Final report:
Ultra Rapid Crack Propagation
(This is a draft)

*

What is URCP?
The Ultra Rapid Crack Propagation (URCP) phenomenon is a rare but significant phenomenon that may result in catastrophic damages. As the name suggests it results in a rapidly progressing crack in the PE tubes & pipes. The pressures (stresses) are usually distribution system pressures, below 100psi (as per CSA Z662). These cracks are initiated by a sudden or intense impact which propagates a crack in existing flaws such as a scratches, grooves or dents. The cracks are known to travel at speeds from 100-300 m/s and travel long distances. The main factors associated with URCP are susceptible material, stresses, temperature and environment. Since the stresses (pressures) required for the initiation of URCP are very small, the effect of predicting failures are very hard which leads to the URCP phenomenon resulting in more catastrophic failures.

The characteristic of the crack follows a sinusoidal crack path with hackle marks that indicate the direction of the crack in the longitudinal crack path. Often times a crack may split in to more than one branch.

Figure #2: URCP Crack

Historical Issues

As mentioned previously since the use of PE tubes & pipes has been a large part of the industry, the URCP phenomenon is becoming a more and more common problem in the industry. Over the course of a couple of decade the following issues have been documented. * In October 1985, an incident related to URCP occurred in a PE gas system which was reported in Szeged (Hungary) over a length of 700 m. Other long running cracks were also observed on the tubes & pipesline system. The crack was initialed at the time of installation.

* In 1989, an incident occurred in Antwerp (Belgium), a rapid crack occurred in a PE water tubes & pipesline system. The crack was initiated due to a butt fusion coupling. It transitioned from a slow crack to a rapid crack. The tubes & pipes was in service with 2.5 bar of pressure of water at 20°C.

* In 1990, another incident in Antwerp (Belgium) occurred in a PE tubes & pipesline with a water medium.

* Between 1990 and 200, URCP was found in tubes & pipesline system of several gas distribution companies across Europe.

Based on these historical incidents it can be noted that URCP is an eminent problem in PE tubes & pipeslines. This has already been taken into consideration with some standard organizations such as ISO, ASTM and CSA. Due to which measures have been taken to ensure that proper testing is being performed and proper mitigation measures are being taken to prevent any catastrophic failures. Typical environment, service and material conditions for URCP

URCP like SCC, is developed over a period of time depending on many material, service loading and operational conditions. No specific rules have been established to date to predict the occurrence of URCP. However, the research has established some general rules that can serve as measure of probability of occurrence of URCP. These rules vary widely between different materials and even among the same material owing to the micro scale differences in chemical composition and microstructure. The characteristics leading to the URCP of most commonly used material for gas tubes & pipeslines; unimodal and bimodal polyethylene are discussed below.
The URCP of unimodal and bimodal PE tubes & pipess is primarily dependant on the following * Service Temperature * Operating Pressure * Tubes & pipes Wall Thickness * Tubes & pipes Material Characteristics
The table below summarizes the effect of each of these factors on URCP. It may be noted that these general rules are applicable for most materials of the PE family. More specific conditions defining the probability of occurrence in unimodal and bimodal PE tubes & pipess are discussed later in this section.
Table #1: Parameters of URCP Parameters | Relationship with URCP | Service Temperature | The probability of URCP occurrence increases with the increase in service temperature. | Operating Pressure | The probability of URCP occurrence increases with the increase in Operating Pressure. | Tubes & pipes Dia & Wall Thickness | The probability of URCP occurrence increases with the increase in Tubes & pipes diameter and wall thickness. | Tubes & pipes Material Characteristics | Bimodal resins based materials decreases the probability of occurrence of URCP. |

To define service temperature and operating pressure conditions that causes URCP in ideal conditions, the critical values have been developed from experiments and research for different materials. The following terminologies are developed in relation to the diameter of the tubes & pipesline and will be used to describe several operating and service conditions on specific materials.
URCP safe zone

It is the area under the temperature and pressure curves plotted against the tubes & pipesline diameter that signifies no probability of URCP occurrence on specific materials. These sets of service conditions are ideal for operation of gas tubes & pipeslines in relation to URCP.
URCP Possible Zone

This portion of the area under temperature and pressure curves plotted against the tubes & pipesline diameter indicates the set of values of service temperature and operating pressure that are known to cause URCP in combination with corresponding tubes & pipes diameters. It is important to note that the URCP possible zone is a statistical estimate. This means that the set of values for operating pressure and temperature for a given diameter falling under the safe zone may not develop URCP, but it’s extremely probable that URCP can develop at any given time.
URCP Caution Zone

The URCP caution zone represents the area enveloped by both URCP possible zone and URCP safe zone where there is known risk of URCP failure. It could be thought of as a threshold zone URCP failure. The tubes & pipeslines in service under URCP Caution zone requires continuous monitoring to ensure that the operating pressure and temperature do not fall towards the URCP possible zone of the curve.
Critical Temperature and Pressure for given diameters for Unimodal and Bimodal PE tubes & pipess

As an example, the critical temperature and pressure that causes URCP for the Unimodal and Bimodal PE tubes & pipess are discussed below.
Critical Temperature

The following plot represents the critical temperature values for unimodal and bimodal PE tubes & pipess for a given set of tubes & pipesline diameters.

Figure #3: Critical Temperature Plot (Unimodal)

Figure #4: Critical Temperature Plot (Bimodal PE 2708)

The plot indicates that contrary to popular assumption, low tubes & pipes diameter and very high temperature do not cause URCP. On the other hand large tubes & pipes diameters can develop URCP even at very low temperatures. Note that the URCP possible zone disappears in the bimodal PE tubes & pipes indicating its superiority over the unimodal PE tubes & pipess with regard to URCP. URCP event can develop in a piping system manufactured by unimodal PE resins at service temperatures below 60 F. However, URCP is not a concern in piping system manufactured using bimodal PE resins unless service temperatures fall below 28F, all regardless of operating pressure.

Critical Pressure
The plots below depict the effect of operating pressure on unimodal and bimodal PE tubes & pipess for different tubes & pipesline diameters.

Figure #5: Critical Pressure Plot (Unimodal PE 2708)

Figure #6: Critical Pressure Plot (Bimodal PE 2708)

As shown in the plot, URCP events can happen in a piping system made from unimodal PE resins while the operating or testing at pressures greater than 90 psig at 32°F. Also, there exists a cautionary zone between the range 60 to 90 psig. On the other hand, there is no concern of pressure related URCP occurrence in a bimodal piping system as the critical pressure values of bimodal resin is 560 psig at 32°F.

References

1. Dr. Gene Palermo, Dr. Dane Chang. (2002) Increasing Importance of Ultra Rapid Crack Propagation (URCP) for Gas Piping Applications - Industry Status” 2. What you need to know about URCP, http://www.duraline.com/sites/default/files/downloads/DL_PP_URCP.pdf 3. G. Palermom, W. J. Michie, D. Changm “Increasing Importance of Rapid Propagation (URCP) for Gas Piping Applications – Industry Status”, Plastics Tubes & pipess Conference XIV, September 2008 4. R.K. Krishnaswamy, P.S. Leevers, M.J. Lamborn, A.M. Sukhadia, D.F. Register, P.L. Maeger, “Ultra Rapid Crack Propagation (URCP) Failures in HDPE Tubes & pipess Structural-Property Investigations”, a White Paper by: Chevron Phillips Chemical Company L.P 5. P S Leevers, Mechanical Engineering Department, Imperial College, London, UK, Ultra Rapid Crack Propagation : the failure mode that never was? 6. J M Greig, British Gas Engineering research Station, Killingworth, Ultra Rapid Crack Propagation in Polyethylene Gas Tubes & pipess
Resistance to URCP – S4 test PE 100+ Association, http://www.pe100plus.com/PE-Tubes & pipess/materials/Testing-methods,i237.html 7. A. Ivankovic and G. P. Venizelos, Ultra Rapid Crack Propagation in Plastic Tubes & pipes: Predicting Full Scale Critical Pressure From S4 Test Results,