200427 Pressure Surge in Pipelines
Pressure Surge in Pipelines
MARS Report 200427
This article is a part of my (as yet, unpublished) book mainly addressing cargo handling on OBO and tankers. It is concerning the underestimation of a pressure surge in the piping system and aimed to demystify the subject. I hope it will be useful for cargo and deck officers as well as for jetty personnel.
I am serving as a C/O on 50,000dwt oil and chemical tanker. I was always aware of the possibility of a pressure surge as it is described in ISGOTT (International Safety Guide for Oil Tankers and Terminals) and felt that it will never happen to me. Two operational cases had opened my eyes:
We were discharging cargo of gas oil in Panama. Our vessel had 10 cargo tanks and was equipped with 10 Framo deep well submerged pumps. Manifold pressure on the ship's side of the 10" manifold was close to 5 bar. It was simple discharge from all tanks at once. After a few hours we were called by the loading master and informed that he needed to stop cargo operations straight away and expected to resume in 5 minutes. We had reduced the rate of discharge to the minimum, say 180m3/hr. At this point the manifold pressure was approximately 1 bar or less. Then all pumps were stopped more or less at the same time. The manifold remained open as we were ready to resume. After a few seconds the manifold pressure jumped up to 10 bar. As you can see from the table, this was easily predictable. That was my first lesson. I also recall when one of the ballast valves was observed cracked. The history records revealed that the same valve was changed 4 times in 4 years. That was my second lesson.
ISGOTT Chapter 21 gives a brief explanation of this as it describes the "phenomenon" and discusses the ways in which it can be prevented. It is pointing to unlikely events leading to it, thus gives us a false sense of safety. However, we should understand that it is not a 'phenomenon' but a law of nature and be aware that it is all around us. It is present in each stage of cargo and ballast operations, flow change, starting/stopping pumps, opening/closing valves. Misconduct of such routine operations will result in damage to pipelines, machinery and other connected equipment. It should be stressed that any pipeline system, I repeat, any one, is liable for more or less severe pressure surge. So what is a pressure surge?
A pressure surge is an abrupt increase of pressure in the pipeline system when there is a sudden change in the rate of flow of liquid in the line(s). It reaches its maximum when the flow is stopped. The main reason for the increase of pressure is the transformation of kinetic energy of the flow to the potential energy of the liquid brought to rest. When the rate changes, a steep pressure front travels through the pipeline with the speed of sound in the particular liquid. Generally, maximum pressure in each cross section of the pipeline will increase by an amount p (bar):
p = V x Y x a
where
a = the speed of sound in the particular liquid
Y = change of speed of flow
V = liquid density
One question should arise after the careful reading of this article. How much time do we need to spend for rate change to reduce the pressure surge to predictable minimum. My next article should be the answer.
If you are interested in my book or full version of this chapter or have thoughts/comments and would like to share them please send feedback to [email protected]