Basic pipeline/pressure question 1

[SweetDewMe]

Im fairly new to designing pipeline systems, so I have a basic question:

Lets say I have a piping system that has 4000' straight pipe equaling 100' head and I have an appropriately sized pump attached to the system. Now, if I put a throttling valve in the middle of that pipeline, can that affect the static pressure downstream of the valve? If you throttle down on that valve, you will increase your system head, therefor slowing the flow because you've gone to the left on your pump curve. So, your upstream static pressure will increase, but will the downstream static pressure always just be 50' head? Will the static pressure downstream of that throttling valve always just be however much system head there is below the throttling valve? Lets also assume that I don't throttle down too much. We'll stay reasonably close to the BEP.

If anyone can let me know if Im thinking about this correctly, of if Im missing something, I would appreciate it. Thanks.

 

[BigInch]

You must define the boundary conditions for any given segment of pipeline you are considering. With the principal variables, inlet pressure, outlet pressure and flow, each will have its own effect on pressures inside the defined segment of pipe.

You are apparently already making a mistake by considering the segment downstream of the valve as independent of the flow upstream.

You must write an equation for each flow element, pump, pipe or valve, fitting etc. relating its flow to the differential pressure across each element.

Let the suction pressure be one boundary condition, noting that the suction pressure might vary with flow in the suction line. Same for the pipeline outlet pressure, which also might vary with flow in the outlet segment.

If inlet and outlet pressures vary with flow, you will not be able to assume they remain constant and you will need to develop a function describing their pressure with the flow at any given time step.

To take your example,
You have apparently assumed that outlet pressure drops to zero, if the valve is completely closed. Depending on exactly what kind of system is downstream, that may or may not be true. Zero flow in the outlet segment and all downstream flow elements, will reduce the pressure drop in each element, thus it is possible that all pressures downstream might increase. If you define your pipeline outlet pressure as constant, then mathematically, you will prohibit any increase of outlet pressure and the valve differential pressure must increase. That may increase the flow at any given valve position, at least until the valve is completely closed. It may also increase the pump discharge pressure as flows there tend to reduce with the closing valve, but the increased differential across the valve, tend to increase the pump flow too thereby reducing its discharge pressure as it moves out on the curve. As the pump flow equation and the valve flow equation and the pipe segment flow equations are not linear, it requires an iterative solution to arrive at the final steady state flow and pressures at all points, for any given valve position.

Got it?

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"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[djack77494]

SDM,
When you say that your 4000' of straight pipe "equals" 100' of head, I think you're meaning that the hydraulic losses at the flowrate you have in mind will be equivalent to 100' of your pumped fluid. If your throttling valve causes your flowrate to change, then yes, it can affect the downstream static (and dynamic for that matter) pressure. Assuming a fixed destination pressure and elevation and no hardware changes, then the pressure at any location downstream of the throttling valve will NOT change if the flowrate does not change. Doug

 

[SweetDewMe]

Wow, BigInch, thats quite the reply. Thanks.

djack77494, your reply was more what I was looking for. Thanks. I understand that by throttling the valve we increase the system head and therefor travel left on the pump curve, thus decreasing the pump flow. So now with the flow decreased the downstream pressure will decrease because every component of system head is based on flow velocity. That makes sense. So if we put a pressure gage just downstream of the throttling valve, the pressure it will read should equal the downstream system head plus elevation change head plus discharge tank pressure right? Thanks for letting talk through some of this with you guys.

 

[BigInch]

If you play with your outlet boundary condition, you can have whatever pressure you want. When the valve closes and you have zero flow, the entire segment downstream of the closed valve assumes the static pressure of the then isolated downstream system, whatever that may be.

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain