Page 240 - Failure Analysis Case Studies II

Failure Analysis Case Studies II
D.R.H. Jones (Editor)
0 2001 Elsevier Science Ltd. All rights reserved 225

VIBRATION-INDUCED FATIGUE FAILURE OF AN
IMPULSE LINE

K. R. AL-ASMI and A. C. SEIBI*
Department of Mechanical Engineering, Sultan Qaboos University, PO Box 33, AI-Khod, Muscat 123,
Sultanate of Oman
(Received 11 February 1998)

Abstract-This paper presents a case study dealing with the operational failure of an impulse line used to
connect a tapping in a crude oil pipeline header to a pressure transmitter in an oil field booster station. The
3/8 in diameter stainless steel tubing failed where it entered a Swagelok connection to the pressure transmitter.
The investigation was complicated due to the non-availability of information pertinent to the field conditions
and the operational urgency to isolate the case of failure. A combination of scanning electron fractography
and finite element structural analysis showed that the failure was caused by high-cycle fatigue resulting from
the transverse vibration of the tubing. Remedial measures were suggested to reduce the amplitude of the
vibration. 0 1998 Elsevier Science Ltd. All rights reserved.
Keywords: Fatigue, fatigue markings, finite element analysis, mechanical connections, vibration.

1. BACKGROUND

This paper presents a case study dealing with the failure analysis of an impulse line used to connect
a tapping in a crude oil pipeline header, at 70 bars, to a pressure transmitter. The 3/8 in. diameter
stainless steel tubing (type 316) failed during operation at an oil field Booster Station consisting of
eight turbine driven pumps, not all operating at once. The investigation was complicated due to the
nonavailability of information pertinent to the field conditions and the operational urgency to
isolate the cause of failure. The situation was of particular concern as the failed tubing was newly
installed, as part of ongoing revamp programme, and several similar connections were installed and
in service in at least two other pumping stations. The investigation proceeded on X-ray element
analysis and microscopic examination of the failed line, site measurements, structural analysis of
the line using a general purpose finite element programme, and an evaluation of the mechanical
properties of the tubing using tensile and hardness testing.
To understand the sources of this failure, relevant information related to the operating conditions
of the booster station was gathered during a site visit. It was noticeable that vibration was present
in all pipe work, walk ways, and support structures, irrespective of whether the pump was operating
or not and that all pumps are coupled through the discharge pipe network. Figure 1 shows a general
view in the vicinity of typical pressure taps, depicting the connection from the discharge header to
pressure transmitters. Note the large overhanging mass of the valve body and flange, mounted on
the pressure taps (Fig. la). The additional support, seen in Fig. lb, was installed after failure
occurred, to reduce excessive movement of the tubing due to vibration. The tubing was connected
to the pipe and the pressure transmitter using standard ‘Swagelok’ connectors (Fig. 2), where two
wedges are driven inward by the action of the nut. In this type of connection, the inner faces of the
wedges apply a radial pressure on the outer surface of the pipe and deforms it permanently. The
extent of deformation depends on the degree of tightening. This type of connection will, therefore,
cause a defect in the outer wall of the pipe and a possible hardening of the material where the plastic
deformation has occurred.

*Author to whom correspondence should be addressed.

Reprinted from Engineering Failure Analysis 5 (3), 195-204 (1998)