Pipe and Vessel Purging

GoM Region Pipe and Vessel Purging Safe Work Practice (SWP) Page 1 of 4

GoM Region Pipe and Vessel Purging Safe Work Practice (SWP) Document Number: CD # UPS-US-SW-GOM-HSE-DOC-00566-2

Document Authority: GoM Ops & OMS Director

Document Custodian: GoM CoW Authority

Scope: GoM H&S Doc Admin: GoM H&S Document Management Administrator

Issue Date: 06/01/08 Issuing Dept: GoM H&S

Revision Date: 09/15/12 Control Tier: 2 GoM H&S

Next Review Date: 03/01/15 OMS Element: 5.4 Inspection & Maintenance 1 Purpose/Scope This Safe Working Practice (SWP) outlines minimum requirements for Purging Operations that are used either to clear in-service lines of hydrocarbons or for commissioning open lines to evacuate the oxygen prior to introducing hydrocarbons. The most appropriate method shall be chosen from those described below. 2 Key Responsibilities

Position Responsibilities

Isolator / Authorized Employee

Prior to purging operations, shall confirm all affected employees understand the potential hazards associated with purging operations (e.g., the use of natural gas and nitrogen as ways of purging hydrocarbon systems).

Verifying that the equipment to be purged has been de-pressurized and brought to a zero energy state before commencement of work.

Issuing Authority

Confirming the Isolator/Authorized Employee is aware of and understands potential hazards associated with purging operations.

Confirming all equipment and energy sources are properly locked and tagged out prior to starting work.

3 General Requirements Prior to purging operations, all affected employees shall understand the potential hazards associated with purging operations. Some examples are: The use of natural gas to purge systems of oxygen, The use of nitrogen or an inert gas to purge hydrocarbon gas, Pressure of the system being purged, Contaminated fluids being flushed through the system, and Potential for 100% LEL during the purging process.

4 Procedures 4.1 Methods of Purging

Control Tier: 2-GoM H&S Revision Date: 09/15/12 Document Number: UPS-US-SW-GoM-HSE-DOC-00566-2 Print Date: 2/20/2013

PAPER COPIES ARE UNCONTROLLED. THIS COPY VALID ONLY AT THE TIME OF PRINTING.


4.1.1 Pressurization Purging (Blowdown) Pressurization purging (blowdown) is the process of introducing a purge gas to increase the system pressure to 15 psig, followed by depressurization to the atmosphere. The pressurization purging method is considered superior to other purge methods. This procedure empties the vessel and purges the vapor space in the same process (e.g., pressure vessels, manifolds, and entire plant or platform process systems). A minimum of four pressure-depressure cycles at 15 psig will be required to purge a vessel at atmospheric pressure. If a system filled with a combustible liquid needs to be emptied and then purged, a purge gas may be applied to the vapor space at a pressure consistent with equipment design limitations.

4.1.2 Displacement Purging (Slug) Displacement purging (slug) is the process of introducing an inert purge gas until a buffer or separation zone is established, followed by the introduction of a chase gas of a different type. The slug shall be of sufficient volume to prevent the original gas and chase gas from mixing. Slow flow rates are used for the inert gas slug and the first portion of the chase gas. The flow rate can then be increased for the remaining chase gas if laminar flow is maintained to avoid mixing (e.g., pipelines). In-service hydrocarbon gas may be purged by an inert gas slug followed by compressed air. A line that has been opened to air can be purged with an inert gas slug that is followed by natural gas.

4.1.3 Dilution Purging (Sweep-Through) Dilution purging (sweep-through) is the process of introducing a purge gas at one opening and letting the system contents escape through another opening at the opposite end of the system. This is the least economical purging method. Consider siphon purging instead of dilution purging for thin-walled vessels and tanks or pressurization purging for pressure vessels (e.g., produced water tanks).

4.1.4 Siphon Purging Siphon purging is the process of filling the system with a liquid and introducing a purge gas into the vapor space to replace the liquid as it is drained from the system. The volume of purge gas required will be equal to the volume of the system, and the rate of application can be made to correspond to the rate of draining, plus any effects from temperature change (e.g., pressure vessels and tanks).

4.1.5 Water Flush Water flush is primarily used to clean in-service lines prior to Hot Work or system entry. This involves filling and flushing the system with water to remove residual hydrocarbon from the pipe, vessel, or process component walls. Soap may be added as an agent to enhance the removal of the residual hydrocarbon.

4.2 Restoring Service to Equipment after Purging After work is completed, the work area shall be inspected by the Isolator/Authorized Employee to confirm that the work area is cleaned of liquid hydrocarbons.

4.3 Precautionary Considerations Purging of piping open to the atmosphere should continue until the residual air and purge gas has an oxygen level below five percent when using an inert purge gas. When using a hydrocarbon sweep-through purge, the atmosphere shall have an oxygen concentration below one percent.




NOTE: When testing the atmosphere, confirm that all personnel understand the potential for an oxygen deficient atmosphere and other LEL issues when purging with hydrocarbon gas.

Natural gas shall be vented away from fired vessels, compressors, electric motors, etc. Vent gas to the low-pressure vent system or flare to prevent a build-up near equipment or personnel. Purging to a flare requires the use of inert gas to prevent oxygen/hydrocarbon mixtures developing inside the flare header.

NOTE: Inert gas is an asphyxiant. Prior to entry, refer to the Confined Space Entry program for details.

LEL monitors are limited in their ability to accurately read levels of combustible gas above the Upper Explosive Limit (UEL) or in low oxygen levels. Verify that low or zero readings are not being caused by the absence of oxygen or by high combustible gas levels. Channeling can occur in large enclosures. Similar situations can occur in pipelines with elevation changes. When channeling occurs, the purge gas may not sweep the entire vessel or piping system. In order to confirm that channeling is not occurring, sample the effluent at more than one vent point. Residual liquids, rust, scale, or coatings on vessel walls can continuously release vapors into the enclosure. A single test of the atmosphere may indicate a full purge was accomplished, while later readings may indicate high levels. For this reason, continuous sampling shall be conducted at several sampling points. When hydrocarbon gas is used to purge air, always limit the pressure to 30 psig or 1/10 Maximum Vessel Design Pressure, whichever is less. An internal ignition can generate pressures up to 10 times the pre-ignition pressure. When returning equipment to service after purging, pressure staging shall be done gradually, in 30 psi increments, until 200 psi is reached. Thereafter, staging can be done in 100 psi increments until working pressure is reached. Open piping may be purged with hydrocarbon gas when: A safe vent point is available The system being purged has minimal dead zones Compressors downstream are thoroughly purged or were not contaminated with air When using steam as a purging agent, monitor system pressure frequently in order to avoid creating vacuums when the steam condenses. 5 Definitions

Term Definition

Affected Employees

Individual who operates or uses a machine or equipment on which servicing or maintenance is being performed under lockout or tagout conditions or whose job requires them to work in an area in which such servicing is being performed.

Isolator/Authorized Employee

Individual who is authorized to lockout or tagout machines or equipment in order to perform servicing or maintenance on those machines or equipment.

Lower Explosive Limit (LEL)

Minimum concentration of a particular combustible gas or vapor necessary to support its combustion in air.

Lockout The placement of a locking device on an energy isolating device confirming that the energy isolating device and the equipment being controlled cannot be operated until the lockout device is removed.

Upper Explosive Limit (UEL)

Maximum concentration of a particular combustible gas or vapor necessary to support its combustion in air.






Revision Log Revision Date Authority Custodian Revision Details

09/15/12 GoM Ops & OMS

Director GoM CoW Authority

Updated pressure increments language in Section 4.3. Changed Document Authority and Custodian.

06/15/12 GoM H&S Director

GoM Safety Programs Lead

Reformatted document to meet new GoM document control template standardization guidelines.

04/07/12 GoM H&S Director

GoM Safety Programs Lead

Updated pressurizing increments.

06/01/08 GoM HSSE

Director

GoM HSSE Programs Manager

Formatting and minor edits for clarification Key responsibilities revised to be consistent with C0W terminology and roles and responsibilities.

3 General Requirements4.1.1 Pressurization Purging (Blowdown)4.1.2 Displacement Purging (Slug)4.1.3 Dilution Purging (Sweep-Through)4.1.4 Siphon Purging4.1.5 Water Flush4.2 Restoring Service to Equipment after Purging4.3 Precautionary Considerations

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Pipe and Vessel Purging