Spacer flanges serve to add space (or fill space) between flanges when connected systems have dimensions that may have short anticipated or un-anticipated gaps between equipment flanges that must contact. Filling the space with a custom dimensioned spacer flange solves this problem. Spacer flanges have the same appearance as Instrument flanges except they have no ports or taps (see the illustrations below)

Instrument flanges, (sometimes called Bleed Rings or Sampling flanges), allow a means to measure pressure or obtain fluid samples from a pressurized flange connected line. The instrument flange provides a means of having threaded ports in the line without drilling and tapping the principle equipment in the line (The operator can replace the instrument flange if the threaded ports should become damaged or begin to leak). Instrument flanges commonly have 2, 3, or 4 ports.

The two common forms of instrument flanges, the drilled through type, and the double studded type, function interchangeability.

Instrument flanges of the drilled through type usually have the same dimensions as a standard flange, except that they have identical flange sealing surfaces on both sides so that they may fit between two standard flanges. These flanges require longer bolts that reach through three flanges instead of two. The instrument connection(s) will appear on the O.D. of the instrument flange in the form of tapped ports, usually 1/2″ L.P. for working pressures through 10M or 9/16 Autoclave F562C (API 1-1/8-12 UNF) for 15M or higher.

Drilled Through Spacer or Instrument Flange
(Spacer Flanges have no ports)


Double studded instrument flanges can function exactly the same as the drilled through type, but may need greater thickness to accommodate the tap end stud thread engagement requirement.

Double Studded Spacer or Instrument Flange
(same size on each side)

Double studded instrument flange design may allow the special benefit of size and/or pressure crossover in the line as a double studded instrument flange may function as a double studded adapter having a different flange facing on each side.

Double Studded Spacer or Instrument Flange
(as an adapter or spacer having different size and / or
pressure designation on each side)

No advantage exists in the installation of either type as the longer studs required for the through bolt type presents the same problem for installation and removal of the instrument flange as the fixed tap end studs in the double studded type.

Although the two types of instrument flanges function interchangeably, and the double studded type usually costs a little more, the user may find the double studded type advantageous. Manufacturers furnish the studs and nuts installed in double studded instrument flanges, eliminating the need to obtain special bolts of the correct length.

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