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Cement darts contain a bursting disc which bursts under given conditions
at a pressure which is reliably known in advance. Thus cement may be
discharged from coil tubing in a controlled manner. The technique is
not limited to cement and may be used for any fluid or fluidised solids,
but is most usually used for cementing. Cement darts are based mostly
on nickel foils which are not suitable for strong acids - see the Acid
Darts section 6.6 - but acid darts are suitable as cement darts. Darts
may be adapted in various ways to suit customer operating needs, and
often have been.
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Materials: Generally based on nickel foil welded
between stainless steel flanges and with a nose and body
encapsulated in polyurethane, shaped to provide a seal with the tubing.
Some are now based where required on Alloy 600, Alloy C-276 or SS316.
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Stock: We try to keep some of the range
of sizes usually in stock. This means the metal cores carrying the disc,
not the made up dart. This is not entirely practicable as the ranges
have grown. Particularly for the new large sizes for 2 3/8", 2 7/8"
and 3 ½" tubing the cores are not stocked, though for these we keep
standard parts from which to make up the desired cores for the
required pressure.
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