ICS 23.020.10  
 
Reference number  
DRS 128: 20 23 
© RSB 2023 
  
 RWANDA 
STANDARD  DRS 
128 
 
Third edition  
2023-mm-dd 
 
 
Rotational moulded polyethylene water 
storage tanks  — Specification  
   
DRS128: 20 23 
©RSB 20 23- All righ ts reserved                  ii 
 In order to match with technological development and to keep continuous progress in industries, 
standards are subject to periodic review. Users shall ascertain that they are in possession of the latest 
edition  
 
 
 
 
 
 
 
 
© RSB2023 
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized 
in any form or by any means, electronic or mechanical, including photocopying and microfilm, without 
prior written permission from R SB. 
Requests for permission to reproduce this document should be addressed to : 
Rwanda Standards  Board  
P.O Box 7099 Kigali -Rwanda  
KK 15 Rd, 49  
Tel. +250 788303492  
Toll Free: 3250  
E-mail: info@rsb.gov.rw  
Website: www.rsb.gov.rw  
ePortal: www.portal.rsb.gov.rw  DRS128: 20 23 
iii                ©RSB2023- All rights reserved  
 Contents  Page 
1 Scope  ................................ ................................ ................................ ................................ ......................  1 
2 Normative references  ................................ ................................ ................................ ............................  1 
3 Terms and definitions  ................................ ................................ ................................ ...........................  1 
4 Requirements  ................................ ................................ ................................ ................................ .........  4 
5 Quality of finish  ................................ ................................ ................................ ................................ ..... 8 
6 Testing  ................................ ................................ ................................ ................................ ....................  8 
7 Sampling  ................................ ................................ ................................ ................................ ................  9 
8 Marking  ................................ ................................ ................................ ................................ .................  10 
9 Transportation  ................................ ................................ ................................ ................................ ..... 10 
10 Workmanship  ................................ ................................ ................................ ................................ ....... 11 
Annex  A (normative)  Determination of carbon black content  ................................ ................................ .... 12 
Annex  B (normative)  Determination of carbon black dispersion in polyethylene materials  ...................  14 
Annex  C (normative)  Determination of effect of heavy metals on water quality  ................................ ...... 15 
Annex  D (normative)  Determination of resistance to deformation test  ................................ .....................  16 
Annex  E (normative)  Determination of tensile properties  ................................ ................................ ...........  18 
Annex  F (informative)  Recommendations for installation of tanks and fittings  ................................ ....... 25 
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 Foreword  
Rwanda Standard sareprepared by Technical Committees and approved by R wanda Standards Board (RSB) 
Board  of Directors in  accordance with the proce dures of RSB, in compliance with Annex 3 of the WTO/TBT 
agreement on the preparation, adoption and application of standards.  
The main task of technical committees is to prepare national standards.  Final Draft Rwanda S tandards 
adopted by Technical committ ees are ratified  by me mbers of RSB Board of Director s for publication and 
gazettment as Rwanda Standards.  
DRS 128 was prepared by Technical Committee RSB/TC 48, Plastic tanks, pipes, accessories and related 
products . 
In the preparation of this standard, reference was made to the following standard s: 
1) IS 12701:1996 ,Rotational moulded polyethylene water storage tanks  
 
2) ASTM D1998 -21,Standard Specification for Polyethylene Upright Storage Tanks  
3) SLS 1174:1998 ,Polyethylene water storage tanks — Specification  
4) KS 1567:2005 ,Rotational moulded polyethylene water storage tanks  — Specification  
The assistance derived from the above source is hereby acknowledged with thanks.  
This third edition cancels and replaces the second edition (RS  128:2019), clause 4.1.2 , which has been 
technically revised.  
Committee membership  
The following organizations were represented on the Technical Committee on Plastic tanks, pipes, 
accessories and related products (RSB/TC 48) in the preparation of this standard.  
AQUASAN  
Ecoplastics  
Global community  
PROWATER  
ROTASSAIRWA  
ROTO Ltd  DRS128: 20 23 
v                ©RSB2023- All rights reserved  
 Rwanda plastic industries  
SOIMEX Plastic  
TRIBUS SERVICES LTD  
Rwanda  Standards Board (RSB) – Secretariat  DRS128: 20 23 
1                ©RSB2023- All rights reserved  
 Rotational moulded polyethylene water storage tanks  — Specification  
1 Scope  
This Draft Rwanda Standard specifies  requirements, sampling and test methods for rotational moulded 
polyethylene water storage ta nks (closed and open top tank).  
This standard does not cover mobile water tanks, horizontal cylindrical water tanks and underground tanks.  
This standard is applicable to both open -topped and closed -topped water storage tanks subjected to the 
following two conditions:  
a) own hydrostatic head of water;  and 
b) uniform flat base support.  
The nominal service temperature is 1 °C to 50 °C.  
2 Normative refe rences  
The following documents are referred to in the text in such a way that some or all of their content constitutes 
requirements of this document.  For dated references, only the edition cited applies. For undated references, 
the latest edition of the re ferenced document (including any amendments) applies.  
ISO 1209 -2,Rigid cellular plastics — Determination of flexural properties — Part 2: Determination on flexural 
strength and apparent flexural modulas of elasticity  
3 Terms and definitions  
For the purposes of this standard, the follow ing terms and definitions apply.  
3.1 
net capacity (actual)  
net total volume of water contained between the lowest level of the overflow and lowest level of outlet (see 
Figures 1 and 2)  
3.2 
gross capacity (nominal)  
total enclosed volume of the tank including any space, which may not be capable of being filled with water   
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 3.3 
mould parting  
circumferential line visible only on the external surface of the tank corresponding to a parting joint of the mould 
required for rotation al moulding (see Figures 1 and 2)  
3.4 
overall height  
height of the finished empty tank at its highest point, including the top rim of the manhole and lid of the tank 
(see Figures 1 and 2)  
3.5 
effective height  
height of the finished empty tank from its base  to the point where overflow connection is provided for the 
purpose of limiting water storage capacity (see Figures 1 and 2)  
3.6 
overall diameter  
maximum outer diameter of finished empty tank measured at its base (see Figures 1 and 2)  
3.7 
man-hole/hand -hole 
hole of suitable internal diameter provided at the top of the tank for the purpose of inspection of internal 
surface and ent ry into the tank (see Figure 1)  
 
Figure 1— typical perspective view of closed top tank  
RCC — Reinforced cement concrete  
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3                ©RSB2023- All rights reserved  
 PCC — Pre-cast cement concrete  
 
Figure 2— typical view of open top tank  
3.8 
internal diameter of man -hole/hand -hole 
internal diameter of the rim of the manhole measured as the mean of two perpendicular diameters (see 
Figures 1 and 2)  
3.9 
rotational molding  
three -stage commercial process consisting of loading the mold with powdered resin,fusing the resin by 
heating while rotating the mold about more than one axis, and cooling and removing the molded artic le 
3.10 
service factor , n 
number less than 1.0 (which takes into consideration all the variables and degrees of safety involved in a 
polyethylene storage tank installation) which is multiplied by the hydrostatic design basis  to give the design 
hoop stress  
3.11 
tensile  stress  
tensile load per unit area of minimum original cross section within the narrow parallel portion, carried by the 
test specimen at any moment. It is expressed in N/mm2 (force per square millimetre)  
 
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 3.12 
elongation  
increase in distance between references likes on the narrow parallel portion of the test specimen, due to a 
tensile load, and expressed as a percentage of the initial dista nce between the reference lines  
3.13 
yield point  
first point on the load/extension curve at whic h an increase in extension occurs without an increase in load.  
NOTE  In cases where the yield point is not well defined by the load extension curve, it is frequently necessary to 
define an offset yield point. This is done by specifying a point on the load e xtension curve where the curve departs from 
linearity by a specified percentage elongation or extension (see Figure 10).  
3.14 
elastic modulus  
ratio of stress to corresponding strain within the range of the greatest stress that the material is capable of 
sustaining without any deviation of proportionality of stress to strain.  
4 Requirements  
4.1 Materials  
4.1.1  The material for construction of the tank and lid, which comes into contact with water, shall not impart 
any taste, colour or odour to water; they shall neither have any toxic effect nor contaminate water thereby 
making it unpotable.  
4.1.2 Water tanks meant for outdoor use shall be manufactured from virgin polyethylene materials intended 
for the rotational molding process (resin). The addition of 5% to 10 % of the manufacturers own reworked 
material resulting from the manufacture of tanks only according to this standard is permissible. No other 
reworked or recycled waste material from any other source or filler shall be used in the manufacture of tank s 
4.1.3  All polyethylene resin material shall contain a minimum of a U.V. -8 stabilizer as compoun ded by the 
resin manufacturer.  
4.1.4  Each resin used in designing tanks covered by this specification shall have hydrostatic -hoop -stress 
data available.  
4.2 Carbon  black content  
4.2.1  Carbon black pigment if added, to the moulding material for the outer layer or the exposed layer, shall 
be not less than 2.5 % ± 0.5 % when tested in accordance with Annex A  (melt compounding) . 
4.2.2  The carbon black or any other pigment added shall be uniformly dispersed in the material, when 
tested in accordance with Annex B.  DRS128: 20 23 
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 4.3 Catalyst  
The total amount of inorganic present in polyethylene shall not exceed 0.2 % (m/m) when tested by atomic 
absorption method and in accordance with A nnex C. An alternative method of testing for the metals shall be 
by x-ray fluorescence, where the atomic absorption method may not be applicable or just non -operational.  
4.4 Fillers and pigments  
The plastic shall not contain any filler. Pigments may be added a s desired by the customer, or as designated 
by polymer processing companies, but shall not exceed 0.5% dry blended, and 2% compounded in, of the 
total weight.  
NOTE  The excessive use of dry -blended pigments may result in an effect on physical properties, th at is, impact 
strength.  
4.5 Types  
There shall be two types of tanks vis -à-vis:  
a) cylindrical vertical tanks (closed and open -end); and  
b) rectangular loft tanks (see Figure 3 a), and b)  
 
Figure 3a) — Cylindrical vertical with closed top  
 
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Figure 3 b) — Rectangular loft tank  
4.6 Fittings  
It shall be the prerogative of the manufacturer to provide as much information as is possible to the consumer 
regarding fittings.  
NOTE  Refer on Annex H for tank and fittings installations  
4.7 Sizes  
Tank sizes wil l vary in accordance with the customer’s requirements.  
4.8 Net capacity  
When filled to the lowest level of overflow, and the volume attained measured, tanks shall discharge the 
capacity marked on it.  
4.9 Design parameters  
 
4.9.1  The design wall thickness shall be determined by appropriate engineering design methods, which 
may include finite element analysis, for the nominated service temperature .  
4.9.2  The minimum required wall thickness (T) of a cylindrical straight walled shell (unsupported portion of a 
tank) at  any fluid level shall be determined from the following equation:  
T = P X OD/ (2 δ) 
P = 0.0098 × SG × H (MPa)  
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 δ= HDB ×n 
where  
T wall thickness, mm ; 
P pressure, MPa ; 
SG specific gravity of fluid, g/cm3; 
H fluid head, m ; 
OD outside diameter of tank, mm ; 
δ hydrostatic design stress, MPa ; 
n Service factor . 
4.9.3  Hydrostatic pressure value is determined by the height (H) of the liquid container and the density (SG) 
of the liquid. The tank walls have to be made to resist this pressure at different levels of water storage. 
Basically, the taller the tank, the greater the pressure on lower tank walls  
4.9.4  The hydrostatic design stress that is used to determine the minimum wall thickness at any fluid level 
must be based on hoop stress data for the resin. The hyd rostatic design - basis must be reduced by a service 
factor to determine the actual hydrostatic design stress. The maximum service factor shall be 0.5 for wall 
thicknesses less than 9.5 mm. For thicknesses equal to or greater than 9.5 mm, the maximum servic e factor 
shall be 0.475.  
4.9.5  The radius of the bottom knuckle of a flat -bottom tank shall not be less than 25.4 mm for tanks with a 
diameter less than 1.8 m and 38.1 mm for a diameter greater than 1.8 m. The minimum thickness of the 
radius shall not be l ess than the maximum thickness of the cylinder wall.  
4.9.6  Tank hoop stresses shall be derated for service above 23°C.  
4.9.7  The tolerance indicated in 4.10.3 applies to these dimensions.  
4.10 Dimensions and tolerances  
4.10.1  General  
All dimensions will be taken with the tank in the vertical position, unfilled. Tank dimensions will represent the 
exterior measurements.  
4.10.2  Outside diameter  
The tolerance for the outside diameter, including out of roundness, shall be ±3 %.   
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 4.10.3  Shell wall and head thickness  
The tolerance for avera ge thickness at each elevation shall be − 10 % of the design thickness on the low side 
and shall be unlimited on the high side. The tolerance for individual audit  readings shall be limited to − 20 % of 
the design thickness. The total amount of surface area on the low side of the tolerance shall not exceed 10 % 
of the total surface area.  
4.10.4  Placement of fittings  
4.10.4.1  The installations of fittings sh all be done by the manufacturer  
4.10.4.2  The tolerance for fitting placements shall be 12.7 mm in elevatio n and 2°radial at ambient 
temperature  
5 Quality of finish  
5.1 The internal and external surface of water storage tank shall be smooth, clean and free from other 
hidden internal defects, such as air bubbles, pits and metallic or other material inclusions.  
5.2 The mould parting line near top rim of the tank shall be cut and finished to the required level. Defects 
like air bubbles and pits at the parting line and at the top of the manhole shall be repaired by hot air filler and 
rod welding method.  
6 Testing  
6.1 Resist ance to deformation  
6.1.1  When cylindrical vertical water storage tank is tested in accordance with method 1 described in Annex 
D, the difference between the circumferential measurements shall be not greater than 2 % of the original 
measurements.  
6.1.2  When rectangular loft tank is tested in accordance with method 2 described in Annex D, the difference 
between the longitudinal measurements shall be not greater than 3 % of the original measurements.  
6.2 Tensile strength  
The tensile strength at yield shall be determined in accordance with Annex E, and shall be not less than 12 
N/mm2. 
6.3 Flexural strength  
The flexural strength shall be determined in accordance with ISO 1209 -2. The flexural modulus of the wall of 
the water tank shall be not less than 400 N/mm2. The manufacturer and/or importer of the raw polyethylene 
pellets shall be liable to supply those that can provide the above -described quality.  DRS128: 20 23 
9                ©RSB2023- All rights reserved  
 6.4 Hydrostatic pressure head  
The tank shall be filled with water up to the nominal capacity. Thereafter it shall be fit ted with a pump with a 
pressure gauge. The pump shall then be started and run till the nominal pressure level is attained and 
examined for leaks. There shall be no leakage observed. A hydrostatic water test will be conducted if ordered 
by the customer.  
7 Sam pling  
7.1 Lot 
7.1.1  All the water storage tanks of same raw material, same type, size and produced under relatively 
uniform conditions of manufacture shall constitute a lot.  
7.1.2  Sample size shall be in accordance with Table 1.  
7.1.3  These tanks shall be sel ected at random from the lot. In order to ensure randomness of selection, 
tables of random numbers shall be used.  
7.1.4  Each selected tank, as per Table 1, shall be tested for the requirements in Table 2 below.  
Table 1 — Scale of sampling and criteria for conformity for tanks  
S/N Lot size  Sample size  Acceptance number  
i.  Up to 50  1 0 Failure  
ii.  200 2 0 Failure  
iii.  300 3 0 Failure  
iv.  500 5 0 Failure  
v.  501 and above  8 1 Failure  
7.2 Routine test  
The routine tests listed in Table 2 shall be carried out to ensure that satisfactory quality is maintained during a 
production run  in accordance with test methods specified therein . 
Table 2 — Routine test parameters  
S/N Requirements  Test method  
i.  Resistance to deformation  Annex D  
ii.  Hydrostatic pressure head  Clause 6.4  
iii.  Carbon black content  Annex A  
iv.  Carbon black dispersion  Annex B   
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 8 Marking  
8.1 All the plastic water storage tanks shall be marked with the following information:  
c) manufacturer's name and address;  
d) net capacity, in litres;  
e) lot number and batch number;  
f) country of origin;  
g) manufacturer date;  
h) material code; and  
i) recyclable  
8.2 The marking shall be carried out either by painting, moulding or by sticking the identification marks on 
external surface of the water tank. The m arking shall be permanent.  
9 Transportation  
9.1 Since there are variations in methods of transportation and handling, the manufacturer’s instructions 
shall be followed in all cases.  
9.2 A suitable means shall be provided, if required, at the open end of open -top tanks to keep the loaded 
tank rigid.  
9.3 All fittings and flange faces shall be protected from damage by covering with suitable plywood, 
hardboard or plastic securely fastened. Tanks shall be positively vented at all times.  
9.4 Pipe and tubing, fittin gs and miscellaneous small parts shall be packaged. Loose items which might 
scratch the interior surface shall not be placed inside the tank during transportation. Additional protection, 
such as battens, end wrapping, cross bracing, or other interior faste nings shall be used as required to assure 
such individual equipment pieces are not damaged in transit.  
9.5 Upon arrival at the destination, the purchaser shall be responsible for inspection for damage in transit. If 
damage has occurred, a claim should be f iled with the carrier by the purchaser. The supplier shall be notified if 
the damage is not first repaired by the fabricator prior to the tank being put into service. The purchaser 
accepts all future responsibility for the effects of the tank failure resul ting from damage.  
9.6 Fittings and accessories that interfere with tank transportation or could be broken during transportation 
are transported separately.  
9.7 The supplier shall provide a brochure to the purchaser including information on installation ins tructions, 
cleaning procedures, disposal and proper caution and warning signs.  DRS128: 20 23 
11                ©RSB2023- All rights reserved  
 9.8 The brochure shall be in one of three official languages used in Rwanda.  
10 Workmanship  
10.1 The finished tank wall shall be free, as commercially practicable, of visual defect s such as foreign 
inclusions, air bubbles, pinholes, pimples, crazing, cracking and delaminations that will impair the  
serviceability of the vessel.  
10.2 All cut edges where openings are cut into the tanks shall be trimmed smooth.   
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 Annex  A 
(normative)  
 
Determinati on of carbon black content  
A.1 Apparatus  
A.1.1  Combustion boat, made of porcelain or silica, having minimum dimensions of 15 mm length, 9 mm 
width and 8 mm height.  
A.1.2  Combustion tube, made of hard glass of approximately 30 mm diameter and 400 ± 50 mm length . 
A.1.3  Gas flow meter, for measuring and controlling the rate of flow of nitrogen within 1.7 ± 0.3 L/s.  
A.1.4  Thermometer, in the range of 250 °C to 550 °C.  
A.1.5  Furnace, to accommodate the combustion tube to give temperatures up to at least 500 °C.  
A.2 Reagents  
A.2.1  Nitrogen, as technical grade.  
A.2.2  Trichloroethylene  
A.3 Procedure  
A.3.1  Assemble the apparatus as shown in Figure 3. Both cold traps following the combustion tube shall 
contain trichloroethylene, but only the first need be cooled with solid ca rbon dioxide.  Alternatively, the entire 
apparatus may be placed in a fume hood and the two traps followin g the combustion tube omitted. Fill the dry 
tube with anhydrous CaCl2 or other suitable desiccant. Hold between loose plugs of glass wool.  
A.3.2  Heat a clean combustion boat to red heat in a Bunsen flame; then transfer the boat to the desiccant 
and allow it to cool over fresh desiccant. Hold between loose plugs of glass wool.  
A.3.3  Remove the boat from the desiccant and weigh it to the nearest 0.00 0 1 g. Immediately place 1.0 g ± 
0.1 g of the ethylene plastic under test in the boat and quickly weigh to the nearest 0.000 1 g.  
A.3.4  Heat the furnace to a constant temperature of 600 °C. Adjust the rate of nitrogen flow to 1.7 L/min ± 
0.3 L/min. Open t he inlet end of the 2.9 -cm diameter tube, quickly place the combustion boat with the sample 
into the tube at the centre of the furnace, and adjust the thermocouple so that the weld is touching the boat. 
Insert copper plug, if this is used. Quickly close th e furnace and allow heating to proceed for at least 15 min.  
A.3.5  Move the tube or furnace so that the boat is no longer in the heated zone of the furnace and allow 5 
min for cooling, while maintaining the flow of nitrogen. Remove the copper plug, if prese nt and the boat 
through the inlet end of the tube and allow it to cool for at least 30 min. Take care that the boat does not DRS128: 20 23 
13                ©RSB2023- All rights reserved  
 become contaminated from any deposits on the walls of the tube. Then quickly reweigh the boat and its 
contents to the nearest 0.000  1 g. 
A.3.6  Make all determinations in duplicate.  
A.4 Calculation  
Carbon black content, % by weight W= 𝑊𝑠
𝑊𝑟×100 
where  
𝑊𝑟 the mass, in g, of the boat before heating in air;  
𝑊𝑠 the mass, in g, of the boat after heating in air; and  
𝑊 the mass, in g, of  the material taken for the test.  
 
Figure 4 — Assembly of apparatus for carbon black determination  
 
 
  
 
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 Annex  B 
(normative)  
 
Determination of carbon black dispersion in polyethylene materials  
B.1 Procedure  
B.1.1  Place two clean microscope slides on a hot plate maintained at 170 °C to 210 °C. Place three 
specimens of pin -head size, each weighing about 5 mg, and each one cut from a separate granule, on one of 
the hot microscope slides, about 20 mm apart. Place a piece of metal shim  40-mm long, 20 -mm wide and 0.03 
mm thick at each end and cover the whole with the other hot microscope slide. Press the specimens by 
applying eve n pressure for 11/2 to 2 min , to the whole area of the face of the upper slide. After the specimens 
have been placed on the slides, these shall not remain on the h ot plate for more than 3 min . 
B.1.2  When the slides are cool enough to be handled examine the three specimens through a microscope 
at magnification of 200 ± 10 with a field of view of 10.1 mm diameter. N ote shall also be made of any lack of 
uniformity of the background.  
B.2 Reporting  
The carbon black dispersion in the material under test shall be considered to be satisfactory, if the specimens 
show a uniform background free from white streaks.  
  DRS128: 20 23 
15                ©RSB2023- All rights reserved  
 Annex  C 
(normative)  
 
Determination of effect of heavy metals on water quality  
C.1 Method of extraction  
C.1.1  Three test specimens of approximately 500 mm2 in surface area shall be taken from tank.  
C.1.2  Each test specimen shall be pre -washed for  a period of 6 h  using tap water wi th pH of 7 to 8. The 
water shall be passed through the specimen at a velocity of 50 mm/s while the specimen is kept fully 
immersed with water. After washing, the specimen shall be filled with fresh solution of the distilled water 
acidified to pH of 4.5 ± 0 .1 by bubbling a current of carbon dioxide through it, and both ends sealed with a 
material that does not contain any toxic substances nor interfere with the determinations of such constituents 
in the aqueous samples.  
C.1.3  After maintaining the specimens at room temperature for 48 h , the solution shall be decanted into a 
suitable container for analysis as the first extraction.  
C.1.4  The procedure shall be repeated a second and a third time. Retain these samples for the 
deter mination of the amount of metals and other toxic substances as second and third extractions.  
C.1.5  The first extraction and the third extraction shall be analyzed for lead.  
C.1.6  The third extraction shall also be analyzed for dialkyl tin (C4 and above) as  tin.  
C.1.7  When calcium and mercury are present; all three extracts shall be analyzed.  
C.2 Method of analysis  
Analysis for lead, tin, cadmium and mercury shall be carried out using atomic absorption.  
   
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 Annex  D 
(normative ) 
 
Determination of resistance to deformation  test 
D.1 Method 1 for cylindrical  vertical tank  
D.1.1  The water tank shall be placed on a flat level base. A circumferential measurement shall be made 
parallel to the base at a distance of one third the effective height from the bottom. The tank shall be fill ed up to 
the effective height at a minimum rate of 23 L/min with water at temperature of not less than 15 °C.  
D.1.2  A continuous film of polyethylene shall be floated over the whole of the surface of water in tank to 
prevent evaporation.  
D.1.3  The temperature of the tank and water shall be maintained at a temperature not less than 15 °C and 
after 7 days a circumferential measurement shall be made at level referred at level to in D.1.1. The difference 
between the two circumferential measurements shal l be expressed as a percentage of the original 
circumferential measurement.  
D.2 Method  2 for rectangular loft tank  
D.2.1  The rectangular tank shall be placed on a level base. The internal length and width of the tank shall 
be measured on the centrelines, as sh own in Figure 5 at the centre of effective height.  
 
 
Figure 5— Centreline measurements  
D.2.2  The tank shall be filled up to the effective height at a minimum rate of 23 L/min with water at a 
temperature not less than 15 °C. The lid shall close after filling the loft tank.  
D.2.3  The temperature of the tank and water shall be maintained at not less than 15 °C and after 7 days 
measurements of length and width shall be made at previously determined centrelines.  
 
 
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 D.2.4  The deformatio n in each direction shall be calculated as follows:  
100x 212 1
LLW WD−=
 
100x 2 11 2
WWLLD−=
 
where 
DL the deformation of the longer side ; 
DW the deformation of the shorter side ; 
W1 the width at the start of the test ; 
W2 the width at the end of test; 
L1 the length at the start of the test ; 
L2 the length at the end of test .  
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 Annex  E 
(normative)  
 
Determination of tensile properties  
E.1 Apparatus  
E.1.1  Testing machine, with grips that can be separated at one or more (as required) of the rates given in 
E.5. 
E.1.1.1  Grips, for holding the test specimen, one being attached to a fixed or essentially stationary 
member of the machine and the other to a movable member. The grips are self -aligning; that is, they are 
attached to the fixed and movable member respectiv ely in such a manner that they will move freely into 
alignment as soon as any load is applied, so that the long axis of the test specimen will coincide with the 
direction of the applied force through the centre line of the grip assembly. The test specimen is held in such a 
way that slip relative to the grips is prevented as far as possible.  
E.1.1.2  Load indicator, suitable load -indicating mechanism capable of showing the total tensile force 
carried by the test specimen when held by the grips.  
This mechanis m is essentially free from inertia lag at the specified rate of testing and indicates the force with 
an accuracy of ± 1 % of the indicated value.  
E.1.2  Extension indicator (when required), suitable instruments for determining, at any time the test, the 
distance between two fixed points of reference lines located within the middle parallel -sided pan of the test 
specimen. It is desirable, but not essential, that this instrument shall automatically record this distance (or any 
change in it) as a function of th e force on the test specimen or of the elapsed time from the start of the test, or 
both. If only distance as, function of elapsed time is recorded, force -time data are also taken. The instrument 
is essentially free of inertia lag at the specified speed of testing and is accurate to ± 1 % of strain or better.  
The elongations between the reference lines at yield and at break of flexible plastics are measured to the 
nearest 7 mm by any convenient means. For the measurement of modulus of elasticity the instrume nt is 
capable of measuring the distance between the reference lines to an accuracy of ± 1 mm.  
E1.3 Micrometers, dial -gauge micrometer, reading to at least 0.02 mm and provided with a flat circular foot 
that will apply a pressure of 9.81 x 10 -3 to 29.43 x 1 0-3 N/mm2. To the test specimen, for measuring the 
thickness of test specimens of non -rigid materials.  
E.2 Test specimens  
E.2.1  The test specimen is cut from the obtained flat portion of the top of the bank with the knife -edged die.  
Reference lines, if used, are 25 mm ± 1 mm apart and are equidistant from the ends of the middle parallel -
sided part of the test specimen as shown in Figure 5.  
E.2.2  It is necessary to apply reference lines to the test specimens. This is done with ink or other medium 
that will not affect the material being tested. Reference lines are not scratched, punched or impressed, upon 
the test specimen.  DRS128: 20 23 
19                ©RSB2023- All rights reserved  
 E.3 Number  of test specimens  
E.3.1  At least five test specimens are tested for each tank.  
E.3.2  Test specimens that do not break with parallel portion are discarded and further test specimens are 
tested.  
E.4 Speed of testing  
E.4.1  The speed of testing is the rate of separa tion of grips of the testing machine during the test.  
E.4.2  The speed of testing is preferably chosen from one of the following, but in any' case is determined by 
the specification for the material being tested, or by agreement between those concerned (see  also E.5.2.1):  
E.4.2.1  With some types of testing machines the rates of separation of grips are virtually the same 
irrespective of whether the machine is running, idle or under load. With these machines, a ratio of separation 
of grips under load within th e above tolerances can be obtained by setting the speed of the machine when it is 
idle.  
E.4.3  If elastic modulus in tension is being determined, the preferred speed of testing is speed A or as is 
specified. Elastic modulus determinations are made with sep arate test specimens from those used to 
determine tensile strength and elongation whenever the speeds of testing are not the same.  
E.5 Procedure  
E.5.1  Measure the thickness of test specimens at several points to the nearest 1 % with the dial -gauge 
micrometer d escribed in E.2.3. The width is taken to be the width measured to the nearest 0.05 mm of the 
middle parallel -sided part of the cutter shown in Figure 8.  
E.5.2  Place the test specimen in the grips of die testing machine, taking care to align the long axis o f the 
test specimen and the grips with an imaginary line joining the points of attachment of the grips to the machine. 
Tighten the grips evenly and firmly to a degree necessary to prevent the test specimen from slipping during 
the test but not the point wh ere the specimen shall be crushed.  
E.5.3  For the determination of properties other than modulus of elasticity and for all test specimens set the 
speed of testing at the specified value and start the machine.  
E.5.3.1  For the test specimens, some or all of,  the following shall be noted as required by the 
specification for the material:  
a) load at the yield point (see Note 1);  
b) distance between the reference lines at the yield point;  
c) load at break or the maximum load (see Note 2);  
d) distance between the reference lines at break.   
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 E.5.3.2  The above information can be obtained from an autographically recorded stress¬ -strain curve or 
may be directly observed.  
NOTE 1  In cases where the yield  point is not well defined, it is frequently necessary to define an offset yield point. This 
is done by specifying a point on the load/extension curve where the curve departs from linearity by a specified percentage 
elongation or extension (see Figure 10).  
NOTE 2  When testing specimens whose cross -sectional area decreases considerably before break, the force may 
decrease to some extent after having reached a maximum value, so that the maximum force and the force at break are 
not the same. For such test spec imens the force at break cannot be obtained by the use of testing machines that only 
register the maximum force. It is often difficult for the load to be read of the moment of break with sufficient accuracy; it  is 
therefore recommended that the load at bre ak be determined from a force/ extension diagram obtained from a tensile 
testing machine equipped with a recording mechanism.  
E.6 Calculation and expression of results  
E.6.1  The force at yield or the maximum force, or the force at break, as appropriate, is div ided by the, 
original minimum cross -sectional area of the test specimen in square centimetres. The results are expressed 
in N/mm2. 
E.6.2  The extension at the yield point (see Figure 9) or the extension at the moment at break, as 
appropriate, is divided by the original distance between the reference lines and multiplied by 100. The 
elongation at yield or break is reported to two significant figures.  
E.6.3  The elastic modulus is calculated from the initial linear portion of the force Vs extension c urve, (see 
Figure 10) by dividing the difference in stress corresponding to a section of the straight -line by the 
corresponding difference in strain. For this purpose, the difference in stresses is the difference in force divided 
by the original cross -sectional area of the specimen in square centimetres and the difference in strain is the 
difference in extension divided by the original distance between the fixed points used for the measurement of 
extension under force.  
E.6.4  For all five specimens, the arit hmetic mean of all values is calculated to three significant figures 
(except for elongation for which the mean is calculated to two significant figures) and reported as the "average 
value" for the particular property in question.  
E.6.5  If the standard devi ation (estimated) is required, it is calculated as follows and is reported to two 
significant figures:  
( )  −−= 1/nxnx S2- 2
 
where  
S the estimated standard deviation;  
X the value of single observation;  
n the number of observations;  
x
 the arithmetic mean of the set of observations.  DRS128: 20 23 
21                ©RSB2023- All rights reserved  
 E.7 Test report  
The test report shall include the following as appropriate:  
a) manufacturer’s principle dimensions, etc.;  
b) width and thickness of parallel portions;  
c) method of preparation of test specimen;  
d) conditioning procedures used;  
e) atmospheric conditions in test room;  
f) number of test specimen;  
g) speed of testing;  
h) tensile stress at maximum force or at break, or both average values;  
i) elongation at yield or at break, or both average values;  
j) elastic modulus, ave rage value;  
k) standard deviations of the individual test results; and  
l) date of test.  
Dimensions in millimetres  
 
Figure 6: — Type 1 test specimen showing positions of reference lines (if used)  
Tolerance on dimensions: ± 5 %  
Key 
A Overall length, minimum   110 
 
 
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 B Width at ends     20 ± 0.5  
C Length of narrow parallel portion  60 ± 0.5  
D Width of narrow parallel portion  10 ± 0.5  
E Radius, minimum    60 
F Thickness     See text  
G Distance between reference lines  50 ± 0.5  
H Initial distance between grips   115 ± 5  
Dimensions in millimetres  
 
Figure 7: — Cutting edges of die -type 2 test specimen showing positions of reference lines (if used)  
Key 
Tolerance on dimensions ± 5 %  
A Overall length     115 
B Width at ends     25 ± 1  
C Length of narrow parallel portion  33 ± 2  
D Width of narrow parallel portion  6 + 0.4  
E Small radius     14 ± 1  
 
DRS128: 20 23 
23                ©RSB2023- All rights reserved  
 F Large radius     25 ± 2  
G Distance between reference lines  25 ± 1  
H Initial distance between grips   80 ± 5  
I Thickness     See tex t 
Dimension in millimetres  
Tolerance on dimensions ± 5 %     
A  Overall length     110 
B Width at ends     45 
C Thickness at ends    6.5 
D Length of narrow parallel portion  9.5 
E Width of narrow parallel portion  25 
F Thickness of narrow portion   3.2 
G Side radius     75 
H face radius     75 
J End radius     6.5 
 
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 Dimensions in millimetres  
Figure 9 — Grip for type 3 test specimen  
 
Figure 8— Load /Extension diagram  
Tolerance on dimensions ± 5 %  
(a) for a material for which a yield point can be determined  
(b) for a material for which an offset yield point ha d to be determined . 
 
 
 DRS128: 20 23 
25                ©RSB2023- All rights reserved  
 Annex  F 
(informative ) 
 
Recommendations for installation of tanks and fittings  
F.1 Tank installation  
F.1.1 Vent pipe/overflow pipe is provided near the top with mosquito and insect proof cap  
F.1.2 The flat base of cylindrical vertical or rectangular water storage tanks should be fully supported over 
its whole bottom area by a durable rigid, flat and level plat form sufficiently strong to withstand without 
deflection the weight of the tank when filled fully with the water. In case, the tank is placed on a suitable M. S. 
platform then it is essential that the latter is free from sharp edges, corners or surface pro jections and shall be 
corrosion resistant.  
NOTE  The requirements for the construction of the reinforced cement concrete or pre -cast cement concrete depend 
on the capacity of the tank and location. Civil engineering will establish the viability of the geolo gy that the water tank will 
be placed on and this will dictate site preparation and cost. The choice of materials of construction will be impacted by the 
whether the tank is located indoors, outdoors  
F.1.3 Where required the tanks shall be suitably anchore d. The tanks may also be provided with clamping 
devices.  
F.1.4 the pipelines, valves and other fittings should be supported in such a manner that it is aligned properly 
so as not to produce any distortion in the water tank where the fitting is fixed.  
F.1.5 the check nuts of the threaded connection should be placed after placing rubber gaskets and should 
not be over tightened. Under no circumstances should jointing compounds or putty be employed in contact 
with the Polyethylene water tanks. PTFE  (poly -tetra-tluroethylene) unsintered tape may be wrapped around 
the threaded Portion of the valves and connections to act as a sealant  
G.1.6  Circular holes drilled for fixing threaded connections should have a clean edge free from notches. 
Holes can be drilled with a  high speed steel hole saw cutter. Scratching or scoring the wall should not be done 
for setting out holes.  
F.1.7 where the section of water tank has a change in profile which is accomplished with a radius, it is 
essential that the outer extremities of the  threaded connections are clear of this radius.  
F.1.8 the water storage tank should not be installed in close proximity to heaters or other direct sources of 
heat.  
F.2 Fittings  
F.2.1 For providing inlet, outlet and other connections. Usually full threaded G.I.  brass connections are used 
which shall not produce any kind of harmful effect on potable water. Flat Surface may preferably be provided 
to fix outlet pipes at appropriate locations. The different sizes of threaded connections required to be fixed for 
different capacities of water storage tanks may be according to Table H.1.   
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 F.2.2 The overflow pipes should be provided with non -corrodible mosquito -proof device of maximum 
clearance not more than 1.6 mm . 
Table G.1  — Sizes of threaded connections  
S/N Capacity of water storage tank  Nominal bore size of threaded 
connection  
i.  Up to 750 12.5 
ii.  Above 750 and up to 2 000  25 
iii.  Above 2 000 and up to 4 000  40 
iv.  Above 4000 and up to 10000  50 
v.  Above 10000  75 DRS128: 20 23 
27                ©RSB2023- All rights reserved  
 Bibliography  
[1] ISOARMSA South African Tank Standard  
 DRS 128: 20 23 
 
Price based on  26pages  
©RSB 20 23-All right s reserved