Nitrogen Process Safety Considerations

Design, Maintenance and Safety Procedures

Practical Experiences

Hazard of Nitrogen-only atmosphere

• When a person enters an oxygen-deprived atmosphere, the oxygen level in the arterial blood drops to a low level within 5 to 7 seconds. Loss of consciousness follows in 10- 12 seconds and if the person does not receive any oxygen within 2-4 minutes, heart failure and death ensue.

• Moving affected and unconscious persons from a N2 atmosphere into fresh air is not enough to promote recovery, the patient has to be physically resuscitated in order to restore the oxygen supply to the brain.

Nitrogen is part of air

As a gas:

• for inerting equipment to prevent flammable atmospheres.

• for preparing equipment for maintenance by purging out hydrocarbons.

• for removal of air / oxygen in equipment before start up.

• for blanketing tanks to prevent the ingress of air.

• for specific welding operations.

• for “mothballing” equipment to avoid the rusting process.

• for use as fire-fighting agent as it removes air.

As a liquid:

• for cooling purposes in the laboratory, freezing a pipeline, etc.

• for storage and transportation of nitrogen in large quantities.

Uses for Nitrogen

As a gas:

• It can cause suffocation by replacing the oxygen in a confined area.

• False readings occur with explosimeters or flammable gas detectors.

• And, like other compressed gases, there are the risks related to its pressurized

containment when it is stored in high pressure cylinders.

As a liquid:

• The same as the gas, when it evaporates.

• By creating an intense coldness (-196oC) that can cause frostbite, crack steel

equipment and explode tyres.

• It boils at a colder temperature than oxygen thereby condensing the oxygen in

the air (which can then form explosive mixtures with other vapours or cause

a violent reaction in contact with organic substances).

Dangers of Nitrogen

Can you become asphyxiated in open air?

The trouble with this picture? A person

without instruments cannot determine the

safe distance!

Can you see or smell Nitrogen?

Do not work over large openings

where nitrogen is used for

‘blanketing’.

Best Practice?

Complex alignments make additional hazards!

Never assume that conditions are favorable!

There are no second chances after exposure!

Learn from other mistakes. Assume that the same conditions are possible at your site, then prove the conditions wrong before work

Never assume check valves are working

Some devices use gravity; they do not work if installed incorrectly.

The seats of these valves cannot resist

repeated pounding

details are important

The ‘fine print’ specifications for this check valve

show that more than ½ to 1 psig delta p is required to prevent

backflow. All smaller backflows will be allowed by

design.

Study the details of engineered equipment!

To allow the flapper doors to open, some clearance must be left between the center sealing post and the center edge of each flapper door. Backpressure on the springs, and clearance in the pins cause the flapper doors to close on a backflow.

Positively verify your maintenance methodsThis valve stroked

properly from closed signal to

100% open signal. After

failure to protect, a bench test with pressure showed

lack of calibration between internal

stem and actuator.

Complex systems lead to more complex problems!

Even with the appearance of

sophisticated detail, incidents of cross

contamination have occurred.

You may be impacted by conditions outside your control!

200 meters between operating units,

separate operating crews, unclear checking of lineup between units. Cat unit hose blocked

in.

PCU unit purge hose a conduit for backflow of

cat unit gas

This incident could have easily been a fatality. An employee was in a confined space ( a RVF ). The manway watch noticed that atmosphere inside the space was changing ( falling O2, increasing CO2) and notified the employee to exit the space. Upon investigation it was

determined that the utility air header had been contaminated with reactor off gas. The utility air system was driving an venturi type "air horn" which was blowing air into the confined space. Connections between Utility air and Reactor Off gas : At XYZ plant, Utility air is mixed with Nitrogen to supplement the reactor off gas supply. Air is mixed to achieve an O2 concentration of 4-5 % in order to reduce N2 usage. If the reactor off gas pressure is higher than the N2 header or utility air header it is possible to have backflow of reactor off gas into the N2 piping and eventually into the

utility air header. There are check valves but they are not a positive isolation. Plants should check to see if there are similar connections between the utility/instrument air systems and

the reactor off gas / N2 systems. I do not believe this design is unique to XYZ. Connections between air and inert gas systems need to be identified and isolation systems in place. This incident happened during a unit outage where system pressures were not normal. Identify potential connections and perform a HAZOP to determine if there are backflow risks.

Follow-up from a PTA incident: (emphasis is mine)

• Utility air is not breathing air! - The proper use of air horns is to PULL air from a confined space to ventilate it. In this case the horn was blowing into the space. I hear this is not that uncommon (especially during TARs). If you want to blow air into a confined space you should use electric fans or air powered turbines that do not mix the utility air with air being moved.

Utility air should not be allowed inside confined space even on

powered tools since it is not considered a safe source of breathing air.

Manway watches with continuous air monitoring equipment save lives! - Make sure your sites require manway watches to always use continuous air monitoring devices. People need to be trained on how to use and maintain these instruments. At XYZ Plant this training paid off and saved a life.

More Follow-up from a PTA incident:

(emphasis is mine)

Air

Utility Air

Air Horn

Possible Immediate Causes - Actions and Condition

 

1-2 Violation by Group – Safety Regulation #21 requires a PM test of the check valve going into the utility air header and no documentation could be produced to show this test had ever been completed.

 

2-1 Improper Use of Equipment – Using the air horn to supply or blow air into the confined space does not constitute proper use of that type of air horn.

 

5-2 Defective guards or protective devices – The check valve was the only device that kept the off-gas or nitrogen from entering the utility air header and the check valve failed. NOTE: nearly ALL the time the nitrogen and reactor off-gas pressure is higher than the utility air pressure.

 

5-5 Inadequate Warning Systems – There was no indication of reverse flow built into the control strategy for the blending system or an oxygen meter on the utility air system that would indicate a reverse flow situation. Additionally the safety regulation does not require a continuous warning system (gas monitor) for all entries.

Simple near misses can lead to very complex and difficult responses

Consider the following three slides, which are the actions created from the PTA air horn near miss

Possible System Causes - Job Factors and Personal Factors7-3 Inadequate training effort – Confined Space permit issuer training was conducted in 1999 and for permit issuers authorized since then and the training materials do not describe the potential hazards of using an air horn to supply air into a confined space. 8-4 Inadequate identification of worksite/job hazards – Safety regulation #21 describes the piping tie between utility air and reactor off-gas or nitrogen but the potential hazard was not recognized. 10-3 Inadequate Assessment of Potential Failure – Two different HAZOP’s reviewed this system for reverse flow and the potential hazard was not identified. 11-6 Inadequate audit / inspection / monitoring – There was no documented tests of the check valve which prevents reactor off-gas or nitrogen from entering the utility air header. Additionally the physical layout of the check valve and manual block valve would not permit a test without completely de-pressuring the plant utility air and instrument air system. 13-1 Inadequate assessment of needs and risks – Many people have observed air horns being used to supply air to a confined space and did not appropriately weigh the potential risks. 14-2 Inadequate Development of Policies, Standards and Procedures – During review of the Utilities SOP 84-11-10, there were many questions around the adequacy of the manual blocks on the system, steps to take to shutdown, etc… Had a manual block be in place on the gas blending system the incident would most likely not have occurred. 14-3 Inadequate Implementation of Policies, Standards and Procedures – Safety regulation #30 did not specifically prohibit use of air horns to supply air to a confined space and the requirements with regard to only exhausting air inside the space were buried in a paragraph containing multiple requirements. 14-4 Inadequate enforcement of Policies, Standards and Procedures – Safety Regulations #21 which contains the requirement to test the check valve twice/year was not enforced. Additionally there is no formal and systematic audit process to verify requirements like this one are being met.

Corrective Actions:

 1.       Conduct a detailed control system strategy of the air and nitrogen or reactor off-gas mixing system. The purpose of this study is to better understand the workings of the current system.

 2.       Revise safety regulation #30 to require continuous monitoring at all times while confined space entry is in progress and to prohibit air horns from being used to supply air into a confined space. Also review practices around using air driven tools when inside confined spaces.

 3.       Review the appropriateness of having the reactor off-gas or nitrogen and utility air systems piped together and determine if this system will remain in place. If it remains, at a minimum the following issues will need to be addressed:

         Modify piping to allow check valve test without shutting down utility and instrument air systems

         Set up check valve test

         Install a control strategy to identify reverse flow into the utility air header and/or install an oxygen meter to indicate reverse flow.

         Set up alarms so that appropriate action will be taken when reverse flow occurs.

If the gas blending system is determined to be inappropriate, dismantle the system or at a minimum remove a spool piece and blind

 4.       Conduct a survey throughout every unit to ENSURE there are NO other hard piped or “hosed” connections between inert gas (nitrogen or reactor off-gas) and utility air

 5.       Evaluate the supply of ventilation equipment, which is designed to supply air into a confined space. Purchase additional equipment as needed.

 6.       Evaluate the supply of confined space gas monitors and ensure a checklist item to rent/purchase additional monitors is included on the pre-TAR checklists; ensure eexisting supply are all working as designed.

 7.       Provide ventilation training with all applicable operations/Maint/Contract Maint personnel. To discuss these hazards.

 8.       Revise the confined space permit issuer training to include a discussion of air horns being used to supply air into a confined space.

 9.       Review OSBL SOP 84-11-10 for adequacy and revise as needed.

 10.   Communicate this incident very broadly to ensure others do not have a practice of using air horns in this manner or have adequate controls and alarms if any inert gas is tied into plant utility air systems.

 12.   Discuss and reinforce the importance of conducting daily pre-use tests of gas monitoring equipment prior to issuing hot work permits, or confined space entry permits.

 13.   Determine a method of systematically auditing site for compliance with requirements in safety regulations.

Consider an air contamination of the nitrogen distribution system. This is theoretically possible through the connections in the same mixing system that delivers inert gas to the inert gas distribution system.

Consider a process contamination of both the air system and the nitrogen system. This is theoretically possible through the nitrogen connections to the process.

Consider the maintenance and testing of "critical" devices. By "critical", I mean those devices that are your primary defense against contamination. So the anti-backflow control valves are the primary defense against contamination at the mixing station. Positive verification (periodic bench testing, for instance) is your means of assuring the system (the valves) performs as intended.

I recommend you do not rely on check valves for positively assured protection. Their presence in systems is necessary, but their performance cannot be counted on.

I recommend an additional inspection of utility station hose connections to assure that 100% of the stations are installed as designed. We found one with an improper fitting, but did not check the whole system. There was no hose connected, but the potential was there for an improper hose connection.

Investigations, without external experience, may lead to inadequate response and false security

What to do next?• Resources

– http://txc-safetynet.bpweb.bp.com/

• Procedures

– Take these events, study similar events, and realistically consider whether the safeguards at your site are 100% effective

• Design issues

– During hazop and design, know that equipment fails

– There is a very high probability that equipment is not used as designed

• Maintenance Issues

– Maintenance is more hazardous than steady operation

– Never trust the preparations

• Behavior

– Does your organization give healthy respect to nitrogen

– Training, practicing, reinforcement

And finally

• Thank you