The bolt-on sight glass bolt pattern for a vessel holder nozzle is the bolt pattern of the mating ANSI Class 150 flange on the nozzle, not a proprietary pattern on the glass itself; standard commercial units are built to fit nozzles from 2 in up to 8 in with raised-face sealing [S1][S8].
Process engineers specifying a bolt-on sight window on a vessel should therefore pull the nozzle's flange class, size, and facing type first, then verify the sight glass base matches that flange. A 4 in ANSI 150 raised-face nozzle will only accept a sight glass base with a 4 in ANSI 150 raised-face pattern, including its 4-bolt on 7.5 in bolt circle, 0.75 in bolts, and 0.88 in gasket contact width [S8].
Why the nozzle flange dictates the bolt pattern
The sight glass base is a flanged subassembly with a raised face machined to seat against the nozzle flange; the through-bolts clamp the two flanges together with a gasket between them, so every dimension of the bolt pattern (number of holes, bolt circle diameter, hole diameter, facing) is identical to the ANSI B16.5 (or ASME B16.5) spec for that class and size [S1][S8]. A 2 in ANSI 150 unit has 4 bolts on a 4.75 in bolt circle, a 3 in unit has 4 bolts on a 6.00 in bolt circle, a 4 in unit has 8 bolts on 7.50 in bolt circle, and a 6 in unit has 8 bolts on a 9.50 in bolt circle, which matches the standard 150# flange chart for those nominal sizes [S8]. For this reason, the practical question is never "what is the sight glass pattern" but "which ANSI class and size is the holder nozzle tapped for," because the sight glass follows that exactly.
Pad flanges on vessels are a common alternative: a short nozzle neck welded to the shell that gives the shortest possible projection inside and outside the vessel, and the sight glass bolts straight onto the pad flange face using the same bolt pattern as a full-length nozzle of the same nominal size [S7]. The advantage is a cleaner internal geometry, no protruding weldolet, and the same bolt circle as a standard flanged nozzle of the same size, so any ANSI 150 bolt-on sight window in that size will drop on with no adapter spool [S7].
Bolt-on vs weld-on vs sanitary: which one fits the holder nozzle
ARCHON's installation matrix shows four common connection styles: weld-on, bolt-on, screw-on (threaded) and sanitary tri-clamp, with bolt-on specifically aimed at ANSI flange nozzles that the engineer wants to remove for cleaning or glass replacement [S3]. Weld-on units, by contrast, are welded into the vessel wall or pipe end and cannot be removed without cutting; rectangular and longitudinal level gauges are almost always weld-in by design [S3][S4]. Tri-clamp and screw-on are reserved for sanitary and low-pressure service and do not share a bolt pattern with an ANSI flange.
For a vessel holder nozzle already flanged to ANSI 150, the bolt-on sight glass is the only option that uses the existing bolt pattern. A weld-in sight glass would require cutting the nozzle off, welding a sight window in its place, and losing the ability to swap the glass disc without cutting the vessel open [S3]. Bolt-on units are also the only sight glass that gives the engineer a defined pressure rating aligned to the flange class, because the rating is governed by the flange, not the glass disc, and an ANSI 150 sight glass bolted to an ANSI 150 nozzle is rated to the same 150# pressure-temperature envelope as any other 150# flange joint [S5][S8].
Comparing connection options on a vessel holder nozzle
Four common ways to put a viewing window on a vessel holder nozzle line up against four practical decision criteria for the engineer. [S2]
Weld-on sight glasses (ARCHON Model KB and KBWN) offer the lowest cost and the smallest projection, but they cannot be removed for glass replacement and are not an option on a flanged nozzle without cutting the nozzle off [S3]. Bolt-on sight glasses to ANSI 150 (ARCHON Model KBRF, Tank Components circular bolt-on, GlobalSpec listing) bolt directly to the existing nozzle flange, are removable for disc and gasket replacement, and follow the ANSI B16.5 bolt pattern of the mating flange [S3][S8][S1]. Sanitary tri-clamp sight glasses (ARCHON Model SS, SSTC) are sized to 1.5 in to 4 in ferrule standards, give fast clean-in-place removal, but are limited to low-pressure hygienic service and do not match a 150# flange bolt circle [S3]. Rectangular longitudinal level gauges (ARCHON RLGL, ACI Type 330/334/335/338) are typically weld-in, give a tall viewing slot for level reading, and are pressure-rated from 3 bar (low-pressure Type 334/335) to 40 bar (Type 330) to 16 bar in the high-pressure Type 338, with borosilicate to DIN 7080 or DIN 7081 as the standard disc [S3][S4].
The decision rule is straightforward: if the holder nozzle is already a flanged connection, the bolt-on sight glass wins because the bolt pattern is already there; if the nozzle is a plain pipe stub and the service is low-pressure sanitary, tri-clamp; if the engineer needs a continuous level slot rather than a round port, longitudinal weld-in [S3][S4][S5].
Glass disc, gasket, and pressure rating: what the bolt pattern does not cover
The bolt pattern only handles mechanical attachment and sealing surface alignment. The pressure-temperature rating of the assembled unit depends on the glass disc, the gasket, and how the bolts are torqued, not the bolt circle itself. For ANSI 150 service, the typical sight glass disc is borosilicate glass (DIN 7080 / DIN 7081 for the European longitudinal level gauges) with a fused or gasketed construction, and the standard metal-side gaskets are PTFE, klinger, or compressed non-asbestos, with FEP or Halar coatings reserved for caustic media such as sodium hydroxide [S4].
Pipeline sight glass ratings on the same ANSI 150 bolt pattern differ sharply by design: a straight-through inline sight glass is commonly rated to ANSI 150 (about 19.6 bar at 38 deg C for carbon steel, less at higher temperature per ASME B16.5), an angle (90 degree) type is typically limited to 64 bar, a full-view double-window unit to 40 bar, and an armoured or ported unit to 200+ bar where thick glass and a steel guard are mandatory [S5]. For a static vessel holder nozzle, the relevant comparison is usually between an ANSI 150 bolt-on (matched to the nozzle rating) and a 300 or 600 class bolt-on if the nozzle itself is the higher class, and that selection is driven by the nozzle, not the glass [S1][S5][S8].
Sizing the nozzle and sight glass together
Commercial ANSI 150 bolt-on sight glasses are stocked from 2 in up to 8 in to match the most common vessel nozzle sizes, and the bolt pattern is identical to a standard 150# weld neck flange of the same size [S1][S8]. For smaller sanitary or low-pressure tanks, weld-on sight windows ship from 1.5 in to 6 in, which is a separate product line without a bolt pattern at all; the engineer specifies a weld-on only when there is no existing flange to bolt to [S9].
Round vs obround is a separate axis from bolt pattern. ARCHON, LJ Star, and Northeast Controls all stock the round port as default, with obround / rectangular / longitudinal options where the engineer needs to monitor a moving interface, foam line, or colour change rather than a single point [S2][S3][S10]. The bolt pattern of an obround or rectangular sight window is not a standard ANSI bolt circle because the gasket is not circular; those windows are usually weld-in by design and are not what an engineer with an ANSI flanged holder nozzle is buying [S2][S3][S10].
Common specification errors on vessel holder nozzles
Three errors come up repeatedly. First, specifying a sight glass by its outside diameter or its glass disc diameter rather than by the nominal flange size of the nozzle, which gives the wrong bolt circle and the wrong gasket seating diameter [S1][S8]. Second, mating a Class 300 bolt-on sight glass to a Class 150 nozzle, which seats on a different raised-face height and a different bolt pattern for sizes 4 in and above, so the holes will not line up without a spacer ring [S5]. Third, choosing an obround or rectangular sight window for a flanged nozzle, which forces a custom adapter spool and breaks the clean bolt-on benefit; for an ANSI flanged holder nozzle, the round bolt-on is the unit the supplier stocks and the unit that drops on with no fabrication [S2][S3].
For a more disciplined approach to inspection windows and gasket selection on process piping, the pipeline sight glass selection guide on ANSI and DIN connection standards is a useful cross-check, and a vessel maintenance decision matrix helps the engineer decide whether a removable bolt-on glass is worth the extra cost over a weld-in unit on a given vessel. A short primer on sight glass and sight window configurations is also worth pulling before the spec is frozen, because it lists the standard sizes, the connection types, and the typical pressure classes a vessel engineer is most likely to encounter.
For an engineer closing out a spec, the next concrete step is to confirm the holder nozzle's nominal size, ANSI pressure class, and facing type (raised face vs flat face), then pull the matching bolt-on sight glass data sheet from any major supplier (ARCHON, LJ Star, Tank Components, Lumiglas) for that exact size and class, and verify the bolt count, bolt circle, and bolt diameter line up before ordering [S1][S3][S5][S8]. If the nozzle is not flanged, the path forward is a weld-on unit sized to the pipe OD, not a bolt-on, and the bolt-pattern question disappears [S3][S9].
For component-level specifications, see aluminum window door, and system window door.