Scenario: Top-Top hot pump with 36" suction line and 28" pump nozzle. Vertical section of the line is 5-6 m due to layout guidelines and then taking a horizontal turn. Obviously a spring is required at the top elbow to relieve the pump nozzle of the vertical loads.
Now the question is, Will the weight of the liquid in the vertical portion of the line be considered to be supported by the spring or by the pump impeller?
If yes, then we have to change the liquid density to zero for the vertical portion, otherwise the spring selection will be erroneous.

Please read...if supported by the impeller then..

Yes fluid wt. should not be considered in that portion otherwise it will over-design your spring.

Now why the max. load will go to the pump support can be explained froma simple analogy. Imagine the pump support as a spring. This spring will be parallel with your elbow spring. Since in case of parallel springs the load distribution will be as per relative stiffnesses, the pump spring being much stiff in comparision to the pipe spring will share the max. load.

However ignoring the fuid wt. matters when the pipe size is big enough ( as is your case). Othwise the effect is very small.

Most consultancies follow this procedure.

Anindya Bhattacharya

Only liquid over pump nozzle will be supported by "impeller" ie. 28". The difference acts on the walls of the reducer and onto the pump nozzle.

Even so, my first attempt would be to size the spring to give zero load on the flange in the installed case in order to give the installer something to check against. ie. With the flange bolts loosened, does the pipe float above the pump. Better to have a real load that is correct rather than than a lower theoretical load which is wrong. Although Caesar calculates loads to the Newton, the real loads are nowhere near that accuracy. The pipe tolerance will be up to 10% for a start. (Maybe not so high in this case if the pipe from plate). I always feel sorry for the apprentice spring manufacturer stamping 19347N on a spring casing rather than 20kN. He's probably spent the last hour trying to find a #7 stamp.
ps. The installed case I referred to above is the WNC Weight No Contents case

In some pumps, specially for big dia. top suction / discharge pumps using heavy springs , there is a tendency for the pumps to lift up
( sensor readings reported at site ).
Also we should not forget the WNC case when the line is empty but spring is unlocked ( starting of pump , maintenance etc. ), in that case an over designed spring will cause a lot of upward lifting force on the pump nozzle.
In our case the reducers are located at the top elbow approx. 4-5 m vertically up from the pump nozzle. This is quite normal as most vendors require straight spool pieces of pipe before connection to the pump nozzle.

Rajesh,

Substituting "common practice" with "common sense" is a risky proposition. Unless you're the principal stress engineer at Technip with authority to depart from common practice, to omit the water weight in the spring support (i.e., common sense to you) may get you into real troubles.

Years ago working at Lummus I asked my boss, Rex Evans, about this same question and he told me a story: During the start-up of a plant, the support beam of a large pump had been found bent and the millwright couldn't get the pump shaft aligned, or vice versa. Also found out was that the pipe spring had ignored the water weight. That stress analyst was later fired. Any surprise?

Remember, this pump spring question embodies an age-old dilemma. Every stress engineer has one time of another pondered over it. Eventually all seem to have come to terms with what the common practice stands.

Again, UNLESS you're the principal stress engineer at Technip, my advice to you is to abide by the common practice--let the spring bear the water weight, however unwise in your opinion.

Be also aware that CAESAR II program applications and its seminar teaching adhere to common practices. You'll find an example on the use of pump spring in Tutorial A, Chapter 8, Application Guide. The spring is sized to carry the water weight.

By the way do you know Rex Evans? Umpteen years ago he broght the first copy of CAESAR II program to Malaysis, plus many other countries. He had a stroke on October 28, and now is recuperating. If you're among his friends, his children ask your prayer for a speedy recovery.

That is why, as a first choice, I would size the spring to put zero dead weight on the flange in the installed condition. It allows an easy check by the installer. If the flange bolts are loosened and the pipe floats rather than being launched skyward, then spring carries the real load. If not, the installer should ask if he can reset the spring (under supervision) until the pipe does float on the nozzle. This might not be the lowest operating load that can be achieved in theory but it is the only one that can be easily guarateed to be 100% accurate and to not affect shaft deflection. (A load other than zero installed load can be achieved by adjusting for zero installed nozzle load and THEN adding / subtracting the required difference).

When the flange bolts are retightened the pump casing should not be carrying any unplanned load and shaft algnment should be as near that of the bare pump as is practicable.

For very heavy loads and those using constant effor springs, the only practical method is hire a load cell, weigh the pipe at the hanger position as-installed and THEN factory set the spring. Pipe stress calcs are probably subject to more tolerance than the pipe weight.

Does somebody have a comparison between installed pipe support loads and calculated pipe support loads in a real pipe system ?

The alternative is for the installer to tighten the hanger rod until the spring indicates the theoretical reading. Bending something or failing to achieve shaft alignment or both is virtually guaranteed.

Darren,

I thought we had established from the responses in this thread that it was common practice to no include the fluid weight in the segment above the nozzle. I wouldn't call it common sense as I think it is something just about every stress analyst ends up being taught the first time they have a pump system checked by a senior analyst.

As far as floating the flanges for install, that's fine as long as the installers don't pull the travel stops while they're doing it. The travel stops should not be released until the pump flanges are bolted up.

As I said above, the floating check should be with the flange bolts loosened. The pin must be out of the spring otherwise there is not much to check. I would only suggest loosening the bolts by a couple turns max otherwise there is a strong chance of actually being launched into orbit if somebody got his calcs seriously wrong. Probably better load the spring with the flange bolts backed off a couple of turns in the first place to maintain a feel of load as the spring pot is being loaded.

We have a CAESAR II user out in Colorado who works with turbine systems. He makes a point to run both a hot load design and a cold load design on his springs, iterating until he can get the same set of springs to suit the hot load and the true structural load at installation. Ideally, that flange face floats free with no net deflection or rotation. This way he can unlock the springs at installation and bolt up the flanges. Provided there isn't much pulling to align the flanges, there is no unaccounted for load introduced in the piping system due to bolt up. The disadvantage to this approach is that, at startup, all springs must be reset to their designed hot load. The advantage is that no unintended pulling loads sneak in the system.

Klein and David,

Glad you're stepping into this hornet's nest.

Should the spring, supporting the riser directly on top of a pump nozzle, INCLUDE or EXCLUDE the water weight projecting through the ID of the nozzle? That is the question, isn't it?

I'm saying that, it makes a lot of sense (i.e., common sense) not to include, but common practice does include! Why? Well, to be sure the latter option will put the pump/pipe flange joint under a tensiion force, but the pump assembly as a whole is rid of the hefty piping water weight. Remember, the focus of nozzle allowables is mostly on the alignment problem between pump shaft and of its driver as it is running. Upsupported water weight can load the pump shaft out of it.

Which concern to us stress engineers is more concerned? A pump/pipe flange pair under tension or a potential misaligment of the pump which we are supposed to dearly protect. Asides, why are we so afraid of a pulling pump flange pair DURING OPERATION? Yes, we need to line them up good during the mechanical completion check, as half of the discussions here indicate. The extra water weight through the spring WON'T leak the flange joint--whatever the hydrostatic pressure head resulting from the water height is already accounted for in the flange class rating. But misalignment of a SPEEDING pump shaft is a complete different story, as told above.

Actually, Klein, you're getting me curious if your opinion reflects S&B's position. I had worked as Lummus and M.W. Kellogg's principal engineer for many years, eon ago though. No, I am not representing the current position of either company (although neither is using the name anymore, what a shame). But I do sometimes teach CAESAR II seminars under David Diehl and Bill Evans' purview. Believe me, I love to know the outcome of this open discourse. Is there any principal stress engineer from E&C company out there listening? Weigh in!

The basic principal is that the weight of water in the vertical portion will not be carried by the spring but supported by the pump impeller/casing.
I don't know how to call it..."comon sense " or
" common practice " because it's governed by the laws of physics.
For smaller or even medium size pipes it's not a big botheration because the difference in spring settings is not much. But for cases such as 36" pipe the difference we are talking about is 15KN which is significant.
Also by not taking the weight of water into consideration we are safeguarding our WNC case wherin just before the start up the line is empty but the spring locks are removed.For big bore piping it could lead to large upward thrust effecting the pump alignment. Could be I guess...!

I can not fault most of the discussions above (there are some very good points here that are often overlooked), but no one has yet to answer the original question which is found in both common practise and common sense.

Should the weight of the pipe contents in the vertical section above a pump be included in the nozzle and spring load calculation – YES

Yes the physics of the problem are such that the weight of the pipe contents in the vertical section above a pump will NOT be carried though the pump flange. However this load is still applied to the pump casing and must be considered in the allowable loads.

As discussed above the acceptance criteria for pump nozzle loads are based on alignment of the rotating equipment. It does not matter how the load is transmitted into the casing (by the pipe flange or by the fluid weight) the resulting effect on machine alignment will be the same.

I would also strongly agree that all cases must be considered when designing the system; I have seen the effects of large springs bending empty pipes. The problem you have is a particularly difficult one. I have no direct experience with such a system but would suggest the following approach may be a solution. Any comments out there gratefully appreciated.

1/ design a spring with no contents; This spring can have the stops removed and the piping checked with the pump flange bolting loose. This will ensure correct piping installation as suggested by Andrew above.

2/ then do not change the setting of this spring, but add another spring to carry the weight of the pipe contents and optimise the nozzle loads as much as possible. In the field once the piping is checked for alignment with the first spring and the pump flange made up then this second spring should be installed. You must then check all cases for loads on the pump.

3/ any out of the ordinary installation as I have suggested here must be fully documented to ensure your procedure is used for the life of the pump.

Nigel,
Thanks for your input. But correct me if I am wrong :
Suppose weight of pipe = 25 KN
Weight of fluid = 15 KN
Are we assuming that by providing a spring of 40KN capacity we shall be able to carry the weight of fluid content by the spring?
My understanding is that since the casing is bolted to the pump support pedestals however high we set the spring the load will still go to the bottom of the casing/impller.If the casing was a floating body then it was possible to avoid weight of fluid acting at the casing.
Also for these big size pumps having requiremnt of a 4-5M vertical straight spool piece above the nozzle I believe vendors are aware of the weight of liquid column acting on the casing/impeller.
What we are planning to do is to design and set the spring in such a way ( with no weight content in the vertical portion ) that even with weight content the nozzle loads are within allowable.

Question was: Should the weight of the fluid in the vertical section above a pump be included in the nozzle and spring load calculation ?

Answer is: NO

Fluid weight of vertical column acting on pump casing shall be added to the foundation loading and has no impact on nozzle loads. Therefore, no need to consider it for checking nozzle allowable loads. If the load is substantial(as a percentage of self weight of the pump), then foundation design should take that into account.

A further note to stir up the hornets nests further….

Note that generally the pump distortion is governed by the base frame not the pump casing; therefore one should include the weight of the vertical column of water as this is a variable load that will cause misalignment of the pump once the system is filled with water. Generally the stresses within the pump casing are not of concern.

However in extreme cases such as this one, with very large nozzles and loads that could be transmitted thought the pump casing, the manufacture must be involved in clarification of the nozzle load acceptance criteria. I would suggest that quoting both the nozzle flange loads and pump casing support loads would give the pump manufacturer the information they need.

I would be very interested in a pump design/manufacturers opinion on this issue.

The final say on what loads you should include will ultimately rest with your pump manufacturer.

I agree with the suggestion from Somnathbasu. The weight of fluid in question is predominantly in the vertical direction over the pump casing. The structural engineer designing the pump foundation should be aware of this load and he/she should add this load to the pump foundation design. Pump manufacturer should also be aware of this unbalanced vertical load which will be basically borne by the casing.Shaft alignment is sensitive to externally imposed unbalanced piping moments.As long as the total live weight of pump and this extra weight from the fluid column is accounted for in the pump frame design there should not be any concern if this weight is not included in the design of spring hanger.It should also be noted that seemingly significant load should not be that significant after all when we are talking about a large size pump with a 36" suction piping!

In this scenario suppose we have a case where there are more than one pumps connected . In case of discharge if there is a non return valve mounted on the pump nozzle and we are checking the pump which is idle( not the one which is running). Here the load of fluid column will get transmitted to pump nozzle flange of idle pump. Since the pump is idle and the spring is designed without taking into account the fluid load do we have to plan a support which will share this load of fluid ? Since the vertical leg will have less expansion as the pump is idle this support can be a +Y so that it takes load in this idle case .Any clues ?
Regards,
Sunil