Minimum straight length requirements before suction nozzle
Minimum straight length requirements before suction nozzle
(OP)
We have two identical centrifugal pumps (same model,size 4x3x10; 4" suction, 3" discharge)installed in our power plant. Suction line is 6". We intend to interconnect the suction lines (6") of the two pumps for redundancy in our operation. Because of the space restriction we are considering to join the two 6" suction lines with a 4" line. There won't be any problem from NPSH point of view (it is flooded suction and I have done the calculations).
Any advise on minimum straight length requirements before suction nozzle. In our present configuration, suction nozzle is app. 24.5" upto the valve in suction line (including valve length, valve is diaphragm type. There will be another 3"-4" upto the new branch connection.
Also, can there be any problem with increased velocity close to the suction because of reduction in size from 6" to 4".
Thanks in advance for your advise.
Any advise on minimum straight length requirements before suction nozzle. In our present configuration, suction nozzle is app. 24.5" upto the valve in suction line (including valve length, valve is diaphragm type. There will be another 3"-4" upto the new branch connection.
Also, can there be any problem with increased velocity close to the suction because of reduction in size from 6" to 4".
Thanks in advance for your advise.





RE: Minimum straight length requirements before suction nozzle
If you use high in line velocities you are more likely to have problems during pump start up and shut down phases because of transient phenomena such as hammer effect and cavitation pocket generation.
RE: Minimum straight length requirements before suction nozzle
RE: Minimum straight length requirements before suction nozzle
And secondally in cases where 5D is not practical, what sort of calculations to do to justify?? Calculating the velocity??
Thanks
RE: Minimum straight length requirements before suction nozzle
Regards checman
RE: Minimum straight length requirements before suction nozzle
RE: Minimum straight length requirements before suction nozzle
The concept is simple though, provide a smooth laminar flow with clean flow lines onto the impeller eye.
When in doubt, increase the straight run if possible before any other method such as flow straightening vanes. Use vibration analyses before and after, while operating under continuous maximum flow conditions to see if you caused a problem.
PUMPDESIGNER
RE: Minimum straight length requirements before suction nozzle
Earlier I was thinking to connect these two lines with 4" line but now after thinking about the possible problems with change in velocity, I am considering it to be 6" by adding 6 x 6 tee. Because of space restraints this tee has to immediately before the valve.
The flow will now have app. 28" of (sort of) straight portion after sharp turn of 90 deg. in tee. 28" includes reducer length and valve legth. Theoritically speaking this 28" is not straight portion because there is flow pattern disturbance in valve and reducer. But is much better situation as compared to immediately after any bend. And 28" is app. 7 times the suction nozzle dia OR app. 4.5 times the suction pipe dia.
Any thoughts/suggestion???
Thanks in advance for your inputs.
RE: Minimum straight length requirements before suction nozzle
Pumpdesigner, this is neither possible nor desirable for most applications. Most piping is designed for turbulent flow, including pump inlets. At best, one should try for an concentrically uniform, when mathematically averaged, velocity profile.
RE: Minimum straight length requirements before suction nozzle
I am aware of the field problems of applications, and I certainly do not condemn those who struggle with this issue due to lack of space, retrofitting, etc.
I helped a guy last year with a 150hp fire engine pump problem.
No way could they get anything close to a flow run onto the impeller eye, looked like a sardine cannery put that pump in. Tight 90-degree ell right up against the suction nozzle. I could not have made it better, no room on the truck. There are many situations where you are correct.
But, we get the job done in our equipment frequently. We get our long term customers to have their architects right at the very front end of the job give us ample room, instead of shoving us into a closet. We also use a lot of turbines, which we package in our own flow tubes, and the architects just have to give us what we need.
The payoff is huge, so we never stop trying because it is a real pleasure to have a pump operate silky smooth from low to high flow, no vibration, no bearing problems, no motor problems, for many many years.
PUMPDESIGNER
RE: Minimum straight length requirements before suction nozzle
I think it is important to get the terminology correct regarding laminar flow. The historic definition of laminar flow exists in fluid with an associated Reynolds number less than or equal to 2000. I would not want to see anyone trying to resolve a pump problem for most fluids by trying to get a Reynolds number less than 2000. To put this in perspective, a 4” sch 40 pipe would have an upper velocity limit of less than one inch per second to be considered laminar flow for water at room temperature. This would be less than 3 gallons per minute for the 4” pipe. This low velocity is neither practical nor desirable for most piping applications.
For most water applications the design velocity is somewhere between 3 to 8 feet per second. A four inch pipe flowing water with a Reynolds number at 2000 is 36 times slower than the typical lower limit of a design velocity of 3 feet per second. Do not try to fix pump problems by limiting flow to the laminar flow velocities.
RE: Minimum straight length requirements before suction nozzle
I think what Pumpdesiner is saying is - straightened flow-ie, have enough straight pipe after bends, elbows, valves etc to at least allow the flow to get straighten out a bit before the pump inlet.
In the case where you cannot afford or don't have the luxury of a straight run of pipe, then flow straightening at the pump inlet must be used.
Naresuan University
Phitsanulok
Thailand
RE: Minimum straight length requirements before suction nozzle
You are correct about the term laminar flow, I misused the term, thank you for pointing that out.
So the correct statement would be something like,
Obtain the lowest possible practical velocity and clean flow lines into the impeller.
PUMPDESIGNER
RE: Minimum straight length requirements before suction nozzle
thread407-106384
thread378-78272
thread407-129352
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RE: Minimum straight length requirements before suction nozzle
RE: Minimum straight length requirements before suction nozzle
What is the maximum Suction Specific Speed for Hydrocarbons on a Vertical Canned 4 Stage Pump-Suction Nozzle 30" in diameter."
You should start a new post for this - not add it to this posting.
Naresuan University
Phitsanulok
Thailand