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Rubber gaskets for ductile iron pipe systems
Which came first – the pipe or the seal?
by Felice Pavan
1 Introduction
It may have been pipes which existed first but
it was only with seals that they could be connected
into pipelines. The story of the transporting
of drinking water is therefore also the story
of sealing. In 1724, in his textbook on hydraulic
engineering, Leupold [1] described the problem
of making a permanently watertight connection
between pipes. In the early days the gap at the
socket was sealed with a type of permanently
elastic putty, but later on the gap was packed
with hemp and lead (Fig. 1). A further development
was gaskets made of vulcanised natural
rubber.
30
Packed socket
Lead Hemp
Fig. 1:
Section through a packed socket joint
Today, it is only gaskets made of synthetic rubber
which are used. EPDM (ethylene propylene
diene monomer) is used for drinking water and
NBR (nitrile butadiene rubber) for sewage.
2 Requirement to be met by rubber gaskets
2.1 Mechanical and physical requirements
For its durability in operation, the most important
characteristic of an elastomer gasket is its
long-term elasticity. This characteristic is determined
in what is known as compression set in
accelerated tests.
In these tests, a rubber specimen is compressed
by a fixed amount and is allowed to age in this
state at various temperatures for 24 or 72 hours.
When the compression is released, the thickness
to which the specimen recovers is measured
and the remaining loss in thickness is
related to the original thickness.
The requirements given in EN 681-1 [2] for the
compression set of gaskets of the hardness class
of TYTON® joints in water pipelines (Fig. 2) are
■ 72 h at 23 °C: compression set < 12 %
■ 24 h at 70 °C: compression set < 20 %.
Further measures of resistance to ageing are the
increase in hardness and reduction in tensile
strength and elongation at break as compared
with the original state, in a seven-day accelerated
ageing test at 70 °C.
FGR® / EADIPS® 46