Process Piping - Key Facts on Process Piping Design
Whenever I heard the word "piping" the images of arrays of pipes arranged in a haphazard manner appeared in my mind until I started working on my final year project, that was about piping. I came to know of so many interesting things and I realized that piping is not a simple thing. Piping just doesn't mean only pipes. It also includes valves, fittings, strainers etc. Everyone has seen piping in one's kitchen or toilets. That piping is indeed very simple. On the contrary process piping is quite complex and a lot more things are taken in to consideration while designing and running a piping system successfully.
Lets see, by examples, how interesting and complex process piping really is:
The petrol that millions of cars burn daily in the engines is a result of different processes carried out on crude oil. It is pumped through pipelines to various equipments. Crude oil is so viscous that it is impossible to pump the fluid at room temperature. Its viscosity is first decreased by increasing its temperature. While in pipeline, it has to be maintained at a particular high temperature so that its viscosity stays within the required limits. This is done by heat tracing. Either electrical resistance wires are "wrapped" around the piping or steam is used to keep the fluid in the pipe at the required temperature. In electrical resistance heating, current is passed through the wires wrapped around the pipes. When current experiences resistance in the wire, energy is dissipated in the form of heat which is conducted by the pipe. Steam tracing systems employ many different approaches. One simple approach is to pump steam through a pipe adjacent to the pipe containing process fluid (the steam pipe and the pipe containg process fluid are in direct contact). The pipe containing steam transfers the heat to the pipe containing process fluid, thus keeping it hot.
Process Piping needs Fire Safety
Fire Safety is one of the most important considerations while designing a piping system. Special measures are taken to prevent fire. Three things are necessary to start a fire: heat, oxygen and source. For a fire to start in a piping system, heat can be provided by static electricity discharge and source is obviously the inflammable liquid. Since we can't eliminate the source i.e. the inflammable liquid we eliminate the other two causes.
Static charge is generated whenever two surfaces contact and rub against each other. You must have seen images of Red Indians rubbing a twig against other twigs to light up a fire. Similarly static charge is generated when liquid flows through the pipe. This charge is gathered in the piping components. Accumulation of charge over time can cause sparks with energies that can be strong enough to start a fire if inflammable liquid is being transported through the pipe. Therefore proper grounding has to be provided for the pipe and other components to safely dissipate it to the ground.
Another way to prevent fire - that can very easily turn into a conflagration - is to remove the second cause i.e. oxygen. Air can leak into pipe through the joints in the piping. If pressure in pipe is lower than the atmospheric air can leak into the system. In order to prevent it either welded joints are used or if flanged joints are being used gaskets with very good leakage integrity are used.
Types of Joints
Apart from corrosion occurring due to moist environment there is another form of corrosion that occurs when two dissimilar metals are in electrical contact with each other. The more reactive of the two gets corroded. This is called galvanic corrosion. In case of piping, galvanic corrosion can occur due to contact between flanges and pipes. Flanges are used to connect the pipes. The materials used for making flanges and pipes are different.
In order to solve this problem cathodic protection is used. Extremely reactive metals like zinc, magnesium are used to protect the pipe. When a metal that is more reactive (than pipe's metal) is brought in to electrical contact with the pipe, it becomes anode while pipe becomes cathode. As a result the anode (also known as 'sacrificial anode') is eaten away over a period of time while saving the pipe from corrosion.
Have you ever heard before that boiling can occur at temperatures much smaller than 100 degree Celsius? If not, I am going to tell you how this can happen. It can occur when pressure at any point within the fluid drops to vapor pressure at the same temperature. This phenomenon is know as cavitation. Cavitation is characterized by the formation of bubbles. And you must be wondering how does the pressure drop down to vapor pressure? The answer is that it happens where fluid is moving at significantly high velocities. That is why pumps are the most likely places where cavitation occurs. The reason for being so "obsessed" with the formation of bubbles is that they can damage the pump and the pipe very badly. These bubbles are carried with the fluid and they later implode in high pressure regions. These implosions have the ability to form small pits in the metal. Cavitation can slowly destroy the pump and pipe till total failure occurs. Pumps are selected such that no cavitation occurs.
So, next time when you see an image of an oil refinery(or any process plant) and you see the complex arrays of pipes, do not underestimate the efforts of designers and engineers who made its successful construction and operation possible.
- Munson, Bruce, Donald Young, and Theodore Okiishi. Fundamentals of Fluid Mechanics. Singapore: John Wiley & Sons, Inc., 2002.
- Mohinder L. Nayyar. Piping Handbook. New York: McGraw-Hill.
- "Steam/Oil tracing" Heatflow B.V. 1 December 2008 <http://www.heatflow.nl/steam_oil_tracing.htm>
- "static Electricity" 2 December 2008. Wikipedia. 1 December 2008 <http://en.wikipedia.org/wiki/Static_electricity>
- Wendy. "Grounding & Bonding Applications". Newson Gale Inc. 1 December 2008 <www.etaassociates.com/images/Newson%20Gale%20G%20&%20B%20Booklet%2007.pdf>
Top Most image Courtesy: zenobia joy at Flickr.com