharge pressure.

Third Example:  Discharge header full with a negative pressure.

1. Choose the lineup called Third Example.  Notice the main sump has a boundary pressure set to a 0 psig boundary pressure.  This simulates fluid flowing by gravity from the main sump to the discharge header.
   
2. Leave "full pipe 1/2" open, and "overflow pipe 1/2" closed.
3. Click the Calculate button.  Notice the pressure at Node 2 is about  –3.83 psig.  This is a negative gage pressure, but it is well above the absolute vapor pressure of water at 60 °F.  This is similar to a siphon in the discharge header.  
   The flow rate through pump 1 is about 1,364 US gpm and through pump 2 is about 1,170 US gpm.  With a negative gage pressure at node 2 the flow rate through pump 2 is greater in this lineup than either of the other two lineups.