Fitting distance from Pump
Fitting distance from Pump
(OP)
I read from a pump manual that we should be avoid any fittings, including bends, compensators, valves in the area of 5 X DN suction and 5 X DN discharge.
I guess if before suction, the fitting will make the flow less smooth and can make cavitation. But what is the purpose of this 5 x DN?
Thanks alot,
MW
I guess if before suction, the fitting will make the flow less smooth and can make cavitation. But what is the purpose of this 5 x DN?
Thanks alot,
MW





RE: Fitting distance from Pump
Any good hydraulic book will give you information on this subject.
Naresuan University
Phitsanulok
Thailand
RE: Fitting distance from Pump
PUMPDESIGNER
RE: Fitting distance from Pump
http://www.irrigationcraft.com/flow_run.htm
PUMPDESIGNER
RE: Fitting distance from Pump
RE: Fitting distance from Pump
If you have bad inlet conditions, this alters the actual fan/pump curve. So the whole thing can be de-rated by possibly up to 30%. Thats why you should have a straight inlet. We tend top say about 1.5D upwards but 5D obviously better.
The flow leaving the fan/pump is swirling and if a fitting is introduced too close, the swirl will create a greater fittings loss hence the reason for a straight outlet.
The efectes of distance between pump/fan and fittings is handled in articles covered by ASHRAE (I think its called system effect or something very similar) There is a calculation procedure to take into account location of fitting near equipment.
The effect is more pronounced on fans as they only generate small pressures to start with and any loss causes problems. The effect is also worse with axial or tube axial fans and much less with backward curved impellers.
On this basis, pumps having backward curves would probably be fairly tolerant to having a bend close to the inlet or outlet. (I'm talking about small/medium heating and chilled water pumps and not monster high pressure pumps which might not like being 'strangled' in this way)
Friar Tuck of Sherwood
RE: Fitting distance from Pump
High pressure occurs right prior to the blade passing the opening.
Low pressure occurs about when the mpeller blade has passed the discharge opening and the discharge opening is in the middle between two impeller blades.
These low and high pressure points will cause a discharge pressure wave pattern in the discharge piping.
The closer you have valves and other items are near the discharge of the pump the better these waves will be reflected and disturb the flow pattern of the liquid through the pump. You need distance between the pump and the first accessories such as valves etc so you get some mass of liquid that can somewhat attenuate this pressure wave.
5 x DN is a rule of thumb where things start to be OK. I have seen in the past that 10 x DN was recommended.
As you start discharge throttling down the flow throught a pump. The more you close your valve, the more your liquid gets trapped between pump discharge and discharge valve and the more the trapped liquid gets pressurized the more energy it contains, the better the pressure waves will start to resonate in this liquid, the more the pressure waves will start to influence the flow going through the pump. Finally flow will be disrupted and the pump will go in minimum flow.
The longer the distance between pump and valves, the better the waves will be attenuated and the longer it will take before the pump goes in min flow. (min flow will be lower). However the longer the distance, the bigger the mass of liquid trapped between pump and valve and the more energy the liquid in the discharge piping contains at min flow.....more energy....more damage at min flow.
Place valves etc as close to the pump as possible but respect the manufacturer's recommendations. Do a min flow test on the valve to determine where the min flow point is and make sure you do not get there. The pump manufacturer can help yoy with this.
If you do a min. flow test in the presence of the manufacturers rep you can even palace valves closer than the 5 x DN provided you ensure that the unit cannot operate at lower than min. flow.
Best Regards.
Scalleke
RE: Fitting distance from Pump
Scalleke
RE: Fitting distance from Pump
Here are some of the things to know:
If pump has a diffuser, then flow run may not be necessary.
Speaking specifically of pumps with Ns<6000, no diffuser, and discharge connection is close to the impeller, such as scroll type single and double volute pumps, they usually require some flow run.
Straight volute pumps (with no diffuser) have very high velocities at the discharge. Energy transfer from the pump shaft into the fluid is almost entirely by an increase in velocity, and then this high velocity energy must be converted into pressure energy by slowing the fluid down. Now the little known fact, this "slow down" or conversion of energy, from velocity to pressure, in many cases is accomplished after the fluid leaves the pump and enters the pipe system which is most often larger size than discharge nozzle, i.e. system resistance in the pipe system causes reduction in velocity. If you allow a fast turn just after pump discharge the high velocity fluid hits that ell or tee and can not only eat out that fitting, but also fluid may re-enter the discharge nozzle of the pump, called discharge recirculation and also discharge recirculation cavitation. Heck, even with a flow run you can have discharge recirculation at low flow rates.
A video for discharge recirculation at this link:
http://www.irrigationcraft.com/cavitation.htm
PUMPDESIGNER
RE: Fitting distance from Pump
RE: Fitting distance from Pump
RE: Fitting distance from Pump
Tired of architects assigning a closet for your stuff to go into? Have some fun, check out this link to a photograph our production manager made up:
http://www.irrigationcraft.com/download3.htm
PUMPDESIGNER
RE: Fitting distance from Pump
Seems like there is plenty of room left for expansion at some later date.
Naresuan University
Phitsanulok
Thailand
RE: Fitting distance from Pump
Complaints, complaints, we all got 'em.
So you are one of them witty guys too huh?
You would fit right in here. About five years ago that photograph appeared on my desk top while we were doing that project.
vanstoja, do you think your references supply enough information that a spreadsheet could be built to enable one to figure out intake and discharge piping minimums for scroll, center discharge, and turbine volutes? If so, is there a single source I could purchase those references from? I am in the middle of building a worksheet to calculate radial thrust (looking for more references on that too), but your references for decay rates look interesting for my next project possibly.
PUMPDESIGNER
RE: Fitting distance from Pump
Like you, I've been there and done that, it made life a little frustrating at times but interesting.
Nothing like an impossible problem to solve to keep you on your toes and prove the experts wrong.
Naresuan University
Phitsanulok
Thailand
RE: Fitting distance from Pump
All the references I have seen on swirlflow are for piping mostly with 90 degree bends and do not address pump casing types or design features. I think the people now using pump CFD analysis will have to develop the data for pump casing discharge distortion control. We got into it experimentally in the 1970's to save downstream swing check valves hammering their disk bushings and pivot pins to destruction due to swirling/distorted pump discharge flows. We used 5-hole pitot tubes to measure discharge pipe velocity profiles in manufacturers water test loops and in an airtest rig using sheet metal ducting where we went inside the wooden pump casing to find secondary backflows from diffuser vane channels nearest the exit nozzle. What is a "turbine volute"?
On centrifugal pump impeller radial hydraulic thrust, I have many of the classic papers since the 1960's and some very limited test data but I think you should open up a separate thread on that subject.
Loved your picture of a typical pump piping shoehorn job.It seems like the pump piping is always the last gasp of the space cadets and the consequences can be monumentally bad. I suffered through several years of grief helping to resolve one such operational crisis.
RE: Fitting distance from Pump
Will start a thread on radial thrust when I get the chance.
PUMPDESIGNER