Pump Discharge - Centerline or Tangential?
Pump Discharge - Centerline or Tangential?
(OP)
From a pump user's standpoint, which is preferred and why? Most chemical (read: ANSI, ISO) pumps are of centerline discharge design (the discharge flange center is in the same vertical plane as the pump's centerline). The theory, or at I've been told, is that it allows for:
(1) self-venting or free passage of air
(2) pipe designers prefer working with centerlines
(3) equally distribute pipe weight to both feet
However, you lose efficiency by diverting the flow away from its desired (tangential) path. Most small and general industrial pumps are tangential.
Any other considerations?
(1) self-venting or free passage of air
(2) pipe designers prefer working with centerlines
(3) equally distribute pipe weight to both feet
However, you lose efficiency by diverting the flow away from its desired (tangential) path. Most small and general industrial pumps are tangential.
Any other considerations?
RE: Pump Discharge - Centerline or Tangential?
Loss of efficiency is minor, and actually there may be no loss of efficiency in many cases.
Speaking only about scroll pump with no diffuser:
The fluid in the scroll just before the discharge opening connection, is undergoing an energy transformation (with associated losses) from velocity energy to diffusion energy, in other words, high velocity fluid coming off the impeller is being slowed down. The tangenital discharge actually allows this slow down to occur in the pipe manifold, so there is some loss of efficiency in the pipe system near the pump.
On a centerline pump the fluid is slowed down more inside the pump than a tangenital discharge, as the fluid turns the corners to get into the centerline discharge connection.
One quick way to look at it is this, the fluid has to be slowed down. The tangenital discharge "cheats" by moving that energy conversion process (and associated losses) out of the pump into the discharge manifolding.
Therefore, when considering the pump AND the discharge manifold, there may be no difference at all in efficiency between centerline and tangenital.
PUMPDESIGNER
RE: Pump Discharge - Centerline or Tangential?
The directional flow changes, as that required for centerline discharge consumes energy (therefore a loss). Any benefit of this loss as it relates to the diffusion process, I'm not sure. Is this what your saying?
I tried to evaluate the difference while referring to Stepanoff and Kurassik, but all of the designs they show are tangential. Centerline is predominantly a "recent" thing, evolved from ANSI B73.1m requirements. And as far as I know, mainly a need to freely vent air w/o other means.
RE: Pump Discharge - Centerline or Tangential?
"Any benefit of this loss as it relates to the diffusion process, I'm not sure. Is this what your saying?"
Yes, that is what I am saying.
Do you think that the results will vary depending on pipe size and high flow versus low flow? In other words, will losses in high flow conditions be transferred from the pump into the pipe manifold on tangenital pumps? I have seen many high flow tangenitals where the pipe is immediately blown up after the pump, i.e. 6" discharge connection on pump but 8"-12" discharge pipe is connected to the pump.
PUMPDESIGNER
RE: Pump Discharge - Centerline or Tangential?
For a given flowrate (say 1000 gpm), the average velocities would be the same at the plane of either pump's discharge (assuming same discharge size). If an 8" pipe is mated to the pumps 6" discharge, the losses upon exit would (?) be a sudden enlargement and with the same velocities the losses would be (v1-v2)^2/2g - at least theoritically for either pump.
I must admit I'm a bit lost on this one. I must not be thinking on the same level.
RE: Pump Discharge - Centerline or Tangential?
The cut-water / commencement of the discharge from the volute of either a tangential or a top centre line pumps would be the same assuming the same impeller /volute design, the only difference is the tang. discharge will have a straight expanding discharge branch as compared to the top centre-line which will have a curved expanding discharge branch. As I see it, but not being a theoretical pump designer I could well be far off the point, any difference in energy recovery (if any) would be inmeasurable for anything other than a purely academic study.
For me as a practical pump guy, give me the top centre-line discharge pump everytime in preference to a tangential discharge pump. Less air entrapment,better distribution of pipe loads (which should be avoided anyway) easier to lay out and pipe upto etc etc.
International College
Naresuan University
Phitsanulok
Thailand
RE: Pump Discharge - Centerline or Tangential?
Yep, we are talking small amounts, I think HomeMadeSin might agree with that also, but he will have to make that statement for I do not speak for him.
HomeMadeSin,
I am thinking out loud too, although I know that both of us could sit down and do a serious analyses I for one am not motivated to do so at this time, not interesting, probably because of what Artisi said.
Continuing discussion,
On tangenital pump with 8" pipe connected to the 6" pump discharge, fluid will leave the pump at high velocity and slow down in the 8" pipe, thus transferring the energy losses into the pipe system (losses occur when energy is transformed from velocity energy into head energy).
If I exaggerate the situation to clarify, if we have an open discharge with no pipe connected, the fluid leaves the pump at very high velocity but much energy is lost the moment the fluid clears the discharge of the pump.
Do you agree with some or all of what I just said?
PUMPDESIGNER
RE: Pump Discharge - Centerline or Tangential?
RE: Pump Discharge - Centerline or Tangential?
Keep them coming.
International College
Naresuan University
Phitsanulok
Thailand
RE: Pump Discharge - Centerline or Tangential?
RE: Pump Discharge - Centerline or Tangential?
A concentric volute can still be either tangential or centerline discharge, and is a different beast altogether.
And while a concentric volute does reduce the radial load, the efficiency difference is much more pronounced than 1%.
RE: Pump Discharge - Centerline or Tangential?
You are headed in the right direction with the radial loading concept.
I do not know an easy way to numerically state the issue exactly because there are too many variables including fluid mass, manufacturing differences, and the affect is heavily dependent upon the flow rate (above or below BEP).
The fluid travels further and turns a corner in a centerline discharge vs. tangenital pump,
Therefore, under some conditions at high flow rates around or above BEP, more fluid energy will be transformed from velocity into diffusion energy, resulting in a more stable environment around the impeller not unlike what a diffuser accomplishes.
PUMPDESIGNER
RE: Pump Discharge - Centerline or Tangential?
However, it seems this is not the subject at all. The location of the outlet nozzle is. And I'm sorry for not understanding the query and my straying away from the original topic.
RE: Pump Discharge - Centerline or Tangential?
Using CFD analysis that includes part of the discharge piping, the exit flow profile from the pump casing can be optimized for either a tangential or centerline discharge nozzle.
RE: Pump Discharge - Centerline or Tangential?
I liked your analyses.
I agree that CFD will change things, but CFD will not change the existing pumps quickly due to cost.
It was a discussion in theory I suppose.
PUMPDESIGNER
RE: Pump Discharge - Centerline or Tangential?
However, just for the sake of exactitude...
Sam Yedidiah in his Centrifugal Pumps User's Guidebook: Problems and Solutions (Chapman and Hall), Chapter 27 titled "Modifying the Casing Geometry", when referring to remachining the part of the volute adjacent to the volute tongue to make part of the casing concentric with the impeller, clearly says, I quote: "Depending on the thickness of the casing walls, this procedure usually allows us to make about 5% to 20% of the volute perfectly concentric with the impeller axis. Such a change usually produces a significant reduction in the radial loads. While such a change in the volute usually reduces the efficiency, this reduction is rarely more than 0.25% to 1.5%. Unquote. Boldfacing is mine.
To PUMPDESIGNER, thanks for your comprehensive comments. The same book by Yedidiah in Chapter 10, titled "Axial and Radial Thrust and Balancing", considers ways to reduce radial thrust and even presents a graph of thrust as a function of flow rate, and a formula to estimate the (maximum) thrust at shut-off.
RE: Pump Discharge - Centerline or Tangential?
RE: Pump Discharge - Centerline or Tangential?
1) Only the true concentric is freely self-venting.
2) Pipework designers/pipefitters/and mechanical fitters will always prefer to work on centre discharge systems due to ease of design and installation.
3) Never allow pipe systems to be supported by any end suction pump.
In the majority of cases any variation in efficiency or thrust forces etc between similarly sized tangential and centre discharge pumps is merely nominal. As far as I am concerned apart from our aged concentric pumps I have no design preference either way.
RE: Pump Discharge - Centerline or Tangential?
Good, very good comments.
I would give my left arm for any long term data you have maintained.
Perhaps you do realize how hard it is for many of us to obtain such data, almost impossible.
Seldom does anyone publish such things, perhaps they think it not valuable, but it is.
PUMPDESIGNER
RE: Pump Discharge - Centerline or Tangential?
RE: Pump Discharge - Centerline or Tangential?
Ahhh,the wonderful art of engineering,
Applying theory in novel situations,
Satisfying to the soul.
While I strive constantly to rid myself of some of the labor (and subsequent satisfaction) by systemetizing the knowledge to the point where I do not have to work so hard.
Good when possible I suppose.
PUMPDESIGNER
RE: Pump Discharge - Centerline or Tangential?
Using quark's idea on adding post-scripts:
Believe it or not the human body has 656 muscles. A smile puts in commission 17, and a frown 43.