Stresses in Piping systems

Hi all,

This is one of my favourite topics, since I had a large number of systems piping failures related to stress, mostly external.

One of the flange connections of the fire main systems on board a ship, couple of bolts used to break at the head frequently; more often if the ship had done some high-speed manoeuvres.  Initially, we used to change the bolt and live with it, but a nagging doubt came into our minds and we did a root cause analysis. What we finally found was that, the last pipe that fitted into the system was a little oversized and had been forced into its slot using a chain pulley, thus leading to a stressed piping section. Whenever high vibrations occurred, the stress used to increase, thus resulting in the flange securing bolts at one end shearing.

I came across a nice article on the topic. Please read and be enlightened on the topic.



Pipe Connections – Care in alignment


This one is from experience on board ships.

While serving on one of the ships, we had a recurring problem of bolts shearing on a fire main header connection to the ring main.

Root Cause Analysis

A large number of pipes in this section of the fire main were replaced with new ones during the last ship refit since inspection revealed extensive internal corrosion and wall thinning by way of bends. What all had happened till then?

The system is a little complicated since it crosses a few decks and watertight bulkheads through watertight glands. There were a few bends and joints in the way. Where, How, Why?


The new pipes were made using the existing ones as template or sample. A few of the older pipes found serviceable were also used. What, Where?

While fitting out the pipes onboard, with the combination of old and new elements, the pipe alignment had gone haywire (the magnitude could be in millimeters per instance though). The pipes were fitted as it is and the cumulative misalignment at the last flange to be connected must have been high.What, Where, How, Why, Who?

The last two flanges were brought together face to face using force and buttoned up, with gaskets and gasket eliminator paste. No leaks were reported during the ensuing trials.How, Who, Why, Where?


The bolt shearing problem started the moment the ship started sailing. This could have been due to the induced vibration on the system pipe line aggravating the strain on the bolts (already stressed due to the forced connection).

Immediate Remedial Measure.

The last section of pipe was removed. A template was made to remake a new pipe to exact dimensions. The system was buttoned up using the newly made pipe. No more bolt shearing…….wow.

How to avoid recurrence?

The above mentioned scenario is applicable to any piping connection. In pipe laying, it is essential that all pipes are made as per a layout diagram. The last pipe connecting to more rigid members such as a pump or gland or fixed flange need to be made as per a template with accurate measurements.

In some cases with Copper and Aluminium pipe systems, age hardening occurs. Both Copper and Aluminium pipes are amenable to damage while in use or in storage. Alignment check before closing the last element of pipe is essential to avoid flange stress. The larger the pipe diameter, the more pronounced the problem.


Process Pigging – For cleaning system pipes

Hi all,

The phrase “Process Pigging” took me completely by surprise. I dug up some material to understand the process.

System pigging is a cleaning process using a plug introduced into the system pipe and launched using some high pressure means. The plug moves through the system including bends. It will carry off dirt and scales from the pipe internal walls as it is propelled forward.

Please go to the three linked pages to understand more. It is not known whether the equipment and plugs are available in India, but the process seems to be interesting in some of the application such as fire main pipe, compressed air pipes, fuel pipes, etc.