●
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.
Table 5-2
Classes of piping systems as per DNV rules
Class III
Class II
Class I
Media
°C
MPa (bar)
°C
MPa (bar)
°C
MPa (bar)
and < 170
< 0.7 (7)
and < 300
< 1.6 (16)
or > 300
> 1.6 (16)
Steam
and < 60
< 0.7 (7)
and < 150
< 1.6 (16)
or > 150
> 1.6 (16)
Flammable fluid
and < 200
< 1.6 (16)
and < 300
< 4 (40)
or > 300
> 4 (40)
Other media
Wärtsilä 31 Product Guide - a1 - 18 October 2016
5-3
5. Piping Design, Treatment and Installation
Wärtsilä 31 Product Guide
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