Every time i do these things i change my opinions, so

I've read:
*the articles in the Caesar Dynamics seminar notes,
*Casti
*Non man regs.
*Brandmaier/Knabel

I'm intrested in the difference between the two thrust figures given by the caesar relief load program.

The use of umbrella fittings is not normal here at JE, so i am curious as to why we have two figures for relief loads from the caesar output where no umbrella fitting is used.

I must admit that the paper by Brandmair is a little over my head.

Anyway, i figure that as the momentum increases and the pressure decreases, the over all reaction degrade from momentum and pressure thrust to mainly pressure thrust only at the vent exit.

so for my relief system which is fully piped in a single diameter with multiple elbows, the reactive force upon relief degrade logrithmically from Momentum and pressure thrust at the orifice to mainly pressure thrust at the vent exit.

or am i talking tripe?

Tim

Umbrella Fitting = Drip Pan Ell commonly used on steam in Power plants

The CAESAR II Relief Load Thrust generator assumes you are venting to the atmosphere. If you're venting to a closed system, don't use this.

The attached image shows a couple of sketchs from the CAESAR II Dynamics Seminar Notes. As indicated above, an umbrella fitting assumes the valve pipe is not connected to the stack. The CAESAR II Relief Load Thrust generator always computes both forces, use whichever one is appropriate for your situation.

Tim,

Have you looked at B31.1, Appendix II (Roman numeral)? It need to be rewritten but still there is some help there.

Regards, John.

Gents,
Probably some misunderstanding here.

I have frequently discussed my techniques for relief analysis with my piers,(including Dave)and they seem to be inline.
Yes i know about drip pan elbows, but they are not used in Ireland.We simply bolt an elbow to the relief valve and pipe to atmosphere. (except once..........)
The scetch shows an increase in pipe diameters after the valve in both cases.

Now when i do relief work, if my elbows are more than 10 pipe diameters or 3m apart i start to wonder about the out of balance forces wizzing through the pipe.

In our relief systems, we always vent to atmosphere, or a header which has a considerable free volume and thus is assumed to be atmospere. but too get there we use bends.
Know i usually used the higher of the figures form the relief program to add to the analysis in elbow pairs.
but i get to thinking if this is stricktly correct, and why we have to different figures.

The unbalanced force that exists between elbow pairs is a major component in closed relief piping loading. As the pressure wave proceeds down the piping sytstem each elbow pair receives an unbalanced load which is decreased and negated finally when the upstream elbow achieves a pressure similiar to the inlet elbow.

Usually because the flowing fluid / gas is moving quickly the shorter the distance between elbow pairs the less...
1)Unbalanced force is applied (because the relief opens over a period of time if your travel time is less than the opening time you can't get to the maximum pressure/unbalanced force).

2)Participation by the elbow pair its loaded and unloaded before it can ramp up its response.

So what does this mean in a general sense?;
1)Shorter pairs are not a problem.... (How short is short depends on V and To (valve opening time)

2)Longer elbow pairs should be restrained.

3)and finally the actual outlet if it is open to atmosphere will always experience an unbalanced jet or rocket type thrust load.

I suggest a time force study would reveal some of the effects more clearly....

good post john, thanks.

So far, i agree with all comments, and they are in line with what i practice(mostly)
(phew!)

But,

I mean to ask what exactly those two different thrust forces are telling me in the thrust generator from Coade?

I want to try to moderate the forces from the generator if possible.

Tim

Ahhh well thats another entirely murky story... The force at the PSV outlet vs the end of the run is what your talking about...

The "corrct" answer is somewhere between those two numbers.... I would say design for the larger one and call it a day...


As A.R. Markl once stated... "The code represents the compromise between scientific truth and day to day practicality."

Without intending in the slightest to disparage the vast contributions made by Mr. A.R. Markl to the body of knowledge in the field we all work in, I feel it is incumbent to make a couple of points.

1. When Mr. Markl was publishing groundbreaking papers in the early 1950's, a digital computer was defined as counting on your fingers. Slide rules and log tables were the high-end computing tools. We have made great strides in what can be done, both in computing and in the fields of metallurgy, welding, fabrication, and other areas. "Day to day practicality" is not a static concept.

2. The Code represents a vast body of practical experience, dating from far before Mr. Markl's time. If piping systems are designed, fabricated, tested, operated, and maintained under the rules applicable to the installation in question, things hardly ever fail. It's when we err that we get into trouble, generally speaking.

3. The marketplace is hell-bent on using the latest and geratest stuff, believing every tomfool claim of the marketing folks. Thus we rush new fabrication techniques, new materials, and the like into service because we can build things stronger, faster, and cheaper. To some extent, this is a worthwhile goal. I recommend a book titled "To Engineer Is Human" for interested readers.

4. The long-suffering folks on the Code Committees, and other learned types in the field, are charged with walking an at-times invisible tightrope spanning the abyss between "If it ain't broke, don't fix it!" and "Stronger, Faster, Cheaper!" The Codes they produce are their best attempts at incorporating the diametrically opposed viewpoints of these two constituencies.

5. Oftentimes, those of us who have chosen to make our livings trying to reconcile the same two camps for our particular projects are caught between an expensive design that covers the "worst case" scenario, and a cheaper design using less demanding assumptions. Mr. Luf is correct in that the answer is usually somewhere out in the middle, accessible only by tightrope.

In the end, it seems that we can solicit all the free advice on topics such as this as we are able to absorb, and it still does little to keep us from lying awake all night pondering. I had an issue like that last week, and was fortunately able to use the wisdom of others, combined with the understanding of my project team, to pass the burden onto those who could bear it.

The hardest part is usually not finding a solution, it's explaining the problem and your recommended solution to those who have no clue about what we do. This becomes considerably easier for those of us who have less, or whiter, hair than we used to. I'm not sure if it's because we have learned to express ourselves better to the non-technical types over the years, or because they have pity on us old folks.

That being said, I am on the other end of the camp from Mr. Luf in theory. At one time, I worked on the design of aircraft engines for a living. (I am not sure I am in a more or less stressful business now.) Exhaust forces are usually overestimated by those of us in this discipline, just like they are usually underestimated by those in the business of getting the most thrust for a buck. This is, I presume, a result of both groups wanting to meet or exceed the expectations of the non-techies that they have to sell their product to.

But Mr. Ay's explanation seems best, I would just follow it, even though his answer is in Mr. Luf's camp. I'd rather spend a little extra time dealing with an overestimate of an occasional load than go through the hell of trying to find an alternate cause of a failure initially blamed on my underestimate. Don't go there, it's not fun.

Wise men have fear, fools are never afraid....

Frohle Weinnachten