DIN 7060 Curcular sight glass.pdf

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May 2005


DIN 7080

Supersedes DIN 7080:1999-05

ICS 71.120.10

Circular borosilicate sight glasses with unlimited pressure resistance at low temperatures

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Runde Schauglasplatten aus Borosilicatglas für Druckbeanspruchung ohne Begrenzung im Tieftemperaturbereich

Document comprises 14 pages

Translation by DIN-Sprachendienst. In case of doubt, the German-language original should be consulted as the authoritative text.


No part of this translation may be reproduced without prior permission of DIN Deutsches Institut für Normung e. V., Berlin. Beuth Verlag GmbH, 10772 Berlin, Germany, has the exclusive right of sale for German Standards (DIN-Normen).

English price group 10 www.din.de www.beuth.de

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DIN 7080:2005-05

Foreword This standard has been prepared by Technical Committee FNCA AA 1 Anlagenteile of the Normenausschuss Chemischer Apparatebau (Process Engineering Standards Committee). Annexes A and B are informative. Amendments This standard differs from DIN 7080:1999-05 as follows: a)

Subclause 4.5.2 has been corrected in accordance with Corrigendum DIN 7080:1999-05.


Subclause 4.5.4 has been corrected.


“Permissible working pressure” has been replaced by “maximum permissible pressure”.


The standard has been editorially revised.

Previous editions

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DIN 7080: 1943x-10, 1965-07, 1975-09, 1996-02, 1999-05



This standard applies to thermally prestressed circular borosilicate sight glasses that can be continuously subjected on one side to liquid or gas pressures at up to 280 °C and, in exceptional circumstances, to 300 °C (cf. subclause 4.3.2). The sight glasses can only be used at pressures and temperatures up to those specified if they are installed as described in clause 10. There is no limit on the use of sight glasses conforming to this standard at temperatures below ambient temperature.


Normative references

The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. DIN 28120, Circular sight glasses with case in main power connection DIN 28121, Circular sight glasses with case in metal to metal contact type flanged joint DIN ISO 695, Glass — Resistance to attack by a boiling aqueous solution of mixed alkali — Method of test and classification DIN ISO 718, Laboratory glassware — Thermal shock and thermal shock endurance – Test methods DIN ISO 719, Glass — Hydrolytic resistance of glass grains at 98 °C — Method of test and classification DIN ISO 1776, Glass — Resistance to attack by hydrochloric acid at 100 °C — Flame emission or flame atomic absorption spectrometric method DIN ISO 2859-1, Sampling procedures for inspection by attributes — Part 1: Sampling schemes indexed by acceptance quality limit (AQL) for lot-by-lot inspection DIN ISO 7991, Glass — Determination of coefficient of mean linear thermal expansion


DIN 7080:2005-05


Dimensions and designation Dimensions in millimetres

Key 1 a

Seal Chamfered edge (cf. Table 1) Figure 1 — Sight glass

Designation of a circular sight glass having a diameter, d1, of 100 mm and a thickness, s, of 15 mm:

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Sight glass DIN 7080 — 100 × 15 The relationship between maximum permissible pressure and dimensions is shown in Table 1. Table 1 — Dimensions Maximum permissible pressure, in bar Diameter d1 45 (50) (60) 63 80 100 125 150 175 200 250 135 265

Diameter d2







10 12 15 15 20 25 — — — — —

12 — — — — — — — — — —

— —

— —

Thickness s

32 — — — — 35 — — — 10 45 — — 10 12 48 — — 10 12 65 — — 12 15 80 — — 15 20 100 — 15 20 25 125 — 20 25 30 a 150 — 20 30a 25 25 30 — 175 20a a 30 — — 225 25 Only applicable to containers made of enamelled steel 110 — — — 25 240 30 — — —

Diameters in brackets are not for use in new designs. a

The safety factor for these sizes is between 4,5 and 4,9.


DIN 7080:2005-05





The material used shall be thermally prestressed borosilicate glass conforming to the requirements specified in subclauses 4.2 to 4.5.


Mechanical strength

Unprestressed glass having a (tested) flexural tensile strength of about 40 N/mm2 shall be thermally prestressed until the compressive prestress at the surface is not less than 100 N/mm2 (see Annex A.1).

4.3 4.3.1

Thermal properties Mean coefficient of linear thermal expansion

The mean coefficient of linear thermal expansion, α (20 °C, 300 °C), determined as specified in DIN ISO 7991 shall not exceed 5,0 × 10–6 K–1. 4.3.2

Permissible working temperature

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The continuous working temperature of the sight glass shall normally not exceed 280 °C. It should be borne in mind that, at temperatures above 280 °C, a permanent decrease in the prestress is likely to occur (see Annex B [1]). For example, at a working temperature of 300 °C, the prestress will probably be only 90 % of the initial value after the sight glass has been in service for 300 hours. However, use at temperatures between 280 °C and 300 °C is permissible provided suitable precautions are taken, such as ⎯ protecting the sight glass with mica and ⎯ restricting the cumulative working time above 280 °C to a maximum of 300 hours. 4.3.3

Thermal shock resistance

The thermal shock resistance of the sight glass determined as specified in DIN ISO 718 and subclause 7.4 shall be 230 °C.


Chemical resistance

The chemical resistance of borosilicate glass shall comply with the following: ⎯ hydrolytic resistance class HBG 1 as in DIN ISO 719; ⎯ acid resistance class as in DIN ISO 1776 not exceeding 100 μg Na2O/100 cm2 ; ⎯ alkali resistance class A2 as in DIN ISO 695 (see also Annex A.2).


DIN 7080:2005-05


Material defects



Bubbles, striations, bands of striations, viscous knots and crystalline inclusions shall not exceed the values given below and shall not affect the mechanical strength or have a technically significant effect on transparency (see Annex B, [2]). 4.5.2


The diameter of spherical bubbles in the glass and the average of the length and width of oval ones shall not exceed 2 mm. Tapered bubbles are not permitted. The number of bubbles in a given size range shall be within the limits specified in Table 2. Table 2 — Permissible number of bubbles Bubble diameter, d3

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d3 < 0,3


Permissible number of bubbles 3 per cm2 of sight glass

0,3 ≤ d3 ≤ 0,5

10 per sight glass

0,5 < d3 ≤ 1

4 per sight glass

1 < d3 ≤ 2

2 per sight glass

Viscous knots, striations and bands of striations

There shall be no viscous knots, sharp striation or bands of striations that are visible to the naked eye. 4.5.4

Crystalline inclusions

The diameter of any inclusions in the glass shall not exceed 0,2 mm, there shall not be more than three inclusions per sight glass and the distance between them shall not be less than 10 mm.



The sight glass shall be thermally prestressed (see subclause 4.2 and Annex A.1), its surfaces shall be ground and polished and, at the manufacturer’s discretion, its rim shall be cold-pressed and/or ground and its edges chamfered by pressing or by rubbing down. For manufacturing reasons, the rim may be inclined at 1,5°, with d1 being taken as the largest external diameter.


DIN 7080:2005-05


Geometrical tolerances

Figure 2 gives the notation for the dimensions referred to in the tables below.

Key a b

Edge dimension (see Table 5) Pressure side Figure 2 — Shape tolerances, positional tolerances and edge tolerances

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Limit deviations

Table 3 shows the allowable tolerances in the diameter and thickness of a sight glass. Table 3 — Limit deviations Dimensions in millimetres Diameter d1


Thickness s

d1 ≤ 135

150 ≤ d1 ≤ 200

d1 > 200

10 ≤ s ≤ 20

s > 20

± 0,5

± 0,8


+ 0,5 − 0,25

+ 0,8 − 0,4

Shape tolerances and positional tolerances

The maximum allowable deviations of the faces from flatness and parallelism are shown in Table 4. Table 4 — Flatness and parallelism Dimensions in millimetres d1 ≤ 100

100 < d1 ≤ 150

150 < d1 ≤ 200

d1 > 200

Flatness e





Parallelism p


Diameter d1




DIN 7080:2005-05


Edge tolerances

The edge finishes resulting from the chamfer are shown in Table 5. Table 5 — Edge tolerances Dimensions in millimetres

7 7.1

Diameter d1

Edge dimensions

≤ 100

− 1,0 − 0,3

> 100

− 1,5 − 0,3

Testing General

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The manufacturer shall test the sight glass for conformity with the specifications given in clauses 3, 4, 5 and 6 and shall confirm that the requirements have been fulfilled by marking the sight glass as specified in clause 8.


Qualitative prestress assessment by photoelastic analysis

Every sight glass shall be tested by viewing it perpendicularly to the flat glass surface. Several clearly distinguishable circular lines closed upon themselves (isochromatics) shall be visible at the rim (see Figure 3). The line pattern described indicates the presence of compressive prestress.

Figure 3 — Sight glass viewed in polarized light


Quantitative prestress assessment by random burst pressure tests

Depending on batch size, random sample tests shall be performed on every (prestressed) batch produced in accordance with the following scheme.


DIN 7080:2005-05

Number of samples Batches of less than 20:

1 sample

Batches of 20 to 50:

2 samples

Batches of more than 50:

3 samples


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Samples shall be taken from every prestressed batch of finished sight glasses in accordance with the above scheme.

Key 1 a b

Plastic ring Ground Pressure side Figure 4 — Testing device

Before testing, one of the two flat sides of the sight glass shall be spoilt beforehand by grinding a central circular area having a diameter of 0,5 d1 in all directions with a circular movement using wet grade 220 emery paper. In doing this, it is important to avoid anomalous individual scratches by drawing off the wet emery paper over a metal edge. To prevent a wide variation in the burst test measurements, it is advisable to work with emery paper from a single batch. After being spoilt, the sight glasses shall be burst-tested with cold water in the device shown in Figure 4, with the side spoilt by grinding facing away from the water. The latter should have film glued to it beforehand to prevent injury due to splintering. The bursting pressure, pBG (the pressure at which the sight glass shatters), in bar, shall be measured and used to determine the bursting stress, σBG, in N/mm2, for every thermally stressed sight glass using equation (1):

σ BG =

pBG ⎡ C × d m ⎤ ×⎢ ⎥ 10 ⎣ s ⎦



Every sample shall fulfil equation (2): σBG – σBU = σDV ≥ 100 N/mm2



DIN 7080:2005-05

where in equations (1) and (2) σBU

is the bursting stress of sight glasses that have not been thermally prestressed (i.e. are stressrelieved), in N/mm2;


is the compressive prestress determined by a bursting test, in N/mm2;


is the pressure at which the sight glass shatters that have not been thermally prestressed (i.e. are stress-relieved), in bar;


is a calculation coefficient (here, 0,55);


is the mean seal diameter, in mm, calculated using equation (3): d + d2 dm = 1 ; 2


is the thickness as in Table 1, in mm.

dm and s are to be substituted in equation (1).

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The value of σBU in equation (2) shall be determined in the same way by burst-testing sight glasses that have been stress-relieved by heating and slowly cooling. It can be taken as the mean of current operating tests, but no test value shall be more than six months old. The value of σBU substituted in equation (2) shall not be less than 35 N/mm2.


Thermal shock test

Random samples shall be taken from every prestressed batch in accordance with the sampling scheme in subclause 7.3, heated to 250 °C and completely immersed in flowing water at 20 °C for at least one minute. The failure criterion shall be complete shattering.


Tolerance checking

Tolerance checks shall be performed on random samples taken from every prestressed batch. The single sampling plans for normal inspection in accordance with DIN ISO 2859-1 test level I apply to each attribute (diameter, thickness, flatness, parallelism and edge tolerances), the acceptance quality limit (AQL) being 10. All the attributes may be tested on a single test piece.


Test report

For every prestressed batch, the manufacturer shall document the results of the tests specified in subclauses 7.2 to 7.5 and, if requested to do so by the client, make them available for inspection. The test report shall include the following details: a)

prestressed batch marking;


the values of σBU, σBG and σBG – σBU;


confirmation that a photoelastic test has been performed on every sight glass in the batch;


thermal shock test result;


confirmation that every sight glass has passed the visual test for material defects;


dimensional test results.


DIN 7080:2005-05



The following permanent markings shall appear in the area shown in Figure 5 on the smaller face, i.e. the one remote from the pressure, of every sight glass with the dimensions shown in Table 6: ⎯ DIN 7080; ⎯ the numerical value of the permissible pressure as specified in Table 1, in bar. The following shall be marked on the cylindrical glass rim: ⎯ the manufacturer’s logo or trademark; ⎯ the prestressed batch mark in such a way as to permit clear traceability of the documented test results.

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The manufacturer’s logo or trademark (e.g. XYZ) can appear in the area shown in Figure 5 with the dimensions shown in Table 6.

Figure 5 — Marking areas



The packaging design may be freely chosen by the manufacturer provided that every sight glass is protected against impact and scratches during transportation, including interdepartmental transportation, and storage at the client’s premises.

10 Installation and safety requirements The maximum permissible pressures shown in Table 1 will only be attained if the sight glasses are installed in such a way that internal plant pressure is applied to the larger face and the sight glasses are uniformly clamped using sight glass fittings with the dimensions, seals and numbers and sizes of screws specified in DIN 28120 or DIN 28121 if sealing takes place on the flat glass faces. Other fittings may be used if their suitability is demonstrated. Sight glasses shall be fitted only by personnel who have been given detailed instructions on the following: — how to handle sight glasses with care;


DIN 7080:2005-05

— how to clean the fittings, sight glasses, seals and shims, i.e. remove foreign bodies, such as splinters, from them, before installation; — how to tighten up the fixing screws uniformly. Sight glasses removed after being in service shall not be reused. Table 6 — Marking dimensions

Dimensions in millimetres Diameter d1










(50) (60) 63


80 100


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125 135 150 175


200 250 265


DIN 7080:2005-05

Annex A (informative) Explanatory notes

A.1 Mechanical strength The simultaneous exposure of circular sight glasses to severe thermal, mechanical and chemical conditions necessitates the use of heat-resistant borosilicate glasses having high mechanical strength and also the highest possible resistance to chemical attack. Such glasses can withstand the liquid or gas pressure acting on one side, the clamping forces and the thermal stresses resulting from temperature differences only if their normal tensile strength is increased by thermal prestressing. The crucial quality criterion is that the compressive surface stress produced by prestressing is not less than 100 N/mm2, thereby increasing the normal flexural tensile strength of about 40 N/mm2 by a factor of at least 2,5. It is essential that no tensile stresses occur after installation. The level of the surface stress resulting from the liquid or gas pressure acting on one side will then remain below the value of the compressive prestresses.

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Assuming a minimum value of the compressive prestress, σDVzul, of 100 N/mm2 and a safety factor, S, of 5, the thicknesses were calculated so as to fulfil the following inequality: s ≥ C × dm ×

p×S 10 × σ DV zul


where p is the maximum permissible pressure, in bar. As the flexural tensile strength increases, the prestressing also results in an increase in the thermal shock resistance, the value of the latter being primarily determined, for a given temperature gradient, not only by the elastic properties, but also by the heat conduction and the coefficient of expansion. The safety of sight glasses when under pressure is crucially dependent on the design of components used to fit them and the installation procedure. The safety factor, S, of 5 will apply only if the manufacturer’s installation and operating guidelines are adhered to.

A.2 Chemical resistance The extent to which glass in boiler sight glasses is attacked when exposed to aqueous media will normally increase exponentially as the temperature and pH of the medium rises and the attack may be considerably accelerated by chemical additives such as those used in various ways and amounts in water treatment plants. The resultant severe erosion and consequential dimensional changes may endanger the working safety of the sight glass and may result in rupture. The most important influencing variables are, however, the pH and temperature, but, for other complex reasons, the temperature plays a major role at temperatures markedly above 200 °C. To illustrate this, the figure below shows the erosion of a glass surface at 170 °C for two different pH levels, while the broken line shows the effect of increasing the temperature to 220 °C at a pH of 10 under laboratory conditions.


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DIN 7080:2005-05

Figure A.1 — Sight glass erosion


DIN 7080:2005-05

Annex B (informative)

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DIN ISO 13715, Technical drawings — Edges of undefined shape — Vocabulary and indication on drawings


DIN ISO 1101, Technical drawings — Geometrical tolerancing — Tolerancing of form, orientation, location and run-out — Generalities, definitions, symbols, indications on drawings (withdrawn June 2005, superseded by DIN EN ISO 1101:2005-06)


DIN EN ISO 1302, Technical drawings — Method of indicating surface texture (identical with ISO 1302:1992) (withdrawn June 2002, superseded by DIN EN ISO 1302:2002-06)


Novotny, V. and Kavka, I. Stress relaxation in toughened glass. Glass Technology, 1977: 18, No. 5.


Jebsen-Marwedel, H. and Brückner, R. Glastechnische Fabrikationsfehler (Glass manufacturing defects), Berlin, Heidelberg, New York: Springer Verlag, 1980.


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