Pressure Class

these class done according to british standard due to maximum test pressure,for ex-

if your system working pressure 10 bar ( 147 psi ) so test pressure will be 15 bar so nearest class for this system will be class 300 for economical reason .

how pressure is releated to class.is it a norm or have some releation/

These pressure classes or ratings 150, 300............... represents the "Maximum allowable non shock working pressure" expressed as guage pressure for the valves and other fittings such as flanges at their tabulated temperatures.

As a general rule we always try to go to the next higher pressure rating to be safe. if the system is over 150 then we would use a 300 lb. flange/valve etc. This is not always the case though, sometimes it is advisable to use a 150 lb rating materials on higher than 150 lb. pressures. This is usually based on the temperature of the piping and what the material is that is in the pipe. On a water line that is lower to ambient temp we might use the 150 lb rated materials up to 200 lb pressure or more after everything is calculated out. There are books that tell you exactly how much pressure each rating can hold safely and at what temp. Try this link it might give you some guidlines/help. http://www.engineeringtoolbox.com/temperature-pressure-ratings-valves-d_1182.html

pipewelder

I'd like to clear that the pressure class or rating (e.g. 150#, 300#, .... etc.) doesn't mean that the valve can withstand a pressure of (150 PSI, 300 PSI, ...etc.). Only, it is a categorization, and the actual pressure in which the valve is capable to withstand is a function of the design pressure and its design temp. for certain material, named Pressure-Temperature Rating. These data can be found easily through Tables of ASME B16.5.

Example:

Determine the class of flange rating (either for a pipe flange or valve flange) of a piping system with the following design conditions :

a. Pipe material : Carbon steel A106-B & Valve material : carbon steel A216-WCB

b. Design Pressure (not operation) : 200 PSIG

c. Design Temperature (not operating) : 500 ^oF

Solution:

From ASME B16.5, the carbon steel material is located at Table 2-1.1, we find that at 500 ^oF the flange rated 150# shall withstand only a pressure of 170 PSI (< 200 PSI), so rating 150# is not good.

Therefore we have to select the higher rating 300# which withstand a pressure of 600 PSIG.

If we lowered the design temp. to be 300 ^oF instead of 500 ^oF (at the same material and same design pressure) we shall discover that class 150# is OK which will withstand a pressure of 230 PSIG (> 200 PSIG).

tq 4 the good info..

You are Welcome.

Abdel, are the temperatures and pressures related to the design of the process (fluid) or to the piping material? How to select the adequate pipe for a system knowing operating conditions (including minimum/maximum possible values)?

SaC.

Yes Jolsac, the pressure and temperature are related to the fluid itself. We have to select the proper piping material to withstand both pressure and temp., in addition to withstand corrosion, erosion, ... etc. Such example for calculating the pipe wall thickness as derived before through CR4 by me.

Example for Determination of pipe wall thickness.

The pipe wall thickness is determined from the applicable formula of the pertinent ASME B31.1, Power Piping (not B31.3, Process Piping).

Given : Code : ASME B31.1

Design Pressure (P) : 13 bar (188.5 psi), [oper. press. 10 bar (145 psi)]

Design Temperature : 180 ^oC (356 ^oF), [oper. temp. 150 ^oC (302 ^oF)]

Material : ASTM A106 Grade B, Seamless

Nominal Pipe Size : 3 inch

Find : Minimum Theoretical Wall Thickness, t_m & Minimum Nominal Wall Thickness, t

Data : t_m = P D_o / (2 S + 2y P) + C (ASME Code B31.1)

Corrosion Allowance, C = 3 mm (0.11811 in.)

Factor, y = 0.4 (for carbon steel) [ASME Code B31.1, Table 104.1.2(A)]

Joint efficiency, E = 1.0 (for seamless pipe E=1)

Pipe Outside Diameter, D_o = 3.5 inch

Allowable Tensile strength, S @ 356 ^oF=15000 psi (ASME B31.1,Table A-1)

Wall Thickness Tolerance = 12.5% (ASME B36.10 & ASTM A106)

Solution : By substitution into equation,

t_m = P Do / (2 S + 2y P) + C

= (188.5 psi)(3.5 inch)/(2*15000 psi +2*0.4*188.5 psi) + 0.11811 inch

= 0.14 inch (3.6 mm)

Which is the theoretical minimum for wall thickness without allowing for wall thickness tolerance.

Thickness adjusted for wall thickness tolerance, t = t_m / (100%-12.5%)

= 0.14 inch/(1-0.125)=0.14/0.875= 0.16 inch (4.06 mm)

The next greater commercial wall thickness is found from ASME B36.10 to be 0.216 inch (5.49 mm) which corresponds to 3" Sch. 40.

In other words 0.216 inch nominal pipe wall thickness is the commercial wall which, when reduced by the full tolerance of 12.5%, satisfies the code formula for t_m.

Note. There are a lot of ASME B31 piping Codes such as :

♦ ASME B31.1 Power Piping.

♦ ASME B31.3 Process Piping

♦ ASME B31.4 Pipeline Transportation Systems for Liquids Hydrocarbons and Other Liquids.

♦ ASME B31.5 Refrigeration Piping

♦ ASME B31.8 Gas Transmission and Disribution Piping Systems.

Hello, Abdel.

You have been very clear. Thanks.

The ASME code, do you have it yourself?

SaC.

Sure, I have a copy at my office and another copy at home.

Abdel, is possible to you to share it? I mean, if you have this code in e-format (pdf), and you don't have problem with doing it.

SaC.

Yes Jolsac, please send your e-mail, I will try to help you.

Abdel, I will send you my e-mail.

Many thanks.

SaC.

Hi! mister, I´m surprised by you send an ASME code trougth e-mail (it´s a joke for you who always helps somebody as a proffessional engineer as you are. Please laugh)

"I will try to help you"

This promise does not mean that I will send ASME code through e-mail, may be any other document(s). Can you please laugh?

Abdel,

I sent you my e-mail to your CR4 e-mail box, please check it out. My advice to you is do not pay attention to such an envying comment. It is clear that you are a professional.