Menu

Piping and Vessels Preparation for Commissioning - Part 5


Continue of Part 4 - Chemicals Loading of Vessels

5. Inert gas purging 

5.1 General

Inert gas purging is most frequently required to remove air from petroleum processing equipment in order to avoid the risk of forming an explosive mixture of oxygen and gas.

In some cases inert gas purging is employed to avoid the ingress of air into equipment containing sensitive catalysts, or wet air into equipment which must be kept as dry as possible, such as refrigeration units.

Air contains 21% oxygen (O2) and therefore when mixed with a petroleum gas all the essential ingredients of an explosive mixture are provided.

The preparation and execution of the inerting of all oil and gas process equipment prior to first oil or gas is a Commissioning responsibility.

Commissioning should especially select the best adapted inerting method and issue detailed procedures with support drawings and sketches. 

5.2 Types of inert gas  

There are several gases which are considered as "inert" but for all practical purposes only three are discussed herein. They are as follows:

5.2.1 Nitrogen (N2

This is usually supplied as a gas in high pressure bottles but may also be obtained in liquid form for subsequent vaporization and compression. Nitrogen can also be supplied by molecular sieves and cryogenic units (i.e. "Air Liquid" type). Impurities consisting of traces of other gases such as oxygen (O2) and carbon dioxide (CO2) may remain in the nitrogen in concentrations ranging from 1000 ppm to 1% depending on the manufacturing process.

High purity N2 is obtainable upon request but is hardly ever used in the petroleum industry except as a "standard" calibration gas for laboratory use.

5.2.2 "Inert gas" (N2/CO2 mixture) 

This is generally produced in combustion-type units which may be portable or permanent installations.

Typical inert gas analysis would be N2 + CO2 in proportions of 88% N2 and 12% CO2.

This gas normally contains minor traces of carbon monoxide (CO) and hydrogen (H2), O2 content must be 0.5%.

5.2.3 Steam 

Steam is commonly used in the petroleum industry for purging air from lines and vessels.

Steam has the advantage of being easy to produce with relatively simple equipment and may often be logistically cheaper than inert gas generators or other sources of inert gas. Temporary steam producing units with various output rates are readily available.

5.3 Inert gas purging operations 

5.3.1 Methods 

Four different methods can be considered:

  • Continuous flow or "sweeping"
  • Pressurization/depressurization
  • Water displacement
  • Steam-out.

5.3.2 Continuous flow or "sweeping" 

This method should generally be used for small, low-volume systems such as piping systems up to 4" line diameter. The principle involved is injecting sufficient quantities of inert gas to create a turbulent flow in the piping which pushes the air ahead of it thus displacing it.

When sweeping piping with inert gas (subsequently, inert gas should be the term employed to describe either nitrogen, or inert gas manufactured on site unless otherwise stated) a high flow rate is required and has to be turbulent. Laminar flow is to be avoided since it should result in poor purging and possible hazard.

The Reynolds number has to be higher than 4000 to create the turbulent flow required. As a general guideline, a velocity of 10 feet per second is an acceptable mean value.

It may be deducted from this that continuous sweeping of large diameter piping is unsuitable due to the very high flow required to produce the ideal velocity.

When considering inert gas purging, the system in question must be studied from a practical viewpoint. Most installations are unable to furnish very large flows of inert gas, due to unit design, and there is little point in concerning oneself with complicated velocity calculations if the equipment to provide sufficient inert gas does not exist.

Proceed as follows:

  • Assuming the system to be at atmospheric pressure, close all unnecessary vents and drains and install plugs and caps except on vents and drains which are to be used for O2
  • Ensure the system to be purged is isolated from associated systems, either by valves or blinds. If blinds are to be installed complete a system blind list.
  • Select an inert gas injection point or points and a vent point at the farthest end of the system. If the inert gas arrives via utility hose, ensure the hose is in good condition and that the injection point valve is open before opening the inert gas header valve. Sufficient injection points should be used to achieve the maximum flow available.
  • Open injection point valves and begin the purge. Ideally the atmosphere in the piping should be analysed when the inert gas volume used is approximately three times the system volume. Before testing simultaneously close inert gas injection valves and vent valves to maintain the system under positive inert gas pressure.
  • Sample for O2 at several points, especially on complicated piping systems, i.e., fuel gas headers. Check for dead ends.
  • When O2 analyses are satisfactorily, stop the purge whilst maintaining system pressure at approximately 0.1 to 0.2 bar or as required. A suitably-calibrated pressure gauge must be installed where easily visible and used to control the system pressure which may require "topping-up" from time to time.

Important notes:

  • This method must not be used for critical equipment requiring very low O2. It is normally suitable when criteria are not severe, i.e. O2 maximum content 2%.
  • A secondary problem with this method is that it may prove to be somewhat wasteful of inert gas and where logistical problems arise the following method may be preferable, even for relatively small system.

5.3.3 Pressurization/Depressurization method 

This is the preferred method of inert gas or nitrogen purging for reasons of economy and safety and is used for both vessels and associated piping:

  • Assuming the system to be at atmospheric pressure, close all unnecessary vents and drains and install plugs and caps except on vents and drains which are to be used for O2 sampling and venting.
  • Ensure the system to be purged is isolated from associated systems, either by valves or blinds. If blinds are to be installed complete a system blind list.
  • After ensuring that the inert gas supply is available, check the integrity of pressurizing hose (if used) and system pressure gauges. Gauges should be situated so as to be easily visible from the injection point valves.
  • Pressurize the system to between 1 and 2 barg (15 and 30 psig) depending upon the system design pressure. Never exceed system design pressure even if safety valves are in service.
  • Let the system stand for approximately 30 minutes to allow the gases to become homogenous. Following this 30 minute period, depressure the system as rapidly as possible via the largest diameter vent or vents on the system.
  • Repeat this operation but stop depressuring at about 0.1 to 0.2 bar in order to take an O2 analysis sample. The O2 at this point should, theoretically, be in the region of 4 to 5%. Samples must be taken from several points, depending upon the magnitude of the system, and averaged.
  • According to the O2 content requirement, repeat the pressurization/depressurization operation until the specification is met.
  • When O2 analyses meet the ongoing specification, proceed to maintain the system under a positive inert gas pressure.

Notes:

  • It is better to pressurize/depressurize 5 times up to 1 or 2 barg each time, than to pressurize/depressurize once up to 5 barg. In the first case the theoretical final oxygen content is 2= 32 times less than the original one. In the second case the theoretical final oxygen content is only 5 + 1 = 6 times less.
  • Purging can be speeded up by pulling a vacuum on the system before pressurizing. However, attention must be paid to the equipment design before this operation may be considered.
  • Under no circumstances should a vacuum be applied indiscriminably to equipment without first checking that the equipment is capable of withstanding vacuum conditions.

5.3.4 Water displacement purging 

This in an extremely simple and economical method of purging but is only applicable where the equipment to be purged is capable of supporting its own weight when water-filled and when the presence of residual free water should not consequently affect the plant operation or cause corrosion problems.

In certain applications inhibited water may be required, i.e. the system contains stainless steel equipment, etc. This should be taken into account when deciding which should be the safest, most economic, and rapid method of purging to be employed.

Special procedures to suit particular cases should be issued but the general procedure is outlined as follows: 

  • The system is assumed to be under atmospheric pressure and isolated by valves or blinds from all other associated systems. If blinds are to be installed, a system blind list must be completed. All system drains must be closed and the system high point vents must be opened. Ensure adequate vents are available to ventilate the system whilst filling with water so as to avoid overpressuring. Adequate venting also ensures pockets of air not trapped.
  • Connect the water supply system to the system to be purged and commence filling. Do not fill too rapidly. When the system is full, check any dead end points for trapped air. Close vents.
  • Connect the inert gas supply to the highest point or points on the system. Check that the unit drain system can dispose of the volume of water to be dumped. Begin inert gas injection whilst simultaneously opening one low point drain.
Warning 

The flow of inert gas into the system and the flow of water out of the system must be regulated so that the system is always under a positive pressure. If care is not taken to do so, there is a high risk of pulling both the system and the inert gas supply system under a vacuum and equipment damage may subsequently result.

This should also prevent air from being accidentally drawn into the system.

  • Gradually increase the inert gas injection rate and draining rate. When water draining ceases and inert gas begins to issue from the drains, close all drains and allow the system pressure to build up to at least 0.2 to 0.3 barg then stop nitrogen injection.
  • Analyse the system atmosphere for O2 content. Normally this should be very low if the procedure has been correctly followed.

5.3.5 Steam-out

The use of steam for purging of petroleum processing equipment is widely accepted due to its overall economy and ease of operation.

In some processing installations certain vessels and systems may be provided with permanent steam-out lines and this greatly assists in purging operations.

Petroleum processing installations do not always have steam raising facilities and in this case temporary steam generators should be used. Notwithstanding this, the general procedure is identical. No specific procedure is given herein but the following points should be noted by the commissioning team if steam is to be used for purging:  

  • Steam is obviously unsuitable for purging equipment which must remain "dry".
  • Following steam-out the system is normally "gassed up" by backing in process gas a fuel gas. Inert gas could equally be used. In all cases it is most important to ensure an adequate supply of gas is available, otherwise, during cooling, the system may be pulled under a vacuum and damage to the equipment may result.
  • Steam injection lines should preferably be in suitably-rated screwed piping. If flexible hoses are used, ensure they are adequately armored and passed for steam service. Air and water hoses must not be used.
  • Always ensure the system equipment is capable of withstanding the steam temperature.

5.4 Inerting criterion

The content to be achieved depends upon the type of installation.

It should therefore be determined on a case by case basis.

For traditional oil and gas processing plants, the maximum acceptable O2 content should be no more than 2%.

5.5 Safety aspects

Inert gas is dangerous. It should always be remembered that an atmosphere containing less than 21% oxygen does not support life. "Entry Permit" procedure is applicable should vessel entry be necessary.

Always carry out a careful scrutiny of flexible hoses used in purging operations.

If an inert gas distribution system is used, it is normal practice to turn closed any spectacle blinds following use of the injection point. Failure to do so may result in petroleum products entering the inert gas distribution system.

Steam is potentially very dangerous and can cause severe burns.

About two thirds of the total heat in steam is taken up in the change of state from boiling water to steam, and hence when steam condenses, two thirds of its total heat is given up to the body on which it condenses. This is what makes steam such an effective heating fluid, and this is also what makes it very dangerous if it condenses on the human body. Hence to get a part of the body in a jet of steam is many times worse than in a corresponding flow of hot air at the same temperature.

The following precautions should be observed:

  • When blowing steam from drains or vents makes sure that everyone is clear of the area. Also a minimum of personnel should be present on site during the operation.
  • Ensure piping insulation is as complete as possible before introducing steam.
  • Personnel must always be adequately clothed when working in the proximity of steam lines. Shorts and tee-shirts should be forbidden.

5.6 Reports

The inert gas purging procedures, with support drawings and sketches, all reports and results should be filed in the corresponding subsystem Commissioning Dossier.

Each purging operation should also be supported by a specific Commissioning Inerting report form.


Continue to Part 6:  Subsea Piping and Vessels Preparations



Tags: Piping Vessel Commission