Pressure vessel basic questons. 3

[mrpv]

Hi everyone.

I am not very well versed in pressure vessel design, however as part of my job scope, i need to oversee some pressure vessel design.

Using ASME VIII as reference,

1.I'm a bit confused, according to UG 99(b), "the vessel designed shall be subjected to a hydrostatic test (HT) pressure......is at least equal 1.3 times the max working pressure". A footnote also states that "the max allow. working pressure may be assumed as the design pressure, when calcs are not made to det. the max allow. working pressure.
Question: How would the vessel pass the HT if the design pressure is say for eg. 30 bar, but tested at 1.3 times the design pressure?

2. How do i calculate Max Working Pressure?

3. Is there a relationship between Max Working Pressure and design pressure?

4.Is there a rule of thumb in choosing a head for the vessel? When do we use flat, hemispherical or ellipsoidal?

5. Is there a guide when choosing a nozzle type;i.e must we used flanged type nozzel or can we use socket (welded) nozzle?

 

[metengr]

mrpv;
I have carefully read thru your post. I would strongly suggest that you get on the asme.org web site and look for a training class on pressure vessel design using ASME Section VIII, Div 1. Many of these courses are taught by personnel experienced in pressure vessel design and application of codes.

For a quick reference, I would suggest using the CASTI web site below. The book on Section VIII pressure vessels contains good information.

http://www.casti.ca/

 

[adrianviorel]

Q 1: Asme algorithms are containing safety coefficients that covers the 1.3, i.e. theoretically, vessel will be calculated for a pressure that clearly overcomes the HT one.
Q 2: Usually, max working pressure results from process data.

 

[jte]

mrpv-

I agree with metengr's suggestions. Don't forget to specify which Division you are using, though we can tell that you are involved with Section VIII Division 1 from your UG reference.

Here's my shot at your questions:
1. As adrian points out above, there is a design margin built in. A vessel is not expected to burst when you go 1 psi over the MAWP. No sane engineer would ever design something which would fail at a loading 1% over the design load.

2. UG-27 etc provide equations to determine a MAWP given a thickness. Your engineer who is competent in vessel design will know which formulas to use. The engineer will calculate every component of the vessel and the lowest calc'd pressure will govern. Alternately, the vessel thickness is not optimized and the process design pressure is stated as the MAWP per the footnote you read.

3. Only that MAWP should be greater than or equal to the design pressure determined by the process engineer. Where this could be confusing to a newcomer is that they don't have to be equal. The process engineer determines what design pressure and temperature he needs and forwards that to a mechanical engineer. The ME plugs the DP into the appropriate equations, adds corrosion allowance, and determines a required plate thickness. Say it comes to 0.36 inches. The ME rounds up to the next commercially available thickness, 3/8" or 0.375". Sometimes (IMHO regrettably) the process stops here. Sometimes (IMHO well done) the ME optimizes his design by one or a combination of three things: Given the extra 0.015" the MAWP can be increased, the design temp can be increased, and/or the corrosion allowance can be increased. My bias is towards increasing the CA. At this point nozzle reinforcement etc can be determined. Do not let nozzle reinforcement or flange rating govern MAWP. Just good practice.

4. Heads are most often going to be of the 2:1 SE variety. Flat heads will only be practical in very small diameters. Hemi-heads make more sense in high pressure applications. If you're involved with something for which a hemi-head makes sense, make sure an engineer competent in vessel design is involved.

5. I suppose it varies by industry and personal bias. I've dealt with too many issues with couplings to recommend them. I'd go with weld neck flanges on your nozzles.

As a final note: Welcome to the industry! Remember that your designs can fail catastrophically and with fatal consequences. Seek mentorship to develop your knowlege and do not attempt a design by yourself until you are at a point where the judge would consider you competent in the field. From the ASME Ethics Policy: 2. Engineers shall perform services only in the areas of their competence.

jt

 

[JStephen]

Other factors being equal, the head style will be chosen for lowest cost, and style will vary according to size, pressure, and head availability. I've seen 80:10 heads used quite a bit, for example. If you're specifying the vessel and there are no other criteria driving the head shape, leave it to the fabricator.