SylvanLMP
Rodney BECAREFUL.
I normally agree with my good friend Harold but there is a lot more involved then just replacing pipe and fittings.
Circulating systems.
The most common failure seen in circulating
systems is erosion/corrosion. Erosion/corrosion is a mechanically induced
failure which may be caused by any or all of the following conditions:
1. Water at high velocity. High water velocity may be caused by an
undersized piping system or an oversized pump. One of the interesting myths
here is the notion that a return line failed because Type M copper was used,
so the repair is to install Type L copper. Since Type L copper has a smaller
ID than Type M copper, the velocity in the system is actually increased,
raising both the likelihood and rate of erosion that may take place. The
real solution is to lower the water velocity. Installation of a smaller
capacity pump or a throttling bypass on the existing pump should help in
lowering the velocity of the water in the system. The relationship is as
follows: Reduce pipe size, increase velocity; increase pipe size, reduce
velocity. Recommended velocity for hot water in a copper tube system is 4 –
5 feet per second (fps). If systems are designed to respect these velocities
the return lines will last as long as the rest of the system. The practice
of reducing tube diameter for the return line run is counter-productive and
is the main cause of high velocity in these systems. Since there is no
sizing guide in the Model Codes for return lines, there is little guidance
for the plumber on how to size this portion of the system. In the course of
installing or replacing circulating lines, some contractors have been
increasing the size of the return loop piping to 1″ or at least the same
diameter as the hot water supply out of the water heater.
2. Numerous, abrupt changes in direction in the piping system. Where
structural conditions cause numerous directional changes, long radius (1.5
Diameter) fittings should be used to minimize the interruption of laminar
flow. Pump manufacturers also have recommendations that limit the number of
changes in direction near the pump and the minimum distances these can be
installed from the pump. This is to protect the pump from these same
erosion/corrosion processes (cavitation). This is the main reason why you
would not use a corrugated connector on these pump installations. Flow
characteristics, the number and severity of the bends that could be made in
these connectors may be detrimental to the pump.
3. Lack of reaming the tube ends. Burrs left on the ID of the tube can
cause interruption of laminar flow resulting in localized high water
velocity and cavitation.
4. Protrusions into the flow stream caused by excessive lumps of
solder/brazing material, improperly fabricated tees (branch protruding into
the run pipe), etc. These protrusions can also cause the
interruption of laminar flow resulting in localized high water velocity and
cavitation.
5. Excessive water temperature. Heating water above 140 degrees Fahrenheit
can accelerate the process of erosion/corrosion. As the temperature
increases, the velocity should be lowered. At 180 degrees Fahrenheit, the
velocity should be down around 2 – 3 feet per second (fps).
Bear in mind that these conditions are not peculiar to copper tube, but can
affect other materials as well. However, when erosion/corrosion does occur
in copper tube it is readily identifiable by the horseshoe shaped pitting
throughout the inside of the tubes. Identification OD these pits or grooved
corrosion-product-free areas on the inside of the tube can help you
determine that water velocity is too high and that the reduction of velocity
can correct the condition.
I would strongly suggest you vist Harolds web site of contact your local Licensed Master plumber to properly design these systems.
[Edited by Moderator on 26 September 2000]
