Dear All,

Question: 1

I referred COADE book and CAESAR-II help file.
For water hammer- Entire force F is applied only in the Flow direction.
For the Slug flow – Force is resolved into axial and orthogonal. Means 0.707*F applied in that two directions.

What is the reason?

Question: 2

For water hammer- If the valve is closed suddenly, then a pressure wave travels in opposite direction also. That force is not considered why?

Richard/Dave

Could please clarify this doubt.

Any comments please.

Durga,

For Q2, Water hammer generates shock wave, and it travels back and forth several times in the pipe for few seconds. We can treat water hammer as occasional due to its short duration.
First impact force is the largest, and therefore you don't have to consider it since the pipe system can withstand first one, it'd endure the second one too.

For Q1, I'd like to know it too!

yes, My question is that first impact whether we have to consider the force in flow direction or in the reverse pressure wave direction.

In CAESAR-II all examples, they consider only the fluid flow direction, why like that??

why we are neglecting the force acting in the opposite side of the flow direction means in pressure wave direction..


problem is reg.
1. application of force direction on the bend
2. how many forces will apply at bends one or two. means only flow direction force or pressure wave direction or both forces


for these two methods what is the best way to perform the water hammer analysis

1. pseudo static water hammer analysis
2. spectrum analysis

please share your comments.

Yes, the hammer load can echo back and forth through the system. CAESAR II examples focus on CAESAR II modelling and data development and evaluating those "echos" do not add much to the data development (although those echos may add to the current response if the timing is right).
I see the hammer load as a pressure wave travelling through the system. This pressure wave pulls the pipe along its axis due to the differential pressure on upstream and downstream surfaces. The slug load is due to a change in momentum (same velocity with a different mass). This change in momentum pulls the pipe away from each elbow center, one at a time (although there may be interaction to the current response as mentioned above).

Thanks Dave for your valuable reply,

Problem:

In one of the running project, for some systems water hammer anaylsis has to be performed.

So we thought we go with pseduo static water hammer analyis approach

Force we applied in the pressure wave direction along the axis only.

Client asked a query there will be a force "IN THE FLOW DIRECTION" also.

Finally, he is saying apply two forces at each bend, means in the FLOW DIRECTION and the PRESSURE WAVE DIRECTION.

Is it really a right procedure to apply two forces at a time at each bend ?

I'm not a transient hydraulics expert but I would agree that these hammer loads "reverberate" through the system.
This link shows a good example:

https://www.youtube.com/watch?v=oZbguheiVs4&feature=related

You see two "positive" impulse loads followed by two negative impulse loads at the changes in direction. The elbow view in the video shows this the best.

If I was running a hammer load using a static equivalent, I would run it both positive and negative - if the system is nonlinear.

Dave Diehl, Thanks for your valuable reply.

"If I was running a hammer load using a static equivalent, I would run it both positive and negative - if the system is nonlinear"

Means..

Case 1. positive and negative forces at a time, means in CAESAR-II both positive and negative forces in vector F1 only. and in next elbow both forces in vector F2 and so on ...

In load cases:

Check 1: F1, F2 and so on.. check individually with two forces acts on each elbow. and

check 2: F1 + F2 + F3 + .. case also considered. means at a time two forces are acting at all elbow in the entire piping system.

Case 2. Positive force in vector F1 and negative force in vector F2.

In load cases:

Check 1: F1, F2 and so on.. check individually with one force on each elbow

Check 2: F1 + F2 + F3+ .. case also considered. means at a time two individual forces are acting at all elbow in the entire piping system.

In both cases which one is correct method to safe the piping system in water hammer effect.

Durga,

Case 2 is more accurate.

For Case 1, you cannot use the same force (F1 for example) for forces impacting on elbow/closing valve/tee along liquid flow and reverse liquid flow because these forces will cancel with each other.

For code compliance check, just check F1, F2, and so on only. No need to combine all forces because you may get a very conservative result. Plus transient is not happening at the same time. the wave will travel from its originated point so better to treat the forces individually.

Noorsyam,

Thanks for your reply.

Iam taking about vector F1 in CAESAR-II. not the force F.

what i understand you are saying, apply flow direction force and reverse flow force individually and check the stresses. Is my understanding is correct.


If you check the hammer spectrum analysis Caesar-ii example they did the combination also. Pls check once.

if the fluid is not in single phase, i think forces cannot be canceled then that case , going with case 2 is ok or not

Dave Diehl,

Could you please clarify these doubts.

"If I was running a hammer load using a static equivalent, I would run it both positive and negative - if the system is nonlinear"

Means..

Case 1. positive and negative forces at a time, means in CAESAR-II both positive and negative forces in CAESAR-II vector - 1 only. and in next elbow both forces in CAESAR-II vector - 2 and so on ... same as Slug force application of force.

In load cases:

Check 1: F1, F2 and so on.. check individually with two forces acts on each elbow. and

check 2: F1 + F2 + F3 + .. case also considered. means at a time two forces are acting at all elbow in the entire piping system.

Case 2. Positive force in vector 1 and negative force in vector 2.

In load cases:

Check 1: F1, F2 and so on.. check individually with one force on each elbow

Check 2: F1 + F2 + F3+ .. case also considered. means at a time two individual forces are acting at all elbow in the entire piping system.

In both cases which one is correct method to safe the piping system in water hammer effect.

The hammer hits each change in direction sequentially. If you assume these are independent events (that is, they do not build upon one another) each event would be evaluated alone.
If you expect any events to interact, a time history evaluation may be more fruitful.

Thanks Dave for your reply.

Yes I go case 2.

Dear Dave,

From june 94- Water Hammer analysis example -SNFAIL (Time history Analysis)

The first force acts @ Node 30 - Angle valve.

I agree the flow first first hits on the angle valve. But our general consideration is force applied on the bends only.

If I am doing Pseduo static, Spectrum analysis and Time history analysis, Whether I have to consider the force on the angle valve or not??

Until that pressure wave front, passes through the first elbow, you will have an unbalanced pressure load on the valve face.
Please note that the newsletter example only considered the first pass of this pressure imbalance. This wave will echo back and forth (and the load on the last elbow will NOT be a 60% load).

Thanks Dave,

Please check my summary of understanding from this discussion.

In summary:

For pseduo static and response specturm method for surge force analysis::

1. I will apply force on the face of suddenly stop valve (Axially only)
2. I will apply force on all upstream elbows in two directions (Axial and orthogonal)seperately in vector 1 and vector 2 (NOT AT A TIME)

Please correct me if my understanding is not right

Thanks for your help.

The surge "hits" the downstream surface at the "end" of each straight run of pipe. These are individual event that occur one after the other. They may be linked dynamically or the response to each event may be isolated. The first "end" is the valve face and the event duration (the imbalance) is the length of the straight run divided by the speed of sound in the fluid, when the pressure wave front passed around the upstream elbow, the first event ends and the second event begins. This second event lasts until the pressure wave front enters the next change in direction. and so on.
Don't think of "two directions" at elbows. The two directions are two separate events. We are working with delta P here not change in momentum. (The loads associated with a change of momentum will have the "two directions" working almost simultaneously.)

Thanks Dave for your reply,

Agree with you dave. Two directions are two seperate events. First direction force is may be positive impulse load and second direction may be negative impulse load.

If we are considering the both positive impulse load and negative loads we have to apply two direction forces at seperately on the elbows these are due to pressure immbalance between elbow to elbow pairs.

This is clear. But after reading your old post title SURGE LOAD, Date- 10/15/07 " I seems to me that either approach would work - either define the calculated load on every elbow where only one elbow load changes at a time or just apply the imbalance on each elbow-elbow pair. The first approach uses the steady-state plus the transient and the second applies only the transient. I think the second is simpler."

Please find the attachment of the sample calculations of forces from both approaches.

In my calculation, first and second Approach force values are varies.

Please correct me, Where my calculations/understanding gone wrong.

Any Help?

Another observation is from the first approach the forces are resolving and applying on eblow at a time

where as the pressure immbalance method the force is applied seperately.

The result in CAESAR-II also varies??

According to Leisher ASME paper stated 1st approach - resolved forces magnitudes are delta p * A -- not (p+delta-p) * A

Please correct me If Iam wrong.