Becoming a second engineer is a big duty as the person is no longer just an engineer officer but a management level officer who has to look after the affairs of the engine room and also share the burden of the chief engineer. A 2nd engineer has to look after his own machinery, assign work to junior engineers, and attend to other important machinery. There are several important aspects of the duties of second engineer as he is someone who s in charge of all the operations of the engine room. In this series of articles, we will be discussing an important aspect of the engine room – pipes and bends. This is the first part of an essential series for second engineers.
In case of new fabrication of a pipe line, repair work, or making dry dock specification, the second engineer must have good knowledge of piping and fittings in the engine room of the ship.
Pipes vs. Tubes – What is the Difference?
There is always a doubt among several marine engineers about difference between pipes and tubes. Several consider them as synonyms and even think that they are interchangeable. However, it is to note that there is a difference and it is based primarily on the rules of nomenclature of the pipes and the tubes. In the following points we endeavor to clarify the issue.
- Pipe is rigid and resistant to bending whereas some tubes such as copper tubes and brass tubes can be flexible. However, in structural projects tubes are rigid.
- Pipes are classified by schedule and nominal diameter. For example, a 250mm nominal diameter and schedule 80 pipe.
- Tubes are classified by outside diameter and thickness. For example 10mm copper tube 2 mm thickness.
- In pipes, all the fittings can be matched by nominal size and schedule. For example a schedule 40 one inch pipe will have fittings specified by same name. These pipe fittings would not fit a 1” tube.
- Pipe is always round or cylindrical. Tubes may be square, rectangular and cylindrical.
- Pipes generally start from ½ inch to very large sizes. However, tubes are generally of small diameter only. We use a 10 inch pipe but not 10 inch tube.
- Tubes are used in applications where the outside diameter must be precise, like in cooler tubes, heat exchanger tubes, boiler tubes etc.
- Pipes are generally used to carry fluids and must contain them and have pressure rating and hence are scheduled.
- In tubes the thickness increases in standard steps like 1 mm thick, 2 mm thick etc. In pipes however the thickness depends on schedule of the pipe and there is no fixed step.
- Pipe joining is more time consuming like welding, threading, flanges with bolts etc. The tubes joining are faster like flaring, brazing, couplings etc.
- Tube dimensions are actual dimensions. Whereas the pipe dimensions are only nominal. That means that a 1” tube will have actually OD as 1”. The pipes on the other hand are named nominally, which means only for name. The 1” schedule 40 Nominal size pipe has an ID of 1.049”, OD of 1.32” and a wall thickness of 0.133”.
- Tube fittings are compression fittings like ferrule and union nut, flared fittings, biting fittings, mechanical grip type fittings. The pipe fittings on the other hand are pipe to pipe butt welding, threaded pipe fitting connectors, flange to flange bolted fittings etc.
What is Nominal Diameter?
The dictionary meaning of nominal is, “existing in name only”. For example a 250 A nominal size pipe has an ID of 242.8 mm and OD of 273 mm and as per schedule 80. But instead of saying 242.8 mm IDF pipe with wall thickness of 15.1 mm, we say 250A, SH80. It is easier to say and easier to remember.
Nominal diameter is the approximate inner diameter of the pipe it is a rounded figure easier to use and remember. By prescribing the nominal size of the pipe all the different fittings can be selected based on the same nominal diameter, without physically checking the dimensions and compatibility of each component.
Nominal diameter is not internal diameter but similar to it. With reference to the above example the nominal bore of the pipe is 250 mm, but the ID varies from 266.2 mm to 222.3 mm depending on schedule of the pipe.
Schedule of Pipes: What does it mean?
In marine field we generally use schedule 40 for light duty and schedule 80 for heavy duty. There are however many other schedules which have been incorporated due to improvement in metallurgy and requirements due to increased pressure demands.
Basically the schedule of a pipe refers to its pressure rating. The higher the schedule the higher pressure it can contain. The schedules are normally 5S, 10S, 10, 20, 30, 40S, 40, 60, 80 100, 120, 140 and 160. As the schedule increases the wall thickness increases and the ID deceases.