Engineering task and calculation objective
This module calculates the cover (flange ring) of a sight glass fitting to DIN 28120 — the German standard for circular sight glass fittings on pressure vessels. The bolted cover ring presses the sight glass through the gaskets against the base flange; the module verifies that the available flange height of the cover is sufficient for the load cases installation, operation and pressure test.
To this end, the bolt forces for all three load cases are determined from the operating gauge pressure, test gauge pressure and calculation temperature — taking into account the gasket geometry (mean gasket diameter, effective gasket width) and the gasket parameters. From the bolt forces, the required flange height follows for each load case and is compared with the available remaining height of the recessed cover. On the material side, yield strength, safety factor and modulus of elasticity for operating and test conditions enter the calculation.
This calculation is needed wherever processes are monitored visually: sight glasses on stirred vessels, columns and piping in the chemical, pharmaceutical and food industries. Since the brittle glass tolerates no bending constraint, the stiff, sufficiently tall cover ring is decisive for tightness and glass safety.
Standard and calculation basis: DIN 28120
Calculation workflow
- Define load cases and geometry: The inputs are the operating gauge pressure, test gauge pressure and calculation temperature as well as the cover geometry: outside diameter, bolt circle diameter, recess diameter, inside diameter, bolt hole diameter, flange height and depth of the recess. The load-bearing remaining height of the ring follows from the flange height and the recess depth.
- Assign material and gasket parameters: For the flange ring, yield strength, safety factor and modulus of elasticity are applied separately for operation and pressure test. For the gasket, the outside and inside diameters and the gasket parameters are recorded; from these the module calculates the mean gasket diameter and the effective gasket width.
- Calculate the bolt forces per load case: The bolt forces are determined for installation, operation and test: in the installation condition from the required pre-deformation of the gasket, in the operating and test conditions from the pressure force on the cover area plus the residual force needed to maintain the gasket seating pressure.
- Verify the required flange heights: From the bolt forces and the lever arms between bolt circle and gasket, the required flange height of the cover ring follows for each load case. The verification is fulfilled when the available remaining height covers all three required heights; the largest value governs.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Operating pressure (g) | pB | N/mm² |
| Test pressure (g) | pP | N/mm² |
| Calculation temperature | TB | °C |
| Outside diameter | D | mm |
| Bolt circle diameter | k | mm |
| Diameter of insertion | d4 | mm |
| Inside diameter | d1 | mm |
| Bolt hole diameter | d6 | mm |
| Flange height | h1 | mm |
| depth of insertion | t1 | mm |
| Remaining height | t2 | mm |
| Material flange ring | Flanschring | - |
| Yield strength operation | KB | N/mm² |
| Safety factor | SB | - |
| Modulus of elasticity | EB | N/mm² |
| Yield strength test | KP | N/mm² |
| Safety factor | SP | - |
| Modulus of elasticity | EP | N/mm² |
| Gasket material | Dichtung | – |
| Outside diameter gasket | DDa | mm |
| Inside diameter | DDi | mm |
| Characteristic | k1 | mm |
| Characteristic | ko*kd | N/mm |
| Outside diameter sight glass | d3 | mm |
Frequently asked questions
Why is the recess depth subtracted from the flange height?
The sight glass sits in a recess machined into the cover ring. In the weakened cross-section, only the remaining height carries the bending load from the bolt forces. Calculating with the full flange height overestimates the load capacity of the ring — the recess is the critical location of the cover.
Why are the installation, operating and test conditions verified separately?
The three conditions load the cover differently: at installation, the full preload acts on the gasket without any relieving internal pressure; in operation, pressure force and temperature-reduced material properties combine; during the pressure test, the increased test pressure acts with room-temperature properties and a lower safety factor. Which case yields the largest required flange height cannot be predicted in general.
Does the calculation also cover the sight glass itself?
No. The module verifies the cover of the fitting. The glass disc itself is covered by the standardized pairing of sight glass (e.g. to DIN 7080 for borosilicate glass) and fitting to DIN 28120 for the assigned pressure/temperature ratings. It is important that the glass lies plane-parallel between two gaskets and experiences no metal contact or bending constraint — the installation instructions and tightening torques must be observed.
What role does the effective gasket width play?
The gasket seating pressure does not develop uniformly over the full gasket width; the calculation uses an effective width at the mean gasket diameter. It determines both the required seating force in the installation condition and the residual sealing force to be maintained in operation — and thus directly the bolt forces and the cover loading.