Piping/Pump + elevation question
Piping/Pump + elevation question
(OP)
Here's the piping/pump problem...
I'm leaving out some details in order to simplify the problem in order to explain it.
I have 3 heat exchangers that have cooling water provided from one pump. These heat exchangers are at different elevations, for simplicity say they are at elevation 10ft, 20ft and 30ft above the centerline of the pump discharge. Currently all three heat exchangers discharge into a common discharge line located at elevation 15ft.
If I were to add a section of pipe within the common discharge line but before the pipe exit point (i.e. the point of discharge is still at 15ft) that resembled a upside down U, with the top of the U at elevation 25ft, would the pump see an increase in the system's pressure if I disregarded frictional losses because they are negligible.
The "loop" would start at the 15ft elevation climb vertically up to the 25ft elevation and back down to the 15ft elevation then to the exit point.
I am unsure as to how much this will affect the system pressure that the pump sees. Will it add 10ft H2O of backpressure to the pump? Or can I neglect the height downstream of the highest point (30ft) and assume it is free flowing at that point? Any ideas would be appreciated. Thanks.
I'm leaving out some details in order to simplify the problem in order to explain it.
I have 3 heat exchangers that have cooling water provided from one pump. These heat exchangers are at different elevations, for simplicity say they are at elevation 10ft, 20ft and 30ft above the centerline of the pump discharge. Currently all three heat exchangers discharge into a common discharge line located at elevation 15ft.
If I were to add a section of pipe within the common discharge line but before the pipe exit point (i.e. the point of discharge is still at 15ft) that resembled a upside down U, with the top of the U at elevation 25ft, would the pump see an increase in the system's pressure if I disregarded frictional losses because they are negligible.
The "loop" would start at the 15ft elevation climb vertically up to the 25ft elevation and back down to the 15ft elevation then to the exit point.
I am unsure as to how much this will affect the system pressure that the pump sees. Will it add 10ft H2O of backpressure to the pump? Or can I neglect the height downstream of the highest point (30ft) and assume it is free flowing at that point? Any ideas would be appreciated. Thanks.





RE: Piping/Pump + elevation question
The 30ft thingy lost me. Believe, You will find the fluid static head pressure at the pump will be at the greatest height above the pump.
At 74th year working on IR-One PhD from UHK - - -
RE: Piping/Pump + elevation question
Is it a closed loop , ie recirculating
RE: Piping/Pump + elevation question
The balance to do is to compare the pressure at the base of the second leg with the height of the U. Say, for example, that the pressure at the base of the second leg was 20' due to friction in the pipe from the U to the discharge point and any static height change. This would mean that the down leg of the U would run full and that you would recover the full 10' of static height that you lost in the up leg.
On the other hand, if the inverted U was near the end of the line and the pressure at the base of the down leg was only 2' it would not be necessary for the downleg to be full to provide this pressure. The pressure at the top of the inverted U could actually go below atmospheric and depending on the temperature of the water you could get boiling and cavitation. As soon as you get boiling you start to lose pressure recovery and the pressure the pump "sees" will increase.
In this sort of installation you will often see a breather pipe or valve (syphon break) above the top of the U to prevent the pressure going below atmospheric (and to prevent the resulting cavitation and vibration). If a breather is installed then there cannot be full pressure recovery.
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Piping/Pump + elevation question
RE: Piping/Pump + elevation question
RE: Piping/Pump + elevation question
At a simplistic level, whatever the pressure drop was from the start of the common line to the discharge point, you have now added in additional static head that cannot be fully recovered. It is likely that your pump will see some additional head but without more detailed info nobody can say for sure how much.
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Piping/Pump + elevation question
Here is a diagram of what is going on.
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All three heat exchangers "thrust bearing", "lower guide", "turbine bearing" are simply water coils running within an oil tub to cool the various bearings on a hydro turbine generator. The "thrust bearing" (30ft) does not have vacuum relief. The inverted U on the second diagram does have vacuum relief. All three lines going to the 3 heat exchangers have flow setters to adjust flow. It is also safe to assume that flow through the 3 bearings on the second diagram can be adjusted to the flows on the first diagram.
Would it be conservative to assume a 10ft increase in static head based on this diagram?
RE: Piping/Pump + elevation question
The only difference I see will be an increase in friction head due to the extra pipe-run bought about by the inverted U, this could effect the output of the pump in terms of the flowrate at the new total head. However - you say to neglect friction - so I don't see any problem.
RE: Piping/Pump + elevation question
Offshore Engineering&Design
RE: Piping/Pump + elevation question
This was my initial thought, however I was second guessing myself.
"The obvious thing to do is to arrange your piping , so that the pump discharges to the highest elevation and cascades to the lowest heat exchanger. Otherwise you will short circuit, if you flow the bottom/up arrangement. "
There are flow setter valves to control all the flows through all three heat exchangers, thus short circuiting can be prevented. Cascading the heat exchanger flows is not possible due to requirements of the heat exchangers.
RE: Piping/Pump + elevation question
I dont think you will make much difference though with the U shape upside down as the head you gain going down the U, you loose going up it in the first place...
so i think you'd end up with less pressure than you initially had...
Please do drop more information or knowledge you have on this subject they will all be taken in and appreciated.
David O.
Mechanical Engineer
Graduate as of: 2007
Oil, Gas & Power
RE: Piping/Pump + elevation question
Once you have solved the question of whether you get full recovery or not you can determine the pressure change immediately upstream of the inverted U. If the pressure at this point increases does it mean there is an increase in the pressure the pump must deliver? To answer this question you need to investigate the positions (percentage opening) of the flow setting valves on the heat exchangers, and also you need to check whether there is pressure recovery in the down leg from the thrust bearing.
Even though the diagrams look simple, this is not a trivial problem to solve rigorously.
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Piping/Pump + elevation question
I'm going to throw another assumption into the equation. If none of the pressure is recovered on the downstream side of the U (due to the vacuum relief valve), does that mean that there would be, at a maximum, an increase of 10ft of static pressure on the pump?
I just need to be appropriately conservative, not necessarily exact.
Thanks
RE: Piping/Pump + elevation question
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Piping/Pump + elevation question
- Vapor pres at top of loop (Katmar explained it well).
- How the outlet of the pipe is sealed (is it below a liquid level, or above it in vapor/air).
- Velocity in the downward section of the loop.
Thus, if the outlet is not sealed in liquid, and the velocity is below self venting flow, then you will not recover the head in the downward section and the pipe will probably vibrate (as it sucks in and then blows out air).