Why do we use SCH 40 HVAC System ?

[Mosarof]

Hi,

In our country generally , we use ASTM A53 SCH 40 SEAMLESS MS PIPE in HVAC (Chilled Water Piping , Cooling Water Piping) System .

What's the actual guideline or reason for this ? Can we USE SCH 20 MS Pipe or GI Pipe ?

 

[LittleInch]

We can only speculate that sch 40 is able to be screwed, bent, fixed in position better than sch 20.

sch 20 is rather thin and could easily fail if not properly supported or has other forces on it.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Mosarof]

@LittleInch Thank you for your reply .

But if we ensure Proper Support & Handle with care , then we can proceed with SCH 20 right ?

 

[LittleInch]

There's no special reason why not, but it's not my decision and you haven't provided any information wrt design code, pressure, temperature, material properties ("MS" doesn't mean anything), connection type (welded/screwed/glued/press fit???) or requirement for corrosion allowance

This sounds like an exercise to reduce initial CAPEX at the expense of long term reliability / lack of leaks??

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[hacksaw]

what size piping are being used?

 

[Mosarof]

@LittleInch

For Pipes we are using Mild Steel 40 SCH Pipe [The Term MS is fairly popular & refers to Mild Steel] . The Pipe Joints are welded & Valves are Threaded upto 40mm for the rest dia (more than 40mm) we go for Flanged.

For Closed Circuit Chilled Water Circuit , we use maximum 5 bar or 6 bar Head Pumps.

@hacksaw

The headers are usually 350mm to 300mm . Chilled Water Supply Pipeline is between 250-200mm . Rest FCU/AHU Connection depends on Cooling/heating Capacity of the unit

 

[hacksaw]

Stick with SCH40, for service life if nothing else.

 

[MJCronin]

Mosarof...

You can do whatever you want and things will be great .... for a while

Are you performing a "code required" piping calculation of the minimum wall thickness for your specific piping system ? It is quite simple ... What are you using for a corrosion allowance ?

Aerated condensate return piping chews through mild/carbon steel very rapidly ..It would not be unusual to have piping condensate failure in less than five years in thin walled systems.. Many modern specifications recommend SCHEDULE 80 for new condensate systems.

Some corrosion related info here:

https://www.eng-tips.com/viewthread.cfm?qid=12866

https://www.eng-tips.com/viewthread.cfm?qid=177724

https://www.eng-tips.com/viewthread.cfm?qid=45487

For example ......

Here is a modern, well developed specification where schedule 80 carbon steel is required for High, medium and Low pressure condensate systems. ( Section 2.02)

https://www.uh.edu/facilities-plann...3-22-13-steam-and-steam-condensate-piping.pdf

Clearly, the university wants their piping systems to last for a long time



MJCronin
Sr. Process Engineer

 

[goutam_freelance]

In India we usually specify minimum thickness corresponding to sch 20 for 300 mm and above low pressure piping. I found no reported problems from users. If you can calculate the corrosion allowance required and then match the strength requirements I think this is a cheaper option.

Engineers, think what we have done to the environment !

https://www.linkedin.com/in/goutam-das-59743b30/

 

[Musttamoni]

ASME B31.3 Provides formula and guidelines for pipe thickness calculation of pipe under pressure:
1-Seamless Pipes : Design Thickness t = (PD)/2(SE+PY)
2-Welded Pipes : Design Thickness t = (PD)/2(SEW+PY)
Where:
P : Internal Design Guage Pressure
D : Outside diameter of pipe,
ASME B36.10 : Welded and Seamless Wrought Steel Pipe.
ASME B36.19 : Stainless Steel Pipes
S : Allowable Stress value for material:
Allowable stress values for different materials at different temperatures. Provided in Table A-1 of ASME B31.3.

E : Longitudinal Weld Joint Quality factor
Applicable as per ASME B31.3 Table A-1A or A-1B .
2.1For Seamless Pipes.
3.0.60 for Furnace Butt Welded Pipes.
4.0.85 for Electric Resistance Welded Pipes
W : Weld Joint Strength Reduction Factor
1.Applicable as per Para 302.3.5(e) of ASME B31.3
2.It is applicable only for Welded pipes.
3.W is Take as 1 for Seamless Pipes.
4.Value of W is taken as 1.0 at temperatures of 510°C (950°F) and below, and 0.5 at 815°C (1500°F) for all materials.
5.Value is linearly interpolated for intermediate temperatures.
Y : Coefficient from Table 304.1.1,
Calculated Design Wall Thickness should be added with Corrosion Allowance, Mechanical Allowance for Grooving, Threading etc and Manufacturing Tolerance to arrive at final value. Next higher standard thickness value from Pipe Standards such as ASME B36.10 and ASME B36.19 is used.
Required Thickness = Design Thickness + Allowances.


Reference: pipingengineer.Org

 

[TugboatEng]

Seamless in low pressure applications is required for corrosion reasons. The weld seam in carbon steel pipe tends to be very anodic and will waste away at much higher rate than the rest of the pipe.

 

[TBP]

I've never actually seen corrosion issues with seamed sch 40 steel pipe in a typical commercial/institutional HVAC application that could be attributed to anything but an improper/badly managed water chemical treatment program.

 

[TugboatEng]

Yes, with proper treatment there will be no issues.