Pipe Class - Wartsila ?31DF Series Product Manual

Wärtsilä 31DF Product Guide
The pressure in the system can:
● Originate from a positive displacement pump
● Be a combination of the static pressure and the pressure on the highest point of the pump
curve for a centrifugal pump
● Rise in an isolated system if the liquid is heated
Within this publication there are tables attached to drawings, which specify pressure classes
of connections. The pressure class of a connection can be higher than the pressure class
required for the pipe.
Example 1:
The fuel pressure before the engine should be 0.7 MPa (7 bar). The safety filter in dirty condition
may cause a pressure loss of 0.1 MPa (1.0 bar). The viscosimeter, automatic filter, preheater
and piping may cause a pressure loss of 0.25 MPa (2.5 bar). Consequently the discharge
pressure of the circulating pumps may rise to 1.05 MPa (10.5 bar), and the safety valve of the
pump shall thus be adjusted e.g. to 1.2 MPa (12 bar).
● A design pressure of not less than 1.2 MPa (12 bar) has to be selected.
● The nearest pipe class to be selected is PN16.
● Piping test pressure is normally 1.5 x the design pressure = 1.8 MPa (18 bar).
Example 2:
The pressure on the suction side of the cooling water pump is 0.1 MPa (1 bar). The delivery
head of the pump is 0.3 MPa (3 bar), leading to a discharge pressure of 0.4 MPa (4 bar). The
highest point of the pump curve (at or near zero flow) is 0.1 MPa (1 bar) higher than the nominal
point, and consequently the discharge pressure may rise to 0.5 MPa (5 bar) (with closed or
throttled valves).
● Consequently a design pressure of not less than 0.5 MPa (5 bar) shall be selected.
● The nearest pipe class to be selected is PN6.
● Piping test pressure is normally 1.5 x the design pressure = 0.75 MPa (7.5 bar).
Standard pressure classes are PN4, PN6, PN10, PN16, PN25, PN40, etc.
5.4

Pipe class

Classification societies categorize piping systems in different classes (DNV) or groups (ABS)
depending on pressure, temperature and media. The pipe class can determine:
● Type of connections to be used
● Heat treatment
● Welding procedure
● Test method
Systems with high design pressures and temperatures and hazardous media belong to class
I (or group I), others to II or III as applicable. Quality requirements are highest on class I.
Examples of classes of piping systems as per DNV rules are presented in the table below.
Gas piping is to be designed, manufactured and documented according to the rules of the
relevant classification society.
In the absence of specific rules or if less stringent than those of DNV, the application of DNV
rules is recommended.
Relevant DNV rules:
● Ship Rules Part 4 Chapter 6, Piping Systems
● Ship Rules Part 5 Chapter 5, Liquefied Gas Carriers
DBAE248994
5. Piping Design, Treatment and Installation
5-3