abstract
bolt
business
chemical
conduit
connection
construction
control
drain
element
energy
engineering
equipment
factory
fuel
gas
gasoline
icon
illustration
industrial
industry
isolated
line
metal
natural
object
oil
pipe
pipeline
piping
plumbing
power
pressure
pump
shadow
sign
single
steel
stencil
symbol
system
technology
transportation
tube
valve
water
white