Insta-Plug™ New low-pressure Feedwater heater and condenser tube plug Not to leak or fall out for the life of the exchanger
American Power SystemsDan Bina
Hollaender Enterprises, Inc.Roger Adelman
Plugging tubes: A problem with many solutions
Metallurgical Plugs Metallurgical bonding mechanisms:
Welded, brazed, soldered Explosive (kinetic)
Mechanical Plugs Friction/interference mechanisms:
Tapered cylinders Tapered cylinder in mating sleeve Taper around a cylinder
(Screws/Compressors)
Mechanical PlugsPrimary Advantages:
Quick to install Simple to install Relatively low cost Effective for lower pressures
Primary Disadvantages:Failure, particularly with cycling
Why Plugs Fail (1)
Forces on a stuck mechanical plug
Ff1 = μN1
N1
2θ
Ff = μN2N2
BlowoutAlternativeBlowout
Why Plugs Fail (2)
Friction and normal force competition
Ff1 = μN1
N1
2θ
Ff = μN2N2
BlowoutAlternativeBlowout
Why Plugs Fail (3)
Friction and normal force competition
Ff1 = μN1
N1
θ
Ff1 = μN1
N1
θ
1. As installed 2. Possible In use
Why Plugs Fail (4)
Friction loses the competition
Ff = μN
N
θ
Nx >Ffx , even the slightest amount and plug slips
FfxNx
Ny
Ffy
Why Plugs Fail (4)
How can that happen?
Differential behavior of tube and plug Changes in coefficient of friction Mechanical dynamics Infiltration of intermediate material Other (micro cracking, etc.)
IT IS A FAIL “UNSAFE” CONDITION:
Once it begins it tends to continue to loosenOnce it begins it tends to continue to loosen
A different mechanical approach:InstaPlug™
InstaPlug™ A non-wedge based plug
NITINOL REPRESENTATION
0
10,000
20,000
30,000
40,000
50,000
60,000
70,000
0 0.02 0.04 0.06 0.08 0.1
Strain (in/in)
Str
ess
(psi
)
unique one-piece shape + a material with very special properties
NiTiNol or “Memory Metal”
Nickel/Titanium alloy Exists in two allotropic forms (atomic structures)
Martensite (at lower temperatures) Austenite (at higher temperatures)
Special processing gives two important properties Ability to “remember” a first shape after being in second Superelasticity – almost six times “normal” metals
Inert enough to be widely used in medical implants Retains large elastic energy when press-fit
Simple positive process (1) Create the plug and shrink it
As machined d+~0.004” Reduced diameter for installation
Heat treatment
Simple positive process (2) Install into well prepared tube/tubesheet
a. Place in clean tube end
b. Heat to restore “memorized”
interference fit
Insta-Plug common sizes
Engineering and Design
NOT the simple analysis used for exposition here Sophisticated Finite Element Analysis for
Deflection Force Stress/strain distribution Thermal analysis
Conclusion
We believe that we now have cost effective, and reliable means in the Insta-Plug™ that will:
not fail in service regardless of the cycling conditions provide for installation that is fast, safe and simple be cost competitive with other mechanical plugs account for real world service demands e.g. thermal cycling
We intend that Insta-Plug™, like our experience with explosive welded plugs for high pressure feedwater heaters, will “last the lifetime” of the heat exchanger.
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