Hi all ,

I have got one question .There are three pumps two working and one stand by. the nozzle loads of the standby pump is above allowable in cold condition, what to do??? Is the mechanical strength of the nozzle in cold condition high then the hot case and what is the limit of those reactions (in cold condition), how to find out. please help me out.

Dear Techy06,
The forces acting on the nozzle shouldn't exceed the allowable in any condition, either hot or cold. The allowable values for API 610 pumps can be obtained from the code itself where as most of the other pumps you might have to contact the pump vendors to get the allowable.

In you're case I would suggest that you 1st check the forces on the nozzle in sustained condition. this is to make sure that you're initial supports are good enough to cater for the dead loads and there are no "see-saw" effects. if loads are OK, then you're piping routing maybe doesn't have enough flexibility and the hot portion of the system is effecting the cold portion of the system. so, you have to either re-route the system or try to isolate it by using restraints.

try attaching a picture of you're system in this thread. that'll make us beter understand you're predicament.

just my 2 cents.

Most of the time, when we have multiple items of rotating equipment, getting the stresses and loads within compliance during the run/standby cases is among the most difficult things we do. This is because we usually don't put enough space between the equipment items to allow us to put in a lot of flexibility between the header and each individual piece of equipment.

A usual "fix" is to start backing the headers away from the equipment until the individual connections become sufficiently long that you can design in enough flexibility to reduce your loads. Your rotating equipment guys will love you, but the process people responsible for head loss calculations will probably put a price on your head.

Hi techy,

It is indicated in the contract if your allowable loads during standby will be increase or same as it is running(I'd seen contract increase there allowable to 150% when shafts are not rotating). You can also verify your friction requirements. Other than this as mention above, add flexibilty but watch-out for process requirements.

Regards!

An increased allowable when the pump is not rotating is rather odd. What will happen when the pump begins to rotate and may be significantly overloaded. Would this not risk damage to internals or the coupling ?

A further problem with larger allowed loads when a pump is not operational is that whatever effect caused those loads may make removal or reinstallation of the pump impossible.

Dear,

I'd like to share my experience that if it's over allowable nozzle load but it's lower 2 times. we can apply the appendix f. for evaluated the actual nozzle load.
But if it's over 2 time of nozzle load. We have to modify routing and supports in order to balancing the expansion value of piping..

TH Engineer

I have seen some EPC companies that I have worked for, specify 50% higher allowable for standby pumps , even though nobody , either from stress or from equipment could give me a logical reason for the same. It eventually boils down to your stress and equipment specification and how much your equipment group and vendor will oblige you.

Regards

The reason for the extra allowable on non-running pumps is because those allowables are based on shaft deflection, they are not reflective of stress limits of the equipment. Over time, high shaft deflections during operation can lead to premature seal failure and thus higher maintenance cycles. So, don't go crazy with the loads, but don't waste time and pipe for a pump in the standby condition. You're going to have enough trouble finding a layout that works with the pump running and that layout may make it prohibitive to meet allowables at idle.

I couldn't agree with you less Ed, and that's saying something since you're a practical fellow just like me. Suppose we have three pumps; two running and one idle. Now suppose we have an overstress [I mean overlaod] on the one idle pump, and we hand-wave it away as you suggest.

What, pray tell, should we do when the plant is running at reduced capacity and we have one pump running and two pumps idle? Note that the load on any given pump nozzle in a group of parallel connected pumps is governed by two, usually conflicting, phenomena. (1) The headers are expanding from their last horizontal restraints or anchors that, in general, create a horizontal load set where the sum of the loads on the pumps and on the last restraint are zero. (2) The differential expansion of the hot and cold drops to the pumps is creating localized loads on the nozzles that, in general, sum to zero.

If the drops to the pumps are sufficiently stiff, then (2) can dominate over (1). This will make it very, very difficult to cope with the run/stop cases. A good design, however, has the drops to the pumps much less stiff than the header, so that (1) dominates over (2). When you reach this state of nirvana, your run/stop cases become much easier to manage.

One practical fix for this for a three pump system is to have the piping for the last pump on the header to have an expansion loop in both the suction and discharge drops. [This drives process engineers crazy (another reason for doing it!) because the flow is no longer "balanced" among the group of pumps. Sure. Right. Every pump that comes off the assembly line has EXACTLY the same head-flow characteristic, and every spool of pipe and every fitting has the exact same bore diameter.] But if you make the connections to the third pump flexible enough, the header can now squirm enough to balance the loads on the first two pumps without messing up the loads on the third pump.

With more than three pumps, it's even easier since you can group them in rows of pairs and triplets, with a main header and sub-headers. Again, this will drive your process engineers crazy, but it's the right thing to do.

Well said CraigB.

I came across a hook-up of five large vacuum distillation bottoms pumps, the calcs for which were accepted on the 150% cold load basis. One pump was removed for maintenance and could not be re-installed becuase the pipe had moved too far to pull in. The unit had to be shut down to get the job done. Not clever.

In this context I have have a question referring to earthquake.
What about the allowable loads of a pumpnozzle with earthquake loads.

I have calculated a three pump system and the loads in all my loadcases are o.k.

Only with earthquake-loads one force or one moment at each nozzle is max. 35% bigger then the allowable loads.

I think that this is not critical for the nozzles but I am interested in the opinion of the stress community.

Regards
___________________

I am relatively new to this field.
And maintenance of pump specially in suction is not a big problem in my analysis since as per observation in my last project(Company standard) different type of strainer installation are selected for each type of pump assembly.. Saying for example a temporary strainer is only for one line pump and Y-type, bucket and etc are for multiple pump.

I think this problem is covered by Piping Designer. IMO when you remove the whole assembly of strainer then you just have to shut down the line or install maintenance support also i would like to point-out on how would you remove your bolting when you have those reactions....

Regards!

CraigB, I think you misunderstand me. First of all, this is not a stress issue. It's purely an allowable load issue. I'm not suggesting at all that the idle pump line be allowed to overstress and "hand wave" it away. As you well know, it tends to be difficult to approach allowable stresses for the piping system when you are limited by the pump nozzle allowables.

My point is that pump allowables are not based on the stress capabilities of the material of the pump, but on limiting distortion of the pump case and deflection of the pump shaft. Long term exposure to excessive load on a running pump leads to premature seal failure.

Nor am I saying that you just ignore the allowables when looking at pump loads for the pump idle scenarios. Quite frankly, it's a rare thing that I run into the need to consider going over for an idle pump. More often I find that the idle pump ends up with lower loads than when it is running.

My point is that, particularly when you have three or more pumps, and you been tearing your hair out for days working on a routing that satisfies all the running pumps - and you are still a bit over on a component or two for a pump in an idle case - that it's time to stop. Too much flexibility creates issue of it's own.

Regarding MoverZ - I think your example is a bit of a red herring. Particularly with a decent sized API pump, you can still have quite a bit of load on the idle pump nozzle at the allowable limits. As such, you are still likely to need a come-along to get the pipe back into alignment if you pull the pump away and it's no longer is place to hold the load. Unless you want to set a criteria that the loads on the idle pump are minimized while the other pumps are running (and make all us stress guys go bald in the process ) so that they can be easily taken and out and reinstalled, I don't see that changing.

Ed,

First, I apologize for using the word overstess (I meant to say overload) in the first paragraph of my post.

Second, I have no reason to disregard your judgment in specific cases; in fact I would tend to trust it based on your many posts here.

Third, I would expect that you, personally, seldom see cases where pump overloads exist in the run/stop cases. I would expect that because you are a veteran hand at this. You doubtless build in what your experience tells you is sufficient flexibility for multiple pump or compressor systems from day 1, and so you avoid most of your later problems.

Fourth, I agree 100% that when rotating equipment allowable loads are the issue, they are MUCH more restrictive than the Code stresses.

But you have to remember, probably 90% of the readers of this forum are young, and don't have the kind of experience that you and I have. I don't think it's prudent to tell them that things like this can be hand-waved away.

Run/stop loads for rotating equipment are some of the most important and difficult things that we do, and to try to give new hands the idea that it is otherwise is very misleading. I have seldom been in an operating powerplant or refinery where they didn't have a pump somewhere having new bearings or seals installed, most likely because some analyst years ago didn't think the run/stop load cases were worth worrying about.

Many of the old hands here, especially including me, are stunned that so many young people are trying to learn this discipline on their own. I estimate that fewer than 5% of those who do not have a senior engineer readily available as a mentor will be in this business five years from now, either out of frustration or as a consequence of their having overlooked something that causes a major problem. I, for one, regard my role here as trying to guide the newbies through the minefield of things not explicitly covered by the piping Codes.

My understanding is like this:

API 610 Nozzle loads are set based on the following two parameters:

1) Casing distortion.

2) Shaft deflection.

It can be expected ( although I feel that always need not be)that casing deflection and shaft deflection will be more when the running pump interacts with the piping loads than when the piping loads are imposed on idle pumps. But how to quantify? Hence , I feel that just because a pump is idle , higher loads can be imposed on them ( that too by a specified %) is not justified.

Regards

Regards

Just want to share something. In my previous case which affected by soil settlement especially at the dummy support which overloaded the pumps, people at site operation told me that they were often to repair pumps shaft-bearing due to the miss-alignment.

Fair enough, Craig. As I look a back on my first posting in the thread, I can see that I probably should have been more explicit in my caveats. I, too, am quite concerned by the trends that we see just here on the forums of "kids" being thrown a computer and Caesar and told to "run the stress" of a piping system.

The thing that I try to bring across most to green stress analysts that come one board is that, Caesar doesn't do the work for you - it's just provides data to help you make a judgment. I've seen it happen a number of times where people get caught up in do loops running Caesar over and over trying to hit some magic numbers without really understanding what the numbers even mean. Addressing that was my intention.

I would like to thank both Craig and Ed for the insightful discussion.

Old people rock !!!

This is also our big problem in our current project, same situation with Techy06. It is for cooling water pump rated at 150lb. Finally my Senior Stress Engineer give up and decided to use tied expansion joint on each suction side instead we are restricted just because our materials for piping were already purchased. I talked to one equipment engineer here, he said there is a better idea. According to him, he would try to choose pump with higher rating with higher allowed loads instead of using expansion joint which the is much expensive.
I would try to convinced with my Lead Stress Engineer but I need also your opinion guys before him.

Regards,
Machoguy
"Do not postpone until tomorrow what you can do today"

Hi Machoguy

As far Api 610 is concern nozzle allowable has no relation with pressure rating like static equipment. Only different specific vendor may agree with higher allowable ( vendor specific).

Just to add one point in this discussion topic.
people consider different temperature profile for idle pump.

1)header to isolation valve average of (ambient + line temperature), since flow is stagnant then valve to pump nozzle ambient temperature.

2) header to isolation valve line temperature. valve to pump nozzle ambient temperature.

Forum member can put some light on the issue.

Regards

Habib