Slug Catcher Forces

[aseeng]

I am designing a foundation for a finger type slug catcher consisting of 36 in diameter pipe. I have done these in the past by using typical friction forces on the support but I have had one client say that a slug catcher moved quite a bit on startup. Prior to design he insisted that there were no unusual forces associated with this type of equipment. Is there a method to predict any lateral forces on these that should be accounted for in the support design?

 

[C2it]

Could be thermal expansion or pressure expansion or both.

Most likely the dynamic effect of an incoming liquid slug impact ?

 

[rbel038]

We look at the two phase flow regime for the pipe , and design the supports to withstand a slug at the expected pressure and velocity at the support location.

 

[aseeng]

Thanks for the replies. I would expect that the pipeline designer should know the velocity and pressures at the inlet? I'll see if I can get that info and compare against the friction forces due to expansion.

 

[LittleInch]

Does depend on the orientation of the inlet line. If it comes in from the side then there may be some reaction force, but if they design the inlet pipework right it should disipate the fluid flow before it getws to that point. However there are many poor designs of slug catcher so your client may have had experience of one of those.

If you can post what your inlet pipework configuration looks like then you may get some more informed comments.

However as noted in onother psot somewhere, slug catcher diesign and loadings are more art than calcualtion. You can get it a magnitude out if you calculate it wrong....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[aseeng]

They decided on moving the inlet to the vertical position. 12" dia. inlet comes down to a 36" dia. manifold that splits to (2) 36" dia. sloping fingers. They described a loop or some routing to achieve flexibility prior to reaching the vertical inlet that would avoid any horizontal forces. I calculated a very small force (500 lbs) due to the predicted 15 ft / sec inlet velocity and came up with a very large force ( 33 kips) due to the 300 psi pressure. This sits 10 ft up in the air on 36" dia. drilled piers. My foundation is definitely strong ( and hopefully strong enough).

 

[BigInch]

Slug forces = Mass of slug X acceleration, not P*A

Independent events are seldomly independent.

 

[LittleInch]

I hate term "slug" as it always gives people the impression that there is this solid thing hurtling down the pipe, when what it really is is an increase in liquid flow in a two phase system which will always have some gas in it. I always ask the FA wonks what the liquid fraction is of the "slug", which usually gives you a much more balanced view of the whole thing. It's always less than 1 and often less than 0.5.

When it comes to forces I agree that Purdue has nothing to do with it. I usually go for a change of momentum calc for forces on a bend based on the additional flow that arises during a liquid surge event (aka slug). Be careful about the velocity you use and mass of the surge volume. For piping, there are some increases in load which can be significant to move pipe which is only simply supported, but there's no way it's 15 tonnes force if you've only got an inlet velocity of less than 5 m/sec. Moving the inlet vertical is a good idea.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[aseeng]

I agree Biginch, f = m x a. Now getting the acceleration is the trick. The vertical inlet seemed like a good idea too. I designed for horizontal friction forces due to thermal growth, coefficient of friction steel / steel = 0.5 and ended up with design force = 7.5 kips. Client felt that would cover the slug forces. Thanks for all the info.

 

[BigInch]

Not possible. Your client is not correct. Slug forces are additive to thermal growth, plus they are dynamic, so ultimate load concrete design requires factors of 1.4 for thermal growth and 1.7 for slug forces.

Independent events are seldomly independent.

 

[aseeng]

Hmmmm.. that's food for thought BigInch. Yes, slug force would be dynamic. We have always considered thermal forces ( on typical piperack design) to be dissipated and to not act concurrently with seismic forces ( also a dynamic force). For current ACI code we use 1.2 DL + 1.6 LL. I consider the thermal growth as LL as it is transient. I guess it could be argued that it is a function of DL and you could justify the lower load factor. Don't know of any impact factor in the ACI code other than the engineers judgement to increase it.

 

[BigInch]

Slugs are dynamic.
And ...
Within a typical pipe racks only 10% of the weight of the pipe is used (refinery design practice of major EPC companies) as a net thermal load in the longitudinal direction, because it is assumed that 45% of the pipe is moving one way and 55% of the pipe is moving the other, thus cancelling out 90%. Not 100%

Thermal is not live or transient, unless it is the result of a diurnal heating of the pipe, or something similar. But that is not the only criteria used to determine the load factor. The degree of accuracy to which the load can be calculated is also a consideration. For example, water loads can be live loads, yet a factor of 1.2 is applied to those, yet wind is 1.6 due to the relatively unknown maximum velocity and buffeting effects. Pipe stress from plant starts and stops is generally considered a dead load. Furthermore it is usually a self-limiting load, that if reached may impart some damage, but probably would not cause the collapse of the structure. Usually by that time the pipe had buckled anyway. Calculate that buckling load first. 1.6 seems high under those circumstances. This reference, and IMO ACI318 itself, suggests 1.2

http://faculty.delhi.edu/hultendc/AECT480-Lecture 2-REV.pdf

Independent events are seldomly independent.