OK short course, I hope.
Thrust blocks can also be required for normal operating pressures. Example
2500 PSF soil will not move with 10 psi of pressure. (How many of your
underground systems only have normal 10 PSI?) but at 50 PSI can that ground
support the normal thrust? NO! that almost 7200 PSF of thrust.( 144 SI = 1
SF)(Over simplification I know, but basically you can't rely on normal push
in or Mechanical Joints for much restraint. So you're looking for the Dirt
to restrain the pipe.) Look at water hammer.
How is water hammer (Or any other type of thrust) expressed in a line of
pipe with let's say elbows on each end and a closed valve after one of those
elbows? (Water hammer requires a closed system.) Velocity then is changed to
thrust which is measured in PSI. This thrust is exerted on the pipe
typically most severely on the lineal axis's. The thrust is exerted to the
sides as well, but that is not a great concern unless the PSI exceed the
pipe rating.
X= closed gate valve
<= direction of thrust
^
X I
I I
< I_______________________I>
V v
There is a sine wave thing we could look at but I find when explaining it to
fitters, it's easier to keep it simple, and being a long ago fitter myself
this is how I learned it. Water hammer is a high velocity wave that hits a
dead end. Some of that pressure is reflected back along the path of origin.
But most of that pressure is transferred into increased pressure in the
direction of the path of travel for that water.
This increased pressure will try to push in both directions along the main
as indicated by <----> there is also pressure ^ and V but let's focus on the
main. What that pressure tries to do is push the two ends apart.
If your pipe is TYTON joint (push in Gasket type) Or normal MJ, without
joint restraint, and no thrust blocks are installed at the <or> locations a
joint can separate. 2500 PSF soil with pipe (will only support a 17-18 PSI
thrust) on the end of the elbow, without giving. (I know the surface area of
the el is greater than the pipe, and the piece 90' to the main will also
provide some restraint. But just to keep it simple.) So we provide thrust
blocks or joint restraint per chapter 10 of 13 or NFPA 24. The increased
pressure doesn't have to be from water hammer. It could be a failed Pressure
reducing valve or station, thermal expansion of a closed system, back
pressure or several other situations.
Basically what we're trying to do with thrust blocks is restrain the pipe
from pulling/pushing the joints apart. Restraint flanges such a MEGA LUG use
set screws. There is a new Tyton push joint with a restraint device in the
gasket that has metal fingers that allow the pipe to be pushed in but bite
into the pipe if it tries to push out. Thrust blocks were just one of the
first methods we developed for joint restraint. Grooved joints is another,
Roding between fittings is another, and mega lugs are yet another. Are there
other means and methods? Sure! And an engineer can create even more.
Thom McMahon, SET
Firetech, Inc.
2560 Copper Ridge Dr
P.O. Box 882136
Steamboat Springs, CO 80488
Tel: 970-879-7952
Fax: 970-879-7926
Subject: RE: Grooved Underground, Thrust Blocks Required?
Perhaps this is semantics. Or perhaps I admittedly know very little about
UG because we sprinkler guys don't do this in this neck of the woods. You
said
"So unless you are connecting to a pipe, fitting or joint type that can come
apart under increased pressure, lack of compaction or some type of movement,
you don't need thrust blocks."
I thought thrust blocks (or restrained joints) were to deal with the force
generated by the moving mass of water changing direction at a fitting. Not
really a pressure thing. Footings (?) would be to deal with soil compaction
or support issues?
Chris Cahill, P.E.
Fire Protection Engineer
Sentry Fire Protection, Inc.
763-658-4483
763-658-4921 fax
Email: [email protected]
Mail: P.O. Box 69
Waverly, MN 55390
Location: 4439 Hwy 12 SW
Waverly, MN 55390
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Thom McMahon
Sent: Friday, March 26, 2010 10:54 AM
To: [email protected]
Subject: RE: Grooved Underground, Thrust Blocks Required?
The question of would thrust blocks be required is almost self answering.
Why do we use thrust blocks? To prevent pipe movement.
Why do we want to prevent pipe movement? So the pipe joints don't come
apart.
Do Groove joints come apart by increased pressure or lack of compaction of
soils? No.
So unless you are connecting to a pipe, fitting or joint type that can come
apart under increased pressure, lack of compaction or some type of movement,
you don't need thrust blocks.
Thom McMahon, SET
Firetech, Inc.
2560 Copper Ridge Dr
P.O. Box 882136
Steamboat Springs, CO 80488
Tel: 970-879-7952
Fax: 970-879-7926
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Vince Sabolik
Sent: Friday, March 26, 2010 9:52 AM
To: [email protected]
Subject: Re: Grooved Underground, Thrust Blocks Required?
No, this is a minor amount of pipe, connecting one part of a
system to another. I'm sure that this has been done by most everyone here
at one time or another. My question was "Would thrust blocks be required"?
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