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Safety Moment #13: Common Process Safety Hazards (Part 2)

Safety Moment common hazards

Safety Moments

We regularly publish Safety Moments for use in the process and energy industries. (Please visit our page Safety Moments for the Process and Energy Industries to learn more about the philosophy behind them.)

This particular Safety Moment is one in a series to do with “Common Hazards”. It can be found at Safety Moment #13: Common Process Safety Hazards (Part 2). It is taken from the 2nd edition of the book Plant Design and Operations.

Common Process Safety Hazards

One of the philosophies that lies behind Process Safety Management (PSM) is that each chemical process is unique. Therefore it is not possible to have a prescriptive standard that tells operating companies what to do. Instead, companies have to identify the hazards associated with their facility, and then implement corrective actions based on a risk-ranking methodology. For this reason, facilities covered by PSM standards have to conduct a series of Process Hazards Analyses (PHAs), often using the Hazard and Operability (HAZOP) methodology.

Yet many hazards, particularly to do with utilities, are really not all that different from one facility to another. Therefore, in order to save time during the PHA and also to improve the quality of the analysis, it is useful to list and evaluate some of these common hazards before the PHA meetings start. Our first post in this series was Safety Moment #10: Common Process Safety Hazards (Part 1) in which we discussed hazards associated with utilities, including common cause failures, process contamination, electrical power failure and the use of nitrogen.

We continue the discussion in this post with some additional thoughts to do with the hazards of utilities.

Reverse Flow to a Utility Header

The hazards associated with (unexpected) reverse flow from the process into a utility header can be very serious. This scenario is illustrated in the sketch, which shows two lines. The top line is a utility such as nitrogen, steam, or service air. The lower line shows a process stream containing a hazardous chemical. In normal operation, the utility, which is at a higher pressure than the process, flows into the process through a check valve (with block valves on either side of it).

Reverse flow of hazardous chemicals to a utility header

The hazard scenario is as follows:

  • The pressure in the utility header falls due to an operating upset so that its pressure is lower than the pressure in the process line.
  • The check valve fails to fully close.
  • Process chemicals flow into the utility header from the process line.
    Process chemicals are then distributed to many other locations in the facility via the utility header.

To make matters worse, this scenario is ‘memory-less’, i.e., once the pressures revert to normal there is no indication as to what happened. Identification of the source of the contamination can be particularly difficult if the process chemical that has entered the utility header is used in many parts of the overall process.

Survivability of Utilities

Many utilities must survive a catastrophic event such as a major explosion or fire so that emergency response systems continue to function. For example, during a fire it is often important to keep cooling water flowing (to cool process operations), to maintain electrical power for critical pumps and compressors, and keep the steam header pressure up — once more to keep critical turbines running. Yet such an incident can destroy critical utilities header containing electrical cables, cooling water lines, and steam pipes. This is a common cause effect that can trigger domino events that make the original problem much worse. The effect is magnified if emergency systems such as the firewater header are also damaged.

Survivability of Utilities

Process safety utilitiesMany utilities must survive a catastrophic event such as a major explosion or fire so that emergency response systems continue to function. For example, during a fire it is often important to keep cooling water flowing (to cool process operations), to maintain electrical power for critical pumps and compressors, and keep the steam header pressure up — once more to keep critical turbines running. Yet such an incident can destroy critical utilities header containing electrical cables, cooling water lines, and steam pipes. This is a common cause effect that can trigger domino events that make the original problem much worse.

Survivability of Emergency Systems

Common cause effect process hazards analysisIf an explosion or large fire occurs it is even more important that the emergency systems, particularly firewater and backup electrical power, remain operable. (For this reason firewater headers are often buried.)

The common cause effect that destroyed both the operating equipment and the backup systems was a major factor in making the Fukushima-Daiichi event so serious, as discussed in Safety Moment #10.

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Safety Moment #10: Common Process Safety Hazards (Part 1)

Common Process Safety hazards

Introduction

One of the philosophies behind Process Safety Management (PSM) is that each chemical process is unique. Therefore it is not possible to have a prescriptive standard that tells operating companies what to do. Instead, companies have to identify the unique hazards associated with their facility, and then implement corrective actions based on a risk-ranking methodology. For this reason, facilities covered by PSM standards have to conduct a series of Process Hazards Analyses (PHAs), often using the Hazard and Operability (HAZOP) methodology.

Yet many hazards, particularly to do with utilities, piping, valves and hoses, are really not all that different from one facility to another. Therefore, in order to save time during the PHA and also to improve the quality of the analysis, it is useful to list and evaluate some of these common hazards before the PHA meetings start.

This Safety Moment is the first in a series that describes some of these common hazards. The topic is “Hazards of Utilities”. The discussion includes:

  • Common cause effects;
  • Electrical power failure;
  • Chemical contamination; and
  • Nitrogen.

Future safety moments will consider additional issues to do with the hazards of utilities.

Citation

The material for this set of safety moments is taken from the 2nd edition of the book Design and Operations.

Storage Tanks in the Process Industries

Tank-8
Storage tanks are widely used in the process industries to store liquids that are below their boiling point at atmospheric pressure and temperatures (some tanks may be insulated and they may have heating or cooling coils to adjust the temperature of the liquid that they are storing). Typically, the tank is either open to the atmosphere or to a system such as a flare or vent header that is at atmospheric pressure (this does not apply to floating roof tanks). Unlike pressure vessels, storage tanks cannot handle either high pressure or vacuum conditions.

We offer a video package to do with the design and operation of storage tanks. The package, which is priced at $31.50, includes:

  • A video (also available as a free YouTube download);
  • An ebook;
  • A set of checklist questions; and
  • A test

Further information to do with tanks is available here.

Link

Plant Design and Operations: Welcome

Plant Design and Operations

Please visit the site www.iansutton.com for the latest information on Plant Design and Operations.

Welcome to our blog Plant Design and Operations. This blog is part of the The PSM Report group. The blogs are:

The posts here are based on the book Plant Design and Operations, published by Elsevier. Details to do with the book are provided here; the Table of Contents (.pdf format) is here; and purchasing information is here.

The posts that we have published so far are shown below, organized by chapters of the book.

Chapter 1 — Operations

Chapter 2 — Maintenance and Inspection

Chapter 3 — Energy Control Procedures

Chapter 4 — Occupational Safety

Chapter 5 — Chemicals

Chapter 6 — Personal Protective Equipment

Chapter 7 — Health and Industrial Hygiene

Chapter 8 — Human Factors and Ergonomics

Chapter 9 — Firefighting

Chapter 10 — Safety in Design

Chapter 11 — Siting and Layout

Chapter 12 — Equipment

Chapter 13 — Piping and Valves

Chapter 14 — Safety Instrumentation

Chapter 15 — Transportation

Chapter 16 — Security

Chapter 17 — Common Hazards

Blinds in Process Piping

Plant Design and Operations

The material in this post, which describes the use of blinds in process piping systems is extracted from Chapter 13 of the book Plant Design and Operations

Blinds

Blinds—also known as blanks or spades—provide positive isolation between sections of a process. The following guidance is provided to do with the location, installation, and use of blinds.

  • At battery limits in all process, utility, relief, and blowdown lines;
  • As required for inspection, maintenance, testing, or alternative operation of
    equipment, such as vessels, heaters, rotating equipment, or exchangers;
  • Where segregation of fluids is required;
  • Blinds should be installed in horizontal lines where possible. Doing so makes
    handling and installation easier and reduces the chances of damaging the
    gaskets during installation. Also, blinds in vertical lines may trap liquid above
    them. For this reason, blinds should not be used in vertical water or steam
    lines where there is a potential for freezing;
  • Blinds for rotating equipment and the tube side of shell-and-tube heat exchangers should not be located at the equipment flanges;
  • Piping at locations where blinds and their associated spool pieces should be arranged so as to permit the removal of the bolting for the blinds, and to allow space for swinging the blind once it is unbolted;
  • Supports to maintain piping alignment when blinds are being installed or removed are required if the piping or other items, such as valves, are located at or near the blind location;
  • When selecting a location for a blind to be inserted it is important to make sure that sufficient space is available not only to insert the blind but also for the equipment needed to lift the blind into place;
  • When it is expected that a blind will be inserted and removed on a regular basis, platforms should be provided;
  • Permanent handling equipment shall be provided for all blinds weighing more than 45 kg (100 lb).

Most blinds are either of the line or spectacle type.

Line Blinds

A line blind —also known as a spade, paddle, skillet, pancake, or slip  blind — consists of a solid metal disk with a thin length of metal attached to it. The metal of the blind should be rated for full process pressure on one side and atmospheric pressure on the other side. The handle should be long enough so that it can be seen through insulation and any other materials that might be covering the flange.

Spectacle Blinds

If a flange is to be routinely blinded, a spectacle blind (also known as a figure of eight blind, disk and donut, or spec blind) can be installed. This type of blind looks like a pair of spectacles or the number 8, with one section closed and the other open.

When the flange is broken and the line cleared, the blind can be rotated around the one bolt that is left in the flange. It is simple to ensure that the blind has been installed because the open part of the blind will be sticking out from the flange face.

If spectacle blinds in horizontal pipes are insulated, the spectacle blind should point downwards at an angle of 45º to avoid water leaking into the insulation.

Plugged Lines

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.

Isolation Methods

Double-Block-Bleed-1

Double Block and Bleed Valves

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

Positive isolation methods are those which remain effective even if there is equipment failure or operator error. These techniques apply not only to vessels, piping and tanks but also to pneumatic and hydraulic equipment.

Figure 3.2 shows some of the various isolation techniques that can be used to protect workers in the process industries. The process containing toxic or flammable chemicals under pressure is on the left; the open system, where the workers are present, is on the right. The order is from the least to the most secure.

Figure 3.2
Isolation Methods

Isolation-Methods-1

Isolation Methods

Level 1 — Closed Valve

The use of a single closed valve is rarely acceptable as a means of isolation in the process industries — except in the most benign services — for two reasons. First, it is very easy for someone else to inadvertently open the valve. Second, the closed valve may leak.

One company places the following conditions on the use of a single block valve to provide isolation.

  • The isolation block valve closes tight and does not leak;
  • It is locked closed and tagged;
  • The job is continuous and uninterrupted;
  • The work is conducted during daylight hours and;
  • No confined space entry or hot work is involved.

If a valve is to be used for isolation, it should generally be a gate, ball, plug or needle valve. In some instances, butterfly valves are allowed in non-hazardous services. Where actuated valves are used, the actuator mechanism must be isolated from all possible supply sources. Check valves, control valves and relief valves are not acceptable as isolation devices.

Types of valve that can be used for single-valve isolation include: gate, ball, plug, and needle. Butterfly valves may be allowed in non-hazardous services. Valves specified for control or throttling service (choke or control valves) should not be used for isolation. Check valves and relief valves are not acceptable for designing means of isolation. In all cases, the valves chosen must be designed to provide a positive shutoff seal for the inventories and pressures involved.

If actuated valves are used then the actuator mechanism must be isolated from all possible supply sources before work commences, and before the valve can be considered secure.