5 replies to this topic

[Saml]

First a little bit of background (and sorry for the long post)

 

We have a process waste heat boiler. It produces steam above 80 bar pressure. The manufacturer (we contacted them over the phone) do the mechanical designs to ASME VIII and depending on the jurisdiction and customer request they provide relief to ASME I (considering it an unfired boiler) or they stamp to ASME VIII per UG-1-(g)(2)(a and b ) and let the customer take the responsibility for the relief. The last one is our case.

 

ASME VIII has legal status in our province.

 

There are multiple valves in the steam drum. The valves calculations show that any of of the valves can be blocked /removed  and still have enough relief capacity.

 

They do not have block valves and on occasions, leaking valves have caused an unplanned shutdown. So, a couple of years back, the Maintenance department, having a budget surplus, decided to buy gate valves (Class 1500) to install under the PSV's . And then asked the Process Engineering group to "do the papers" and also specify the trapped key system to avoid more than one being isolated at a time.

 

Problem is: the valves are conventional port, not full port. So for some time we, the process engineers, have refused to "do the papers" stating that the ASME code UG135 b 1 states that "the opening of all pipe,fittings....shall have at least the area of the pressure relief valve inlet", in Appendix M calls for "full area stop valves" and the guidance on API 520 part II further clarify that "The opening through all pipe and fittings (including stop valves) between a pressure vessel and its PRV shall have at least the area of the PRV inlet connection"

 

Up to here is the plain letter of the code.

 

However, the PSV has a small oriffice for the body size. This is because, being high pressure steam what is vented, the limitation is on the discharge nozzle and / or piping. So, we've got 6X8 valves with an oriffice that would normally fit in 4x6 valves.  A 4" inlet connection would ensure less than 3% unrecoverable pressure drop. If a 4x8 PSV were available that would be good solution

 

So, if I follow the plain letter of the code, I should stick to my guns and recommend that those valves are not used. However, all the calculations show that there should not be any problems with the PSV operation with a conventional port, since a 4" inlet line would be adequate. 

 

Yes... I know... cannot buy anything without specification and then ask others to circumvent the code.

Yes...there are no consequences for me for saying no. I am not under pressure.

I just want to be sure that I am doing the right thing by recommending that those 20kUSD a piece valves are scrapped. And honestly, while I know that the code is written for a reason, and that many problems have arisen from people believing that exceptions were reasonable, it is also clear that the code cannot cover any imaginable case and that I should use my engineering judgment on each specific case. And that is telling me that these conventional port valves should work.

 

So, what I am asking is your point of view on this.

 

Thank you very much.

Edited by Saml, 06 April 2017 - 09:42 PM.

[Bobby Strain]

Maybe there are some cases such as yours where ASME has ruled. Look for them. If you don't find any, consult with the people who enforce the code in your jurisdiction.

 

Bobby

[Saml]

Thank you Bobby.

 

The only case I found was case 2254 about a changeover for boiler, applicable to Section I

 

Another section, different pressures, changeovers instead of gate valves. The only point is that they focus on performance, They are not prescriptive regarding "full port".

 

Other than that, I could not find any related case on the ASME cases.  And I will take your advice and  suggest that we involve our regulatory affairs people.

[Napo]

Saml:

The API Standard 520, Part II, Sixth Edition, March 2015, in 5.2 Minimum Diameter says:

"The nominal size of the inlet piping and fittings shall be the same as or larger than the nominal size of the pressure relief valve inlet connection as shown in Figure 2 and Figure 3.

For inlet piping to multiple relief valves, the piping that is common to multiple valves shall have a flow area that is at least equal to the combined inlet areas of the multiple PRVs connected to it."

The Standard says: Nominal size.

Napo.

[Saml]

Point 5.2 is what you mention. It says nominal size and refers to two drawings that do not have valves included.

 

Point 8.3.1 is the one I cited above, that it is more specific about having the same area.

 

Taken into account that the later is on a section specifically dedicated to block valves,  I find it difficult to make the interpretation that point 5.2 allows for a nominal (but not full port) block valve.

[Napo]

Saml,

The following link has related information:http://www.1gatevalv...ate-valves.html

Napo.

Edited by Napo, 10 April 2017 - 12:21 PM.