We have often had to deal with vertical displacements of more that 12" (sometimes up to 22") but we do not buy off the shelf constant weights. For these large movements the commercial ones tend to be very large and often do not fit in with the piing design, if you can find one that will handle the range required.
We design our own constant weights for these situation. One design we often use is a sew-saw type support. You can use this design to support above or below your pipe depending on available space. Or if you are in an envirnment where you can get away with a very basic design we have seen ones with a 40 gallon drum full of concrete attached to hte pipe via some pulleys and cables, you can alter the concrete weight to get your required weight.
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Miss Itchy