Description of the incident

A drilling contractor was flushing a rotary percussion drilled borehole when the pressure relief valve on the rig mounted compressor was blown off and separated from the compressor. This caused a mist of compressor oil to spray onto hot parts of the engine which caused a fire.

The drill crew activated the automatic fire suppression system, which shut down the rig and compressor and extinguished the fire.

Apparent causes

The investigation revealed that:

•  The Driller had recently replaced the pressure relief valve but after installation, the relief port was facing towards hot components of the engine and so he instructed an offsider to turn the pressure relief valve half a turn anticlockwise so that the relief port was facing away from potentially hot components.

• The thread on the pressure relief valve and the thread on the reducing bush on the compressor were different and so, although the pressure relief valve was screwed into the reducing bush, the integrity of the connection was insufficient to hold the force exerted by the compressed air.

• The Driller applied a thread sealing compound to the threads on the pressure relief valve but the thread was loosened half a turn after the sealant had begun to harden and so it is suspected that the sealant did not set as envisaged.

Comment and recommendations

The investigation identified that the pin thread on the pressure relief valve was an NPT (National Pipe Thread) and the thread on the reducing bush on the compressor was a BSP (British Standard Pipe) thread. While both of these are common thread forms used to connect pipes and pipe fittings, NPT and BSP threads are not compatible. It would seem therefore that even if the offsider had not loosened the valve half a turn as described, the valve would have been blown off the compressor at some time. Similarly, it would appear that applying a sealing compound to the threads would also not have prevented the valve from being blown off.

This incident has clearly brought into focus how critically important it is for drill crews and maintenance crews to be made aware of the type and dimensions of fittings associated with critical safety equipment. The accident also highlights the requirements for repairs and mainetence of critical safety equipment to be carried out only by competent persons.

BSP and NPT threads are both standardised thread forms that are widely used; NPT thread forms are commonly used in the United States and Canada, and BSP is most commonly used in the United Kingdom, Australasia, in European countries and in many African countries.

The essential differences between BSP and NPT threads are:

• NPT threads are pointed in the peaks and valleys, whereas BSP threads are rounded. 

• Most notably, the NPT thread angle is 60 degrees versus the BSP 55-degree angle.

To highlight these issues, an article explaining how to identify the thread type on a pressure relief valve is included in this edition. 

This article was originally published by Asset Management Engineers (Pty) Ltd of Perth, Australia.