RE: [PipingDesign] pump selection, Allowable nozzle loads

From: <>
Date: Tue Jan 21 2003 - 20:42:00 EST

Well said, Sir,
I will add something to it.

  1. On what basis API , NEMA, and other codes decide the allowable nozzle loads ? They have specified allowable loads irrespective of rating, material of construction etc. Is it convincing that 150# and 600# nozzles, with different MOC and temperature have the same allowable loads !! Many times we ask vendor for higher allowable and they easily agree to give 2 times and sometimes 3 times the API allowable loads. In case of critical equipment like fired heater (API 560), manufactures allow upto 9-10 times the API allowable loads. What does this indicate ?
  2. What about allowable nozzle loads for occasional cases. B 31.3 has increased allowable stress value for occasional cases , but both piping codes and equipment standards are silent about allowable nozzle loads.

I know the common answer to above questions is we have to follow the rules and regulations set by the code. In my opinions, these are the reasons why we end up with what we call over-designing and over-consumption of manhours. API nozzle loads should be the minimum requirement for the equipment designing and should not be imposed as the governing loads for piping design. Equipment manufacturer should give their own actual allowable loads based on their FEM after keeping suitable factor of safety.


Gang Cvg <> on 01/21/2003 12:58:35 PM

Please respond to

cc: (bcc: TUSHAR K RAJYAGURU/L&T-CHIYODA) Subject: RE: [PipingDesign] pump selection

Dear piping design engineers,
I think, it is the right time to remind all the fraternity, that the rotating machines (pumps and compressors) form the most severe group of equipments which causes serious concerns to the stress engineer. Often, the cases are such that, the piping systems have heavy piping, on account of the size, as well as, wall thickness. The reason could be anything attributed by the process designer, may be low fluid velicity, less of pressure drop loss, long straight runs, no pockets, etc. etc, or whatever it may be. The system on analysis will come out with some (reasonably handsome) values of forces and moments at the terminal nozzles. One often ends up with a very small sized nozzle on the equipment side.(They have their own reasons for that). API or NEMA stipulates the limits on the allowable limit of the imposed loads, which has some relation with the size of the nozzle only. It is also seen that the same size nozzle with low as well as high pressure class rating declares the same value for forces and moments. Though it may look ridiculous, this is often, the rule of the game. As Ralph rigthly puts it, this information comes at a very late stage in the project too; by the time the tress engineer must have finished his part of the stress analysis study. Now the ball is in his court! He has to sit late nights to solve the puzzle and bring the results to some acceptable levels. The Project team does not appear to see these aspects. Well, this is the stark reality. So folks, be prepared for this situation. The caution is that, it may consume a good amount of engineering man-hours, much much more than originally budgeted for and much to the displeasure of the schedule man.
Good Luck!!!
C. V. Gangadharan
--- "SARE, RALPH H." <> wrote:
> Hi animesh_singh.
> in most of the projects that ive been involved, I
> have always defer to the
> judegement of the rotating equipment engineer when
> it comes to the
> selection/installation/commissioning of pumps .
> During the selection
> process, the pump nozzle orientation is not much of
> a big deal to the
> rotating equip engineer since he's more concern of
> ensuring that the pump
> type/performance and the process system
> characteristic matches, its suitable
> for service and that the LCC is acceptable.
> This has always presented a problem to the piping
> designer like you since
> chances are that on several occasions the
> suction/discharge piping has
> laready been laidout but the selection of pump has
> not been finalized
> (information you get are mostly limited to dimension
> without the allowable
> loads). The problem is compounded with working with
> a mirror-image pump and
> a spare. In this scenario, the piping designer has
> to consider all possible
> spare pump operating combination. one such example
> is the behaviour of the
> piping when the piping is attached to a spare pump
> that is cold while the
> rest of the system is hot (common arrangement for
> critical equipment).
> Sure, the type of pump (which then determines the
> nozzle orientation)-
> whether its vertical in-line, horizontal pumps with
> end suction and top
> disch nozzles or with side suction/ side disch has
> significant effect on the
> nozzle loadings going by API 610. And in majority of
> cases, the
> manufacturers allowable loads are incomprehensibly
> low. Best approach is to
> discuss the issue with the rotating equipment
> engineer and raise your
> concern.
> One execellent source that may help present your
> case better on this very
> subject is Chapter 23 Review of Mechanical
> Structures and Piping for
> Machinery from the book Machinery reliability
> Assessment 2nd ed by Heinz
> Bloch.
> Good luck. Ralph
> -----Original Message-----
> From:
> []
> my querry is to you all
> who is responsible for the selection of pump nozzle
> orientations, and what
> are the criteria for its design input.
> I had raised this query to many experts but till
> date no body is able to
> reply me back, here I am putting in this forum ..and
> really expect comments
> from you experinced people.
> rgds

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