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When to use more than two saddles?

When to use more than two saddles?

When to use more than two saddles?


I've been looking at some old drawings of refrigerated CO2 storage vessels, with capacities ranging from 500- 600 tons (DIA 3000-3800, length (30-50m), and all of them had 3 saddle supports.

In the searches I made, I've seen that it is not a good practice to use more than 2 saddles, and saw some people saying that have designed longer vessels with only 2 saddles.

I'm studying a case for a 3800mm diameter vessel , 44000mm long, with hemispherical caps and design pressure of 2.4 MPa, using ASME VIII-2.

My thinking is that a first try with two saddles should be made, and if the stresses fail the stress analysis (Per Part 4 of ASME-2), I should add an extra saddle to lower the vertical loads and see the results. Of course there will be the problem of differential settlement of the saddles, which will cause different loadings, but I guess that these loads can be factored towards safety (any thoughts on the factors to use?)

I also wonder if there are any practical rules of thumb to quickly evaluate the need of more than two saddles?

RE: When to use more than two saddles?

Not a new question


What I did not see in these links is a discussion on the what happens when the vessel bends under thermal loading. There could cooler fluid on the bottom than the top and this will bend the vessel and completely off load one of the supports. You can see this happen with large pipe and short spans.

RE: When to use more than two saddles?

Hi Kevin,

I'm aware this is not a new question, but in my searches never find a good answer, lots of different opinions. Thank you for the links, I was studying the method of EN 13445 which incorporates multiple saddles, but I'm trying to use only ASME VIII-2

From my studies, Part 4 is only applicable for two saddles, for multiple saddle configurations it refers to Part 5. If I do carry a FEA study, I see myself with two options

1 - Model the vessel without the supports, imprinting the face that is contact with the saddle and then fixing it.

2- Model the vessel with the saddles.

My questions:

In the first scenario, I'll not be accounting the flexibility of the saddle. From your perspective, is this considered a bad practice?

On the second scenario, Flexibility of the vessel will be achieved, but since I'm conducting a elastic-plastic analysis, I won't know if failure is occurring at the saddle on at the vessel. Also, I have no acceptance criteria for this kind of analysis for the saddle (can anyone recommend any literature on this?)

RE: When to use more than two saddles?

See Pressure Vessel Design Manual, by D. Moss, for optimum tank proportion, and ovoid more than 2 saddles.
Also avoid PWHT.


RE: When to use more than two saddles?

Remember, if you have to hydrotest the tank in the field, then you should crib the center of the tank if you have only two saddles. I suspect a hydro will be in order therefore the middle saddle can be a substitute for cribbing the tank. Just make sure that the soil bearing pressure has the same value for uniform settlement under the three footings. I have seen, only one propane tank with three saddles, over a period of 45 years in the Northeast but this tank was well over 50 feet long.

RE: When to use more than two saddles?

If you use a crib the test is false.
Hydrostatic test shall be at the shop, unless special case for horizontal pressure vessel.


RE: When to use more than two saddles?

I assume that you are going to be using some kind of Austenitic Stainless Steel based on the low temperatures necessary to keep CO2 in the liquid tank. Is this true ?

I question your wisdom in designing such an extremely long tank (44000mm), with three saddles that would be subject to high axial expansion and contraction. You are asking for trouble ...

Stick with a more conventional proven design. Discuss your specific needs with experienced vessel fabricators.

Does anyone else have an equally long tank in similar CO2 service ?

Are you specifying a unique tank ?

Could your needs be met with two (or more) smaller/shorter tanks ?

Please keep us informed about your final design .....

Sr. Process Engineer

RE: When to use more than two saddles?

Hi MJCronin,

I`m not designing this vessel for any client, as I said I'm just studying the case based on some old drawing I found with little information on them.

Material is SA-516 gr70N, CO2 is kept liquid at a temperature of -20 - -29 at pressures from 16-24bar

I`m aware it's not good practice to use more than two saddles. But for such a long vessel, my initial thought was that it might be the case.

I've seen long CO2 storage vessels supported by 3 saddles, but I reckon it is not common at all.

Still studying this case, when and if I have a final position I shall post here :)

RE: When to use more than two saddles?

Do a simple hand calculation to establish how much the vessel can flex without exceeding its allowable stress.

You will find the maximum force from this simple calc which can then be plugged into the zicks calc.

Costly FEM avoided.

RE: When to use more than two saddles?

R6155, if you do not support the tank in its center line, it may collapse when it is full of water when supported on two piers.. I am not sure how it would be a false test because the hydro is to detect leaks around welded joints and to test the integrity of the tank. A sagging tank when full of water is not part of an integrity check.

RE: When to use more than two saddles?

chicopee. You are wrong 100%

Please see UG-99 in full and
The loadings to be considered in designing a vessel
shall include those from:
(a) internal or external design pressure (as defined in
UG-21) ;
(b) weight of the vessel and normal contents under operating
or test conditions;


RE: When to use more than two saddles?

r6155, I disagree with you,

UG-22 says "weight of the vessel and normal contents under operating or test conditions".

It says "normal contents" under test conditions. It doesn't say "test contents" under test conditions.

UG-22 also states that the following loads be considered:
(j) test pressure and coincident static head acting during the test
If there was a need to consider the weight of "test contents", then it would be specified here.

We apply UG-22 to a vessel that has temporary saddles (or sand bags) during hydro testing all the time.

Your interpretation of UG-22 is overly restrictive and just makes no sense.

RE: When to use more than two saddles?

Quote (r6155)

chicopee. You are wrong 100%

I disagree... There is nothing to say that a vessel must be supported during hydrotest in exactly the same way as it will be during operation.

Take the extreme example of a tall distillation column that will operate in gas service. These are routinely laid down on their sides and supported by dunnage during the hydrotest. If your argument is that only a vessel's permanent supports are allowed during a hydrotest, how do you then propose to hydrotest tall distillation columns? In the vertical position?

RE: When to use more than two saddles?

I am referring to a horizontal pressure vessel which needs to be hydro tested in the field, not in the shop. I personally would not want this vessel filled with water without cribbing the middle section of the tank. The conditions that require the vessel to be hydro tested in the field vary such as the client purchasing a second hand vessel with unknown history, vessel had undergone major repair in the field, exposure to a nearby fire, accidental contact between vessel and vehicle.

RE: When to use more than two saddles?

More reading see:
1) Pressure Vessel Design Manual, D. Moss
Vertical Load per Saddle

For loads due to the following causes, use the given formulas.
• Operating weight.
Qo =Wo / 2

• Test weight.
QT= WT / 2 QT = load per saddle, test, lb

2) Pressure vessel design Handbook, H. Bednar
Shop test dead load of the vessel consists only of the weight of the vessel, after all welding is finished, filled with test liquid.

3) Full design example calculation for horizontal vessel
PD CEN/TR 13445-101:2015
Unfired pressure vessels
Part 101: Example of application

EN 13445 -3
5 Calculations and design
5.1 General
In this Clause 5 of this report, design by formulas proposed by EN 13445-3:2009 will be used to determine the
required thicknesses and therefore the nominal thicknesses of pressure vessel’s components.
Verification of opening and of the resistance of the shell on two symmetrically saddle supports is also performed.
Normal operating load case and test load case are taken into account.

NOTE W is the total weight of the pressure vessel including the content. To calculate W, a density of
7 900 kg/m3 for steel have been taken. For the content (fluid weight WF), a density of 540 kg/m3 have been taken
for normal operating load case and 1 000 kg/m3 for testing load case


RE: When to use more than two saddles?

There are many references that provide methods for calculating the loads on saddles during a hydrotest. If there is minimal design impact in doing so, it can be a nice approach and it may actually be simpler than coming up with alternate hydrotest support methods.

What we have to ask though, is what does the official design code require. In this case we are discussing an ASME VIII-1 application, and UG-99 does not state anywhere that a vessel must only sit on its permanent supports during the hydrotest.

As I stated previously, this would be an impossible requirement for tall process columns. They are normally tested in the horizontal position.

Our shop also works in high-value materials such as Titanium. These can be large thin-walled horizontal vessels supported on two saddles that are only subject to gas service (no liquid). We would likely have to triple the wall thickness of some of these vessels if they had to support the hydrotest weight on the two saddles alone. When you are talking about expensive materials like Titanium, this cost would be atrocious!

RE: When to use more than two saddles?

The post is for CS horizontal pressure vessel, please do not talk about tall tower
The post includes british standard. Also ASME VIII Div 1 includes BS as reference
Hydrostatic test also is to verify the saddles, not only pressure parts

For large horizontal pressure vessel I recommend Acoustic Emission Test during hydrostatic test.

In your case (Titanium) you can use pneumatic pressure test, but this is another post, IMO - See Appendix G


RE: When to use more than two saddles?

If it is too dangerous to conduct a pneumatic test, it is a waste to scrap a perfectly good pressure vessel.

The purpose of the pressure test is to assess the integrity of the vessel (including saddles) with 1.3x design internal pressure and the 'operating' contents (not test contents). UG-22 makes this clear.

If the operating contents is air and you decide to emulate the weight of this operating contents by using water, then the saddle loading will be 10 times or more than during design or operating conditions. Testing at 10 times design saddle loading is extreme considering that the test pressure only needs to be 1.3x design pressure.

You interpretation means that the vessel needs to be designed around the hydrotest, which is non-sensical.

RE: When to use more than two saddles?

Hydrostatic pressure test also is dangerous. See “Case 102: Use caution when specifying a safe hydrotesting distance”, Hydrocarbon Processing magazine August 2018.

Acoustic Emission test may be used as a complementary measure for pneumatic test and combined
hydrostatic/pneumatic test.

Shop test dead load of the vessel consists only of the weight of the vessel, after all welding is finished, filled with test liquid.


RE: When to use more than two saddles?

ASME div 2 section 8.1.1 says that a pneumatic test can be substituted for a hyrotest under the following condition:

"(1) The vessel is constructed and supported such that the weight of the hydrostatic test fluid could cause permanent visible distortion."

Hydrotesting is always the preferred choice. Only if after adding additional temporary test supports to a vertical or horizontal vessel the stresses are still calculated to be too high, the clause above can be used.

For a hydrotest you need to stand back a few meters to avoid getting a high pressure squirt of water in your eye.

For a pneumatic test of the same vessel, you need to stand back a few hundred meters even with an Acoustic Emission test, for obvious reasons.

RE: When to use more than two saddles?

See ASME PCC-2 Article 5.1, Mandatory Appendix III
Safe Distance Calculations for Pneumatic Pressure Test


RE: When to use more than two saddles?

Like I said...... Maybe multiple vessels are a better choice !

Sr. Process Engineer

RE: When to use more than two saddles?

Maybe consider using a soil bearing foundation for long bullet vessels, similar to a mounded LPG bullet (EEMUA 190), rather than resorting to 3 or more saddles (?).

RE: When to use more than two saddles?

I found in my library 2 horizontal large pressure vessel with 2 saddles

ID= 6000 mm TL-TL= 56000 mm Ellip. heads
Empty= 355000 kg
Operating= 2716000 kg
Field hydrotest= 3634000 kg
Specific gravity of fluid= 0,534
Shell thk= 30, 32 and 34 mm

ID= 7500 mm TL-TL= 56000 mm hemispherical heads
Empty= 875000 kg
Operating= 4098000 kg
Field hydrotest= 5771900 kg
Specific gravity of fluid= 0,460
Shell thk= 62 mm


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