Hi,

Please let me know how to calculate the maximum and minimum slug length for a slug load stress analsis?

Thank you

There is no calculation. This is an educated guess only.

Hi Richard,

Thank you for your reply.

Could you give me some idea with this regard.

Regards

Here's one: say you have a fluid line you shut down. MAYBE a certain slug can't be longer than a certain expansion loop?

Like I said, every situation is different, and there is experience involved.

Provided that the liquid slug fills a bend, I suggest that it doesn't really matter how long a liquid slug is after that. It's the change of state form vapour to liquid filled that is most important in terms of dynamic loads.

Slug length may be calculated as

Liquid volume fraction X Length between two elbows

Ankit,

Sorry, I think your calculation is meaningless. There is no generalisation for evaluating slug length. Any analysis would have to address diameter, pressure, viscosity, velocity and a number of other factors too.

The length between elbows is not relevant to slug length.

I wonder why you posted your comment, nearly three years after the previous one ?

Hi MoverZ,

I'd request you to be a bit more clear with your intent while writing "Provided that the liquid slug fills a bend, I suggest that it doesn't really matter how long a liquid slug is after that"
or re-consider your statement while quoting the above.

Reason being,
Slug being a impulsive force, when analyzed dynamically, have somewhat trapezoidal load-time waveform with
1) defined RISE TIME/FALL TIME - depending on RATIO of ARC LENGTH OF ELBOW (when elbow is considered as change in direction) & slug velocity and
2) defined IMPACT DURATION - depending on RATIO of LENGTH OF SLUG & slug velocity.

Now as you may be aware, or can be deduced from above relation, for a defined arc length (i.e.elbow), if slug length is
a) small - duration of impact decreases, thus decreasing the impact (impulse)overall time.
b) large - duration of impact increases, thus increasing the impact(impulse) overall time.

In both the cases, what changes is the length of constant force in the trapezoidal waveform with respect to rise/fall time thus leading to change in DLF computation and hence, the overall applied loading at the change in direction.

Dinesh,

Simply put, the force that results from changing the direction of a fluid column is a function of of its mass and velocity and angle, as governed by the laws of change of momentum. (rho x v^2 x A)

Due to the volume of an elbow, you can only change the direction of a given amount of fluid, that being the filled contents of the elbow in a given time. Once that is achieved, it does not matter if the succeeding slug of liquid is 1m diameter or 1000m long. the force due to change of momentum remains the same.

Your comment about trapizoidal loads is correct but only applies to the intial change of state. I was referring to the general, established flow case, in relation to Ankit's erroneous comment.

See also
http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=showflat&Number=60278

MoverZ..
i agree what you say.. all the parameters are needed to do the slug ANALYSIS... I have said "SLUG LENGTH".. and i also agree that there is no effect on slug force due to slug length..

Slug length is useful while doing spectrum analysis.. i.e. TIME HISTORY..

Hi,

Attached presentation titled "03_Ernstsen_Jensen_Slug force presentation" further emphasizes the importance of slug length into consideration. However it is based on slug length reasonably small than compared to the length of the elbow (i.e. the entire elbow is only partially filled with the slug).

This again is quiet different when the SLUG FULLY FILLS THE ELBOW WHILE ITS TRAVEL. As answered by Moverz, "once the filled contents of the elbow in a given time is achieved, it does not matter if the succeeding slug of liquid is 1m diameter or 1000m long. The force due to change of momentum remains the same". Its true that it doesn’t matters MUCH (except some changes in DLF value, while performing spectrum analysis), but what if the given bend turns out to of non- regular size i.e 5D, 10D or at times 40D as is the common case in large pipelines? In this case unless slug length is known, we don’t know whether the bend is partially filled or fully filled. For example, a 20” pipeline with 10D bend 5m length of slug is required to completely fill the elbow and slug length less than this only partially fills it.

As illustrated in the attached presentation by Mariog, if the impact force calculation is done by calculating the ANGLE THE SLUG ACTUALLY ‘FILLS’ the bend (sheet 18 of ppt.) (which again is dependent on length of slug) in the relation Fr = dp / dt =ρv2 A [2(1 - cos θ)]1/2 given in CAESAR II user guide, and derivative of the same in the ppt itself, the slug force reduces drastically.

As known and can also be calculated from CAESAR II spectrum generator, for the partially filled slug, peak DLF is low than that when the bend is completely filled with slug.

Based on the attached ppt. by Mariog can this be interpreted as “BY INCREASING THE BEND RADIUS, SLUG IMPACT FORCE CAN BE MINIMIZED KEEPING THE SLUG LENGTH CONSTANT?”

Dear Forum members,

Do you have any idea or suggestion regarding the above posted question in relation to the effect of INDUCED forces with change in bend radius?

Awaiting responses!

Bend radius affects the rise and fall time of the momentum impulse on the bend. The length of the slug will affect the overall duration of the pulse. A tighter bend will shorted the rise time. A quicker rise time will increase the high frequency response. A longer duration increases the lower frequency response. ...in general.

Hi Dave,
Thanks for your response!

However, I’d be a lot more thankful for a response seems to address the concern in entirety.

If I read your statement this way that TIGHTER BENDS and SHORT SLUG DURATION increases the high frequency response, I can assume that the vice versa i.e. longer bends (long radius) and longer slug duration, increases the lower frequency response.

This though points in direction where slug INDUCED forces can be reduced by increasing the rise time what I mean is that, has it been practically considered in industry to INCREASE THE BEND RADIUS AS A MEASURE to decrease the slug loads on the piping system?

Changing bend radius will change the attack time of the slug impulse. But the induced load is a function of both the system timing (i.e., natural frequency & mode shape) and the event timing (attack time & duration). Without considering system timing, I do not believe you can categorically say that the induced load will always fall if attack is slower.

Thanks a lot Dave for your response!