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Paper Prepared for
Conference of World Water Day –
Organized by National Water Supply and Drainage Board (NWSDB)
on
Problem Analysis and Possible Precautions
on Selection and Installation of PVC / PP full encirclement Clamp Saddle
Prepared by:
Eng. H.M. Nimal Jayantha
Consultant Engineer
Date: December, 2013
Problem Analysis and Possible Precautions
on Selection and Installation of PVC / PP full encirclement Clamp Saddle
Executive Summary
National Water Supply and Drainage Board (NWS&DB) has revealed that the present potable water loss in distribution system due to wastage and over usage is about 30 to 40%. It was also found that the main contributor to the water losses is leakages through service connection. The present practice of service connection is connecting the service line to the mains through full encircled PVC or PP clamp saddled on which copper alloy ferrule is fixed.
The problems encountered in the field at present are not directly related to clamp saddle. It is integrated problem of assembled unit of clamp saddle and the ferrule buried in the ground. The problems relating to PVC / PP -clamp saddle must therefore be analyzed by taking it as a whole unit when it is selecting (choosing), assembling, fixing on the mains and, covering by the earth-filling.
The most of the causes of the problems identified are occurred due to one or more of the following reasons.
I. Mismatching of dimensions and thread type
II. Excessive external load exerted on the saddle
III. Exceeding fatigue strength limits of PVC / PP material when it undergone to fluctuation of pressure in the pipe line (mains)
IV. Poor code of practices followed by designers and technicians during design and installation of mains and service line
V. Usage of non standard clamps saddle and ferrules
Out of these reasons dimensional and thread type mismatching is the task of designers and users in selecting and using correct clamp saddle and ferrule which could not be a difficult task. Addressing the rest of the reasons is the challenges for users will have to face today. The paper analyzes the prevailing problems of clamp saddle and indicates the directions to be followed on rectification of those problems. The effective activities identified and discussed briefly in the paper are development of product standards and testing methods for PVC / PP clamp saddle unit. The common practice in the world relating to the product standard of the clamp saddle and the ferrule is application of individual standards for material, service connection, safety and influence of water polluting. There is no single product standard for the clamp saddle and the ferrule practiced in ISO systems or BS EN standards. By considering this fact, engineers of Greater Kandy Water Supply Project (GKWSP) have already developed draft standards for the clamp saddle and the ferrule and code of practice for their assembling and installation. The GKWSP has already taken steps to establish the testing facilities for testing of the clamp saddle and the ferrule including endurance testing.
The main problems discussed in the paper are as follows.
I. Splitting (Cracking) or breaking of Boss (protruded neck) of the saddle
II. Leakage through interface of service connector and clamp-boss (protrude portion)
III. Leakage through clamp
IV. Breakage of saddle (See photograph below)
Data and information gathered from field visits of meeting technicians and users at the locations where the problems are raised are given indication that the modes of failures (splitting and breaking) occurred frequently are due to the results of improper thread type of ferrule and saddle boss and poor practice of installation. In some cases the technicians / pipe fitters themselves have developed their skills in certain areas such as maintaining tightening-torque on ferrule at correct level in the saddle without using torque wrench which is one of correct and essential tools required for installation of service connection. The breakage of the saddle found in the field is may be due to external load exerted on the saddle or due to pressure fluctuation in the mains which leads fatigue failures. The information available at present is not adequate to make clear and definite conclusion on experience of fatigue failures.
The GKWSP has now taken initial steps to rectify the problem faced in domestic service connection of potable water line. The principle of them are preparation of product standards of ferrule and PVC / PP clamp saddle and development of testing facilities for clamp saddle including endurance test facilities.
Field Problem Analysis on PVC / PP full encirclement Clamp Saddle and Possible Precautions
1. Introduction:
The Clamp on Saddle is a device which is used as adaptor for connecting service line to supply pipe line (main line). It is a full encirclement saddle and is in two halves which are encircled around the mains (supply pipe) and is tightened with help of bolts or “V” grooved clamps. It is consisting of one side outlet or two outlets in both pieces. The service line could be connected by means of solvent cement socket, threaded joint or compression fit.
The problems encountered in the field at present are not directly related to clamp saddle. It is a combine problem of assembled unit of clamp saddle and the ferrule buried in the ground. The problems relating to PVC / PP -clamp saddle must therefore be analyzed by taking it as a whole unit when it is assembling, fixing on the mains and, covering by the earth-filling. The most common breakage problems found in the field are shown in the photographs below.
[pic]
2. Common failure modes of PVC clamp saddle in potable water supply:
The following table summarizes most common types of field failure modes in water supply mains. The table has been developed on the observations of field officers.
Problems associated with Clamp Saddles and their causes
|Problem Experienced |causes |
|1 |Splitting (Cracking) or breaking of Boss |Due to unbearable external load |
| |(protruded neck) of the saddle (See |Over tightening the service connector |
| |photograph below) |Mismatching thread of service connector with thread of saddle-boss |
| | |Too much of entangling thread seal around the service connector |
| | |Non standard product and their material |
|2 |Leakage through interface of service |Connection made by mismatched mating threads |
| |connector and clamp-boss (protrude portion) |Improper /inadequate sealing material on thread |
| | |Non standard product and their material |
|3 |Leakage through clamp |Under size “O” ring |
| | |Inadequate tightening of clamp saddle |
| | |Non standard material |
| | |Non standard product and their material |
|4 |Breakage of saddle (See photograph below) |Due to unbearable external load |
| | |Mismatching pipe size with dimensions of saddle due to wrong selection|
| | |of products |
| | |Due to trapping/insertion of debris in interface between saddles and |
| | |main (pipe) |
| | |Over tightening the clamps |
| | |Fatigue failures due to pressure fluctuation in the pipe |
| | |Non standard product and their material |
The most of the causes identified in the above table are occurred due to one or more of the following reasons.
I. Mismatching of dimensions and thread type
II. Mismatching of length of shank of the ferrule with depth of threaded hole of the boss of the saddle
III. Excessive external load exerted on the saddle
IV. Exceeding fatigue strength limits of PVC / PP material when it undergone to fluctuation of pressure in the pipe line (mains)
V. Poor code of practices followed by designers and technicians during design and installation of mains and service line
VI. Usage of non standard clamps saddle and ferrules
3. Analysis of failures and causes:
1. Dimensions mismatching:
1. Thread type of ferrule inlet connection:
Example referring to specific of product
|Pipe Dimensions |Clamp Dimensions |Ferrule Dimension |
|BS EN 1452-1 & 2 - Imperial |BS EN 1452-1 & 3 - Imperial | |
|Nominal size – 4” |PVC clamp –Imperial 4” X | |
|Outer diameter- 114.1 mm |Inner diameter of the saddle -114 mm | |
|114.4 mm | | |
|BS EN 1452-1 & 2 - |Din 8061 – Metric pipe | |
|Din 8061 – Metric pipe |PVC clamp – metric 110 X | |
|Nominal size – 110 mm |Inner diameter of the saddle - 110 mm | |
|Outer diameter- 110.0 mm | | |
|110.3 mm | | |
| |Threads: |Threads: |
| |BS EN 10226-1:2004 (Parallel thread) |BS EN 10226-1:2004 (Taper thread) |
| |Thread per inch for 1” BSP – 11 |Thread per inch for 1” BSP – 11 |
| |Pitch - 2.309 mm |Pitch - 2.309 mm |
| |Threads: |Threads: |
| |NPT (ANSI standard B1.20.1) – |NPT (ANSI standard B1.20.1) – |
| |Thread per inch for 1” - 11½; |Taper Thread per inch for 1” - 11½; |
| |Pitch - 2.208 mm |Pitch - 2.208 mm |
If the boss of the saddle having female parallel thread (G –thread) of standard BS EN 0228-1:2003 (Parallel thread) is over tighten with ferrule having tapered thread (R-thread) of EN 10226 or NPT (ANSI standard B1.20.1) the saddle boss could be split as seen in photograph above. It is also possible to split the boss even with the ferrule having parallel thread of BS EN 0228-1:2003 (Parallel thread) if it is tightened with excessive layers of thread seal tape wrapped around the ferrule shank.
2. Inner Diameter of saddle straps with outer diameter of the pipe:
The saddle straps manufactured on metric dimension can’t be fixed on the PVC pipes of the standard BS EN 1452-2 ; Annex B –imperial size pipes, because inner diameter of the saddle is usually smaller than the outer diameter of the pipe (e. g. saddle straps of nominal size 110 mm of which inner dia. 110 mm with the pipe of nominal size 4” of which outer dia. -114 mm). In this case if the saddle-halves wrapped around the pipe and are tightened together it is possible to break the saddle along the corner between the collar and the circular body. This type of breakage sample is shown in the photograph above.
2. Pipe deflection due to excessive load:
The PVC pipes are flexible to certain degree without breaking the pipe so that buried pipes are progressively deflected with soil weight until the point where soil stiffness become equal to lateral forces experienced due to deflection of the pipe (Fig 1). When the pipe is loaded externally from soil weight or soil weight together with other external forces such as vehicular traffic to exceed the limit of soil stiffness the diametric deflection of the pipe will be progressed further resulting in the breakage of pipe longitudinally. Maximum recommended diametric deflections are as follows.
• PVC pressure pipes - 5%
• PVC sewer / Drain pipe - 7½%
• PVC electrical conduits - 5%
The effect of soil weight together with external loads such as vehicular traffic against soil type and buried depth is expressed by following equation named as Lowa equation.
[pic]
Where PS is pipe stiffness in psi
P is soil density in lb/ft3
w’ live load factor in lb/ft3
E’ is degree of compaction of pipe zone on soil type in lb/ft3
Clamp deflection:
Flexibility of PVC or PP clamp is lower than PVC pipe due to its shape and material so that lateral soil resistance on the clamp gained due to soil stiffness is less. It is therefore tendency to exceed the limit of diametric deflection of the saddle resulting longitudinal breakages as shown in photograph of failure samples. Forces acting on the clamp saddle are shown in Fig. 2 when it mounted on the pipe.
[pic]
Forces acting on one piece of the saddle are shown in Fig. 3 as follows.
[pic]
3. Fatigue failures
There is a tendency to break the clamps on fatigue due to prolong cyclic load experienced on it. But identification of pattern of cyclic load experienced on the clamp saddle is not easy. Some research finding has suggested that the safety factors to be incorporated in pipe laying design approximately according to the table below on the shape of the pressure variation as shown in Fig 4.
[pic]
Recommended fatigue cycle factors for PVC-U, PVC –M and PVC –O are included in following table.
|Total cycle |Approx No. Cycles/day for 100y life |Fatigue Cycle Factors, f |
| | |PVC -U |PVC -M |PVC - O |
|26,400 |1 |1 |1 |1 |
|100,000 |3 |1 |0.67 |0.75 |
|200,000 |5.5 |0.81 |0.54 |0.66 |
|500,000 |14 |0.62 |0.41 |0.56 |
|1,000,000 |27 |0.50 |0.33 |0.49 |
|2,500,000 |82 |0.38 |0.25 |0.41 |
|5,000,000 |137 |0.38 |0.25 |0.41 |
|10,000,000 |274 |0.38 |0.25 |0.41 |
This analysis can’t be applied to the assembled unit of clamp on saddle fixed on the water pipe as their geometrical shape and material properties differ from the pressure pipe alone. The fatigue failures of the clamp unit could be occurred in two possibilities as follows.
➢ According to the different variable soil load or vehicular traffic exerted on the clamp saddle vertically
There is very less possibility of repeating variable external load due to vehicular traffic and soil load over the working clamp saddle buried in soil by the side of road. The assumption of expecting this type of fatigue failure is therefore, not reasonable.
➢ According to the different magnitude of soil resistance experienced on the clamp saddle vertically and horizontally on variation of internal pressure of the pipe
The soil resistance generated vertically against the pressure variation inside the pipe is greater than the resistance generated horizontally. The clamp assembled unit is therefore deflected diametrically outward in horizontal direction and the magnitude will be varied according to pressure variation inside the pipe. Accepting this phenomena in the working clamp unit, the limitation of fatigue failure in number of cycles deflecting within the accepting magnitude of diametrical deflection of the clamp saddle can be find out by experimentally in the laboratory.
4. Product Standard:
The drafts of standard for both products the PVC u and PP clamp saddle and copper alloy ferrule have already been developed and forwarded to Sri Lanka Standard Institute (SLSI) for review and implementation. (Annex 1- Product standards - Draft)
5. Testing of PVC u and PP clamp saddle and copper alloy ferrule:
Design of the test apparatus are based on the proposed standards and fabrication and installations are now in progressed. (Annex 2- Design of test apparatus)
Conclusion:
It is necessary to conduct further surveys to analyze the problem in details and maintain proper data-base in order to validate and confirm the above findings. It is a challenge and duty of engineers of NWS&DB.
References:
BS EN ISO 228-1:2003
Pipe threads where pressure-tight joints are not made on the threads. Dimensions, tolerances and designation
BS EN 10226-1:2004
Pipe threads where pressure tight joints are made on the threads. Taper external threads and parallel internal threads. Dimensions, tolerances and designation
Status : Current Published : July 2004
BS EN 10226-2:2005
Pipe threads where pressure tight joints are made on the threads. Taper external threads and taper internal threads. Dimensions, tolerances and designation
Status : Current Published : January 2006
ISO 7-1:1994
Pipe threads where pressure-tight joints are made on the threads -- Part 1: Dimensions, tolerances and designation
BS EN 10226-3:2005
Pipe threads where pressure-tight joints are made on the threads. Verification by means of limit gauges
Status : Current Published : September 2005
ISO 7-2:2000
Pipe threads where pressure-tight joints are made on the threads -- Part 2: Verification by means of limit gauges
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