Actually i am doing analysis of a Hot Reheat line 24inch 585 Degc39bar press

of boiler at saftey valve discharge my boiler vendor has given a horizontal force of 20KN to be act at the saftey valve discharge also the vertical force will be 2oKN while applying the both in Operating + Saftey relive valve case i found the straess in occasional condition goes high that is 97% i want to limit it by 85% .

Can any one tell me as per my knowldge 20KN load in the horizontal direction in saftey vale discharge is too high also tell me how to reduce the stress at the stub on which saftey valve is mounted i have already tried this with fabricated TEE still it fails in stress i cant put Reinforce pad because its P-91 material please help me

adi singh,

Look at ASME B31.1 Appendix II Non-mandatory Rules for the Design of Safety Valve Installations.

See Fig. II-2-2, Consider how the loads are acting on the discharge elbow and the possible duration and sequence of loads for the horizontal and vertical loads that the boiler vendor has given. In particular consider Fig II-6-1 (b) to restrain loads from discharge pressure that cause bending loads at the stub. Instead of fabricated tee, evaluate use of a thicker wall for welding tee, or possibly thicker wall for stub, like shown in Fig.II-7-2.

Other branch designs to consider might be from the IBR Chapter VIII para 365E, Fig.365 / 7,8,9,15,18,or 25, which show full penetration groove welds for the stub attachment weld.

Richard

Having worked with IBR for about 5 yrs myself and moved on to ASME...I'm really impressed you are able to quote Both!!

sn_idea,

The education into IBR was result of Reliance Jamnagar project. I wish I had been able to discuss it with someone for questions of IBR scope a year ago, when all the reports and documentation were being assembled for IBR review. At least the IBR is 'English'. I was trying to relate something that might have been in the context of adi singh's experience.

If adi singh responds, then I might also tell him how he might reduce the loads from the relief valve - by making the relief valve smaller size.

Hi Richard and Sn_idea
Thanks for replying .
I will be more Curious and thank full, if u will give me details/procedure to reduce the forces.

Richard Yee
hello can you please mail this to me
om-prakash.mohanty@siemens.com

adi singh,

It is good to hear that you are curious. First, you probably understand a smaller relief valve could result in smaller forces and reduced stresses. But how does one design the reheat system and piping to allow using a smaller relief valve? I suggest first reading ASME Sec I PG 68.4 for reheat relief valve requirements. This will make one think more in terms of the system design and not just running a stress analysis. I am not going to be able tell you simply a procedure or details, because there is lacking some more information, as well as a need for some education.

There is not much information regarding the relief valve. You do mention the 'Reheat line 24inch 585 Degc39bar press', force of 20 kNt (4500 lbs), and the P91 material. What are the relief valve(s) inlet / outlet sizes? Flow required thruogh the relief valve? What direction is the relief valve discharge pointed? How does the relief valve discharge geometry compare to the typical geometry shown in ASME B31.1 Appendix II Fig. II-6-1 a,b,c,d ? What supports are there on the discharge piping?

Branch connection stresses usually can be reduced by providing an enlarged size of connection diameter compared to the piping (branch) size. That is, the equipment nozzle (heat exchanger or vessel) is sometimes made to a larger diameter (pipe size plus 1 or 2 sizes larger) to spread the loads over a larger cross section. If the relief valve could be selected smaller, and then a reducer fitting provided from the original branch connection size to the smaller relief valve size, there should be benefits of lowered stresses.

There could also be lowered cost(s) of relief valve(s). Let us know what you conclude fron reading of the ASME Sec I PG 68.4 rules.

The orifice size in a relief valve must be sized to be able to pass the required flow, it really can't be reduced "to reduce thrust forces." Sometimes, relief systems for utility boilers are provided with several relief valves to help to cope with this issue. There are other issues as well.

How are you going to support the relief valve discharge line and elbow?

What are you going to do about the moment load exerted on the relief valve inlet due to the lateral thrust in the relief valve discharge?

Is your support design sufficient to cope with the expansion that will occur when the relief valve pops open and the temperature of the discharge line and its support goes from 50 C to 600 C in about a half a second?

This is not a topic for beginners. As many of us have posted many times on this site, please discuss your problem with a senior engineer that you work with on a daily basis who is familiar with the problem. If you don't have that opportunity, I strongly urge you recommend to your management that they hire a consultant with the necessary experience.

CraigB,

You are correct that the orifice in a relief valve is sized to pass a required flow amount. Sonic flow condition at the throat of orifice is a physical limitation that can't be changed. However, there is a system design consideration that you mention in "providing several relief valves to cope with this issue".

There was an old post by 'engmostafa' on eng-tips forum, where he questioned why that on reheater piping, there were 2 or 3 multiples of safety valves on the reheat Inlet piping -compared to the reheat Outlet piping. His information from examples
:Boiler 700 MW(BABCOCK AND WILCOX) 6 SAFETY VALVES ON COLD REHEAT LINE,3 SAFETY VALVE ON HOT REHEAT LINE.
:BOILER 371MW(FOSTER WHEELER) 3 SAFETY VALVES ON COLD REHEAT LINE,1 SAFETY VALVE ON HOT REHEAT LINE

The ASME Sec I PG 68.4 sets the requirement of one or more relief valves for total capacity equal to the reheater flow. Plus the additional requirement of a 15% minimum of total flow in a relief valve on the outlet of reheater. Why is this?

The minimum flow of 15% through the reheater could save the tubes from the excess temperatures that would result from turbine trip valves blocking further outlet flow from reheater. Why not put all the safety valves on outlet? The outlet conditions of reheater are typically 1000 deg F, or in adi singh's case 585 C ( 1185 deg F). The material would be P91 or P22. The cost of the relief valves would be many times more than commom carbon steel relief valves. The better system designs put more of the required relief capacity into carbon steel relief valves on the inlet piping of the reheater, and smaller relief valve(s) of P91 / P22 material of the outlet piping of the reheater. (There might be a relief valve available in P91, or not?)

If the reheater system that adi singh is evaluating, has already placed the inlet piping relief valves and a miniimum sized reheater outlet piping relief valve, then it is a good system design. The outlet piping relief valve will need some more design improvements to deal with the 20 kN loads. The 20 kN load is not that extreme, 4500 lbs force, which makes it seem the relief valve is already sized small, compared to the Nps-24 reheat line.

The solution to branch connection / nozzle load stress problems would be similar to solving an equation with multiple unknown variables. Sometimes both sides of the equation need to be manipulated. Sometimes another equation is needed to solve for the variables. And almost always there is not one unique solution - just some solutions that are better (less cost, more robust).