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Increase pipe schedule - influence on pump ?

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cedeng

cedeng

Materials
Mar 23, 2005
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CA
Hi

We have some corrosion problem in my acid piping and I would like to increase the pipe schedule from SCH 10 to SCH 40 for 3'' and 6'' diameter (SS316L).

I want to know if the increase in pressure drop (lowering the ID) will affect my pump (total Head). Is it significant ?

How can I confirm if I can do the change for a 700 ft long corroded piping line? Do I have to calculate all my system and simulate the modification ? It should exist a table of something to see if there is a significant influence. I don't even think that SCH was original on the design...

Thanks for you input

Cedric
 
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Go to the tables and get the new ID's. Then do the pressure drop calculations based on this new diameters and you'll have your answer

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>
 
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Pressure drop will change inversely as the square of the diameter change.
For example: new diameter .9 times original, new pressure drop 1.23 times original pressure drop.

Ted
 
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But is there any rules that allow me to make the change without any more calculations ?

It is a 1000 ft line in the aire with difficulty of access. I want to make the calculations only if necessary.
 
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If you do not want to do calculations just change and see what happens. That might be more difficult than just measuring (estimating) I think.
I do not understand your last post

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>
 
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Artisi - Would you mean (known pressure loss)x(dia A/dia B)^5 = New pressure loss??

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>
 
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I can't comment on the hydraulics.

However, increasing the schedule of piping connected to a pump will adversely affect mechanical loads imposed on the pump.

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

"All the world is a Spring"

All opinions expressed here are my own and not my company's.
 
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A temperature change of T1 to T2 produces a stress, which when multiplied by cross-sectional area of steel, equals thermal axial forces, so a schedule change makes for possible increases in pipe loads to equipment and other points of resistance to thermal growth. If changes are significant to the existing pipe configuration, flexibility may have to be increased, anchors moved or new ones installed. A new stress run might be required to verify continued compliance.

"If stupidity got us into this mess, then why can't it get us out?" - Will Rogers (1879-1935) ***************
 
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Pumps carrying hydraulic and thermal thrusts might be ok in the oil industry with centrelinre mounted pumps etc - but in the process industry the aim is to eliminate all thrust from the pump casings as they are not realy designed for carrying anything other than nominal external loads.

Further, lowering the friction loss which was the original post by increasing pipe dia. will also have the effect of lowering discharge pressure.

Don't have time to check the relative thrusts -- increased dia./ lower pressure / smaller dia./ higher pressure, too busy playing builder renovating my apartment.
 
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Small allowable loads are exactly the problem. Pumps can be very sensitive to any increase in load, however small. Thermal stress is usually the controlling factor, being many times larger than pressure stress.

I'm not going to check it here either, primarily because nobody asked about it.

"If stupidity got us into this mess, then why can't it get us out?" - Will Rogers (1879-1935) ***************
 
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No, the axial thermal stress is dependent on temperature change, coefficient of thermal expansion [&alpha;] (alpha), modulus of elasticity, and the degree of fixity (netspring constant) of the axial restraint on the pipe. Since dT is Fº[&alpha;] is in units of in/in/Fº and E is psi, you are left with lbs force.

[;sigma]t = dT * [&alpha;] * E

F = [&sigma;] * x-sect Area
and imparts compressive stress on the pipe when temperature is increased.

****************************
Pressure stress in the axial direction Sa = Poisson's ratio (v = around 0.25) * pressure (or hoop) stress,
Sa = v * PD/2/t, which acts to shorten the pipe length or cause an axial tensile stress, if the pipe is restrained in the axial direction. So it tends to reduce the axial thermal stress, if that stress was caused by the usual increasing temperature.

If hoop stress is limited to say, yield stress / 2, pressure stress = 0.25 * YS/2, or 0.125 * YS
Steel pipe thermal stress is about 0.000006/Fº * E * dT, or
0.000006 * 30,000,000 = 180 psi/Fº

Take a 6" discharge pipe change from sch 10 to 40, dT=50 Fº
disch pressure 700 psig. Axial pressure stress reduces to less than 1/2 with the heavier WT, but axial thermal stress increases by 2 from 26 Kips to 54 Kips with the x-s area, if fully axial restrained. Since pressure stress, when restrained, is tension any compressive thermal stresses are reduced. The net effect to the above being that the 6" sch 40 net axial load is almost 3 times the axial load of a 6" sch 10, in a fully restrained case, holding discharge pressure equal in both cases. However, if he doubled the discharge pressure, the increased axial tension would reduce the compressive thermal load in the sch 40 such that the net axial compressive load becomes only 2 x that of the sch 10. Hopefully when the discharge pressure is reduced, the pipe cools off.

The most critical stresses can be generated in cold piping, where axial tension from both thermal and pressure cases add together and total shear stress is increased. The only good thing about that is that net axial tension does not buckle pipe.

Thermal stress * x-sect area is the largest component of change.





"If stupidity got us into this mess, then why can't it get us out?" - Will Rogers (1879-1935) ***************
 
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