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Plugged Lines

July 29, 2014

Plant Design and Operations

The material in this post is extracted from Chapter 3 of the book Plant Design and Operations.

Process pipe partially restricted.

Process pipe partially restricted

Process lines, piping and valves frequently become plugged. Various techniques for avoiding the formation of pluggage and for removing pluggages safely are discussed in this section.

If line pluggage is a recurring problem it is best to try and identify ways in which the problem can be prevented from occurring. If that solution is not possible then valves, drains, tees and connections should be designed so that it is possible to remove the pluggage safely and with minimum time and expense.

Prevention of Pluggage

It is obviously much better to prevent pluggage from occurring — all of the techniques for unplugging lines pose safety risks, and the time for which the line is out of service will normally lead to production losses. The following techniques can help prevent pluggage.

  • Install filters and strainers, and ensure that they are replaced or cleaned on a regular basis.
  • Install low point drains to remove water, thereby reducing the chance of hydrate formation. The water should be drained on a regular basis. Also, use antifreeze agents, such as methanol or ethylene glycol, to prevent hydrate formation.
  • Drain and flush pipe branches that have no normal flow with an inert, non-corrosive liquid on a regular basis.
  • Maintain adequate flow in lines that contain materials that will solidify at low temperatures. These lines should also be insulated, and heat traced as appropriate.
  • Maintain adequate flow/minimum velocity in lines that contain materials that could settle out.
  • Provide and maintain bleed gas or flushing oil flows to instrument taps that are prone to plugging.
  • Piping or hoses used with compressed gas should be properly anchored.
  • Consider installing a relief valve downstream of the pressure regulator to protect the equipment in case the regulator fails.
  • Where possible, any hazardous or reactive material that may be in the plugged equipment should be removed before the pressure is increased.
  • Vent all gas pockets since they will contain considerable energy when under pressure.
  • Do not leave open drain valves or open-ended piping unattended at any time during unplugging operations.
  • Provide a place to bleed off pressure upstream of the obstruction, particularly if the obstruction is not cleared.
  • Provide containment for the system contents once the line becomes unplugged.
  • Identify locations to which the plug could travel and take precautions to protect personnel and equipment in those areas.
  • Flush idle lines to remove materials that could clog piping.
  • Install adequately sized filters, strainers, and knock-out drums.  Change out, flush, or drain these devices as needed.
  • Water in a hydrocarbon stream can cause plugging by increasing corrosion rates, forming hydrates, or freezing.  Install and use low point drains.  Draw water regularly from vessels and tanks.  Automatic draws may be appropriate for some problem areas.
  • Water washes can be used to remove salts that will cause plugging.
  • Pipe branches that frequently experience no flow are highly susceptible to plugging.  Employ a self-draining design to avoid the collection of water and/or sediment.  When practical, locate valves in horizontal sections of piping to avoid the accumulation of water and/or sediment above the valves.  The use of “ram type” drain valves have been successful in keeping drain valves clear of ice and debris.
  • Maintain adequate flow in (or insulation and heat tracing around) lines that contain materials that will solidify at low temperatures.
  • Maintain adequate flow/minimum velocity in lines that contain materials that could settle out, e.g., catalyst in decanted oil.
  • Provide and maintain bleed gas or flushing oil flows to instrument taps that are prone to plugging.

Unplugging a Line

Pluggages can be removed in the following ways:

  • Water wash to remove salts that are causing the problem.
  • Disassemble and replace the affected piping.
  • Hydroblast.
  • Applied heat to melt the obstruction. The heat can come from electric coils, steam tracing, or live steam.
  • Chemicals to dissolve the obstruction. For example, methanol can be used to melt the ice in the hydrates that block subsea lines.
  • Mechanical.
  • Differential pressure.

When unplugging underground lines, a line rupture may give the appearance that the plug is dislodged by a sudden drop in pressure. The clearance should be verified by flowing gas or liquid through the line.

The following general precautions should be observed:

  • All piping and hoses should be properly anchored.
  • Always wear appropriate eye protection when applying pressure to a piece of equipment.
  • Wear appropriate gloves, hoods, and clothing to prevent exposure to any thermal or chemical hazards.
  • Air-supplied respirators are required when using an inert gas in a confined space or when unplugging equipment that could contain a toxic material, such as hydrogen sulfide.
  • Identify locations to which the plug could travel and take precautions to protect personnel and equipment in those areas.
  • Provide a place to bleed off pressure upstream of the obstruction. Release of pressure should be verified before disconnecting the piping, hose, etc.
  • Reduce the likelihood of utility system contamination by providing a check valve whenever a utility stream is used to unplug process equipment.
  • Temporary connections should be disconnected when the unplugging work is complete, even if they may be needed again at a future date.
  • Precautions should be exercised when:
  • Rapidly pressurizing/depressurizing to bump free the obstruction.
  • Applying pressure to the opposite side of the obstruction.
  • Striking the plugged equipment to jolt the obstruction loose.
  • Any piping or hoses used with compressed gas should be properly anchored.
  • When unplugging underground lines, a line rupture may give the appearance that the plug is dislodged by a sudden drop in pressure. The opening should be verified by flow in and out of the line.
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